Chapter 21 Ingestion Toxicity Assessment

Table of Contents

21.1 Introduction ............ ............ ............ ............ ............ ...... 1

21.2 Hazard Identification ............ ............ ............ ............ .......... 2

21.3 Predictive Approach for Cancer Effects ............ ............ ............ ........ 2

21.3.1 Determining the Point of Departure (POD) ............ ............ ........... 2

21.3.2 Deriving the Human Equivalent Dose ............ ............ ............ ... 2

21.3.3 Extrapolating from POD to Derive the Oral Cancer Slope Factor ............ ..... 3

21.4 Dose-response Assessment for Derivation of a Reference Dose ............ ............ . 3

21.5 Sources of Human Health Reference Values for Risk Assessment ............ ........... 4

References ............ ............ ............ ............ ............ ............ . 5

a

See http://www.epa.gov/ttn/atw/toxsource/summary.html for an up-to-date list of dose-response values.

April 2004 Page 21-1

Risk = ƒ (metric of exposure, measure of toxicity)

Toxicity Assessment is a Two-Step Process:

1. Hazard Identification –What types of effects does the

chemical cause? Under what circumstances?

2. Dose-response Assessment – How potent is the chemical

as a carcinogen and/or for noncancer effects?

21.1 Introduction

As described previously in Chapter 12, the purpose of the toxicity assessment is to weigh

available evidence regarding the potential for toxicity in exposed individuals (hazard

identification) and to quantify the

toxicity by deriving an appropriate

dose-response value (doseresponse assessment). Toxicity

assessment is the second part of

the general risk equation. The

toxicity assessment is

accomplished in two steps: 

hazard identification and doseresponse assessment. Although

the toxicity assessment is an integral and important part of the overall air toxics risk assessment,

this is usually accomplished prior to the risk assessment. EPA has completed the toxicity

assessment for all HAPs and has made available the resulting toxicity information and doseresponse values, which have undergone extensive peer review (see Appendix C).(a)

This chapter focuses on toxicity assessment for the ingestion (oral) pathway. Dermal toxicity

assessment is described in detail in several EPA guidance documents.(1) The ingestion pathway

uses the same general types of studies, hazard and dose-response information, and dose-response

methods to assess toxicity as those used for the inhalation pathway (see Chapter 12). The

discussion in this chapter focuses on the unique features of toxicity assessment for the oral

pathway.

Ingestion Dose-Response Values(a)

Oral Cancer Slope Factor (CSF): An upper bound, approximating a 95 percent confidence limit, on

the increased cancer risk from a lifetime exposure to an agent. For ingestion, this estimate is usually

expressed in units of amount of risk per amount of intake and is written as risk per mg/kg-day or

simply (mg/kg-d)-1.

Reference Dose (RfD): An estimate (with uncertainty spanning perhaps an order of magnitude) of a

daily oral exposure to the human population (including sensitive sub-populations) that is likely to be

without an appreciable risk of deleterious effects during a lifetime. Generally used in EPA’s

noncancer health assessments. RfDs are usually given in units of intake per day on a body weight

basis (written as mg/kg-d).

(a)The phrase “dose-response” is used generally here and elsewhere in the document. EPA’s values for ingestion,

however, are related to oral intake rather than dose. Consideration of the relationship between exposure

concentration, dose, and dosimetry (what happens to a chemical in the body once it is ingested) may be

considered, depending on data availability in the derivation of these values.
bNote that the corresponding value for inhalation exposures is the benchmark concentration (BMC).

cA model that estimates the dose to a target tissue or organ by taking into account the rate of absorption into

the body, distribution among target organs and tissues, metabolism, and excretion.

April 2004 Page 21-2

21.2 Hazard Identification

The hazard identification process for the ingestion pathway is identical to that for the inhalation

pathway, although the specific toxic effects of concern and details of the toxicity studies are

derived from feeding a chemical to animals (either in food or drinking water) rather than on

having the animals inhale the chemical. As with inhalation, the hazard identification step

includes consideration of various types of studies (e.g., feeding, in vitro, etc.) and the resulting

weight of evidence with regard to potential for carcinogenicity and identification of critical

effects. See Part II, Chapter 12, for information on the hazard identification step.

21.3 Predictive Approach for Cancer Effects

The approach to dose-response assessment for cancer effects is identical to that for the inhalation

pathway discussed in Chapter 12, including:

• Determination of the point of departure (POD);

• Duration adjustment of the POD to a continuous exposure;

• Extrapolation of an animal study POD into its corresponding Human Equivalent Dose

(PODHED); and

• Low-dose extrapolation from the PODHED to lower doses for the purposes of deriving the

oral cancer risk estimate.

As with inhalation, the first three steps are also performed in the derivation of reference values

for ingestion, such as the oral RfD. In addition to the steps shown above, the derivation of RfDs

are followed by the application of uncertainty factors (see Section 21.4). Additionally, the use of

tools such as pharmacokinetic modeling, which go beyond these default approaches, may

facilitate the accomplishment of several of these steps.

21.3.1 Determining the Point of Departure (POD)

The process for determining the POD for ingestion exposures is identical to that for inhalation

exposures. The POD may be the no-observed-adverse-effect level (NOAEL) or lowest-observedadverse-effect level (LOAEL), or it may be a benchmark dose (BMD) for noncancer effects.(b)

21.3.2 Deriving the Human Equivalent Dose

The optimal approach for extrapolating from an animal study to a human dose-response

relationship is to use Physiologically Based Pharmacokinetic (PBPK)(c) modeling. When such a

model us used, the duration adjustment step is incorporated into that model. Otherwise, any

duration adjustment, if necessary (e.g., when the exposure is not via daily feed), would be

accomplished by deriving an average daily dose for the exposure period (e.g., two years in an

animal cancer bioassay).
dAt the time of publication, an Agency activity is underway to “harmonize” the cancer assessment and RfD

development methods with regard to the method employed for interspecies scaling, which may result in the use of

body weight scaling in the development of the RfD.

April 2004 Page 21-3

For purposes of cancer assessment, an animal to human body weight-based scaling factor is

applied to the oral study POD (duration-adjusted if applicable) to extrapolate to a human

equivalent oral exposure.(2) The default scaling factor is based on the body mass raised to the 3/4

power of the test animals relative to humans. This step stems from the consideration of various

studies of the species differences in toxicity of certain compounds, including data collected on

chemotherapeutic agents.(3) These data served as the principal basis for the use of a body surface

area or metabolic rate scaling as the default method in cancer risk assessments. Empirically, the

best estimate of surface area scaling is BW2/3 and for metabolic rate scaling is BW3/4.

(4) These

findings reflect general expectations of more rapid distribution, clearance, and metabolism by

smaller animals.

In the case of the RfD, a scaling factor is not currently applied. Instead, the interspecies

uncertainty factor is intended to account for potential differences in sensitivity of humans

compared to the test animal, including this consideration.(d)

A PBPK model can accommodate adjustments for metabolic rate as well as other species-related

dosimetric variables such as liver perfusion rates. The model therefore provides a more accurate

estimate of steady-state target site concentrations than use of default methods. EPA’s preferred

approach for calculating a HED for oral exposures is to use a chemical-specific PBPK model

parameterized for the animal species and body regions (e.g., of the gastrointestinal tract) involved

in the toxicity.

21.3.3 Extrapolating from POD to Derive the Oral Cancer Slope Factor

As with inhalation, extrapolation from the PODHED to lower doses is usually necessary and, in the

absence of a data set rich enough to support a biologically based model (e.g., a PBPK model), is

conducted using linear extrapolation or a nonlinear extrapolation using a Reference Dose

approach. 

The Cancer Slope Factor (CSF) for oral exposures is derived in a similar way as the unit risk

estimate for inhalation (URE) (see Chapter 12). The CSF is derived using the upper bound

estimate of risk. In other words, the true risk to humans, while not identifiable, is not likely to

exceed the upper-bound estimate (the CSF). The CSF is presented as the risk of cancer per mg of

intake of the substance per kg body weight per day ([mg/kg-day](-1)).

21.4 Dose-response Assessment for Derivation of a Reference Dose

The oral reference dose is expressed as a chronic dietary intake level (in units of mg of the

substance per kilogram body weight per day, or mg/kg-day) for the human population (including

sensitive sub-populations) that is likely to be without an appreciable risk of deleterious effects

during a lifetime. In other words, exposures at or below the RfD will probably not cause adverse

health effects, even to sensitive sub-populations. While the RfD is routinely employed for
April 2004 Page 21-4

noncancer effects, it may be inclusive of cancer for those pollutants for which a nonlinear (e.g.,

threshold) mode of action has been demonstrated consistent with the Cancer Guidelines.

As with the derivation of an inhalation reference concentration, the reference dose is derived by

dividing the POD by one or more uncertainty factors (UFs). EPA includes with each RfD a

statement of high, medium, or low confidence based on the completeness of the database for that

substance. High confidence RfDs are considered less likely to change substantially with the

collection of additional information, while low confidence RfDs may be especially vulnerable to

change.(5)

The UFs are applied to account for recognized uncertainties in the extrapolations from the

experimental data conditions to an estimate appropriate to the assumed human scenario. As with

the derivation of RfCs, a UF of 10, 3, or 1 is applied for each of the following extrapolations:

• Animal to human;

• Human to exposed sensitive human populations;

• Subchronic to chronic;

• LOAEL to NOAEL; and

• Incomplete to complete database.

The UFs are generally an order of magnitude (10), although consideration of available

information on the chemical may result in the use of reduced UFs for RfDs (3 or 1). It is noted

that as there is currently no default dosimetric adjustment for the oral route. The uncertainty

factor for extrapolation from animal to human data is usually the full 10, as compared to the

reduced factor of 3, routinely used for RfCs which employs an interspecies dosimetric

adjustment. Additional discussion on the application of uncertainty factors is provided in Section

12.4.3.

21.5 Sources of Human Health Reference Values for Risk Assessment

Appendix C provides a current listing of chronic oral dose-response values (i.e., RfDs and CSFs)

for HAPs. Chapter 12 describes additional sources of human health reference values for risk

assessment for the ingestion route.
April 2004 Page 21-5

1. U.S. Environmental Protection Agency. 2001. Risk Assessment Guidance for Superfund

Volume I: Human Health Evaluation Manual (Part E, Supplemental Guidance for Dermal

Risk Assessment) Interim Review Draft - For Public Comment, Office of Emergency and

Remedial Response, Washington, D.C. EPA/540/R/99/005, available at:

http://www.epa.gov/superfund/programs/risk/ragse/index.htm.

U.S. Environmental Protection Agency. 1992. Dermal Exposure Assessment: Principles and

Applications. Office of Health and Environmental Assessment. EPA/600/6-88/005C.

2. U.S. Environmental Protection Agency. 1999. Guidelines for Carcinogen Risk Assessment.

Review Draft. Risk Assessment Forum, Washington, DC. NCEA-F-0644.

U.S. Environmental Protection Agency. 1986. Guidelines for Carcinogen Risk Assessment. 

Federal Register 51(185):33992-43003.

U.S. Environmental Protection Agency. 2003. Draft Final Guidelines for Carcinogen Risk

Assessment (External Review Draft), Risk Assessment Forum, Washington, DC,

NCEA-F-0644A.

These documents are available at http://cfpub.epa.gov/ncea/raf/rafguid.htm.

3. Freireich, E.J., Gehan, E.A., Rall, D.P., et al. 1966. “Quantitative comparison of toxicity

anticancer agents in mouse, rat, hamster, dog, monkey, and man.” Cancer Chemother Report

50:210:244.

4. U.S. Environmental Protection Agency. 1992. Request for Comments on Draft Report of

Cross-species Scaling Factor for Cancer Risk Assessment. Federal Register 57:24152.

5. U.S. Environmental Protection Agency. 2002. A Review of the Reference Dose and

Reference Concentration Process. Risk Assessment Forum, Washington, DC. 

EPA/630/P-02/002F. Available at:

http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=55365.

References
Chapter 22 Multipathway Risk Characterization

Table of Contents

22.1 Introduction ............ ............ ............ ............ ............ ...... 1

22.2 Cancer Risk Estimates ............ ............ ............ ............ ......... 2

22.2.1 Characterizing Individual Pollutant Ingestion Risk - Scenario Approach ............ 2

22.2.2 Characterizing Risk from Exposure to Multiple Pollutants - Scenario Approach ...... 3

22.2.3 Combining Risk Estimates across Multiple Ingestion Pathways - Scenario Approach . . 4

22.2.4 Evaluating Risk Estimates from Inhalation and Ingestion Exposures ............ ... 4

22.3 Noncancer Hazard ............ ............ ............ ............ ............ . 4

22.3.1 Characterizing Individual Pollutant Hazard - Scenario Approach ............ ..... 5

22.3.2 Multiple Pollutant Hazard ............ ............ ............ ............ 6

22.3.3 Evaluating Hazard Estimates From Inhalation and Ingestion Exposures ............ 6

22.4 Interpretation and Presentation of Risks/Hazards ............ ............ ............ . 7

References ............ ............ ............ ............ ............ ............ . 8

April 2004 Page 22-1

Steps in a Multipathway Risk Characterization

1. Organize outputs of the ingestion exposure and toxicity assessments.

2. Derive cancer risk estimates and noncancer hazard quotients for each pollutant in each pathway.

3. Derive multiple pollutant cancer risk estimates and noncancer hazard indices for each pathway.

4. In consideration of target organ, develop target organ specific hazard indices, if appropriate.

5. As appropriate, combine information on cancer risk and noncancer hazard from the ingestion

analysis with appropriate risk information from the inhalation analysis to derive a total estimate of

cancer risk and noncancer hazard.

6. Identify key features and assumptions of exposure and toxicity assessments.

7. Assess and characterize key uncertainties associated with the assessment.

8. Consider additional relevant information (e.g., related studies).

The risk characterization should be written consistent with EPA guidance and policy, including a risk

summary and risk conclusions that are complete, informative, and useful for decision-makers, and

which clearly identify and discuss the major uncertainties associated with determining the nature and

extent of the risk.

22.1 Introduction

The last component of risk assessment, Risk Characterization, integrates the information from

the exposure assessment (Chapter 20) and toxicity assessment (Chapter 21), using a combination

of qualitative information, quantitative information, and a discussion of uncertainty.(1) Risk

assessors should present the risk characterization and its components so that they are transparent,

clear, and consistent with EPA guidance and policy, and thus components should support the

conclusion that the analysis is reasonably conservative enough for its intended purpose. The risk

summary and risk conclusions must be complete, informative, and useful for decision-makers. 

Major uncertainties associated with determining the nature and extent of the risk should be

identified and discussed. 

Risk characterization for the multipathway risk assessment is performed using the same approach

as described for the inhalation pathway (Chapter 13), except that risks for both inhalation and

ingestion are considered. As for inhalation-only analyses, most multipathway risk assessments

for air toxics will focus on estimating individual risk and hazard. This chapter focuses on the

unique features of risk characterization for multipathway analyses. This chapter also assumes

that the inhalation risk characterization has been completed, as described in Chapter 13.

The general process for characterizing cancer risks and noncancer hazards for multipathway

analyses can be thought of as developing information to fill in a matrix similar to that shown in

Exhibit 22-1 (which presents cancer risks for a group of chemicals; a similar matrix can be

developed to present noncancer hazards [see Exhibit 22-2]). A table like this would be

developed for each of the types of receptors being evaluated in the study area (e.g., adult farmer –

high-end exposure; adult farmer – central tendency exposures; child resident – high-end

exposure). This type of presentation format shows the total risk by chemical, pathway, and

across all pathways. In addition, this format allows one to quickly identify both the individual

chemicals and pathways that contribute most to the total risk estimate. The following sections

describe how to develop the numbers to fill in such a table for both multipathway cancer risk

estimates (Section 22.2) and multipathway noncancer hazards (Section 22.3). The focus of this
April 2004 Page 22-2

chapter is on developing risks and hazards for the ingestion pathways; procedures for developing

inhalation risk estimates have previously been provided in Chapter 13.

Exhibit 22-1. Example Matrix for Estimating Excess Cancer Risks for Multiple Chemical

Exposure through Multiple Ingestion Pathways for a Particular Exposure Scenario

Pathway 1

(Vegetable

Ingestion Risk

Estimate)(a)

 Pathway 2 

(Fish

Ingestion Risk

Estimate)(a)

Pathway 3 

(Egg Ingestion

Risk

Estimate)(a)

Pathway 4

(Beef 

Ingestion Risk

Estimate)(a)

Aggregate

Chemical

Ingestion

Risk Estimate (a)

Chemical 1 1 × 10-6 3 × 10-4 9 × 10-8 8 × 10-5 4 × 10-4

Chemical 2 4 × 10-7 4 × 10-6 4 × 10-8 4 × 10-7 5 × 10-6

Chemical 3 4 × 10-9 7 × 10-7 3 × 10-8 9 × 10-9 8 × 10-7

Chemical 4 9 × 10-7 1 × 10-6 6 × 10-7 6 × 10-7 3 × 10-6

Cumulative

Ingestion

Pathway

Risk

Estimate (a)

3 × 10-6 3 × 10-4 7 × 10-7 8 × 10-5 4 × 10-4

(a) Standard rules for rounding apply which will commonly lead to an answer of one significant figure

in both risk and hazard estimates. For presentation purposes, hazard quotients (and hazard indices)

and cancer risk estimates are usually reported as one significant figure.

22.2 Cancer Risk Estimates

As discussed in detail in Chapter 13, estimated individual cancer risk is expressed as the

probability that a person will develop cancer as a result of the estimated exposure over a lifetime. 

This predicted risk is the incremental risk of cancer from the exposure being analyzed, which

are in addition to other risks due to any other factors (e.g., smoking). Due to default assumptions

in their derivation, cancer slope factors (CSFs) are generally considered to be “plausible upperbound” estimates, regardless of whether they are based on statistical upper bounds or best fits. 

As noted in Chapter 13, risks may be estimated for both the central tendency (average exposure)

case and for the high-end (exposure that is expected to occur in the upper range of the

distribution) case, or probabilistic techniques can be used to develop a distribution of estimated

risks.

22.2.1 Characterizing Individual Pollutant Ingestion Risk - Scenario Approach

The first step in characterizing individual pollutant risk for an exposure scenario (e.g., a

recreational fisher) is to quantify risk for each ingestion exposure pathway being evaluated. In

this step, cancer risks for individual pollutants are estimated by multiplying the estimate of the

lifetime average daily dose (LADD) for each ingestion exposure pathway by the appropriate CSF

to estimate the potential incremental cancer risk:

Risk = LADD × CSF (Equation 22-1)
April 2004 Page 22-3

where:

Risk = Individual cancer risk (expressed as an upper-bound risk of contracting cancer

over a lifetime) for each pollutant via the ingestion pathway being evaluated

(unitless);

LADD = Lifetime Average Daily Dose for the pollutant via the ingestion pathway being

evaluated (mg/kg-d); and

CSF = Cancer Slope Factor for the pollutant via the ingestion pathway being

evaluated [(mg/kg-d)-1]

Estimates of cancer risk are usually expressed as a probability represented in scientific notation

as a negative exponent of 10. For example, an additional risk of contracting cancer of 1 chance

in 10,000 (or one additional person in 10,000) is written as 1x10-4. Because CSFs are typically

upper-bound estimates, actual risks may be lower than predicted (see Chapter 12) – note that the

true value of the risk is unknown and may be as low as zero.(2) These statistical projections of

hypothetical risk are intended as screening tools for risk managers and cannot be used to make

realistic predictions of biological effects.

Risks are generally evaluated initially for individuals within the potentially exposed population. 

Population risks for the exposed population may also be estimated, which may be useful in

estimating potential economic costs and benefits from risk reduction. Sensitive subpopulations

should also be considered, when possible. Estimates of incidence also are possible, although

there are some caveats associated with these measures (see Chapter 13).

For carcinogens being assessed based on the assumption of nonlinear dose-response, for which a

reference dose (RfD) was derived that considers cancer as well as other effects, the hazard

quotient approach will be appropriate for risk characterization (see Section 22.3).

22.2.2 Characterizing Risk from Exposure to Multiple Pollutants - Scenario Approach

For each exposure pathway of a scenario, exposure may be to multiple chemicals at the same

time rather than a single chemical; however, CSFs are usually available only for individual

compounds within a mixture. Consequently, a component-by-component approach is usually

employed.(3) The following equation estimates the predicted cumulative incremental individual

cancer risk from multiple substances for a single exposure pathway, assuming additive effects

from simultaneous exposures to several carcinogens:

RiskT = Risk1 + Risk2 + .... + Riski(Equation 22-2)

where:

RiskT = Cumulative individual ingestion cancer risk (expressed as an upper-bound risk of

contracting cancer over a lifetime); and

Riski = Individual ingestion risk estimate for the ith substance.

In screening-level assessments of carcinogens for which there is an assumption of a linear doseresponse relationship, the cancer risks predicted for individual chemicals may be added to

estimate cumulative cancer risk for each pathway. This approach is based on an assumption that
April 2004 Page 22-4

Aggregate vs. Cumulative Risk

Aggregate risk refers to risk attributed to a

single chemical across multiple pathways/routes

Cumulative risk refers to risk attributed to

simultaneous exposure to multiple chemicals via

a single or multiple pathways/routes

the risks associated with individual chemicals in the mixture are additive. In more refined

assessments, the chemicals being assessed may be evaluated to determine whether effects from

multiple chemicals are synergistic (greater than additive) or antagonistic (less than additive),

although sufficient data for this evaluation are usually lacking. In those cases where CSFs are

available for a chemical mixture of concern, risk characterization can be conducted on the

mixture using the same procedures used for a single compound.

For carcinogens being assessed based on the

assumption of nonlinear dose-response, for which an

RfD considering cancer as well as other effects has

been derived, the hazard quotient approach will be

appropriate (see Section 22.3).

22.2.3 Combining Risk Estimates across Multiple

Ingestion Pathways - Scenario Approach

To evaluate risks associated with the aggregate

exposure across multiple pathways of a given scenario, the individual pollutant cancer risk

estimates may be summed for each chemical across the multiple ingestion pathways assessed. 

Additionally, a cumulative multi-pathway risk estimate may be derived by summing cumulative

(multiple pollutant) cancer risk estimates across the multiple ingestion pathways.

22.2.4 Evaluating Risk Estimates from Inhalation and Ingestion Exposures

Depending on the ingestion scenario, the inhalation pathway will also have been assessed. In

such cases, the inhalation exposures must be presented along with the ingestion exposures to

provide an overall estimate of risk across the multiple pathways. When there is a compatibility

in the exposure scenarios, inhalation and ingestion risk estimates can be combined. Essentially,

an additional column for inhalation can be added to Exhibit 22-1 to achieve this result. 

Regardless, when both routes are assessed, risk estimates for both routes of exposure should be

presented, along with descriptions regarding the populations assessed for all pathways and routes,

thereby clarifying any differences in populations.

It is important to note, however, that the methods and assumptions used to derive the inhalation

and ingestion risks may not always yield compatible exposure scenarios. This is particularly

important when population-level (versus individual) risk estimates are being developed. For

example, a scenario-based ingestion exposure assessment will not be easily amenable to

producing estimates of numbers of people at different risk levels, while a population-based

inhalation assessment may be more appropriate. In addition, it would generally not be

appropriate to add an inhalation risk that presumes a 70-year exposure duration with an ingestion

pathway that presumes a 30-year exposure duration. Any matching of exposure durations among

pathways in a multipathway assessment should be carefully considered.

22.3 Noncancer Hazard

For noncancer effects (as well as carcinogens being assessed based on the assumption of

nonlinear dose-response), ingestion exposure concentrations are compared to RfDs, which are

estimates (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to
aAlthough subchronic RfDs are not routinely developed by EPA, ATSDR develops MRLs for

“intermediate” exposures and describes them as being relevant to exposure durations on the order of weeks to

months (i.e., >14 days to 364 days).

April 2004 Page 22-5

the human population (including sensitive sub-populations) that is likely to be without an

appreciable risk of deleterious noncancer effects during a lifetime (see Chapter 21).

As with carcinogens, the development of hazard quotients (HQs) for ingestion typically is

performed first for individual air toxics. Then, hazard indices (HIs) may be developed for

multiple pollutant exposures and summed across pathways to develop multiple pathway

cumulative hazard estimates. An additional step in the multipathway analysis is to evaluate

combining both ingestion and inhalation hazard estimates. These steps are described in separate

subsections below.

22.3.1 Characterizing Individual Pollutant Hazard - Scenario Approach

The first step in characterizing individual pollutant hazard for an exposure scenario (e.g., a

recreational fisher) is to quantify hazard for each pollutant being evaluated. For ingestion

exposures, noncancer hazards are estimated by dividing the estimate of the Average Daily Dose

(ADD) by the chronic oral RfD to yield an HQ for individual chemicals:

HQ = ADD ÷ RfD (Equation 22-3)

where:

HQ = Hazard Quotient for the pollutant via each ingestion pathway being evaluated

(unitless);

ADD = Estimate of the Average Daily Dose for the pollutant via the ingestion pathway

being evaluated (mg/kg-d); and

RfD = Corresponding reference dose for the pollutant via the ingestion pathway being

evaluated (mg/kg-d).

In screening assessments, the chronic exposure estimate is commonly based on a simplifying

assumption of continued similar conditions for a long-term period (for example, that the

maximum annual average modeled concentration remains constant during the full course of the

exposure duration). A more refined assessment might consider how concentration changes with

time over the exposure duration. In both cases, it is important to match the type of RfD value to

the specific exposure scenario. For example, for childhood scenarios (e.g., ages 0-6), risk

assessors commonly use chronic RfDs (rather than subchronic). Subchronic RfDs(a) are more

commonly used to evaluate exposure scenarios that last a year or less (e.g., a construction worker

who is exposed for 6 months). For exposure durations of a few years, both chronic and

subchronic values may be considered, with chronic values commonly being used, particularly in

screening assessments, with explicit recognition of the decision and its basis. Acute toxicity

values are for exposures that are much shorter in duration (usually 24 hours or less); however,

such exposures generally are not evaluated in a multipathway air toxics risk assessment.

Based on the definition of the RfD, an HQ less than or equal to one indicates that adverse

noncancer effects are not likely to occur. With exposures increasingly greater than the RfD (i.e.,
April 2004 Page 22-6

HQs increasingly greater than one), the potential for adverse effects increases, but we do not

know by how much. An HQ of 100 does not mean that the hazard is 10 times greater than an HQ

of 10. Also an HQ of 10 for one substance may not have the same meaning (in terms of hazard)

as another substance resulting in the same HQ.

22.3.2 Multiple Pollutant Hazard

Noncancer health effects data are usually available only for individual compounds within a

mixture. In these cases, the individual HQs can be summed together to calculate a multipollutant HI:

HI = HQ1 + HQ2 + ...+ HQi(Equation 22-4)

where

HI = Hazard index; and

HQi = Hazard quotient for the ith air toxic.

For screening-level assessments, a simple HI may first be calculated for all chemicals of potential

concern (COPCs) (Exhibit 22-2). This approach is based on the assumption that even when

individual pollutant levels are lower than the corresponding reference levels, some pollutants

may work together such that their potential for harm is additive and the combined exposure to the

group of chemicals poses greater likelihood of harm. Some groups of chemicals can also behave

antagonistically, such that combined exposure poses less likelihood of harm, or synergistically,

such that combined exposure poses harm in a greater than additive manner, although information

needed to perform such an analysis is generally not available. Where this type of HI exceeds the

criterion of interest, a more refined analysis is warranted. 

The assumption of dose additivity is most appropriate to compounds that induce the same effect

by similar modes of action. Thus, EPA guidance for chemical mixtures(3) suggests subgrouping

pollutant-specific HQs by toxicological similarity of the pollutants for subsequent calculations;

that is, calculating a target-organ-specific-hazard index (TOSHI) for each subgrouping of

pollutants. This calculation allows for a more appropriate estimate of overall hazard.

The HI approach encompassing all chemicals in a mixture may be appropriate for a screeninglevel study. However, it is important to note that applying the HI equation to compounds that

may produce different effects, or that act by different mechanisms, could overestimate the

potential for effects. Consequently, in a refined assessment, it is more appropriate to calculate a

separate HI for each noncancer endpoint of concern when target organs or modes of action are

known to be similar. Refined assessments also may employ techniques more complex than the

HI derived using RfDs.(4)

22.3.3 Evaluating Hazard Estimates From Inhalation and Ingestion Exposures

As with carcinogenic assessments, inhalation hazards must be combined with ingestion hazards

to provide total hazard across all exposure pathways for a receptor. Similar to Exhibit 22-1,

inhalation and ingestion risk estimates can be combined either by chemical across pathways or

across chemicals within a pathway. Essentially, an additional column for inhalation can be added

to Exhibit 22-2 to achieve this result.
April 2004 Page 22-7

Estimating Risk for Drinking Water Sources

In evaluating potential risks associated with drinking water supplies, risk assessors commonly assume

that the drinking water undergoes at least a minimum level of treatment to remove solids (i.e.,

particles in the water which are persistent bioaccumulative hazardous air pollutants [PB-HAPs] or

onto which PB-HAPs may be absorbed). Therefore, the risk assessment commonly focuses on the

dissolved concentrations of PB-HAPs in drinking water sources. In addition, if the drinking water

source is part of a public drinking water system, the risk assessment may also assume that the water is

treated to meet applicable drinking water standards (i.e., treated to maximum contaminant levels or

MCLs, unless study-specific information indicates otherwise) for chemicals regulated under the

drinking water program. National Primary Drinking Water Regulations are enforceable standards that

apply to public water systems. The MCLs are the highest level of a specific list of contaminants

allowed in drinking water (see http://www.epa.gov/safewater/mcl.html).

Exhibit 22-2. Example Matrix for Characterizing Hazard for Multiple Chemical Exposure

through Multiple Ingestion Pathways for a Particular Exposure Scenario

Pathway 1

(Vegetable

Ingestion HQ

Estimate)(a)

 Pathway 2 

(Fish

Ingestion HQ

Estimate)(a)

Pathway 3 

(Egg Ingestion

HQ Estimate)(a)

Pathway 4

(Beef 

Ingestion HQ

Estimate)(a)

Aggregate

Chemical

Ingestion

HQ Estimate(a)

Chemical 1 2 × 10-1 2 × 10-1 4 × 10-2 2 × 10-1 7 × 10-1

Chemical 2 3 × 10-1 7 × 10-1 3 × 10-2 2 × 10-1 1

Chemical 3 1 × 10-1 4 × 10-1 2 × 10-1 4 × 10-1 1

Chemical 4 9 × 10-2 1 × 10-2 1 × 10-1 2 × 10-2 3 × 10-1

Cumulative

Ingestion

Pathway HI (a)

7 × 10-1 1 4 × 10-1 9 × 10-1 3

(a) Standard rules for rounding apply which will commonly lead to an answer of one significant figure

in both risk and hazard estimates. For presentation purposes, hazard quotients (and hazard indices)

and cancer risk estimates are usually reported as one significant figure.

22.4 Interpretation and Presentation of Risks/Hazards

In the final part of the risk characterization, estimates of cancer risk and noncancer hazard should

be presented in the context of uncertainties and limitations in the data and methodology. 

Exposure estimates and assumptions, toxicity estimates and assumptions, and the assessment of

uncertainty should be discussed. Chapter 13 provides more detailed information and examples. 

Part VI of this reference manual discusses risk communication and other elements of the riskbased decision-making process.

Note that multipathway air toxics risk assessments are subject to additional sources of

uncertainty as compared to inhalation risk assessments. The multimedia modeling effort is both

more complex and less certain due to many factors. For example: (1) there are many more

chemical-dependent and chemical-independent variables involved as input values to the models;
April 2004 Page 22-8

1. U.S. Environmental Protection Agency. 2002. Framework for Cumulative Risk Assessment

(External Review Draft). Risk Assessment Forum, Washington, DC, April 2002, available

at: http://oaspub.epa.gov/eims/eimsapi.dispdetail?deid=29570

U.S. Environmental Protection Agency. 1997. Guidance on Cumulative Risk Assessment,

Part 1, Planning and Scoping. Science Policy Council, Washington, DC.

U.S. Environmental Protection Agency. 1984. Risk Assessment and Management:

Framework for Decision Making, Washington, D.C. EPA 600/9-85-002.

2. U.S. Environmental Protection Agency. 1986. Guidelines for Mutagenicity Risk Assessment. 

Risk Assessment Forum, Washington, DC. EPA/630/R-98/003; published in the Federal

Register 51:(1185): 34006-34012, Sept 24, 1986, available at:

http://cfpub.epa.gov/ncea/raf/pdfs/mutagen2.pdf.

3. U.S. Environmental Protection Agency. 2000. Supplementary Guidance for Conducting 

Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum, Washington, D.C. 

EPA/630/R-00/002, available at

http://www.epa.gov/ncea/raf/pdfs/chem_mix/chem_mix_08_2001.pdf. 

U.S. Environmental Protection Agency. 1986. Guidelines for the Health Risk Assessment of

Chemical Mixtures. EPA/630/R-98/002; published in the Federal Register 51 (185):34014-

34025, Sept 24, 1986, available at

http://www.epa.gov/ncea/raf/pdfs/chem_mix/chem_mix_08_2001.pdf.

4. U.S. Environmental Protection Agency. 2002. Guidance on Cumulative Risk Assessment for

Pesticide Chemicals that have a Common Mechanism of Action. Office of Pesticides

Programs, Washington, D.C., January 2002. Available at: 

http://www.epa.gov/pesticides/trac/science/.

U.S. Environmental Protection Agency. 2003. National Center for Environmental

Assessment. Developing Approaches to Estimate Cumulative Risks of Drinking Water

Contaminants. Updated December 30, 2003. Available at: http://cfpub.epa.gov/ncea/cfm/

recordisplay.cfm?deid=18494. (Last accessed April, 2004.)

(2) the models involve analysis of the transfer of air toxics from the air to other media (e.g., soil,

sediment, water), the subsequent movement of the air toxics between these media (e.g., soil

runoff to surface water), and uptake and metabolism by biota; and (3) many variables affect the

ingestion of food, water, and other media by humans and wildlife, and the exposure and risk

estimates may differ considerably as a consequence of the assumptions used to derive intake

estimates. Sampling of biota and abiotic media also may be more complex. Additional

uncertainties are incorporated in the risk assessment when exposure estimates to multiple

substances across multiple pathways are summed.

References
PART IV 

ECOLOGICAL RISK ASSESSMENT

Chapter 23 Overview and Getting Started: Problem

Formulation

Table of Contents

23.1 Introduction ............ ............ ............ ............ ............ ...... 1

23.2 Overview of Air Toxics Ecological Risk Assessment ............ ............ ......... 4

23.2.1 Problem Formulation ............ ............ ............ ............ .... 6

23.2.2 Analysis ............ ............ ............ ............ ............ . . 8

23.2.3 Evaluation of Ecological Effects ............ ............ ............ ....... 9

23.2.4 Ecological Risk Characterization ............ ............ ............ ...... 9

23.3 Planning and Scoping ............ ............ ............ ............ .......... 9

23.3.1 What is the Concern? ............ ............ ............ ............ ... 10

23.3.2 Identifying The Participants ............ ............ ............ ......... 11

23.3.3 Determining the Scope of the Risk Assessment ............ ............ ...... 12

23.3.4 Study-Specific Conceptual Model ............ ............ ............ ..... 12

23.3.4.1 Identifying Receptors of Concern ............ ............ .... 13

23.3.4.2 Identifying Assessment Endpoints and Measures of Effects ........ 15

23.3.5 Analysis Plan and Quality Assurance Program Plan (QAPP) ............ ........ 16

23.4 Tiered Ecological Risk Assessments ............ ............ ............ ......... 21

References ............ ............ ............ ............ ............ ............ 23

April 2004 Page 23-1

Part IV constitutes a snapshot of EPA’s current thinking and approach to the adaptation of the

evolving methods of ecological risk assessment to the context of Federal and state control of air

toxics. While inhalation risk assessment has been increasingly used in regulatory contexts over the

last several years, ecological risk assessment tools are less well developed and field tested in a

regulatory context. Part IV should be considered a living document for review and input. By

publishing Part IV in its current state of development, EPA is soliciting the involvement of persons

with experience in this field to help improve these assessment methods for use in a regulatory

context. EPA anticipates revisions to this draft section of Part IV on the basis of this input.

Trophic Levels and Biomagnification

The trophic level is a way to describe where an

organism may be located within an aquatic or

terrestrial food web. The lowest trophic level consists

of primary producers, the green plants that convert

sunlight into carbohydrates via photosynthesis. The

next trophic level generally consists of primary

consumers, or the organisms that feed directly on

green plants. The next level up, often termed

secondary consumers, represents animals that feed on

primary consumers. The highest trophic level

consists of the top predators in the food web. For

some chemicals, the concentration in biological tissue

can increase as it moves up the food chain, a process

called biomagnification.

23.1 Introduction

Part III of this Reference Manual discusses how to plan for and conduct a multipathway human

health risk assessment when air toxics that persist and may also bioaccumulate (e.g., the

persistent bioaccumulative hazardous air pollutant compounds, or PB-HAPs) in media other than

air and/or biomagnify in food chains are present in releases. For these compounds, the risk

assessment generally will need to include consideration of exposure pathways that involve

deposition of air toxics onto soil and plants and into water, subsequent uptake by biota, and

potential human exposures via consumption of contaminated soils, sediments, surface waters,

and foods. These substances may also pose risks to ecological receptors from direct exposure to

contaminated media or through indirect exposure via aquatic and terrestrial food chains (see

Exhibit 23-1). The preliminary list of PB-HAPs was derived primarily on the basis of exposure

and risk/hazard once HAPs are deposited onto soils, into surface waters, etc. Its derivation did

not consider direct exposures of ecological receptors to air toxics while they are in the air (e.g.,

phytotoxic effects on plants; inhalation by animals). Additional HAPs of potential concern for

ecological risk may be identified as EPA gains more familiarity with ecological risk assessments

for air toxics. Appendix D describes the process by which EPA identified the PB-HAP

compounds.
April 2004 Page 23-2

Exhibit 23-1. Air Toxics Exposure Pathways of Potential Concern for Ecological Receptors

This graphic illustrates some of the potential multimedia pathways of concern for air toxics exposure

to ecological receptors. Air toxics released from a source disperse through the air and eventually fall

to the earth (atmospheric deposition) via settling and/or precipitation. Air toxics deposited to soil may

be absorbed or ingested by plants and soil invertebrates (uptake). Terrestrial animals may be exposed

to air toxics via ingestion of contaminated plants and soil, or by consuming contaminated terrestrial

animals (for those air toxics that bioaccumulate and transfer up the terrestrial food web). Air toxics

deposited to water may be dissolved in the water column and/or may settle and be absorbed into

aquatic sediments. Air toxics in sediments may be absorbed or ingested by benthic organisms

(uptake); those in sediments and the water column may be absorbed by aquatic plants (uptake). 

Aquatic organisms (e.g., fish) may be exposed directly to air toxics in the water column and/or by

consuming contaminated aquatic organisms (for those air toxics that bioaccumulate and transfer up the

aquatic food web). Terrestrial animals may be exposed to air toxics by eating contaminated fish or

shellfish and/or by drinking contaminated water. Note also that, while in the atmosphere, air toxics

may also have direct impacts on plants (direct exposure) and terrestrial animals (inhalation).

This part (Part IV) of this reference manual introduces the basic concepts of ecological risk

assessment and describes their application to air toxics. Several differences of particular

importance are highlighted in a text box on page 23-3. The discussion of ecological risk

assessment follows the same general framework as that presented in Part III since the overall

concept is the same; namely that certain air toxics may move from the air into other media where

exposures to organisms (in this case, non-human organisms) can occur with potentially adverse

outcomes. Readers are strongly encouraged to become familiar with the information provided in

Part III before reading this Part. However, although there are many similarities between
April 2004 Page 23-3

Air toxics may have adverse effects on ecological receptors through direct exposures (e.g.,

inhalation by animals; direct deposition onto plants). However, EPA does not have sufficient

experience with multipathway air toxics risk assessments to identify the circumstances for which

these exposures would represent a potential concern. This reference manual therefore does not

address these additional exposure pathways. The methods for conducting such an analysis are

described in greater detail in EPA’s Guidelines for Ecological Risk Assessment.(1)

Key Ecological Risk Assessment Resources

• NCEA’s Ecological Risk Assessment webpage http://cfpub.epa.gov/ncea/cfm/ecologic.cfm

• The Oak Ridge National Laboratory Ecological Risk Assessment webpage on tools, guidance, and

applications http://www.esd.ornl.gov/programs/ecorisk/ecorisk.html

• The Superfund Ecological Risk Assessment Program

http://epa.gov/superfund/programs/risk/ecolgc.htm

• Navy Guidance for Conducting Ecological Risk Assessments http://web.ead.anl.gov/ecorisk/

• EPA’s Watershed Ecological Risk Assessment program

http://cfpub.epa.gov/ncea/cfm/weracs.cfm?ActType=default

multimedia human health risk assessment and ecological risk assessment (e.g., they may use

the same multimedia monitoring and modeling tools), professional expertise will always be

required to apply the ecological risk assessment principles and tools identified in this

document to specific assessment areas or problems. This document is not a substitute for a

working familiarity with ecological principles, their application, and the field of ecological

risk assessment.

This chapter presents an overview of ecological risk assessment and discusses the initial planning

and scoping activities. The remaining chapters of this part focus on Characterization of

Exposure (Chapter 24), Characterization of Ecological Effects (Chapter 25), and Risk

Characterization (Chapter 26). The discussion presented here is based largely on EPA’s

Guidelines for Ecological Risk Assessment(1) and the Residual Risk Report to Congress.(2) The

Guidelines for Ecological Risk Assessment were developed especially for evaluating ecological

risk. Readers are also strongly encouraged to become familiar with that document for a more

complete understanding of EPA’s recommended approach to ecological risk assessment. 

Interested readers are also referred to EPA’s Ecological Risk and Decision Making Workshop

materials which provide detailed information on the definition of ecological risk assessment, how

it relates to human health assessment, the ecosystem protection place-based approach, and the

bases for ecological protection and risk assessment at EPA.(3)
April 2004 Page 23-4

Some Important Differences Between Ecological Risk Assessment and

Multipathway Human Health Risk Assessment

• Planning and scoping. The ecological risk assessment requires more preliminary analysis and

deliberation regarding endpoints to be assessed and toxicity reference values to be used because

ecological systems are more complex and are not as well understood biologically as human health

systems. The planning and scoping team should include individuals with specific expertise in

ecological risk assessment.

• Assessment area. It may be necessary to evaluate additional portions of the assessment area that

are not of concern from a human health perspective.

• Potentially exposed populations. The focus shifts from potentially exposed individual humans to

potentially exposed populations and species of ecological receptors of concern. In many cases, the

exposure assessment may need to address multiple species and life-stages, many of which have

physiological and biochemical processes that differ significantly from humans. (When threatened

or endangered species are present, the assessment may also include an evaluation of those

organisms as individuals).

• Exposure pathways and exposure routes. It may be necessary to assess different exposure

pathways and routes that are not of concern for human health.

• Ecological effects assessment. Ecological systems have traits and properties that are different

from humans and, thus, the ecological effects assessment (comparable to hazard assessment for

human health) may consider a wider range of potential causal relationships. 

• Risk characterization. While risks may be assessed at multiple levels of ecological organization

(i.e., organism, population, community, and ecosystem), they generally are assessed at the

population level in air toxics assessments. (Nevertheless, when appropriate, consideration should

be given to assessments at high levels of ecological organization, such as at the landscape level).

23.2 Overview of Air Toxics Ecological Risk Assessment

The ecological risk assessment process has three main steps that broadly correspond to the four

basic steps in human health risk assessment methodology (Exhibit 23-2):(1)

• Problem formulation, which corresponds to the problem formulation step of the human

health risk assessment methodology (planning and scoping activities similar to human health

risk assessment are also integrated with this step; however, they are discussed separately

below to maintain the operational structure of the ecological risk assessment as described in

EPA’s ecological risk assessment guidelines);

• Analysis, which corresponds to the exposure assessment and toxicity assessment steps of the

human health risk assessment methodology; and

• Risk characterization, which corresponds to the risk characterization step of the human

health risk assessment methodology.
April 2004 Page 23-5

Exhibit 23-2. Ecological Risk Assessment Framework

Source: EPA Guidelines for Ecological Risk Assessment(1)
April 2004 Page 23-6

23.2.1 Problem Formulation

Problem formulation provides the foundation for the entire ecological risk assessment. This step

includes:

• Identifying risk management goals from an ecological perspective, ecological receptors of

concern (e.g., wetlands, fish populations, keystone species that impact the overall ecosystem),

and assessment endpoints (explicit expression of the environmental value that is to be

protected, operationally defined by an ecological entity and its attributes); 

• Developing the ecological risk part of the conceptual model as necessary to account for

ecological exposure pathways and receptors; and

• If necessary, developing the Sampling and Analysis Plan and associated Quality Assurance

Project Plan to collect data on exposures and measures of effects that are needed to support

the ecological risk assessment. 

As with human health risk assessments, problem formulation is often an iterative process, in

which substantial re-evaluation may occur as new information and data become available. Data

collection in subsequent iterations often is triggered by identification of major data gaps and

uncertainties in the risk characterization that prevent confident decision-making by risk

managers.

The problem formulation process for ecological risk assessment for air toxics focuses on

developing a common understanding of what needs to be done to assess ecological risks

associated with pathways involving deposition; the transfer of compounds to soil, water,

sediment, and biota, and subsequent exposure. While the ecological risk assessment may build

on the foundation of the human health multipathway assessment (e.g., using the same emissions

data and multimedia models), the problem formulation step is particularly critical for the

ecological risk assessment because of the effort needed to understand and identify ecological

receptors, exposure pathways, endpoints, and management goals. The ecological risk assessment

is not simply an “add-on” to the human health multipathway risk assessment. The problem

formulation effort will need to consider a wide variety of possible ecological receptors that are

not similar to humans. For example:

• Different species (and life stages) may have very different responses to the same exposure.

Therefore, knowledge of the exposure-response of many species, including those that may be

particularly sensitive to the air toxic, is needed.

• Ecosystems may show adverse effects at lower exposures than most individual species do

because species that are important in terms of ecosystem function (e.g., energy flow, nutrient

recycling) may also be sensitive to toxic effects. Ecosystem-level metrics such as species

diversity indices may be more sensitive indicators of adverse effects than are toxicological

studies.

• There may be many different types of ecosystems present in the assessment area, and

sensitivity likely varies among them. Therefore, the particular features of the ecosystem(s)

that occur in areas where high exposures are predicted may be particularly important.
April 2004 Page 23-7

An Ecological Risk Assessment Case Study: Ozone Risks To Agroecosystems

The case study summarized here provides an example of how EPA has assessed environmental risks from an air

pollutant (ozone) as part of EPA’s effort to promulgate National Ambient Air Quality Standards (NAAQS) for

criteria air pollutants (see Chapter 2). Note that this example is for ozone, a criteria air pollutant; however, the

concepts presented here are relevant to air toxics risk assessment. In addition, an agroecosystem, such as the

system discussed here, is more of a human construct than a natural ecosystem and is provided here only for

illustration of general principles. An actual air toxics ecological risk assessment of a natural system would have

to consider site-specific characteristics of the system in question.

Problem Formulation. Pursuant to the Clean Air Act (CAA), EPA is required to set NAAQS for “any pollutant

which, if present in the air, may reasonably be anticipated to endanger public health or welfare and whose

presence in the air results from numerous or diverse mobile and/or stationary sources.” EPA develops public

health (primary) and welfare (secondary) NAAQS. According to section 302 of the CAA, the term welfare

“includes ... effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility,

and climate, damage to and deterioration of property, and hazards to transportation, as well as effects on

economic values ....” A secondary standard, as defined in section 109(b)(2) of the CAA, must “specify a level of

air quality the attainment and maintenance of which in the judgment of the Administrator, based on such criteria,

is requisite to protect the public welfare from any known or anticipated adverse effects associated with the

presence of such air pollutant in the ambient air.”

This case study focuses on an assessment endpoint for agricultural crops (e.g., the prevention of an economically

adverse reduction in crop yields). Yield loss is defined as an impairment of, or decrease in, the value of the

intended use of the plant. This concept includes a decrease in the weight of the marketable plant organ, reduction

in aesthetic values, changes in crop quality, and/or occurrence of foliar injury when foliage is the marketable part

of the plant. These types of yield loss can be directly measured as changes in crop growth, foliar injury, or

productivity, so they also serve as the measures of effect for the assessment.

Exposure Analysis. EPA used ambient ozone monitoring data across the U.S. and a Geographic Information

System (GIS) model to project national cumulative, seasonal ozone for the maximum three month period during

the summer ozone season. This allowed EPA to project ozone concentrations for some rural parts of the country

where no monitoring data were available but where crops were grown, and to estimate the attainment of

alternative NAAQS scenarios. The U.S. Department of Agriculture’s (USDA’s) national crop inventory data

were used to identify where ozone-sensitive crop species were being grown and in what quantities. This

information allowed the Agency to estimate the extent of exposure of ozone-sensitive species under the different

scenarios.

Ecological Effects Analysis. Stressor-response profiles describing the relationship between ozone and growth

and productivity for 15 crop species representative of major production crops in the U.S. (e.g., crops that are

economically valuable to the U.S., of regional importance, and representative of a number of crop types) had

already been developed from field studies conducted from 1980 to 1986 under the National Crop Loss

Assessment Network (NCLAN) program. The NCLAN studies also included secondary stressors (e.g., low soil

moisture and co-exposure with other pollutants like sulfur dioxide), which helped EPA interpret the

environmental effects data for ozone.

Risk Characterization. Under the different NAAQS scenarios, the Agency estimated the increased protection

from ozone-related effects on vegetation associated with attainment of the different NAAQS scenarios. 

Monetized estimates of increased protection associated with several alternative standards for economically

important crops were also developed. This analysis focused on ozone effects on vegetation since these public

welfare effects are of most concern at ozone concentrations typically occurring in the U.S. By affecting

commercial crops and natural vegetation, ozone may also indirectly affect natural ecosystem components such as

soils, water, animals, and wildlife.

Source: U.S. Environmental Protection Agency. 1999. Residual Risk Report to Congress. Office of Air Quality

Planning and Standards, Research Triangle, NC, March 1999. EPA-453/R-99-011.
April 2004 Page 23-8

23.2.2 Analysis

Analysis includes two principal steps. Characterization of exposures includes identifying the

contaminants of potential ecological concern (COPECs) that may affect ecological receptors,

characterizing the spatial and/or temporal pattern of stressor concentrations in environmental

media (including certain body burden levels), and analyzing the level of contact or co-occurrence

(exposure) between the stressors and the ecological receptors. This often is done using the

multimedia models identified in Chapter 18; however, different models or approaches may be

appropriate. Characterization of ecological effects includes identifying the types of effects that

different stressors may have on ecological receptors, along with characterizing the stressorresponse relationship (the relationship between the level of exposure to the stressor and the

expected biological or ecological response). A common result is the identification of ecological

toxicity reference values (TRVs), which are concentrations of chemicals in environmental

media (including biota such as fish tissues) below which no significant ecological effects are

anticipated. TRVs are similar, in concept, to RfDs (reference doses) and RfCs (reference

concentrations) for human health noncancer evaluations. TRVs may be screening level (i.e.,

conservative, generic values) or more refined values for use in higher levels of analysis. They

may be point values, ranges, or developed using more advanced probabilistic methods (such as

Monte Carlo techniques). The ecological exposure characterization also is likely to differ

significantly from the corresponding multipathway exposure assessment for human health. For

example:

• In addition to food chain (ingestion) exposures, many ecological receptors can be exposed to

air toxics via direct contact with contaminated soils (e.g., earthworms) or sediments (e.g.,

sediment-dwelling invertebrates, bottom-feeding fish); direct exposure to surface water (e.g.,

free-swimming invertebrates and fish); or direct exposure to contaminated air via inhalation

(e.g., birds), dermal contact (e.g., amphibians), deposition to plant surfaces, etc.

• Particular geographic areas of concern may differ because ecological receptors may occur in

areas rarely used by human populations (e.g., large wetland areas, ponds where people rarely

fish).

• Sampling and analysis may involve a wider range of media (e.g., sediment) and different

types of biota (e.g., earthworms, aquatic invertebrates). Each type of sampling and analysis

has its own methods, protocols, and Quality Assurance/Quality Control (QA/QC) procedures.

• Quantitative metrics of exposure may include both direct and indirect exposures for

ecological receptors. Quantification of direct exposure is similar to human health inhalation

analyses, in which ambient concentrations of COPECs in soil, water, and/or sediment are

compared to corresponding TRVs. Quantification of indirect exposure via ingestion is

similar to that for human health ingestion analyses, except that different food items may be

involved, and the appropriate ecological exposure factors (e.g., diet, body weight) will be

different. As with human health analyses, many exposure factors can be treated either as

constants or as distributions in the exposure assessment. Ecological exposure assessments

for ingestion pathways frequently use bioenergetic models to more explicitly relate intake to

adverse effects.(4)
April 2004 Page 23-9

23.2.3 Evaluation of Ecological Effects

The characterization of ecological effects is similar to a toxicity assessment for human health. It

considers the types of adverse effects associated with chemical exposures, stressor-response

relationships, and related uncertainties. There are two primary differences:

• Adverse effects of concern generally focus at the population, community, or ecosystem level. 

With rare exceptions (e.g., threatened or endangered species), effects to individual organisms

are not the primary concern. Note, however, that ecological risk assessments often use

estimates of impacts to individual organisms (e.g., mortality, reproductive effects) to infer

impacts at higher levels of organization because exposure-response data for populations,

communities, or ecosystems often are lacking. Some approaches are available, however, for

incorporating population-level analysis in ecological risk assessments.(5)

• A distinction is made between assessment endpoints, which are the environmental values to

be protected, and measures of effects, which are the specific measures used to evaluate risk

to the assessment endpoints (assessment endpoints and measures of effects are defined in 

Section 23.3.4.2).

23.2.4 Ecological Risk Characterization

Similar to human health risk characterization, ecological risk characterization combines

information concerning exposure to chemicals with information regarding effects of chemicals to

estimate risks. Human health risk assessments consider health effects in the bodies of individual

people. Ecological risk assessments consider various “health” issues that can range from actual

health effects in the bodies of individual ecological receptors to something more attuned to the

“health” of the ecosystem as measured by species richness and diversity.

23.3 Planning and Scoping

To ensure that the ecological risk assessment will

provide information useful to the risk managers

who will be making the risk management

decisions, EPA’s Guidelines for Ecological Risk

Assessment recommends a planning and scoping

dialogue occur between the risk assessors, risk

managers, and where appropriate, interested

stakeholders at the very start of the risk

assessment process. The outcome of the planning

and scoping phase is an agreement on the basic

goals, scope, and timing of the risk assessment. 

Important goals of the dialogue are the

identification of the risk management goals and

risk management options that the risk assessment

will be designed to inform (see accompanying

text box). This ‘kick-off’ dialogue sets the stage for the problem formulation phase, when the

plans for the ecological risk assessment are finalized. 

Planning and Scoping the Ecological

Risk Assessment

The planning phase is complete when

agreements are reached on:

• The management goals for ecological

values;

• The range of management options the

risk assessment is to support;

• Objectives for the risk assessment,

including criteria for success; and 

• The focus and scope of the assessment,

and resource availability.
April 2004 Page 23-10

When actually performing the problem formulation phase of an ecological risk assessment, the

five-step planning and scoping process identified for human health risk assessments is a helpful

tool to get the right people involved and the risk questions, expectations, and plans in place to

make the overall assessment go smoothly and in a scientifically responsible manner. Similar to

the human health evaluation process, the risk assessment and management team should be

assembled to start identifying the concern, identifying who needs to be involved in the risk

assessment process, determining the scope of the risk assessment, describing why there may be a

problem, and determining how the concern will be evaluated. 

23.3.1 What is the Concern?

In human health risk assessment and risk management, the assessors are dealing with a single

organism (human beings) and the precedent and rationale for specific risk management goals

(such as the 1x10-6 to 1x10-4 cancer risk range) are generally well established. The parallel

process for ecosystems, however, is not as easy to study or as straightforward to manage. To

begin with, it can be difficult to choose which of many organisms in a study area to evaluate. 

Moreover, there is little agreement on which (if any) organisms or ecosystems are important

enough to single out for protection. These factors make planning, evaluation, and management

of ecological risks more complicated and time-consuming (and often, more controversial). 

EPA’s Risk Assessment Forum developed draft guidance(6) to help decision-makers work with

risk assessors, stakeholders, and other analysts to plan for ecological risk assessments that will

effectively inform the decisions they need to make. Planning for ecological risk assessment

includes three primary steps:

1. Defining the risk management decision to be made, the context in which it will be made,

and its purpose. This includes articulating the decision or problem that the risk manager

faces, understanding the social and legal context for the decision, placing preliminary

boundaries on the scope of the risk assessment, and identifying who needs to be involved. 

Appropriately framing the context will help ensure that management objectives are relevant

to the risk manager’s decision and increase the likelihood that the information generated by

the risk assessment will be useful. 

2. Developing objectives. This starts with a clear statement of the problem, issue, or

opportunity identified in the first step and ends with a set of specific objectives which will

guide all of the remaining steps. An important determination is the “what to protect” (i.e., the

assessment endpoint) question for ecological issues and to describe what is at stake. Key

questions include:

– What should be protected? Define the entities, ecological processes, and geographic

areas to be considered. 

– How is “protection” defined? Define the ecological objectives. 

– What are the most important objectives and how can they be achieved? Review and

structure objectives.

In some cases, there is a strong consensus on “what to protect” (e.g., if a commercially

important resource such as a fishery is potentially exposed). In many other cases, it is not

always obvious to a risk manager or the public what features of an ecosystem are of potential

concern or what the broader consequences would be from adverse effects to those features. 
April 2004 Page 23-11

Assessment Endpoints

According to EPA’s Guidelines for Ecological Risk Assessment,

(1) an assessment endpoint is an

explicit expression of the environmental value that is to be protected, and is operationally defined by

an ecological entity and its attributes. For example, a particular area has air toxics releases that may

be affecting area salmon populations that are important for location recreation and commercial

fishermen as well as an important resource for a local Native American tribe. In the study area, the

salmon population is the valued ecological entity; reproduction and age class structure of a salmon

population are some of their most important attributes. An appropriate assessment endpoint for this

study area might be stated as salmon reproduction and age class structure. The ecological risk

assessment for this study area would be structured to evaluate whether this specific salmon population

is at risk from air toxics with regard to healthy reproductive ability and age class structure.

Given the diversity of species and other ecological attributes in almost any study area, the assessors

generally establish at least one assessment endpoint that will, together, provide an assessment of air

toxics impacts on the ecosystem as a whole. More than one assessment endpoint may be necessary at

the ecosystem level.

Developing a consensus on the specific risk management objectives may be a difficult and

time-consuming part of the planning and scoping process.

3. Identifying what information is needed to inform the decision. When identifying

information needs, planners are encouraged to think ahead about everything that will be

needed to decide what to do about identified risks. Ecological risk is part of the picture, but

issues such as feasibility, practicability, cost, and acceptability also need to be factored into

the decision. They should also consider who and what resources are available to perform the

ecological risk assessment. The aim of this step is to narrow down which questions the risk

assessment should address and identify those that will be addressed elsewhere.

The questions identified at this step will be examined during the remainder of the problem

formulation process. Management objectives are by definition closely related to the assessment

endpoints evaluated in ecological risk assessment, and it should be possible to characterize them

using the measures described below.

23.3.2 Identifying The Participants

The participants for the ecological risk assessment may include some of the same people as those

for the human health multipathway risk assessment (e.g., multimedia modelers that understand

how to model for both human and ecological receptors). However,

• Additional risk managers may be involved, including natural resource management agencies

such as the U.S. Fish and Wildlife Service; state, local, or tribal (S/L/T) fish and game

departments; and/or private-sector risk managers.

• The risk assessment technical team will need significantly different experts (e.g., aquatic

ecologists, experienced ecological risk assessors).
April 2004 Page 23-12

• The specific set of interested or affected parties may change or be expanded (e.g., different

environmental groups may be more concerned/involved; local fishermen may become

interested).

EPA’s Public Involvement Policy may be helpful in performing this task (see

http://www.epa.gov/stakeholders/policy2003/index.htm). Part V of this document provides

additional information on community involvement.

23.3.3 Determining the Scope of the Risk Assessment

The scope of the human health multipathway risk assessment may expand to include additional

exposure pathways and exposure routes, and to address ecological receptors of concern.

• The specific chemicals that will be the focus of the ecological risk assessment will generally

be those that persist, bioaccumulate, and biomagnify (the PB-HAP compounds); however, a

different set of PB-HAP compounds may be of more concern for the ecological risk

assessment than for human health risk assessment. As with human health risk assessment,

additional compounds may need to be added to the analysis, depending on study-area specific

considerations.

• The specific sources included in the analysis may be focused on the subset that releases most

or all of the identified COPECs.

• The physical boundaries of the study area may need to expand to include geographic areas

where COPECs may be transported after deposition (e.g., the COPECs may have the

potential to be deposited in a watershed and be carried out of the geographic area defined for

the human health multipathway modeling).

23.3.4 Study-Specific Conceptual Model

A study-specific conceptual model for the ecological risk assessment is developed using the

fundamental elements of the conceptual model developed for the human health multipathway

assessment as a starting point. Steps to develop the study-specific ecological risk conceptual

model include the following:

• Determine whether the set of potential sources and chemicals that were identified in the

human health multimedia risk assessment are appropriate for the ecological risk assessment.

• Consider expanding the set of potential sources, chemicals, and exposure pathways to include

those identified below (potential exposure pathways are listed in Exhibit 23-3).

• Identify ecological receptors of concern (see Section 23.3.4.1).

• Formulate a risk hypothesis that describes possible relationships between emissions of a

chemical, exposure, and assessment endpoint response, including the information that sets

the problem in perspective, as well as an identification of the proposed relationships that need

evaluation.
April 2004 Page 23-13

• Identify assessment endpoints and measures of effects (See Section 23.3.4.2).

Exhibit 23-3. Common Exposure Pathways Considered for

Ecological Air Toxics Risk Assessments

Direct exposure pathways:

air 6 soil 6 soil-dwelling biota

air 6 soil 6 water 6 aquatic biota

air 6 water 6 aquatic biota

air 6 water 6 sediment 6 aquatic biota

air 6 soil 6 water 6 sediment 6 aquatic biota

air 6 vegetation

Indirect exposure pathways:

air 6 vegetation 6 bird/mammal

air 6 soil 6 vegetation 6 bird/mammal

air 6 soil 6 water 6 aquatic biota 6 fish

air 6 soil 6 water 6 aquatic biota 6 fish 6 bird/mammal

air 6 water 6 aquatic biota 6 fish

air 6 water 6 aquatic biota 6 fish 6 bird/mammal

air 6 soil 6 water 6 sediment 6 aquatic biota 6 fish

air 6 soil 6 water 6 sediment 6 aquatic biota 6 fish 6 bird/mammal

Conceptual model diagrams, such as the example illustrated in Exhibit 23-4, are used (along with

the risk hypothesis) to select the pathways to be evaluated in the analysis phase of the ecological

risk assessment, as well as to assist in communication with risk managers.

As with human health risk assessments, the conceptual model for an ecological risk assessment

must provide both a graphical representation of the important exposure pathways that are

presumed to be occurring along with a written description that outlines each element of the

conceptual model. Taken together, these two parts of the conceptual model clearly identify the

sources of concern, the COPECs that will be evaluated, the exposure pathways, and the

assessment endpoints. Similar to conceptual models for human health analysis, the conceptual

model may be modified (perhaps a number of times) as more is learned about the study area.

23.3.4.1 Identifying Receptors of Concern

Ecological receptors of concern are an important part of the conceptual model. These may be

plants, animals, habitats, communities, or larger ecosystem elements. Specific receptors may be

of concern for a variety of reasons, including:

• The receptor (or one of it’s life stages) is particularly vulnerable or sensitive to one or more

COPECs;

• The receptor (usually a species or a community such as a wetland) is listed as endangered or

threatened or is otherwise given special legal protection by the state or federal government;
April 2004 Page 23-14

• The receptor plays an important part in the overall structure or function of the ecological

community or ecosystem;

• The receptor is of particular economic or cultural value to stakeholders.

Exhibit 23-4. Conceptual Model Diagram for Exposure of Piscivorous Birds to Air Toxics

Conceptual model diagrams are used, along with the risk hypothesis, to select the pathways to be

evaluated in the analysis phase of the ecological risk assessment, as well as to assist in communication

with risk managers. The three risk hypotheses in this hypothetical example are:

• Risk Hypothesis 1: Concentrations of chemical X in the surface water column are less than a

level known to cause adverse effects on survival and reproduction in Daphnia

– Mechanism: Chemical X causes mortality and inhibits larval development

• Risk Hypothesis 2: Dietary intake levels of chemical X in lake trout are less than a level known

to cause adverse effects on reproductive ability 

– Mechanism: Due to a lack of enzyme A in lake trout, chemical X rapidly accumulates in lipid

tissues and damages reproductive organs

• Risk Hypothesis 3: Dietary intake levels of chemical X in kingfisher chicks (passed to them by

their parents) is less than a level known to adversely affect their survival

– Mechanism: Chemical X accumulating in egg lipids is a metabolic toxin to the developing

embryo
April 2004 Page 23-15

For taxonomic, physiological, and exposure reasons, it is important to consider a broad range of

potential ecological receptors during problem formulation. For example, the types of adverse

effects that may occur to terrestrial plant communities (e.g., impacts to photosynthesis, nitrogen

fixation, nutrient uptake; foliar damage) are very different than the types of adverse effects that

may occur to terrestrial mammals. Many ecological receptors (e.g., molds, lichens, many

invertebrates) have unique physiological and biochemical features that may make them

particularly sensitive to air toxics. Sensitive life stages often are a particular concern. In surface

waters and sediments, early life stages (e.g., eggs, larvae) may be particularly sensitive to

contaminants due to their small size (e.g., contaminants may readily penetrate cell membranes)

and developmental processes (e.g., major metamorphosis from one life stage to another). Many

terrestrial organisms (e.g., amphibians, dragonflies) have aquatic-dwelling early life stages. In

addition, many invertebrates that can bioaccumulate PB-HAPs (e.g., aquatic dwelling dragonfly

larvae) may be sources of food for sensitive life stages of other species (e.g., nestling birds).

Often it is important to understand the aquatic and terrestrial food webs in the habitats of concern

because these can be important parts of ecological exposure pathways. Top predators are often

of special concern for exposure to PB-HAP compounds.

Ecological receptors for each habitat potentially impacted should be identified to ensure (1) plant

and animal communities representative of the habitat are represented by the habitat-specific food

web, and (2) potentially complete exposure pathways are identified. Screening-level ecological

assessments often focus on the most sensitive organisms within an ecosystem or on the most

sensitive life stages within a species, if these are known. Ecological receptor identification may

need to include species both known and expected to be present in a specific habitat being

evaluated, and include resident and migratory populations. Consultation with ecological experts

is recommended. Potential sources of information include:

• Government Organizations. The U.S. Fish and Wildlife Service has biologists and other

ecological experts and also maintains National Wetland Inventory maps.(7) State Natural

Heritage Programs provide maps or lists of species based on geographic location, and are

very helpful in identifying threatened or endangered species or areas of special concern.

• Private or Local Organizations. Private or professional organizations that are examples of

sources of information include: National Audubon Society, the Nature Conservancy, local

wildlife clubs, and universities.

• General Literature. Monographs, field guides, and other literature describing the flora and

fauna of America and/or a particular region or state may be useful sources of information.

23.3.4.2 Identifying Assessment Endpoints and Measures of Effects

As previously noted, an assessment endpoint is an explicit expression of the environmental

value that is to be protected or is of concern. It includes the identification of the ecological entity

for the analysis (e.g., a species, ecological resource, habitat type, or community) as well as the

attribute of that entity that is potentially at risk and important to protect (e.g., reproductive

success, production per unit area, surface area coverage, biodiversity). The measures of effects

are the measures used to assess these endpoints.(8)
April 2004 Page 23-16

Generally, a manageable subset of the most important assessment endpoints is selected for the

risk assessment, and specific measures of effects that address each assessment endpoint are

identified. EPA guidance documents discuss additional issues that are important in the

identification of assessment endpoints.(9)

Appropriate selection of relevant assessment endpoints is critical so that the risk assessment

provides valuable information for the associated risk management decisions. Assessment

endpoints that can be measured directly are most effective, although assessment endpoints that

cannot be measured directly, but can be represented by measures that are easily monitored or

modeled, may also be used. Additional uncertainty is introduced depending on the relationship

between the measurement and the assessment endpoints. Exhibit 23-5 provides examples of

assessment endpoints, measures of effect, and other elements of the problem formulation phase.

EPA has recently released guidance that describes a set of endpoints, known as Generic

Ecological Assessment Endpoints (GEAE), that can be considered and adapted for specific

ecological risk assessments.(9) The entities and properties comprising the initial set of GEAEs is

presented in Exhibit 23-6. The EPA Guidance defines GEAE further and provides the basis for

the terms assessment community and assessment population, which are used in the definitions. 

In addition, EPA’s Science Advisory Board recently published a Framework for Assessing and

Reporting on Ecological Condition,

(10) which includes a checklist of ecological attributes that

should be considered when conducting ecological risk assessments and developing ecological

management objectives (Exhibit 23-7). Note that many of these GEAEs and attributes focus at

levels of ecological organization higher than organisms (e.g., species richness) or on ecological

processes (e.g., nutrient cycling) rather than attributes of organisms (e.g., growth, reproduction).

It often is useful to summarize the results of the problem formulation process in a problem

formulation summary that lists management objectives, assessment endpoints, and the structure

of the risk assessment from exposure scenarios through risk characterization. Exhibit 23-8

provides an example problem formulation summary.

23.3.5 Analysis Plan and Quality Assurance Program Plan (QAPP)

As noted in Parts II and III of this reference manual, the Analysis Plan and QAPP are formulated

by considering both the the conceptual model and the data quality required for the risk

management decision. The Analysis Plan and QAPP, including data quality objectives, are just

as important for the ecological risk assessment as they are for the human health risk assessment,

and in some cases may be more complex. The analysis plan for the ecological risk assessment

will need to match each of the elements of the conceptual model with the analytical approach that

will be used to develop data about the element, including: sources; exposed populations and

exposure pathways; exposure concentrations of COPEC; exposure conditions; toxicity of

COPECs; risk characterization; QA/QC; documentation; roles and responsibilities; resources;

and schedule.

Because the focus is on ecological receptors, additional types of monitoring (sampling and

analysis) may need to be conducted. For example, it may be important to measure concentrations

of COPECs in the sediments of surface water bodies as part of the analysis of direct exposures

for sediment-dwelling invertebrates as well as bioaccumulation from these invertebrates to

predatory fish through the aquatic food web.
April 2004 Page 23-17

Exhibit 23-5. Example of Ecological Risk Assessment Problem Formulation:

EPA’s Water Quality Criteria

A specific example of elements of the problem formulation step in a national-level ecological risk

assessment can be found in the development of Ambient Water Quality Criteria by EPA’s Office of

Water pursuant to the Clean Water Act (CWA).(11) Water quality criteria have been developed for the

protection of aquatic life from chemical stressors. The following elements of problem formulation

support subsequent analyses in the risk assessments used to establish specific criteria.

Regulatory Goal

• CWA Section 101: Protect the chemical, physical, and biological integrity of the Nation’s water.

Program Management Decisions

• Protect 99 percent of individuals in 95 percent of the species in aquatic communities from acute

and chronic effects resulting from exposure to a chemical stressor.

Assessment Endpoints

• Survival of fish, aquatic invertebrates, and algal species under acute exposure

• Survival, growth, and reproduction of fish, aquatic invertebrates, and algal species under chronic

exposure

Measures of Effect

• Laboratory LC50s for at least eight species meeting certain requirements

• Chronic no-observed-adverse-effect-levels (NOAELs) for at least three species meeting certain

requirements

Measures of Ecosystem and Receptor Characteristics

• Water hardness (for some metals)

• pH

The water quality criterion is a TRV derived from a distributional analysis of single-species toxicity

data. It is assumed that the species tested (which represent a range of taxonomic groups) adequately

represent the composition and sensitivities of species in a natural community.
April 2004 Page 23-18

Exhibit 23-6. Generic Ecological Assessment Endpoints(a)

Entity Attribute Identified EPA Precedents

Organism-level endpoints

Organisms (in an assessment

population or community)

Kills (mass mortality,

conspicuous mortality)

Vertebrates

Gross anomalies Vertebrates, shellfish, plants

Survival, fecundity, growth Endangered species, migratory

birds, marine mammals, bald

and golden eagles, vertebrates,

invertebrates, plants

Population-level endpoints

Assessment population Extirpation Vertebrates

Abundance Vertebrates, shellfish

Production Vertebrates (game/resource

species), harvested plants

Community and ecosystem-level endpoints

Assessment communities,

assemblages, and ecosystems

Taxa richness Aquatic communities, coral reefs

Abundance Aquatic communities

Production Plant assemblages

Area Wetlands, coral reefs,

endangered/rare ecosystems

Function Wetlands

Physical structure Aquatic ecosystems

Officially designated endpoints

Critical habitat for endangered

or threatened species

Area

Quality

Special places Ecological properties that

relate to the special or legally

protected properties

e.g., National Parks, National

Wildlife Refuges, Great Lakes

(a)Generic ecological assessment endpoints for which EPA has identified existing policies and precedents (in

particular, the specific entities listed in the third column). Bold indicates protection by federal statute.

Source: EPA’s Generic Ecological Assessment Endpoints (GEAE) for Ecological Risk Assessment(9)
April 2004 Page 23-19

Exhibit 23-7. Essential Ecological Attributes and Reporting Categories

Landscape Condition

• Extent of ecological system/habitat types

• Landscape composition

• Landscape pattern and structure

Biotic Condition

• Ecosystems and communities

– Community extent

– Community composition

– Trophic structure

– Community dynamics

– Physical structure

• Species and populations

– Population size

– Genetic diversity

– Population structure

– Population dynamics

– Habitat suitability

• Organism condition

– Physiological status

– Symptoms of disease or trauma

– Signs of disease

Chemical and Physical Characteristics

(Water, Air, Soil, and Sediment)

• Nutrient concentrations

– Nitrogen

– Phosphorus

– Other nutrients

• Trace inorganic and organic chemicals

– Metals

– Other trace elements

– Organic compounds

• Other chemical parameters

– pH

– Dissolved oxygen

– Salinity

– Organic matter

– Other

• Physical parameters

Ecological Processes

• Energy flow

– Primary production

– Net ecosystem production

– Growth efficiency

• Material flow

– Organic carbon cycling

– Nitrogen and phosphorus cycling

– Other nutrient cycling

Hydrology and Geomporphology

• Surface and groundwater flows

– Pattern of surface flows

– Hydrodynamics

– Pattern of groundwater flow

– Salinity patterns

– Water storage

• Dynamic structural characteristics

– Channel/shoreline morphology, complexity

– Distribution/extent of connected floodplain

– Aquatic physical habitat complexity

• Sediment and material transport

– Sediment supply/movement

– Particle size distribution patterns

– Other material flux

Natural Disturbance Regimes

• Frequency

• Intensity

• Extent

• Duration

Source: U.S. EPA. 2002. A Framework for Assessing and Reporting on Ecological Condition(10)
April 2004 Page 23-20

Exhibit 23-8. Example Problem Formulation Summary

1. Management Objective

• Bald eagle (entity), local population size (attribute), should be stable (desired state)

2. Assessment Endpoints

• Bald eagle (entity), reproduction (measurable attribute)

• Bald eagle (entity), chick survival (measurable attribute)

3. Exposure Scenario

• Sediment –> pore water –> benthic invertebtrates –> forage fish –> bald eagle

4. Risk Hypothesis

• Dose of chemical X to adult bald eagles from consumption of fish is less than a level known to

cause adverse effects on reproductive ability

– Mechanism: Chemical X damages reproductive organs (or interferes with egg shell

development)

• Dose of chemical X to bald eagle chicks (passed to them by their parents) is less than a level

known to adversely affect their survival

– Mechanism: Chemical X accumulating in egg lipids is a metabolic toxin to the developing

embryo

5. Metrics of Exposure

• Concentration of chemical X in fish

• Dose of chemical X received through consumption of fish

6. Measure of Effect

• TRV for chemical X (NOAEL or LOAEL) where adult reproduction was an endpoint

• TRV for chemical X (NOAEL or LOAEL) where chick survival (mortality) was and endpoint

7. Measure of Characteristics

• Proximity of bald eagle nest site to potentially contaminated foraging areas

• Proximity of alternative (non-contaminated) foraging areas to the nest site

8. Risk Characterization

• HQ = Oral Intake of chemical X/TRV (separate calculations for adults and chicks)
April 2004 Page 23-21

23.4 Tiered Ecological Risk Assessments

One of the key elements in the ecological risk assessment process is deciding if and when further

analysis is warranted. As with human health risk assessment, EPA recommends a tiered

approach to ecological risk assessment.(1) Each of these tiers follows the basic three steps

(problem formulation, analysis, and risk characterization) but with varying levels of complexity

in the assessment and with varying requirements for resources. Examples of the three tiers of

ecological risk assessment approaches are described briefly below.

• Screening-Level ecological risk assessments provide a general indication of the potential for

ecological risk (or lack thereof) and may be conducted for several purposes including: (1) to

prioritize COPECs based on their relative environmental behavior (e.g., relative potential for

bioaccumulation or to exhibit chronic toxicity) or determine their relative contribution to the

overall risk estimate; (2) to estimate the likelihood that a particular ecological risk exists; (3)

to identify the need for additional data collection efforts; or (4) to focus more detailed

ecological risk assessments where warranted. Screening assessments often use simplified

conservative assumptions in place of detailed modeling. For example, concentrations in

aquatic invertebrates or fish might be estimated from the modeled or measured water

concentrations (obtained as part of a multipathway human health risk assessment) and

available bioconcentration factors (BCFs) or bioaccumulation factors (BAFs). Another

example is the comparison of maximum sediment and water concentrations to screening level

TRVs. A screening level assessment, while abbreviated, is nonetheless a complete risk

assessment. Therefore, each assessment should include documentation supporting the risk

characterization and uncertainty analysis. Some examples of screening level TRVs used in

screening level ecological risk assessments are available from EPA’s draft Ecological Soil

Screening Level Guidance (http://www.epa.gov/superfund/programs/risk/ecorisk/

guidance.pdf) and EPA Region 4 (http://www.epa.gov/region4/waste/ots/ecolbul.htm).

• More Refined assessments are generally used to: (1) identify and characterize the current and

potential threats to the environment from an air toxics release; (2) evaluate the ecological

impacts of alternative emissions control or abatement policies; and (3) establish emissions

levels that will protect those natural resources at risk. A more refined assessment may

contain a more intensive evaluation than a screening level assessment, and usually employs

multipathway analysis to estimate if, and to what extent, ecological receptors (e.g., an oyster

fishery, a wild duck population, or a unique wetland community) may be exposed. The

exposure and potential impact are characterized and evaluated against predetermined

assessment endpoints (i.e., edibility of oysters, sustainability of the duck population,

maintenance of the integrity of the wetland community). This tier may be iterative. For

example, a multipathway analysis using conservative assumptions may first be performed to

identify whether any of the COPECs emitted from the sources in an area pose a potentially

significant concern to one or more ecological receptors. If so, a more detailed multipathway

risk assessment, using more site-specific data, may be performed. From this last stage a

detailed characterization of the environmental risks is developed.

• Probabilistic assessments are used to increase the strength of the predictive evaluation of

ecological risks, as well as help better evaluate distributions of observational data for an

ecological risk assessment. Screening-level and more refined assessments usually utilize

simplified point estimates in the development of a risk characterization, while the
April 2004 Page 23-22

Additional Reference Materials

EPA has developed extensive technical and policy guidance on how ecological risk assessments

should be planned and performed. These are available at EPA’s “Tools for Ecological Risk

Assessment” website http://www.epa.gov/superfund/programs/risk/tooleco.htm.

• EPA’s Guidelines for Ecological Risk Assessment, April 1998. This document expands upon and

replaces the earlier 1992 Framework for Ecological Risk Assessment.

• EPA’s Ecological Risk Assessment Guidance for Superfund (ERAGS): Process for Designing and

Conducting Ecological Risk Assessments, Interim Final, June 1997. This document includes

processes and steps for use in ecological risk assessments at Superfund sites. This document

supersedes the 1989 RAGS, Volume II, Environmental Evaluation Manual, Interim Final. 

Supplements to ERAGS include the Eco Updates (Intermittent Bulletin Series, 1991 to present),

which provide brief recommendations on common issues for Superfund ecological risk

assessments. The approaches and methods outlined in the Guidelines and in ERAGS are generally

consistent with each other.

• Risk Assessment Guidance for Superfund (RAGS): Volume 1–Human Health Evaluation Manual

(Part D, Standardized Planning, Reporting, and Review of Superfund Risk Assessments), June

2001. This guidance specifies formats that are required to present data and results in baseline risk

assessments at Superfund sites; many of these formats are useful for air toxics ecological risk

assessments.

• Policy Memorandum: Guidance on Risk Characterization for Risk Managers and Risk Assessors,

F. Henry Habicht, Deputy Administrator, Feb. 26, 1992. This policy requires baseline risk

assessments to present ranges of risks based on “central tendency” and “high-end” exposures with

corresponding risk estimates.

• Policy Memorandum: Role of the Ecological Risk Assessment in the Baseline Risk Assessment,

Elliott Laws, Assistant Administrator, August 12, 1994. This policy requires the same high level of

effort and quality for ecological risk assessments as commonly performed for human health risk

assessments at Superfund sites.

• Policy Memorandum: EPA Risk Characterization Program, Carol Browner, Administrator, March

21, 1995. This policy clarifies the presentation of hazards and uncertainty in human health and

ecological risk assessments, calling for clarity, transparency, reasonableness, and consistency.

• Issuance of Final Guidance: Ecological Risk Assessment and Risk Management Principles for

Superfund Sites. Stephen D. Luftig for Larry D. Reed, October 7, 1999. This document presents

six key principles in ecological risk management and decision-making at Superfund sites; these

principles are also useful for air toxics ecological risk assessments.

probabilistic tier of assessment uses probability distributions as inputs. Therefore, this tier

generally can yield risk estimates that allow for a more complete characterization of

variability and uncertainty. Although probabilistic assessments generally are resourceintensive, they may be especially valuable in situations when the risks are close to a policy

threshold or if the management decisions, if implemented, would require significant

expenditures.
April 2004 Page 23-23

1. U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. 

Risk Assessment Forum, Washington, D.C., April 1998. EPA/630/R095/002F. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12460.

2. U.S. Environmental Protection Agency. 1999. Residual Risk Report to Congress. EPA453/R-99-001. Available at: http://www.epa.gov/ttn/oarpg/t3/reports/risk_rep.pdf. 

3. U.S. Environmental Protection Agency. 1995. Ecological Risk and Decision Making 

Workshop. December 1995. EPA 230/B96/004B.

4. Moore, D.W.J., et al. 1999. A probabilistic risk assessment of the effects of methylmercury

and PCBs on mink and kingfishers along East Fork Poplar Creek, Oak Ridge, Tennessee,

USA. Environmental Toxicology and Chemistry 8:2941-2953.

5. Suter II, G.W., et al. 2000. Ecological Risk Assessment of Contaminated Sites. Lewis

Publishers, Boca Raton, FL. (see pages 228-231).

6. U.S. Environmental Protection Agency. 2001. Planning for Ecological Risk Assessment:

Developing Management Objectives (External Review Draft). Risk Assessment Forum,

Washington, D.C., June 2001. EPA/630/R01/001A. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=20683

7. U.S. Fish and Wildlife Service. 2003. National Wetlands Inventory Maps. Available at:

http://nwi.fws.gov.

8. U.S. Environmental Protection Agency. 1992. Framework for Ecological Risk Assessment.

Risk Assessment Forum, Washington, D.C., February 1992. EPA/630/R92/001.

9. U.S. Environmental Protection Agency. 2003. Generic Ecological Assessment Endpoints

(GEAE) for Ecological Risk Assessment. Risk Assessment Forum, Washington, D.C.,

October 2003. Available at: http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=55131.

10. U.S. Environmental Protection Agency. 2002. A Framework for Assessing and Reporting on

Ecological Condition. Science Advisory Board, Washington, D.C., September 2002. EPASAB-EPEC-02-009A. Available at: http://www.epa.gov/sab/pdf/epec02009a.pdf.

11. U.S. Environmental Protection Agency. 1985. Guidelines for Deriving Natural Numerical

Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses. Office of

Water Regulations and Standards, Washington, D.C. EPA/822/R85/100. Available at:

http://yosemite.epa.gov/water/owrccatalog.nsf/0/3fab714d53e9ae5385256b0600723bd3?Ope

nDocument

References
Chapter 24 Analysis: Characterization of Ecological

Exposure

Table of Contents

24.1 Introduction ............ ............ ............ ............ ............ ...... 1

24.2 Characterization of Exposure ............ ............ ............ ............ .... 1

24.2.1 Quantifying Releases ............ ............ ............ ............ .... 3

24.2.2 Estimating Chemical Fate and Transport ............ ............ ............ 3

24.2.2.1 Physical and Chemical Parameters ............ ............ .... 3

24.2.2.2 Multimedia Modeling ............ ............ ............ . . 3

24.2.2.3 Multimedia Monitoring ............ ............ ............ . 4

24.2.3 Quantifying Exposure ............ ............ ............ ............ ... 5

24.2.3.1 Metrics of Exposure ........................................ 5

24.2.3.2 Dimensions of Exposure ............ ............ ............ 7

24.2.3.3 Exposure Profile ............ ............ ............ ...... 9

24.2.3.4 Evaluating Variability and Uncertainty ............ ............ . 9

References ............ ............ ............ ............ ............ ............ 10

April 2004 Page 24-1

24.1 Introduction

As noted in the previous chapter, the analysis step of ecological risk assessment includes both

characterization of exposures and characterization of ecological effects. This chapter describes

the approaches and methods used for exposure characterization. Chapter 25 discusses the

approaches and measures used for characterization of ecological effects. The discussion in this

chapter is based largely on EPA’s Guidelines for Ecological Risk Assessment.

(1) Readers are

referred to that document for a more complete discussion of available approaches and methods.

24.2 Characterization of Exposure

Ecological exposure refers to the contact of an ecological receptor with an air toxic through

direct or indirect exposure pathways. As with human health risk assessment, characterization of

ecological exposure should initially evaluate (in the problem formulation phase) all exposure

pathways that are potentially complete. Unlike human health exposure, ecological risk

assessments will generally identify a limited number of specific metrics of exposure to actually

quantify since it is not usually possible to evaluate all exposure pathways for all the species or

other ecosystem attributes present in any given study area. Initially the assessors will generally

consider all exposure pathways broadly, but then identify the assessment endpoints which will

lead to a specific and narrowly defined set of exposure pathways to actually study in depth.

Ecological exposure pathways that are generally important for air toxics include all pathways

where contaminants are taken up directly from environmental media (e.g., air, soil, sediment, and

surface or rain water) for lower trophic level organisms (including plants) and ingestion of

contaminated plant or animal food items for higher trophic level receptors. Pathways that may be

important in specific cases include foliar and root uptake by plants, deposition and dermal

exposure pathways, and ingestion via grooming, preening, and food consumption.

Once the specific set of exposure pathways to be studied are determined (and the matching

assessment endpoints that are to be assessed are determined), characterization of ecological

exposure is based initially on information derived from modeling and/or existing monitoring

data. Later, additional modeling and/or site-specific empirical information may be obtained. The

objective of the exposure characterization is to produce a summary exposure profile that

identifies the exposed ecological entity, describes the course a stressor takes from the source to

that entity (i.e., the exposure pathway), and describes the intensity and spatial and temporal

extent of co-occurrence or contact (see Section 24.2.4.3). The exposure profile also describes the

influence of variability and uncertainty on exposure estimates and reaches a conclusion about the

likelihood that exposure will occur. Exhibit 24-1 provides a list of questions that can help define

the specific information needed to characterize exposure.

Exposure characterization includes the following steps, each of which is discussed in a separate

subsection below:(1)

• Quantifying releases of contaminants of potential ecological concern (COPEC);

• Estimating chemical fate and transport via modeling and/or monitoring;

• Quantifying exposure (e.g., exposure concentrations and dietary intakes);

• Evaluating uncertainty; and

• Preparing documentation.
April 2004 Page 24-2

Exhibit 24-1. Questions to Ask Concerning Source, Stressor,

Exposure, and Ecosystem Characteristics

Source and Stressor Characteristics

• What is the nature of the source(s) (e.g., point vs. nonpoint vs. mobile sources)?

• What is the intensity of the stressor (e.g., the dose or concentration of a chemical)?

• What is the chemical form of the stressor and its lability as a function of local physical-chemical

conditions?

• What is the mode of action? How does the stressor impact organisms or ecosystem functions?

• How does the stressor come into contact with a receptor (transport)?

Exposure Characteristics

• With what frequency does a stressor release occur (e.g., is it episodic or continuous; is it subject to

daily, seasonal, or annual periodicity)?

• What is the duration of release and exposure? How long does the stressor persist in the

environment (e.g., what is its half-life)?

• What is the timing of exposure? When does it occur in relation to critical organism life cycles or

ecosystem events (e.g., reproduction, lake overturn)?

• What is the spatial scale of exposure? Is the extent or influence of the stressor local, regional,

global, habitat-specific, or ecosystem-wide?

• What is the distribution? How does the stressor move through the environment (e.g., fate and

transport)?

Ecosystems Potentially at Risk

• What are the geographic boundaries of the study area? How do they relate to functional

characteristics of the ecosystem?

• What are the key abiotic factors influencing the ecosystem (e.g., climatic factors, geology,

hydrology, soil type, water quality)?

• Where and how are functional characteristics driving the ecosystem (e.g., energy source and

processing, nutrient cycling)?

• What are the structural characteristics of the ecosystem (e.g., species number and abundance,

trophic relationships)?

• What habitat types are present?

• How do these characteristics influence the susceptibility (sensitivity and likelihood of exposure) of

the ecosystem to the stressor(s)? For example, what portion of the receptor’s home range is in the

area of impact?

• Are there unique features that are particularly valued (e.g., the last representative of an ecosystem

type)?

• What is the landscape context within which the ecosystem occurs?

Source: EPA Guidelines for Ecological Risk Assessment(1)
a

EPA’s Science Policy Council is embarking on the development of an assessment framework for metals. 

The first step in the process is formulation of an Action Plan that will identify key scientific issues specific to metals

and metal compounds that need to be addressed by the framework, potential approaches to consider for inclusion in

the framework (including models and methods), an outline of the framework, and the necessary steps to complete the

framework.

April 2004 Page 24-3

24.2.1 Quantifying Releases

The process used to quantify releases of air toxics for purposes of ecological risk assessment is

identical to that for the human health analyses (see Chapter 7).

24.2.2 Estimating Chemical Fate and Transport

The process and methods used to estimate chemical fate and transport generally are similar to

those used for multipathway human health risk assessments. Key differences and special

considerations are highlighted in the subsections that follow.

24.2.2.1 Physical and Chemical Parameters

The same physical and chemical parameters identified in Chapter 17 affect the persistence of air

toxics in the environment and their potential to accumulate in ecological food webs. Additional

considerations are specific to ecological risk assessment.

• The bioconcentration factors (BCFs) and bioaccumulation factors (BAFs) used to

characterize ecological exposure may be different than corresponding factors used for the

human health exposure assessment. For example, wildlife may eat different species of

fish/shellfish than humans; these may have different BCFs or BAFs. Also, whole-fish BCFs

or BAFs are used for ecological exposure rather than those specific to the parts of the fish

people normally eat (e.g., fillets).

• Chemical speciation (e.g., for metals such as mercury) may be an important determinant of

exposure and bioavailablity.(a)

• Fate and transport analysis may need to examine a wider range of lower-trophic level

organisms to assess impacts to the communities and ecosystems of interest as well as to

develop exposure estimates for ecological food webs.

24.2.2.2 Multimedia Modeling

As with human health exposure assessment, some combination of multimedia modeling and

monitoring is generally used for ecological exposure assessment. The appropriate mix of

modeling and monitoring will depend on the level of assessment and the risk management goals. 

Modeling is relatively easy and inexpensive to implement and can be used to evaluate not only

risks from current levels of contamination, but also how risks might change over time (e.g.,

concentrations of persistent bioaccumulative hazardous air pollutant [PB-HAP] compounds in

fish may slowly increase over time in the presence of a continuous release) or as a result of
April 2004 Page 24-4

potential changes in land use (a change in land use might alter a number of habitat factors that

influence the number and identity of ecological receptors). The modeling approach, however,

has inherent uncertainties, which may lead to either over- or underestimates of exposure.

Model choices range from simple, screening-level procedures that require a minimum of data to

more sophisticated methods that describe processes in more detail, but require a considerable

amount of data. The same multimedia models used for the multipathway human health exposure

assessment generally can be used for at least part of the ecological exposure assessment (e.g., the

same models can be used to estimate concentrations in abiotic media at specific locations,

whether for human health or ecological exposure assessment). However, choice of specific

exposure points or areas may differ due to the focus on ecological receptors, as will the specific

food webs being evaluated. Specific models may also be configured in ways that facilitate

ecological exposure assessments. For example, TRIM (Total Risk Integrated Methodology)

includes a fate, transport, and ecological exposure model (TRIM.FaTE) which simulates

multimedia pollutant transfers and ecological receptor exposures in an ecosystem of interest (see

Part III).(2) However, other approaches (e.g., Multiple Pathways of Exposure) are not specifically

designed for ecological exposure assessment).

24.2.2.3 Multimedia Monitoring

The term monitoring in ecological risk assessment can also be more broadly used to mean

collection of any type of empirical field data for the assessment (e.g., plant counts and spatial

distribution in an assessment area). The use of monitoring in ecological risk assessment can

serve a number of purposes. For example, if there is a need to reduce uncertainties in the

predictive modeling approach, monitoring can be performed in various media and biota in the

study area. As with human health exposure assessment, monitoring can be used to confirm or

calibrate predictive modeling estimates of contaminant concentrations in media or biota.

For higher-tier risk assessments, monitoring for ecological exposures also may include sitespecific toxicity or bioaccumulation studies, in which test organisms are exposed to the actual

mixtures of contaminants from within the study area to develop site-specific and

chemical-specific toxicological and/or bioaccumulation relationships (See Chapter 25). 

However, poorly designed sampling or toxicological evaluations of environmental media from

the site may not allow a definitive identification of the cause of adverse response. For example,

receptor abundance and diversity as demographic data reflect many factors (e.g., habitat

suitability, availability of food, and predator-prey relationships). If these factors are not properly

controlled in the experimental design of the study (e.g., through use of a comparison site or a

gradient design that examines effects along a two-dimensional gradient downwind of sources),

conclusions regarding chemical stressors can be confounded. In addition, monitoring may not

provide sufficient information to develop estimates of potential risks should land use or exposure

change in the future.

Monitoring techniques for ecological exposure characterization may differ from those used for

multipathway human health exposure assessment. In particular, different species or components

of the food web may be of concern. For example, large invertebrates such as dragonfly larvae

often are a focus for ecological exposure assessments because they are important components of

surface water ecosystems as well as key prey items for both aquatic (e.g., fish) and terrestrial

(e.g., birds) predators. 
b

The TRIM.FATE model(6) can output estimates of ingestion intake at user-designated time points in a

dynamic simulation, and as an average over a user-designated period, as well as estimates for steady-state simulation.

April 2004 Page 24-5

Example Consideration in Monitoring: Soil Sampling for Ecological Risk Assessments

The depth over which surface soils are sampled should reflect the type of exposure expected in the

study area, the type of receptors expected in the study area, the depth of biological activity, and the

depth of potential contamination. For example, if exposures to epigeic (surface dwelling) earthworms

are a concern, concentrations in the first few inches of soil are most relevant. On the other hand, if a

burrowing mammal is of concern, concentrations at a depth of two or more feet may need to be

estimated. Careful consideration of the size, shape, and orientation of sampling volume is important

since they have an effect on the reported measured contaminant concentration values.(3) Selection of

sampling design and methods can be accomplished by use of the Data Quality Objectives (DQO)

process discussed in Chapter 7. Additional soil sampling guidance that may be consulted includes

EPA’s Preparation of Soil Sampling Protocols: Sampling Techniques and Strategies (4) and Guidance

for Data Usability in Risk Assessment.

(5)

24.2.3 Quantifying Exposure

Three elements are important components of quantifying exposure: the specific metrics of

exposures that are to be used, the dimensions of exposure, and the exposure profile. Each is

described in a separate subsection below. These estimates can be produced by some models such

as TRIM.FaTE.(6)

24.2.3.1 Metrics of Exposure

Depending on the specific receptors and pathways of concern, ecological exposure is quantified

generally in one of three ways.(1)

• Exposures to abiotic media may be evaluated using contaminant media concentrations as the

exposure concentrations – that is, concentrations of air toxics in soil, sediment, and/or

surface water at the exposure points. This is because the ecological toxicity reference levels

(TRVs) used to characterize risk are based on laboratory studies that directly relate

environmental concentrations in these media to adverse ecological impacts (e.g., a laboratory

study that dissolves known concentrations of a chemical in water and measures adverse

responses in the invertebrates or fish living in that water - the resulting concentration in water

that shows no effect is then compared to modeled or monitored concentrations of the

chemical in study area surface water).

• Exposures via the ingestion route of exposure may be evaluated using the average daily dose

(ADD), generally expressed as mg of chemical per kg of body weight per day (mg/kg-d). The

general formula

(b) for calculating ADD for ecological receptors is similar to that used for

human health ingestion exposure:(1)

(Equation 24-1)
April 2004 Page 24-6

where

ADD pot = Potential average daily dose, expressed in units of mg/kg-day.

Chemical-related variable:

Ck = Average contaminant concentration in the kth type of food, expressed in units

of mg/kg (wet weight)

Variables that describe the exposed ecological receptor population (also termed “wildlife

exposure factors”):

FRk = Fraction of intake of the kth food type that is from the contaminated area (unitless).

NIRk = Normalized ingestion rate of the kth food type of a wet-weight basis, expressed in

kg food/kg body-weight-day.

m = Number of contaminated food types

Exposure factors can be found in the EPA Wildlife Exposure Factors Handbook.

(7)

Contaminant concentration (Ck) is commonly estimated with the use of multimedia models. 

In some situations (e.g., a higher tier of analysis), Ck in food has been measured directly at the

point of contact where exposure occurs. An example is the use of food collected from the

mouths of nestling birds to evaluate exposure to pesticides through contaminated food.

Although such measurements can be difficult to obtain, they reduce the need for assumptions

about the frequency and magnitude of contact. 

• Exposures to some stressors are evaluated using uptake. Some stressors must be internally

absorbed to exhibit adverse effects. For example, a contaminant that causes liver tumors in

fish must be absorbed and reach the target organ to cause the effect. Uptake is evaluated by

considering the amount of stressor internally absorbed by an organism and is a function of the

following:

– Chemical form of the contaminant (speciation);

– Medium (sorptive properties or presence of solvents);

– Biological membrane (e.g., integrity, permeability); and

– Organism (e.g., sickness, active uptake).

Because of interactions among these factors, uptake will vary on a study-specific basis. Uptake

is usually assessed by modifying an estimate of the exposure concentration indicating the

bioavailable fraction (i.e., the proportion of the stressor that is available for uptake) actually

absorbed (e.g., monomeric aluminum is generally bioavailable to certain aquatic receptors while

polymeric aluminum generally is not). Absorption factors and bioavailability measured for the

chemical, ecosystem, and organism of interest are preferred. Internal dose can also be evaluated

using a physiologically-based pharmacokinetic (PBPK) model or by measuring biomarkers or

residues in receptors.

When using a tiered approach, conservative assumptions generally are used at the screening

level. Exhibit 24-2 presents examples of conservative assumptions; these are described in more

detail in EPA’s Guidelines for Ecological Risk Assessment.

(1)
April 2004 Page 24-7

Exhibit 24-2. Examples of Conservative Assumptions for Ecological Exposure Estimation

Exposure Factor Assumed Value

Area-use factor (factor related to home range

and population density)

100 percent (organism lives completely within area of

highest exposure concentrations)

Bioavailability 100 percent

Life stage most sensitive life stage

Body weight minimum possible

Food ingestion rate maximum possible

Dietary composition 100 percent of diet consists of the most contaminated

dietary component

The use of conservative assumptions should be informed by study-specific information. For example,

assuming 100 percent for area-use factor and diet would not be appropriate if study-specific

information indicates otherwise (e.g., the receptor is only present in the assessment area part of the

year). Similarly, use of the most sensitive life stage would only be appropriate if that life stage were

reasonably expected to be exposed to the chemical.

24.2.3.2 Dimensions of Exposure

Three dimensions are considered when quantifying exposure: intensity, time, and space.

• Intensity. Intensity is generally expressed as the amount of chemical contacted per day. 

Intensity may be affected by a number of factors, including the concentration of the chemical

in various media and biota and chemical form (e.g., speciation), which may affect toxicity,

bioavailabilty, and/or bioconcentration.

• Time. The temporal dimension has aspects of duration, frequency, and timing. For air toxics

assessments, intensity and time may sometimes be combined by averaging intensity over

time. Due to the emphasis on persistence and bioaccumulation, the focus of the ecological

exposure characterization for air toxics is generally on chronic (long-term) exposures. In

using predictive modeling to estimate exposure concentrations, an average annual

concentration generally is sufficient, at least for screening-level analyses. An exception

would include situations where the release and the presence of ecological receptors are both

periodic (e.g., releases are much higher in the spring and summer, when ecological receptors

are more abundant and active). If using predictive modeling to develop estimates of the

average daily dose (ADD), the duration of time modeled generally should be sufficient for

concentrations of air toxics in the media and biota of concern to reach equilibrium. If the

models indicate that equilibrium is not reached, the duration of time modeled generally

should be at least as long as the period of time over which releases are likely to occur (e.g.,

the design life of a specific facility). Timing is particularly important if the exposure

coincides with a sensitive life stage of the receptor organism.
April 2004 Page 24-8

• Space. Space is important because ecological risk assessments generally focus at the

population level or higher (e.g., community, ecosystem). Therefore, space is a measure of the

total fraction of the population, community, or ecosystem that is potentially exposed – a

factor that will impact the overall risk characterization. Space is generally expressed in terms

of areas (e.g., hectares, acres, square meters) that exceed a particular chemical threshold

level. However, another important spatial consideration is the fraction of the overall habitat

type that is potentially affected. At larger spatial scales, the shape or arrangement of

exposure may be an important issue, and area alone may not be the appropriate descriptor. 

Geographic Information Systems (GIS) have greatly expanded the options for analyzing and

presenting the spatial dimension of exposure (see Part VII of this reference manual for more

information about GIS). Several recent papers discuss ways to incorporate spatial

considerations in ecological risk assessments.

(8)

Sometimes, temporal and spacial considerations must both be considered together. For example,

in the case of acidic deposition, the andromous fish species in Maryland and other middleAtlantic states have a special risk scenario. Specifically, their spawning run occurs at the same

time when the weather pattern changes in the late winter and early spring from a coastal to a

continental pattern. This increases acidic deposition to the headwaters where the spawning

occurs and the eggs and hatchlings are at the most vulnerable part of their life cycle.

 Example PATCH Output.

Using Spatial Information in Ecological Exposure Assessment

Many terrestrial organisms that might be evaluated

in an ecological risk assessment are mobile. Where

these populations spend their time depends on the

locations of habitats necessary to provide food,

breeding sites, and protection from predators. 

Behaviors such as migration also affect locations of

receptor populations. Screening-level assessments

usually assume that the ecological receptors of

interest reside at the locations of the highest

exposures modeled. In subsequent tiers of analysis,

the assessor may spatially refine the exposure

estimate by considering the habitat use and foraging

areas of the receptor(s) of interest. GIS land cover

and land use information can be used to estimate

where an ecological receptor is likely to reside or

breed. For example, EPA’s Western Ecology Division of the National Health and Environmental

Effects Laboratory developed a model called Program to Assist in Tracking Critical Habitat

(PATCH), which can be used to generate “patch-by-patch” descriptions of landscapes, assessments of

the number, quality, and spatial orientation of breeding sites, and map-based estimates of the

occupancy rate. In the example output shown here, the medium grey areas denote

significant/acceptable habitat and the lighter gray (or light green) areas denote areas suitable for

breeding. This information can be used to identify where the ecological receptors are likely to reside

or breed, and the modeled exposure concentrations at those locations can be used in the risk

characterization calculations. The PATCH software and user’s guides are available at:

http://www.epa.gov/wed/pages/models/patch/patchmain.htm.
April 2004 Page 24-9

Questions Addressed by the Exposure Profile

• How may exposure occur?

• What may be exposed?

• How much exposure may occur?

• When and where may exposure occur?

• How may exposure vary?

• How uncertain are the exposure estimates?

• What is the likelihood that exposure will occur?

24.2.3.3 Exposure Profile

The final product of the ecological

exposure assessment is an exposure

profile. Exposure is generally described

in terms of intensity, space, and time, and

in units that can be combined with the

ecological effects assessment (see Chapter

25). The exposure profile identifies the

receptor and describes each exposure

pathway as well as the intensity, spatial

extent, and temporal extent of exposure. 

The exposure profile also describes the

impact of variability and uncertainty on exposure estimates and reaches a conclusion about the

likelihood that exposure will occur. Depending on the risk assessment, the exposure profile may

be a written stand alone document or a module of a larger document. In either case, the objective

is to ensure that the information needed for risk characterization has been collected, evaluated,

and presented in a clear, concise, and transparent way. The exposure profile also provides an

opportunity to verify that all of the important exposure pathways identified in the conceptual

model (i.e., those that support an evaluation of the assessment endpoints) were evaluated.

24.2.3.4 Evaluating Variability and Uncertainty

The exposure profile described in the previous section should aid understanding of how exposure

can vary depending on receptor attributes (exposure factors) or stressor levels. Variability can be

described qualitatively, by using a distribution or by describing where a point estimate is likely to

fall on a distribution. EPA policy recommends the use of both central tendency and high-end

exposure estimates.(9)

The exposure profile also should summarize important uncertainties (e.g., lack of knowledge),

including:

• Identification of key assumptions and how they were addressed;

• Discussion (and quantification, if possible) of the magnitude of modeling, sampling, and/or

measurement error;

• Identification of the most sensitive variables influencing the exposure estimate; and

• Identification of which uncertainties can be reduced through additional data collection,

modeling, or analysis (e.g., in a subsequent tier of analysis).

Professional judgment often is needed to determine the uncertainty associated with information

taken from the literature and any extrapolations used in developing a parameter to estimate

exposures. All assumptions used to estimate exposures should be stated, including some

description of the degree of bias possible in each. Where literature values are used, an indication

of the range of values that could be considered appropriate also should be indicated. The

uncertainty and variability associated with ecological effects criteria must also be taken into
April 2004 Page 24-10

1. U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. 

Risk Assessment Forum, Washington, D.C., April 1998. EPA/630/R095/002F. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12460.

2. U. S. Environmental Protection Agency. Total Risk Integrated Methodology (TRIM). 

Documentation is available at: http://www.epa.gov/ttn/FERA/urban/trim/trimpg.html.

3. U.S. Environmental Protection Agency. 2000. Draft Ecological Soil Screening Level

Guidance. Office of Emergency and Remedial Response, Washington, D.C., July 10, 2000.

4. U.S. Environmental Protection Agency. 1992. Guidance for Data Useability in Risk

Assessment (Part A). Office of Emergency and Remedial Response Publication 9285.7-09A,

PB92-963356, Washington, D.C., April 1992.

5. U.S. Environmental Protection Agency. 1992. Preparation of Soil Sampling Protocols:

Sampling Techniques and Strategies. Office of Research and Development, Washington,

D.C. EPA/600/R92/128.

6. U. S. Environmental Protection Agency. 2002. Total Risk Integrated Methodology. 

TRIM.FaTE Technical Support Document. Volume 1: Description of Module. EPA-453/R02-011a; Volume 2: Description of Chemical Transport and Transformation Algorithms. 

EPA/453/R-02/011b. Evaluation of TRIM.FaTE. Volume 1: Approach and Initial Findings. 

EPA/453/R-02/012; TRIM.FaTE User’s Guide. Office of Air Quality Planning and

Standards, Research Triangle Park, NC. These documents and information are available at:

http://www.epa.gov/ttn/fera/trim_fate.html#current_user.

7. U.S. Environmental Protection Agency. 1993. Wildlife Exposure Factors Handbook. Office

of Research and Development, Washington, D.C. EPA/600/R93/187. Available at:

http://cfpub.epa.gov/ncea/cfm/wefh.cfm?ActType=default.

8. Freshman, J.S., and Menzie, C.A. 1996. Two wildlife exposure models to assess impacts at

the individual and population levels and the efficacy of remedial actions. Human and

Ecological Risk Assessment 2:481-498.

Hope, B.K. 2001. A case study comparing static and spatially-explicit ecological exposure

analysis methods. Risk Analysis 21:1001-1010.

Linkov, I., Burmistrov, D., Cura, J., and Bridges, T.S. 2002. Risk-based management of

contaminated sediments: Consideration of spatial and temporal patterns in exposure

modeling. Environmental Science and Technology 36:238-246.

9. U.S. Environmental Protection Agency. 1992. Guidance on Risk Characterization for Risk

Managers and Risk Assessors. National Center for Environmental Assessment. Risk

Assessment Council, Washington, D.C. February 1992.

consideration. A more thorough description of how to deal with variability and uncertainty in the

risk assessment process is provided in Chapter 31.

References
Chapter 25 Analysis: Characterization of Ecological

Effects

Table of Contents

25.1 Introduction ............ ............ ............ ............ ............ ...... 1

25.2 Ecological Response Analysis ............ ............ ............ ............ ... 1

25.2.1 Stressor-Response Analysis ............ ............ ............ ........... 1

25.2.1.1 Ecological Effect Levels ............ ............ ............ 1

25.2.1.2 Selection of TRVs for a Particular Assessment ............ ...... 7

25.2.1.3 Stressor-Response Curves ............ ............ .......... 11

25.2.1.4 Species Sensitivity Distribution ............ ............ ...... 11

25.2.2 Linking Measures of Effects to Assessment Endpoints ............ ............ 12

25.3 Stressor-Response Profile ............ ............ ............ ............ ...... 16

25.4 Evaluating Variability and Uncertainty ............ ............ ............ ....... 16

References ............ ............ ............ ............ ............ ............ 16

April 2004 Page 25-1

25.1 Introduction

As noted in the previous chapter, the analysis step of ecological risk assessment includes

characterization of exposures and characterization of ecological effects. Chapter 24 described

the approaches and methods used for exposure characterization. This chapter describes the

approaches and measures used for characterization of ecological effects. The discussion in this

chapter is based largely on EPA’s Ecological Risk Assessment Guidelines.

(1) Readers are referred

to that document for a more complete discussion of available approaches and methods.

The methodology used to characterize ecological effects is generally similar to that used for

human health toxicity assessment. One of the distinctive features of ecological effects

characterization relates to the more general management goal of protecting a receptor population

or community rather than a single individual. This has led to the development of water,

sediment, and soil quality criteria that are designed to protect the communities of organisms that

inhabit surface waters and soils. It also provides the option of using a distribution or range of

values to characterize chemical toxicity (an option not generally available in human health risk

assessment).

Characterization of ecological effects involves describing the potential effects resulting from

exposure to a stressor, linking these effect to the assessment endpoints identified during problem

formulation, and evaluating the stressor-response relationship (i.e., how the effects will change

with varying stressor levels). The characterization begins by evaluating effects information to

specify the resulting effects, verifying that these effects are consistent with the assessment

endpoints, and confirming that the conditions under which the effects occur are consistent with

the conceptual model. Once this has been done, the effects characterization involves two

additional steps: (1) performing an ecological response analysis, and (2) developing a stressorresponse profile which also contains an analysis of uncertainty and variability. Each of these

additional steps is discussed in a separate section below.

25.2 Ecological Response Analysis

Ecological response analysis examines three primary elements: identifying stressor-response

relationships, establishing causality, and determining the linkages between measurable ecological

effects and assessment endpoints. Each is described in a separate subsection below.

25.2.1 Stressor-Response Analysis

Stressor-response analysis for ecological effects is functionally similar to dose-response analysis

for human health effects (e.g., see Chapter 12). The specific stressor-response relationship(s)

used in a given risk assessment depend on the scope and nature of the assessment as defined in

the problem formulation and reflected in the analysis plan. Three types of stressor-response

relationships are commonly used: point estimates, stressor-response curves, and cumulative

distribution functions. Each of these is discussed in a separate subsection below.

25.2.1.1 Ecological Effect Levels

Ecological effect levels are point estimates of an exposure associated with a given effect (e.g., a

concentration that results in 50 percent mortality in the exposed population, or LC50) used to
April 2004 Page 25-2

compare with an environmental exposure concentration. Data on the toxicity of a chemical is

usually obtained from laboratory studies in which groups of organisms (e.g., invertebrates,

benthic organisms, plants, earthworms, laboratory mammals, fish) are exposed to varying levels

of the chemical, and one or more responses (endpoints such as survival, growth, reproduction)

are measured. Various statistical methods are used to establish thresholds for adverse ecological

effects associated with acute or chronic exposures. Risk assessors often choose no-effect or loweffect levels as screening values. Stressor-response relationships may be relatively simple (as

illustrated in Exhibit 25-1) or may be very complex.

Exhibit 25-1. Hypothetical Simple Stressor-Response Relationship

Hypothetical relationship between intensity of stressor (in this example, concentration of a

chemical in water) and ecological response (in this example, percent mortality of an exposed

population of minnows). Different points on the curve represent, respectively, the

concentration resulting in 10 percent mortality (LC10), 50 percent mortality (LC50), and 90

percent mortality (LC90).

Several specific point estimates are commonly used to characterize ecological effects (Exhibit

25-2):

• Median effect concentrations or doses are those levels that result in effects that occur in 50

percent of the test organisms exposed to a stressor. The median effect level is always

associated with a time parameter (e.g., 24 hours, 48 hours). Because the tests used to derive

median effects levels seldom exceed 96 hours, these values are used primarily to assess acute

(short-term) exposures.
April 2004 Page 25-3

Exhibit 25-2. Commonly Used Point Estimates

Median effect concentrations or doses (acute exposures)

LC50 Concentration (food or water) resulting in mortality in 50 percent of the exposed organisms

LD50 Dose (usually in dietary studies) resulting in mortality in 50 percent of the exposed organisms

EC50 Concentration resulting in a non-lethal effect (e.g., growth, reproduction) in 50 percent of the

exposed organisms

ED50 Dose resulting in a non-lethal effect (e.g., growth, reproduction) in 50 percent of the exposed

organisms

Low- or no-effect concentrations or doses (chronic exposures)

NOAEL no-observed-adverse-effect-level, the highest dose for which adverse effects are not

statistically different from controls

LOAEL lowest-observed-adverse-effect level, the lowest dose at which adverse effects are

statistically different from controls

NOEC no-observed-effect-concentration, the highest ambient concentration for which adverse

effects are not statistically different from controls

LOEC lowest-observed-effect concentration, the lowest ambient concentration at which adverse

effects are statistically different from controls

MATC maximum acceptable toxicant concentration, the range of concentrations between the

LOEC and NOEC

GMATC geometric mean of the MATC, the geometric mean of the LOEC and NOEC

• Low- or no-effect concentrations or doses are derived from experimental data using

statistical estimates. The no-effect level is determined by experimental conditions as well as

the variability inherent in the experimental data. Thus, depending on experimental conditions

(e.g., the range of concentrations tested), two separate tests using the same chemical and the

same organism could result in different no-effect levels. Low- or no-effect levels are used

primarily to assess chronic (longer-term) exposures.
April 2004 Page 25-4

Point Estimates, TRVs, and Benchmarks

The terms Toxicity Reference Values (TRVs)

and Ecological Benchmarks are used to describe

those Point Estimates identified or derived for

use in ecological risk assessments. These

particular point estimates may be derived from a

single study (e.g., an NOEC or EC50) or from the

integration of multiple studies (e.g., water quality

criteria). When TRVs or benchmarks are drawn

from a single study, they are usually set in

consideration of multiple studies (e.g., from the

study most relevant to the purposes and specifics

of the assessment has been selected, or the most

sensitive result among the relevant studies)

A variety of different types of studies can be

used to develop ecological stressor-response

relationships, including field studies,

laboratory studies, and microcosm studies

(Exhibit 25-3).

For air toxics, stress-response analysis can

include both primary and secondary effects.

• Primary effects (e.g., lethality, reduced

growth, neurological/behavioral deficits,

impaired reproduction) result from

exposure of aquatic and terrestrial

organisms to air toxics. An example of a

chronic effect would be reduced

reproduction in a fish species exposed to

air toxics in a surface water body or in a terrestrial bird eating contaminated fish from a small

pond. An extreme example of an acute primary effect might be deaths of birds caused by

inhalation of a particular toxin. Toxic effects on survival, growth, development, and

reproduction might have population-level consequences for a species (e.g., result in local

population extinction over time) and are widely accepted as endpoints for characterizing

ecological risks. In recent years, more subtle effects have been investigated, including those

pertaining to clinical signs of poisoning, immunotoxicity, and even behavioral changes that

might influence survival, growth, development, or reproduction.

• Secondary effects (e.g., loss of prey species in the community) result from the action of air

toxics on supporting components of the ecosystem. These secondary effects occur through

biological interaction of one or more species’ populations with individuals or populations

that have been primarily affected. For example, exposure to an air toxic may adversely affect

one or more species of microscopic algae, bacteria, or fungus, which can adversely affect an

ecosystem’s nutrient cycling and primary production. This can lead to an alteration in the

abundance, distribution, and age structure of a species or population dependent on these

microscopic organisms, which can then lead to changes in competition and food web

interactions in other species. These ecosystem effects can be propagated to still other

populations, affecting their presence or representation within the ecosystem. A relatively

simple example of secondary effects involves the aerial application of pesticides that

dramatically reduced the population of an aquatic insect. This impact to the insect population

indirectly affects wild ducklings in the ecosystem, which depend on the insects as a food

supply.(2) Although it often is possible to identify the potential for secondary effects,

developing stressor-response functions for secondary effects (e.g., in a manner analogous to

that illustrated in Exhibit 25-2) is not an easy task. A recent paper provides one example of

the evaluation of secondary effects in ecological risk assessment.(3)

The use of the point estimate approach has some potential limitations. The most important is

that the point estimate established by a given study depends on both the range of doses tested and

the statistical power of the study (e.g., the ability to detect an effect if it occurs). For example,

studies with low power (e.g., those with only a few test animals per dose group) tend to yield

NOAEL or NOEC values that are higher than studies with good power (those with many animals
April 2004 Page 25-5

per dose group). In addition, the choice of some point estimates (e.g., NOEC and LOEC) is

restricted to concentrations that were tested, which may or may not be close to the

environmentally relevant concentrations, and this uncertainty increases as the interval between

doses increases. Finally, it is not always easy to interpret the significance of an exposure that

exceeds some particular point estimate, since the severity and incidence of response depends on

the shape and slope of the exposure response curve (information that is not captured in a point

estimate).

Exhibit 25-3. Types of Ecological Stressor-Response Studies

• Laboratory Studies. Most information on ecological stressor-response comes from laboratory

ecotoxicology studies using a generic set of species to represent different components of terrestrial

or aquatic ecosystems. For example, the freshwater crustacean Daphnia, is often used as a

surrogate for all small invertebrates that inhabit surface waters, and various species of minnows are

used as surrogates for fish. Laboratory studies are relatively easy and inexpensive to conduct, and

effects can be directly linked to exposure to a single air toxic. There is uncertainty, however, in

extrapolating the results from standard laboratory species to the wide array of species in the

environment or from the controlled laboratory conditions to the complex conditions that occur in

nature. Additionally, in most cases, laboratory studies are not designed to assess effects on

populations, communities, and ecosystems.

• Field Studies. Studies of wildlife, populations, communities, and ecosystems exposed to air toxics

in natural settings can provide valuable information on stressor-response effects. Field data can be

valuable in demonstrating the presence or absence of a cause-effect relationship that can provide a

basis for prioritization or for recognizing the efficacy of a risk reduction action. These studies also

can be used to assess stressor-response relationships for the site-specific mixtures of concern. 

However, the study organisms may be exposed to numerous types of stressors (chemical and nonchemical), and the effects of individual air toxics (and sometimes site-specific mixtures) may be

difficult to isolate. In addition, field studies are conducted infrequently due to the significant time

and resources required. Comparison of the study area to a control area is necessary to evaluate the

potential impact of the chemical release.

• Microcosm Studies. Microcosm studies use assemblages of several different taxa and

environmental media in an enclosed experimental system as a surrogate for natural ecosystems. 

Such studies can control for some of the uncertainty associated with multiple stressor exposure in

field studies. These studies also may provide information about food web dynamics and the

interactions of populations or organisms. As with field studies, microcosm studies are time and

resource intensive and, therefore, may be relatively uncommon for air toxic studies.

A variety of point estimates are used in ecological risk assessments. Some are developed from

acute (short-term) exposures; others are developed from chronic (long-term) exposures. Three

general types of point estimates are available for use in ecological risk assessments:

• Community-level criteria. EPA has developed ambient water quality criteria (AWQC) and

sediment quality criteria for the protection of aquatic communities. These values are based

on consideration of a cumulative distribution function (see Section 25.2.1.4). For example,

AWQC are designed to protect 95 percent of all aquatic species in freshwater or marine

environments. Criteria have been developed for both acute and chronic exposures, although

for a limited number of chemicals.
aNote that some ecological risk assessment guidance refers to the point estimates of ecological effects

selected for a given assessment as Toxicity Reference Values (TRVs), while others use the term ecological

benchmarks.

April 2004 Page 25-6

• Effect levels from laboratory toxicity tests. A variety of aquatic species are routinely used

in ecological toxicity tests, including fathead minnows (a small fish species) and Daphnia (a

tiny freshwater crustacean). Effects of concern can include acute effects such as mortality

(e.g., LD50) as well as chronic effects such as reproduction. Toxicity tests also are available

for terrestrial organisms (e.g., earthworms) and occasionally involve vertebrate species of

wildlife (e.g., the effects of polychlorinated biphenyls (PCBs) have been studied extensively

in mink).

• Effect levels from field bioassays. In some cases, ecological effects are evaluated directly

by exposing test organisms to ambient conditions. This most often is done where complex

mixtures of chemicals are present (e.g., in soils or sediments).

The point estimates employed in ecological risk assessments may be generally termed toxicity

reference values (TRVs).(a) They may be values taken from individual toxicity studies (e.g.,

NOECs or EC50s)or the result of integration of multiple studies (e.g., water quality criteria). 

TRVs may be developed for site-specific ecological receptors, depending on the importance of

those receptors to the local ecosystem, or for an endpoint not previously evaluated. For example,

while some TRVs may be based on survival, growth, and reproductive success of a population,

TRVs protective of a threatened or endangered species, a valuable game species (e.g., trout), or

an ecologically key species (e.g., wolf) might be based on an endpoint that is relevant to

individual organism health (e.g., a neurological deficit) rather than to population maintenance. 

On the other hand, TRVs based on higher effect levels (e.g., 20 to 50 percent or higher of the

population is affected) might be appropriate for species for which great functional redundancy

exists in the ecosystem (e.g., different herbaceous plants).(4)

Derivation of TRVs for pathways involving wildlife ingestion would require information on food

ingestion rates for sensitive and highly exposed animal species and information on the degree of

bioaccumulation in appropriate trophic components. Examples of these derivations for aquatic

systems can be found in the Great Lakes Water Quality Initiative (GLWQI) for mercury,

dichlorodiphenyltrichloroethane (DDT), PCBs, and dioxin (2,3,7,8-TCDD)(5) and for terrestrial

systems in the EPA methods of assessing exposures to combustor emissions.(6) EPA’s Wildlife

Exposure Factors Handbook (7) also provides data, references, and guidance for conducting

exposure assessments for wildlife species exposed to toxic chemicals in their environment.

EPA and other organizations have developed a number of types of TRVs based on data for a

chemical’s toxicity to freshwater or saltwater organisms (see Exhibit 25-4). Toxicity data for

longer term or chronic exposures generally will be more useful for an air toxics risk assessment;

however, short term or acute toxicity data may be used for chemicals that lack or have

incomplete chronic data. EPA has in the past used acute values in conjunction with conversion

factors (i.e., acute-to-chronic ratios) to estimate chronic toxicity values, specifically for the

derivation of chronic Ambient Water Quality Criteria and Great Lakes Water Quality Initiative

criteria for aquatic life.
April 2004 Page 25-7

25.2.1.2 Selection of TRVs for a Particular Assessment

In reviewing toxicity studies for potential use in identifying or developing specific TRVs to use

in a given assessment, the following questions should be considered: 

• What taxa are used in the study?

• Did the study present any significant methodological difficulties?

• Did the study identify a LOAEL?

• Were the adverse effects seen possibly related to growth and survival, or reproduction and

development?

• Did the study identify a NOAEL?

• Was the study duration appropriate to assess potential effects of chronic exposure?

If the test species are not within the taxonomic group of the ecological receptors of concern, the

study may need to be rejected because the test species are too distantly related to assume similar

physiological responses to a toxic agent.

Many studies may be of limited use in selection of TRVs. Potential deficiencies include:

• No control group was analyzed, or there was a high incidence of effects in the control group

(applies to laboratory studies);

• No reference area was analyzed, or there was a high incidence of effects in the reference area

(applies to field studies);

• No statistical analysis of results was conducted;

• In the case of fish/shellfish, body burdens were estimated, not measured;

• In the case of fish/shellfish, only fillet, carcass (guts, gills, and scales removed), or other body

part concentrations were measured, not the whole body;

• In the case of wildlife, insufficient data were provided to calculate the dose to the animal; and

• Multiple contaminants were present in the experimental studies.

Most environmental contamination concerns for air toxics that persist and bioaccumulate will

tend to be long-term and relatively low-level. As such, the most appropriate toxicity studies are

those evaluating chronic (long-term) toxicity or, if chronic studies are not available, subchronic

(medium-term) exposure durations. Although no one definition of “chronic” is accepted by

human or ecological toxicologists, the general concept is that the duration encompasses a

significant portion of the species life span (e.g., ten weeks for birds and one year for mammals).

“Subchronic” is commonly defined as a 90-day or longer study for mammals and 10 weeks or

fewer for birds. For aquatic bioassays, chronic tests may span multiple generations and assess

sensitive growth or reproductive endpoints. In mammalian and avian tests, the term average

daily dietary dose (e.g., expressed as mg/kg-day) generally implies chronic or subchronic

exposure.(8)

In order to develop TRVs (sometimes termed benchmarks) for avian and mammalian receptors,

Oak Ridge National Laboratory’s Toxicological Benchmarks for Wildlife,(11) and some

information from EPA’s Integrated Risk Information System(9) can be used (in a more limited

fashion). Information provided in these sources has to be modified using allometric information

available in EPA’s Wildlife Exposure Factors Handbook(7) to better represent potential wildlife

species sensitivity.
April 2004 Page 25-8

Exhibit 25-4. Sources of Ecological TRVs or Benchmarks

Data Source Available Toxicity

Reference Value(s)

Overview of Data Source and Values

EPA Office of Water

Ambient Water Quality

Criteria (AWQC)

• AWQC Chronic Criteria

• AWQC Acute Criteria

Note: many state water quality

standards are based on AWQC

EPA has developed national recommended water quality criteria for the

protection of aquatic life for approximately 150 pollutants. These

criteria are published pursuant to Section 304(a) of the Clean Water Act

(CWA) and provide guidance for States and Tribes to use in adopting

water quality standards under Section 303(c) of the CWA. 

Source: http://www.epa.gov/waterscience/criteria/aqlife.html

Great Lakes Water Quality

Initiative (GLWQI) Criteria

Documents

• GLWQI Tier I Criteria

• Final Chronic Values (FCVs)

GLWQI Tier I criteria and final chronic values (FCVs) are calculated

under the same guidelines as the Sediment Quality Criteria (SQC). 

Draft GLWQI criteria documents were released for public review and

were revised as necessary before they were published as “final.”

• Tier I Criteria are designed to be protective of aquatic communities

• FCVs are designed to measure chronic toxicity to aquatic organisms

Source: Final Water Quality Guidance for the Great Lakes System.

Federal Register, Mar. 23, 1995, vol. 60, no. 56, p. 15365-15424

EPA Soil Screening Levels • Soil screening levels EPA has developed a methodology and initial soil screening levels

protective of ecological receptors. 

Source: U.S. Environmental Protection Agency. 2000. Ecological Soil

Screening Guidance (Draft). Office of Emergency and Remedial

Response, Washington, D.C., July 2000. 

http://www.epa.gov/superfund/programs/risk/ecorisk/ecossl.htm.

EPA Region 4 Soil Screening

Levels

• Soil screening levels Source: U.S. Environmental Protection Agency. 1995. Supplemental

Guidance to RAGS: Region 4 Bulletins No. 2. Ecological Risk

Assessment. Region IV, Waste Management Division. 

http://www.epa.gov/region04/waste/ots/ecolbul.htm
Exhibit 25-4. Sources of Ecological TRVs or Benchmarks

Data Source Available Toxicity

Reference Value(s)

Overview of Data Source and Values

April 2004 Page 25-9

Ecotox Thresholds ECO

Update and EPA’s

Hazardous Waste

Identification Rule (HWIR)

documents

• GLWQI Tier II Criteria

• Secondary Chronic Values

(SCVs)

The GLWQI Tier II criteria and SCVs have received some peer review

prior to publication, and 12 of them are included in the HWIR, which

underwent public comment before promulgation. The GLWQI Tier II

methodology calculates SCVs in a similar way to FCVs, but uses

statistically derived “adjustment factors” and has less rigorous data

requirements.

• Tier II Criteria are designed to be protective of aquatic communities

• SCVs are designed to measure chronic toxicity to aquatic organisms

Source: Ecotox Thresholds ECO Update (volume 3, No. 2, January

1996, EPA/540/F-95/038).

ECOTOXicology database

(ECOTOX)

• Point Estimates from Chronic

Tests (e.g., EC50, EC10, LC50, or

GMATC)

• Point Estimates from Acute

Tests (e.g., LC50)

ECOTOX is a source for locating single chemical toxicity data for

aquatic life, terrestrial plants, and wildlife. ECOTOX was created and

is maintained by EPA’s Office of Research and Development and the

National Health and Environmental Effects Research Laboratory’s

Mid-Continent Ecology Division. ECOTOX is a source for locating

single chemical toxicity data from three EPA ecological effects

databases: AQUIRE, TERRETOX, and PHYTOTOX. AQUIRE and

TERRETOX contain information on lethal, sublethal, and residue

effects. AQUIRE includes toxic effects data on all aquatic species

including plants and animals and freshwater and saltwater species. 

TERRETOX is the terrestrial animal database. It primarily focuses on

wildlife species but the database does include information on domestic

species. PHYTOTOX is a terrestrial plant database that includes lethal

and sublethal toxic effects data. Source: http://www.epa.gov/ecotox.

Sediment Quality Criteria • Varies EPA and other agencies have developed sediment quality criteria for

the protection of benthic communities. These criteria are highly

specific to regions and bodies of water in the U.S. Regional experts are

the recommended source for appropriate site-specific criteria.
Exhibit 25-4. Sources of Ecological TRVs or Benchmarks

Data Source Available Toxicity

Reference Value(s)

Overview of Data Source and Values

April 2004 Page 25-10

Ecological Structure Activity

Relationships (ECOSAR)

• Estimated Chronic GMATC

• Estimated Acute Data (LC50 or

EC50)

ECOSAR is a computer program that uses structure-activity

relationships (based on available data) to predict the acute and chronic

toxicity of organic chemicals to aquatic organisms. ECOSAR provides

quantitative estimates of chronic values (e.g., GMATC), acute LC50

values, and acute EC50 values for industrial chemicals for several

aquatic species (e.g., fish, daphnia, green algae, mysids). When the

estimated aquatic toxicity value exceeds the water solubility of the

compound, the estimated value is flagged; this situation generally is

interpreted to mean that the chemical has no toxic effects in a saturated

solution. Source: http://www.epa.gov/oppt/newchems/21ecosar.htm

Exposure-Related Effects

Database (ERED)

Tissue-based effects values for fish

and benthic invertebrates

The U.S. Army Corps of Engineers Exposure-Related Effects Database

(ERED) lists toxicity information for a large number and wide

taxonomic range of fish and shellfish. ERED is constantly being

updated. Source: http://www.wes.army.mil/el/ered/

Jarvinen and Ankley

database

Fish and shellfish exposure and

effects information

The authors assembled a database of fish and shellfish exposure and

effect information. Source: Jarvinen and Ankley (1999)(9)

Oak Ridge National

Laboratory (ORNL) Soil

Invertebrate toxicity database

Acute and chronic TRVs for soil

invertebrates and microbial

processes

This report focuses on chemicals found at U.S. Department of Energy

(DOE) sites; however there are overlaps with air toxics (metals and

organics). Source: Efroymson et al. (1997);(8)

http://www.esd.ornl.gov/programs/ecorisk/documents/tm126r21.pdf

ORNL Plant toxicity

database

Acute and chronic TRVs for

terrestrial plants

This report presents a standard method for deriving TRVs, a set of data

concerning effects of chemicals in soil or soil solution on plants, and a

set of phytotoxicity TRVs for 38 chemicals potentially associated with

DOE sites. Source: Efroymson et al. (1997)(8)

ORNL Wildlife toxicity

database

Wildlife NOAEL and LOAELs This report presents both NOAEL- and LOAEL-based TRVs for

assessment of effects of 85 chemicals on 9 representative mammalian

wildlife species and 11 avian wildlife species.

Source: Sample et al. (1996)(10)
April 2004 Page 25-11

25.2.1.3 Stressor-Response Curves

One way to resolve some of the limitations in the TRV approach is to fit a mathematical equation

to the available exposure-response data and describe the entire stressor-response curve. Data

from individual experiments may be used to develop curves and point estimates both with and

without associated uncertainty estimates. The advantages of curve-fitting approaches include

using all of the available experimental data, the ability to interpolate to values other than the data

points measured, and an improved ability to extrapolate to values outside the range of

experimental data (e.g., for a low- or no-effect level). Curve-fitting often is used to extrapolate

from observed effects levels to develop estimates of NOAELs, NOECs, and/or GMATCs. 

Stressor-response curves can be developed using any convenient data fitting software, but EPA

has developed a software package specifically designed for this type of effort. This software is

referred to as the Benchmark Dose Software (BMDS). More information on this software can be

found on the National Center for Environmental Assessment’s webpage.(11) A disadvantage of

curve fitting is that the number of data points required may not always be available (e.g.,

especially for toxicity tests with wildlife species)

25.2.1.4 Species Sensitivity Distribution

In some cases, risk management decisions may also consider community-level effects as well as

population-level or sub-population effects (one example is the Ambient Water Quality Criteria

for the protection of aquatic life discussed in Section 25.2.1.1). That is, a stressor might be

considered to be below a level of concern for the sustainability of a community if only a small

fraction of the total number of exposed species are affected. In this case, toxicological responses

may be best characterized by the distribution of toxicity values across species. This is called a

Species Sensitivity Distribution (SSD). The SSD approach is generally used for communities

of aquatic receptors, since all of the different species that make up the community (e.g., all fish,

benthic invertebrates, aquatic plants, and amphibians that reside in a stream) will be exposed to

approximately the same concentration of contaminant in the water.

The process for generating an SSD consists of the following steps:

(1) Select an appropriate type of endpoint (e.g., lethality, growth, reproduction), and select an

appropriate type of point estimate from the exposure-response curve for each species. For

example, the TRV might be the LC50 for lethality or the EC20 for growth. The key

requirement is that the SSD be composed of TRVs that are all of the same type, not a

mixture.

(2) Collect all reliable values for that type of TRV from the literature for as many relevant

species as possible. When more than one value is available for a particular species, either

select the value that is judged to be of highest quality and/or highest relevance, or combine

the values across studies to derive a single composite value for each species. It is important

to have only one value per species to maintain equal weighting across species.

(3) Characterize the distribution of values across species with an appropriate SSD. Note that

there is no a priori reason to expect that an SSD will be well characterized by a parametric

distribution, so both parametric and empirical distributions should be considered.
April 2004 Page 25-12

Examples of Extrapolations

• Between taxa (e.g., minnow to rainbow trout)

• Between responses (e.g., mortality to growth

or reproduction)

• From laboratory to field

• Between geographic areas

• Between spatial scales

• From data collected over a short time frame to

longer-term effects

Once an SSD has been developed, the fraction of species in the exposed community that may be

affected at some specified concentration may be determined either from the empirical distribution

or from the fitted distribution. These distributions can help identify stressor levels that affect a

minority or majority of species.

A limiting factor in the use of SSDs is the amount of data needed as inputs. SSDs also can be

derived from models that use Monte Carlo or other methods to generate distributions based on

measured or estimated variation in input parameters for the models.

25.2.2 Linking Measures of Effects to Assessment Endpoints

As noted in Chapter 23, assessment endpoints

express the environmental values of concern

for the risk assessment; however they cannot

always be measured directly. For example,

the assessment endpoint may be maintaining a

healthy population of trout in a lake, but

measures of effect (e.g., toxicity tests) were

conducted on different species (e.g., fathead

minnows). Where there is a lack of time,

monetary resources, or practical means to

acquire more data, extrapolations may be the

only way to bridge the gap in available data. Two general approaches are used for such

extrapolations:

• Empirical extrapolations or process models. Empirical extrapolations use experimental or

observational data; process-based approaches rely on some level of understanding of the

underlying operations of the system of interest.

• Professional judgment. This is not as desirable as empirical or process-based approaches,

but it is the only option when data are lacking. However, professional judgment can be

credible, provided it has a sound scientific basis.

One of the most common types of extrapolations is that of effects observed in the laboratory

(e.g., toxicity tests) to those observed in the field. Exhibit 25-5 highlights the general questions

to consider when performing such an extrapolation.

When conducting field sampling or other monitoring studies, it sometimes is difficult to identify

exposure-response relationships. However, there are a number of reasons why a relationship

between a chemical and a toxic response in a natural system may not be apparent (Exhibit 25-6).

Therefore, the lack of an observed exposure-response relationship does not disprove that one or

more air toxics caused an apparent toxic effect. These sources of variation should be considered

during planning and scoping, but may not become apparent until field studies have begun.
April 2004 Page 25-13

Exhibit 25-5. Questions to Consider When Extrapolating from Effects

Observed in the Laboratory to Potential Effects in Natural Systems

Exposure Factors

• How will environmental fate and transformation of the air toxic affect exposure in the field?

• How comparable are exposure conditions and the timing of exposure?

• How comparable are the routes of exposure?

• How do abiotic factors influence bioavailability and exposure?

• How likely are preference and avoidance behaviors in the receptors of concern?

• How does life-stage affect exposure?

Effects factors

• What is known about the biotic and abiotic factors controlling populations of the receptors of

concern?

• To what degree are critical life-stage data available?

• How may exposure to the same or other stressors in the field have altered organism sensitivity?

Empirical approaches are derived from experimental data or observations. They commonly are

used when adequate effects data are available, but the understanding of the underlying

mechanisms, action, or ecological principles is limited. Two types of empirical approaches are

generally used:

• Uncertainty factors are derived numbers that are divided into measure of effects values to

derive an estimated level of stressor that should not cause adverse effects to the assessment

endpoint. An example might be an uncertainty factor of 10 to convert an acute LC50 value

into a presumed NOAEL. Uncertainty factors should be used with caution, especially when

used in an overly conservative fashion, as when chains of factors are multiplied together

without sufficient justification.

• Allometric scaling is used to extrapolate the effects of a chemical stressor on one species to

another species. Allometry is the study of change in the proportions of various parts of an

organism as a consequence of growth and development. Processes that influence

toxicokinetics (e.g., renal clearance, basal metabolic rate, food consumption) tend to vary

across species according to allometric scaling factors that can be expressed as a nonlinear

function of body weight. Allometric scaling factors are commonly used for human health

toxicity assessments (see for example Chapter 12), but have not been applied as extensively

to ecological effects.

When sufficient information on stressors and receptors is available, process-based approaches

such as population or ecosystem process models may be used. Process models allow information

on individual effects (e.g., mortality, growth, reproduction) to be extrapolated to potential

alterations in specific populations, communities, or ecosystems. Such models are particularly

useful in evaluating hypotheses about the duration and severity of impacts from a stressor on an

assessment endpoint (e.g., species diversity) that cannot be tested readily in a laboratory. Two

types of process-based models are commonly used:
April 2004 Page 25-14

Exhibit 25-6. Reasons Why Contaminant Concentrations in Ambient Media

May Not Be Correlated with Toxicity of Those Media

Variation in bioavailability

• Due to variance in medium characteristics

• Due to variance in contaminant age among locations (contaminants deposited to soil and sediments

may become less bioavailable over time due to sequestration)

• Due to variance in transformation or sequestration rates among locations

Variation in the form of the chemical (e.g., ionization state)

Variation in the concentration over time or space (i.e., samples for analysis may not be the same as

those tested)

• Spatial heterogeneity

• Temporal variability (e.g., aqueous toxicity tests last for several days but typically water from only

one day is analyzed)

Variation in the composition of releases (concentrations of components of releases other than the

individual air toxic that is believed to be the principal toxicant may vary over space and time, thereby

obscuring the relationship)

Variation in co-occurring contaminants (concentrations of contaminants from upgradient

[background] sources may vary over time)

Inadequate detection limits (if detection limits are too high, gradients of toxic effect may be

observed even when the chemicals are at the “not detected” levels)

Variation in toxicity tests

• Inherent variation

• Variation due to variance in medium characteristics (e.g., hardness, organic matter content, pH)

Source: Guidelines for Ecological Risk Assessment

(1)

• Single-species population models describe the dynamics of a finite group of individuals

through time. They have been used extensively in ecology and fisheries management to

assess the impacts of power plants and toxic chemicals on specific fish populations.

• Community and ecosystem models are particularly useful when the assessment endpoint

involves structural (e.g., community composition) or functional (e.g., primary productivity)

elements or when secondary effects are of concern.

Exhibit 25-7 provides further discussion of process-based models, highlighting a few models that

have been applied in ecological risk assessment.
April 2004 Page 25-15

Exhibit 25-7. Process-based Model Applications in Ecological Risk Assessment

Process-based models can help the assessor understand the potential significance of toxicant effects to

the population structure, and ecosystem models can help determine whether the effect may result in

secondary effects on other species in the system that are linked in the food web or on overall

ecosystem functions. Pastorok et al.

(12) review a number of population, and community and ecosystem

models, as well as software that implement these models.

Population models typically deal with the dynamics of the abundance or distribution of a single

species, sometimes with explicit descriptions of endpoints in time and space. These models can be

categorized as scalar abundance, life history, individual-based, and metapopulation models. The first

two types of models are highlighted here:

• Scalar abundance models, which represent populations as a single scalar dimension without a

breakdown of population age structure, are frequently used in screening assessments. These

models include Malthusian population growth models and logistic population growth models. 

• Life history models estimate population characteristics such as survival rates and fecundity as a

function of age or size/morphological state. These models are important because toxicants can

have a differential impact on different demographic sections of the same species. These models

include deterministic and stochastic age- or stage-based models, which are implemented in

software by programs such as RAMAS-Age®, -Stage®, -Metapop®, or -Ecotoxicology®; and ULM®.

Community and Ecosystem models are intended to describe ecological systems composed of

interacting species. These models incorporate species dynamics and specific biological interactions

(predator-prey, competition, dependence) to predict ecosystem endpoints such as species richness or

the productivity of a multi-species assemblage. Pastorok et al. categorize these models as food web,

aquatic, and terrestrial models.

• Food web models capture feeding relationships between all or some species in an ecological

community, thus determining population dynamics as well as identifying key exposure pathways

for bioaccumulative chemicals. These models include predator-prey models and populationdynamic food chain models, which are implemented in software such as RAMAS Ecosystem®

,

Populus®, and Ecotox.

• Aquatic ecosystem models are spatially aggregated models that represent biotic and abiotic

structures in combination with physical, chemical, biological, and ecological processes in rivers,

lakes, reservoirs, estuaries, or coastal ecosystems. A number of models exist for each type of

aquatic ecosystem. The standard water column model or SWACOM® requires the use of laboratory

data to predict changes in the parameters of an entire ecosystem. The extrapolation is

accomplished with knowledge of toxicological modes of action, and by simulation of the effects of

a toxic substance across different trophic levels according to the relationship between nutrients,

phytoplankton, zooplankton, and fish. AQUATOX (http://www.epa.gov/ost/models/aquatox/)

predicts the fate of various pollutants, such as nutrients and organic chemicals, and their effects on

the aquatic ecosystem, including fish, invertebrates, and aquatic plants. The Comprehensive

Aquatic Simulation Model (CASM) is a bioenergetics-based food web model that includes

phytoplankton, periphyton, macrophytes, zooplankton, benthic invertebrates, fish, bacteria, and

cyanobacteria.

• Terrestrial ecosystem models represent biotic and abiotic components in deserts, forests,

grasslands, or other terrestrial environments, and often include physical, chemical, biological, and

ecological processes. The primary endpoints of these models include the abundance of individuals

within species or guilds, biomass, productivity, and food-web endpoints such as species richness

or trophic structure.
April 2004 Page 25-16

1. U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. 

Risk Assessment Forum, Washington, D.C., April 1998. EPA/630/R095/002F. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12460.

2. Sheehan, P.J., Baril, A., Mineau, P., Smith, D.K., Harfenist, A., and Marshall,W.K. 1987. 

The Impact of Pesticides on the Ecology of Prairie-nesting Ducks. Technical Report Series,

No.19. Canadian Wildlife Service, Ottawa.

25.3 Stressor-Response Profile

The final product of an ecological response analysis is a summary profile in the form of a written

document or a component of a larger process model. The stressor-response profile should

address the following questions:

• What ecological entities are affected? These may include single species, populations, general

trophic levels, communities, ecosystems, or landscapes.

• What are the nature of the effects? The nature of effects should be germane to the assessment

endpoints. For example, if a single species is affected, the effects should represent

parameters (e.g., growth, reproduction) appropriate for that level of organization.

• Where appropriate, what is the time scale for recovery? Short- and long-term effects should

be reported as appropriate.

• How do changes in measures of effects relate to changes in assessment endpoints (see

Section 25.2.2 above)?

• What is the uncertainty associated with the analysis (see Section 25.4)?

25.4 Evaluating Variability and Uncertainty

The stressor-response profile described in the previous section should include an explicit

description of any uncertainties associated with the ecological response analysis. If it was

necessary to extrapolate from measures of effect to the assessment endpoint, both the

extrapolation and its basis should be described. Similarly, if a TRV was calculated, the

extrapolations, assumptions, and uncertainties associated with its development should be

described. The discussion also should include any information about known or potential

variability in a stressor-response profile (e.g., among different species or taxa).

Professional judgment often is needed to determine the uncertainty associated with information

taken from the literature and any extrapolations used in developing a parameter to estimate

stressor-response. All assumptions used to develop stressor-response relationships and TRVs

should be stated, including some description of the degree of bias possible in each. Where

literature values are used, an indication of the range of values that could be considered

appropriate also should be indicated. A more thorough description of how to deal with

variability and uncertainty in the risk assessment process is provided in Chapter 31.

References
April 2004 Page 25-17

3. Lohman, K. et. al. 2000. A probabilistic analysis of regional mercury impacts on wildlife. 

Human and Ecological Risk Assessment 6:103-130.

4. Lawton, J.H. and Brown, V.K. 1994. Redundancy in Ecosystems: Biodiversity and

Ecosystem Function. Springer-Verlag, Berlin Heidelberg, Germany. pp. 255-270.

5. U.S. Environmental Protection Agency. 1995. Great Lakes Water Quality Initiative Criteria

Documents for the Protection of Wildlife: DDT, Mercury, 2,3,6,8-TCDD, PCBs. Office of

Water, Washington, D.C. EPA/820/B95/008.

U.S. Environmental Protection Agency. 1995. Final Water Quality Guidance for the Great

Lakes System. Final Rule. Federal Register 60:15366, March 23, 1995.

6. U.S. Environmental Protection Agency. 1993. Addendum to the Methodology for Assessing

Health Risks Associated with Indirect Exposure to Combustor Emissions. Office of Research

and Development, Washington, DC. EPA/600/AP93/003.

7. U.S. Environmental Protection Agency. 1993. Wildlife Exposure Factors Handbook. Office

of Research and Development, Washington, D.C. EPA/600/R93/187. Available at:

http://cfpub.epa.gov/ncea/cfm/wefh.cfm?ActType=default.

8. Efroymson, R.A., Will, M.E., and Suter II, G.W. 1997. Toxicological Benchmarks for

Contaminants of Potential Concern for Effects on Soil and Litter Invertebrates and 

Heterotrophic Processes: 1997 Revision. Oak Ridge National Laboratory, Oak Ridge TN.

ES/ER/TM-126/R2. 

9. U.S. Environmental Protection Agency. 2003. Integrated Risk Information System. Office of

Research and Development, National Center for Environmental Assessment. Available at:

http://www.epa.gov/iris/.

10. Sample, B.E., Opresko, D.M., and Suter II, G.W. 1996. Toxicological Benchmarks for 

 Wildlife: 1996 Revision. June 1996. ES/ER/TM-86/R3. Available at: 

 http://www.hsrd.ornl.gov/ecorisk/tm86r3.pdf .

12. U.S. Environmental Protection Agency. 2003. Benchmark Dose Software. National Center

for Environmental Assessment. Available at: www.epa.gov/ncea/bmds.htm.

13. Pastorok, R.A., Bartell, S.M., Ferson, S., Ginzburg, L.R.(eds). 2002. Ecological Modeling

in Risk Assessment: Chemical Effects on Populations, Ecosystems and Landscapes. Lewis

Publishers, Boca Raton, FL.
Chapter 26 Ecological Risk Characterization

Table of Contents

26.1 Introduction ............ ............ ............ ............ ............ ...... 1

26.2 Risk Estimation ............ ............ ............ ............ ............ ... 2

26.2.1 Single-Point Exposure and Effects Comparisons ............ ............ ...... 2

26.2.2 Comparisons Involving the Entire Stressor-Response Relationship ............ .... 3

26.2.3 Comparisons Involving Variability ............ ............ ............ ..... 4

26.2.4 Process Models ............ ............ ............ ............ ........ 4

26.3 Risk Description ............ ............ ............ ............ ............ . . 5

26.3.1 Lines of Evidence ............ ............ ............ ............ ...... 5

26.3.2 Significance of the Effects ............ ............ ............ ............ 6

26.4 Risk Characterization Report ............ ............ ............ ............ .... 7

26.5 Evaluating Variability and Uncertainty ............ ............ ............ ........ 9

References ............ ............ ............ ............ ............ ............ . 9

a

Consistent with EPA’s Guidelines for Ecological Risk Assessment,(1) the term “lines of evidence” includes

a “weight of evidence” in order to emphasize that both qualitative evaluation and quantitative weighting may be

used.

April 2004 Page 26-1

26.1 Introduction

Similar to human health risk characterization, ecological risk characterization combines

information concerning exposure to chemicals with information regarding effects of chemicals to

estimate risks. The major difference in ecological risk characterization is the necessity for

estimating risks based on individual lines of evidence and then combining them through a

process of weighing the evidence.a Another difference is that in human health assessment, we

primarily consider health effects in the bodies of individual people. In ecological assessment, we

consider various “health” issues that can range from actual health effects in the bodies of

individual ecological receptors to something more attuned to the “health” of the ecosystem as

measured by species richness and diversity. This chapter provides an overview of the approaches

and methods used for ecological risk characterization. As before, additional information is

provided in EPA’s Guidelines for Ecological Risk Assessment,

(1) and readers are referred to that

document for a more complete discussion of available approaches and methods.

Risk characterization is the final phase of ecological risk assessment and is the culmination of the

planning and scoping, problem formulation, and analysis of predicted or observed adverse

ecological effects related to the assessment endpoints. It is also based on metrics of exposure and

ecosystem and receptor characteristics that are used to analyze air toxics sources, their

distribution in the environment, and the extent and pattern of contact. Risk characterization is

used to clarify the relationships between stressors, effects, and ecological entities, and to reach

conclusions regarding the occurrence of exposure and the likelihood of anticipated effects. The

results of the analysis phase are used to develop an estimate of the risk posed to the ecologically

valued entities that are the focus of the assessment endpoints.(2) After estimating the risk, the risk

estimate is described in the context of the significance of any adverse effects and lines of

evidence supporting their likelihood. Finally, the uncertainties, assumptions, and qualifiers in the

risk assessment are identified and summarized, and the conclusions are reported to risk

managers.

Conclusions presented in the risk characterization should provide clear information to risk

managers in order to be useful for environmental decision making. If the risks are not

sufficiently defined to support a management decision, risk managers may elect to proceed with

another iteration of one or more phases of the risk assessment process. Re-evaluating the

conceptual model (and associated risk hypotheses) or conducting additional studies may improve

the risk estimate. 

Characterization of ecological risk includes risk estimation, (usually a quantitative risk estimate;

see Section 26.2), risk description (Section 26.3), and documentation of results (Section 26.4).
April 2004 Page 26-2

(Equation 26-1)

26.2 Risk Estimation

Several general techniques are available for characterizing ecological risks associated with air

toxics that persist and bioaccumulate. These are divided broadly into single-point comparisons,

comparisons incorporating the entire stressor-response relationship, comparisons involving

variability in exposure and/or effects, and process models. Each is described in a separate

subsection below. EPA’s Guidelines for Ecological Risk Assessment(1) provides additional

discussion and examples of these techniques.

26.2.1 Single-Point Exposure and Effects Comparisons

The simplest approach for comparing exposure and effects estimates for air toxics ecological risk

assessments is the Hazard Quotient (HQ) approach (also referred to as the “quotient method”),

which is similar to that used for human noncancer health risk assessments (see Chapter 13). In

this approach, modeled or measured concentrations of the chemical in each environmental

medium are divided by the appropriate point estimate for ecological effects to yield a HQ for an

individual chemical.

where:

HQ = hazard quotient

Oral Intake = estimated or measured contaminant intake relevant to the oral intake-based

TRV (usually expressed as mg/kg-day)

TRV = Toxicity reference value. This may be in terms of oral intake, media

concentration, or body burden. As described elsewhere, it may be a result of a

single study (e.g., NOAEL) or the result of integration of multiple studies

(e.g., water quality criterion).

EEC = estimated or measured environmental media concentration at the exposure

point (usually expressed as mg/L for water and mg/kg for soil and sediment)

BB = estimated or measured body burden (usually expressed as mg/kg wet weight)

As with human health assessments, the measure of oral intake, EEC, or BB must be in the same

units as the TRV to which the measure is being compared.

As chronic risk will usually “drive” an ecological assessment, the HQ approach will usually be

employed for chronic exposure scenarios using chronic duration TRVs. For initial screening,

conservative exposure factors may be used (see Exhibit 24-2). As in human health risk

assessment, an HQ greater than one indicates the potential for adverse ecological effects to occur,

but does not predict their occurrence (see Chapter 13).
b

The HI approach is termed the “quotient addition approach” in EPA’s Guidelines for Ecological Risk

Assessment(1)

April 2004 Page 26-3

State Water Quality Standards

Pursuant to Section 303 of the Clean Water Act,

States have developed numerical water quality

standards for the protection of aquatic ecosystems. 

These standards generally are considered regulatory

requirements that must be met, and often are based

on EPA’s Ambient Water Quality Criteria (see

Chapter 25). If persistent, bioaccumulative

hazardous air pollutants (PB-HAPs) enter surface

waters, one way to assess risk is to compare the EEC

to a water quality standard using the HQ approach. 

State water quality standards can be accessed via

EPA’s national water quality standards database at

http://www.epa.gov/ost/wqs/.

When ecological toxicity data for complex mixtures are unavailable, the hazard index (HI)

approachb may sometimes be used in screening assessments, as scientifically appropriate, to

assess potential ecological risks associated with simultaneous exposure to multiple air toxics.(1) 

If the HI approach is used, the assumptions and associated limitations should be clearly

documented. It may often be the case that a single chemical is responsible for the HI exceeding

one, and the assessment can then focus on the HQ for that chemical. In more refined

assessments, an alternative approach may be necessary.

As with human health assessments, a

number of limitations restrict application

of the HQ approach. While a quotient can

be useful in answering whether adverse

effects are likely to occur or not, it may

not be helpful to a risk manager who

needs to make a decision requiring an

incremental quantification of ecological

hazard. For example, it is seldom useful

to say that a mitigation approach will

reduce the value of a quotient from 25 to

12, since this reduction cannot, by itself,

be clearly interpreted in terms of effects

on an assessment endpoint. Quotients

also may not be the most appropriate

methods for predicting secondary effects

(e.g., bioaccumulation, loss of prey species). Finally, in most cases the quotient does not

explicitly consider uncertainty, such as extrapolation from the test species to the species or

community of concern. Some uncertainties, however, can be incorporated into single-point

estimates to provide a statement of likelihood that the effects point estimate exceeds the exposure

point estimate (see Exhibit 26-1).(1)

26.2.2 Comparisons Involving the Entire Stressor-Response Relationship

If a curve relating the intensity or level of the stressor to the magnitude of response is available

(for example, see Exhibit 25-1), the risk characterization can examine risks associated with many

different levels of exposure. These estimates are particularly useful when the risk management

decision is not based on exceeding a pre-determined reference value or regulatory standard (e.g.,

a state water quality standard). This approach provides a predictive ability lacking in the hazard

quotient approach, and it may be used in screening level assessments or subsequent more refined

risk analyses. Because the slope of the effects curve relates the magnitude of change in effects to

incremental changes in exposure, the ability to predict changes in the magnitude and likelihood

of effects for different exposure scenarios can be used to compare different risk management

options. Also, uncertainty can be incorporated by calculating uncertainty bounds on the stressorresponse or exposure estimates. Limitations to this approach may include: lack of consideration
April 2004 Page 26-4

for secondary effects, assuming the exposure pattern used to derive the stressor-response curve is

comparable to the environmental exposure pattern; and failing to consider uncertainties such as

extrapolations from tests species to the species or communities of concern.

Exhibit 26-1. Example Comparison of Point Estimates with Associated Uncertainties

26.2.3 Comparisons Involving Variability

If the exposure or stressor-response profiles describe the variability in exposure or effects, then

many different risk estimates can be developed. Variability in exposure can be used to estimate

risks to moderately or highly exposed members of a population being investigated, while

variability in effects can be used to estimate risks to average or sensitive members of

populations. As an example, exposure can vary by life-stage (e.g., exposure may be greater

during spawning or migration). Likewise, effect may also vary by life-cycle (e.g., hatchlings may

be more sensitive to a chemical than are adults). A major advantage of this approach is its ability

to predict changes in the magnitude and likelihood of effects for different exposure scenarios and

thus provide a means for comparing different risk management options. Limitations include the

increased data requirements compared with previously described techniques and the implicit

assumption that the full range of variability in the exposure and effects data is adequately

represented. In addition, secondary effects are not readily evaluated with this approach. This

risk estimation technique would likely be used in more refined risk assessments. (A discussion

of probabilistic techniques, including Monte Carlo Simulation, is provided in Chapter 31.)

26.2.4 Process Models

Process models are mathematical expressions that represent understanding of the mechanistic

operation of a system under evaluation. They can be useful tools in both analysis and risk

characterization (process models are discussed briefly in Chapter 25). A major advantage of

using process models is the ability to consider “what if” scenarios and to forecast beyond the

limits of observed data that constrain approaches based solely on empirical data. Process models
April 2004 Page 26-5

also can consider secondary effects, and in some cases, the combined effects of multiple

stressors. Process model outputs may be point estimates, distributions, or correlations. 

However, since process models are only as good as the assumptions on which they are based, the

outputs from these models should be interpreted with care. The lack of knowledge on basic life

histories for many species, and incomplete knowledge about the structure and function of natural

ecosystems are some of the many uncertainties that need to be considered. These models are

complex and, are usually reserved for more refined risk assessments.

Risk Assessment Frontiers: Integrating Human Health and Ecological Risk Assessment

Many tribal cultures view ecological and human health in an integrated way such that they cannot be

easily separated. Similarly, there is some effort (especially in Canada) toward an integration of human

health and ecological assessment, as well as decision-making, in a field known as strategic

environmental assessment.

(3) This approach has not been applied widely in the United States, and it

remains to be seen how it will develop in the next few years.

The World Health Organization has published approaches to integrating human health and ecological

risk assessments to improve data quality and understanding of cumulative risks for decision making.(4)

This approach includes an integrated framework (modified from EPA’s guidance)(1) and case studies.

EPA, in its Framework for Cumulative Risk Assessment,

(5) offers a flexible structure for conducting

and evaluating cumulative risk assessment. By “cumulative risk,” EPA means “the combined risks

from aggregate exposures to multiple agents or stressors.” Agents or stressors may be chemicals, but

they may also be biological agents or physical agents, or an activity that, directly or indirectly, alters

or causes the loss of a necessity such as habitat.

26.3 Risk Description

The results of the risk characterization should be documented in the risk description, which

includes an evaluation of the lines of evidence supporting or refuting the risk estimate(s) and an

interpretation of the significance of the observed and/or predicted effects.

26.3.1 Lines of Evidence

The development of lines of evidence provides both a process and a framework for reaching a

conclusion regarding confidence in the risk estimate. Confidence in the conclusions of a risk

assessment may be increased by using several lines of evidence to interpret and compare risk

estimates. These lines of evidence may be derived from different sources or by different

techniques relevant to adverse effects on the assessment endpoints (e.g., hazard quotients,

modeling results, or field observational studies). There are three principal categories of factors to

consider when evaluating lines of evidence:

1. Data adequacy and quality. Data quality directly influences confidence in the results of a

risk assessment and the conclusions that can be drawn from the study. Specific concerns

include: whether the experimental design was appropriate for the questions being evaluated

in the risk assessment; whether data quality objectives were clear and adhered to; and

whether the analyses were sufficiently sensitive and robust to identify stressor-caused effects

in light of natural variability of the attributes of the ecological receptors of concern.
April 2004 Page 26-6

2. Relative uncertainty. One major source of uncertainty comes from extrapolations (e.g.,

from one species to another; from one temporal scale to another; from laboratory to field

effects). In general, the greater the number of extrapolations, the greater the uncertainty.

3. Relationship to the risk hypothesis. Finally, the relative importance of each line of

evidence may be determined by how directly they relate to the risk hypothesis developed

during planning and scoping. For example, lines of evidence based on a definitive

mechanism rather than associations alone are likely to be relatively important.

The evaluation of lines of evidence involves more than just listing the evidence that supports or

refutes the risk estimate. Each factor should be examined carefully, and its contribution in the

context of the risk assessment should be evaluated. For example, data or study results are often

not reported or carried through the risk assessment because they are of insufficient quality. If

such data or results are eliminated from the evaluation process, however, valuable information

may be lost with respect to needed improvements in methodologies or recommendations for

further studies.

When lines of evidence do not point toward the same conclusion, it is important to investigate

possible reasons for the disagreements. A starting point is to distinguish between true

inconsistencies and those related to methodology (e.g., statistical powers of detection). For

example, if a model predicts adverse effects that were not observed in the field, it is important to

determine whether the model predictions were unrealistic, or the experimental design of the field

study was inadequate to detect the predicted effects, or both.

26.3.2 Significance of the Effects

In this step, the significance of the observed or estimated changes in the assessment endpoints is

interpreted in light of the lines of evidence evaluated above. In this context, significance refers to

a conclusion as to whether the observed or estimated changes are considered “adverse.” Adverse

ecological effects represent changes that are undesirable because they alter valued structural or

functional attributes of the ecological receptors of concern (e.g., the loss of a keystone species). 

This determination is difficult and is frequently based on professional judgment. The assessment

of degree of adversity, along with other factors such as the economic, legal, or social

consequences of the ecological change, may be considered in the risk management decision. 

Unless an endangered or threatened species is at issue, society is generally not concerned with the

death of individual plants or animals, and therefore significance is generally assessed at the

population, community, or ecosystem level(s). The following factors may be used to evaluate the

degree of adversity (see also Exhibit 26-2):

• Nature and intensity of effects. This focuses on distinguishing adverse changes from those

that are within the normal pattern of ecosystem variability or that result in little or no

significant alteration of biota. For example, if survival of offspring will be affected, by what

percentage will it diminish, and is that likely to have a major impact on population dynamics? 

It is important to consider both ecological and statistical information in evaluating the nature

and intensity of effects. For example, a small change in a growth rate may not be statistically

distinguishable from natural variation; however, its impact may be more significant for a

population of slowly reproducing fish than for rapidly reproducing algae. When performing a

more refined assessment, it is necessary to compare the potentially impacted ecosystem to a
April 2004 Page 26-7

non-impacted ecosystem (i.e., a “control” site) so there is a basis for statistical comparisons

between the two systems.

Exhibit 26-2. Examples of Considerations for Determining Ecological Significance

• How large is the area where ecological criteria have been exceeded?

• What proportion of the habitat is affected at local, county, State, and national levels?

• Are the exposure concentrations and ecological criteria above background levels for the area of

interest?

• What types of ecological impacts have been associated with this pollutant or similar pollutants in

the past?

• Is the criterion or stressor-responsive curve based on high quality data (i.e., is there a high degree

of confidence in the criterion)?

• Spatial and temporal scale. The spatial dimension encompasses both the extent and pattern

of effect as well as the context of the effect within the broader ecosystem or landscape. 

Factors to consider include the absolute area affected, the percentage of area affected

compared with a larger area of interest, and the relative importance of the affected area(s) to

the ecological receptors of concern (e.g., are they critical breeding or overwintering areas?). 

For air toxics that persist and bioaccumulate, the temporal dimension of concern generally

will be in the years to decades range, although effects in other time frames may be important

in specific cases. Temporal responses for ecosystems may involve intrinsic time lags, so

responses to a stressor (or risk mitigation effort) may be delayed.

• Potential for recovery. Recovery refers to the rate and extent of return of a population or

community to some aspect of its condition prior to exposure to the stressor(s) of concern. 

Because ecosystems are dynamic, even under natural conditions, it is unrealistic to expect

that a system will remain static at some level or return to exactly the same state that it was

before it was disturbed. Thus, the “attributes” of a recovered population, community, or

ecosystem should be carefully defined. In general, changes that preclude recovery or result in

long recovery times are more significant than changes that allow rapid recovery. Note that

different components of a community or ecosystem may recover at different rates. For

example, stream chemistry may recover relatively rapidly after removal of a stressor, but reestablishment of predatory fish populations may take several years or more.

26.4 Risk Characterization Report

The information on estimates of ecological risk, the overall degree of confidence in the risk

estimates, lines of evidence, and the interpretation of the significance of ecological effects

generally is included in a risk assessment or risk characterization report. Exhibit 26-3 lists

the elements that generally are considered in the risk characterization report. A risk

characterization report may be brief or extensive, depending on the nature of and resources

available for the assessment. The report need not be overly complex or lengthy; it is most

important that the information required to support the risk management decision be presented

clearly and concisely. To facilitate mutual understanding, EPA policy(6) requires that risk

characterizations be prepared “in a manner that is clear, transparent, reasonable, and consistent

with other risk characterizations of similar scope prepared across programs in the Agency.” It

describes a philosophy of transparency, clarity, consistency, and reasonableness (TCCR), and
April 2004 Page 26-8

provides detailed approaches to achieving TCRR. Exhibit 26-4 provides an overview of the

TCRR principles (these are the same principles listed in Chapter 13).

Exhibit 26-3. Possible Risk Characterization Report Elements

• Describe risk assessor/risk manager planning results.

• Describe the scope of the assessment.

• Review the conceptual model and the assessment endpoints.

• Describe the measures of effect.

• Discuss the major data sources and analytical procedures used.

• Review the stressor-response and exposure profiles.

• Assign risks to the assessment endpoints, including risk estimates and adversity evaluations.

• Review and summarize major areas of uncertainty (as well as their direction) and the approaches

used to address them:

– Discuss the degree of scientific consensus in key areas of uncertainty;

– Identify major data gaps and, where appropriate, indicate whether gathering additional data

would add significantly to the overall confidence in the assessment results;

– Discuss science policy judgments or default assumptions used to bridge information gaps and

the basis for these assumptions; and

– Discuss how the elements of quantitative uncertainty analysis are embedded in the estimate of

risk.

Exhibit 26-4. Transparency, Clarity, Consistency, and Reasonableness Principles

Principle Definition Criteria for a Good Risk Characterization

Transparency Explicitness in the risk

assessment process

• Describe assessment approach, assumptions,

extrapolations, and use of models

• Describe plausible alternative assumptions

• Identify data gaps

• Distinguish science from policy

• Describe uncertainty

• Describe relative strength of assessment

Clarity The assessment itself is

free from obscure language

and is easy to understand

• Employ brevity

• Use plain English

• Avoid technical terms

• Use simple tables, graphics, and equations

Consistency The conclusions of the risk

assessment are

characterized in harmony

with EPA actions

• Follow statutes

• Follow Agency guidance

• Use Agency information systems

• Place assessment in context with similar risks

• Define level of effort

• Use review by peers

Reasonableness The risk assessment is

based on sound judgment

• Use review by peers

• Use best available scientific information

• Use good judgment

• Use plausible alternatives
April 2004 Page 26-9

1. U.S. Environmental Protection Agency. 1998. Guidelines for Ecological Risk Assessment. 

Risk Assessment Forum, Washington, D.C., April 1998. EPA/630/R095/002F. Available at: 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=12460.

2. U.S. Environmental Protection Agency. 1992. Framework for Ecological Risk Assessment.

Risk Assessment Forum, Washington, D.C., February 1992. EPA/630/R92/001.

3. Bonnell, S., and Storey, K. 2000. Addressing Cumulative Effects Through Strategic

Environmental Assessment: a Case Study of Small Hydro Development in Newfoundland,

Canada. Journal of Environmental Assessment Policy and Management 2: 477-499.

4. World Health Organization (WHO). 2001. Approaches to Integrated Risk Assessment. 

International Programme on Chemical Safety, Geneva. WHO/IPCS/IRA/01/12. Available at:

http://www.who.int/pcs/emerg_main.html

5. U.S. Environmental Protection Agency. 2002. Framework for Cumulative Risk Assessment.

Office of Research and Development, Washington, D.C., October 2002.

EPA/630/P-02/001A.

6. U.S. Environmental Protection Agency. 2000. Supplementary Guidance for Conducting

Health Risk Assessment of Chemical Mixtures. Risk Assessment Forum, Washington, DC.

EPA/630/R-00/002. Available at:

http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=20533.

26.5 Evaluating Variability and Uncertainty

An important part of the Risk Characterization Report is a discussion and assessment of

variability and uncertainty in all aspects of the ecological risk assessment. Note that ecological

risk assessments are subject to additional sources of uncertainty and variability as compared to

multipathway human health risk assessments. In addition to the uncertainties associated with

multimedia modeling and sampling, the ecological risk assessment involves many decisions

regarding choice of ecological receptors of concern and associated assessment and measures of

effect. Some of these may be at levels of organization above individual species (e.g.,

communities, ecosystems), where stressor-response relationships are poorly understood. Because

many different species and higher taxonomic groups may be included in the assessment, selection

of many parameter values such as bioconcentration factors, dose-response values, and dietary

intake is more complex and uncertain for the ecological risk assessment as compared to the

human health multipathway risk assessment.

References
PART V 

RISK-BASED DECISION MAKING

Introduction to Part V

Part V of this Reference Manual provides an overview of three components of risk-based

decision making.

• Risk Management (Chapter 27) refers to the regulatory and other actions taken to limit or

control exposures to air toxics, including the role of risk management in regulating hazards.

• Community Involvement (Chapter 28) is an integral part of many risk management strategies

because good community involvement helps ensure that the strategy selected will have the

highest likelihood of success. Various levels of community involvement are also required by

many laws. 

• Risk Communication (Chapter 29) describes the process of planning the risk assessment

(during scoping) and conveying the results of the risk assessment in a way that meets the

information requirements for the risk management decisions. This chapter discusses the

importance of risk communication, and planning and implementing a risk communication

strategy.

Chapter 27 Risk Management

Table of Contents

27.1 Introduction ............ ............ ............ ............ ............ ...... 1

27.2 Role of Risk Management in Regulating Hazards ............ ............ ............ 1

27.3 Types of Risk Management Decisions Related to Air Toxics ............ ............ ... 4

27.4 Use of Risk Estimates in Decision-Making ............ ............ ............ ..... 5

27.5 Process for Making Risk Management Decisions ............ ............ ............ 9

27.5.1 Define the Problem and Put it in Context ............ ............ ............ 9

27.5.2 Analyze the Risks Associated with the Problem in Context ............ .......... 9

27.5.3 Examine Options for Addressing the Risks ............ ............ .......... 11

27.5.4 Make Decisions about Which Options to Implement ............ ............ . . 12

27.5.5 Take Actions to Implement the Decisions ............ ............ ........... 13

27.5.6 Conduct an Evaluation of the Action’s Results ............ ............ ....... 14

27.6 Information Dissemination ............ ............ ............ ............ ..... 14

References ............ ............ ............ ............ ............ ............ 15

April 2004 Page 27-1

27.1 Introduction

This chapter introduces risk management, focusing on its role in addressing the risks that air

toxics pose. It provides an overview of the types of risk management decisions related to air

toxics, a discussion of how risks to individuals and populations are presented to the public, and

options for implementing decisions (e.g., regulation, voluntary risk reduction activities).

Specifically, risk management refers to the regulatory and other actions taken to limit or control

exposures to a chemical. Risk assessment, on the other hand, is a tool used to support risk

management decisions by providing quantitative and qualitative expressions of risk, along with

attendant uncertainties. Specifically, the risk assessment conveys a quantitative and qualitative

description of the types of impacts that may occur from exposure to an air toxic, the likelihood

that these impacts will occur given existing conditions, and the uncertainties surrounding the

analysis. Risk management considers these principle factors along with a variety of additional

information (which may include the cost of reducing emissions or exposures, the statutory

authority to take regulatory actions, and the acceptability of control options) to reach a final

decision.

27.2 Role of Risk Management in Regulating Hazards

Risk management may include implicit or explicit policy and value judgments. Therefore, one

would expect there to be differences of opinion concerning what represents an appropriate risk

management action. Even the most basic risk management decision can be highly controversial. 

A classic example is the decision(s) needed to answer the question how clean is clean? This

question refers to a risk management decision that must establish a target level to which existing

levels of contamination/pollution should be reduced. Establishing this level is not a trivial

matter. Working through these issues can be complicated by the different values of the

stakeholders and debates over individual perceptions about risk. As discussed below, many

authors and organizations stress the importance of understanding risk management mandates,

options, and concerns throughout the risk assessment process, from the initial problem

formulation steps to the final risk characterization and risk communication. Many of the critical

decisions in structuring the technical risk assessment depend on risk management concerns (e.g.,

what risk management options are feasible, what level of certainty in the risk estimate is

acceptable).

Although the National Academy of Sciences and others stress the distinction between risk

assessment and risk management, they also stress the integration of the two efforts (see Exhibit

27-1). Risk assessments are often designed and conducted with awareness of the risk

management options available to decision-makers and the social, economic, and political context

in which those decisions are to be made. Likewise, periodically reviewing the risk management

options during the risk assessment effort ensures that the results of the risk assessment will

provide meaningful input into the decision-making process. The National Research Council

(NRC) of the National Academy of Sciences (NAS), in their 1983 study entitled Risk Assessment

in the Federal Government: Managing the Process (the “Red Book”),(1) advocated a clear

conceptual distinction between risk assessment and risk management, noting, for example, that

maintaining the distinction between the two would help to prevent the tailoring of risk

assessments to the political feasibility of regulating a chemical substance. However, the NRC

also recognized that the choice of risk assessment techniques could not be isolated from society’s
April 2004 Page 27-2

risk management goals. Ultimately, the risk assessors should be aware of risk management

goals; however, the fundamental science performed in the risk assessment should be impartial

and based on the factual base of information, to the extent possible.

Exhibit 27-1. Illustration of the Integration Between Risk Assessment and Risk Management

Use of the Term “Safe”

Safe: Condition of exposure under which there is a practical certainty that no harm will result

to exposed individuals (as defined in EPA’s Terms of Environment).

Safe: Free from harm or risk (as defined in the Merriam-Webster Collegiate Dictionary).

During government and community interactions and risk communication, it is important to be

sensitive to perceived meanings of the term “safe.” Regulators and scientists are often reluctant to use

the term “safe,” because many people understand “safe” to mean “zero risk.” Ideally, one would like

to eliminate all risks, but this is usually not a realistic expectation. Regulators commonly work to

address the most important risks and decrease them to the level at which they believe the risks are

smaller than the benefits of the activity causing the problem (in this case, risk from exposure to air

toxics). They commonly refer to this level as “acceptably low risk.” 

However, community members may become frustrated with regulators who are reluctant to use the

term “safe,” potentially perceiving the regulators’ choice of words as a dodge of the issue. Therefore,

it is important for government representatives to address perceptions of the meaning of safe during

risk communication and, as appropriate, use risk comparisons to help in communicating the concepts

of safe versus acceptably low risk. Information on risk communication is provided in Chapter 29, and

Section 29.4 provides specific information about risk comparisons.
April 2004 Page 27-3

The NRC, in their 1994 report, Science and Judgment in Risk Assessment (the “Blue Book”),(2)

noted that, while the Red Book emphasized the distinction between risk assessment and risk

management, the purpose of separation was not to prevent any exercise of policy judgment when

evaluating science or to prevent risk managers from influencing the type of information that

assessors would collect, analyze, or present. The Blue Book concluded further that the sciencepolicy judgments that EPA makes in the course of a risk assessment would be improved if there

were more clearly informed by the Agency’s priorities and goals in risk management. Protecting

the integrity of the risk assessment, while building more productive linkages to make risk

assessment more accurate and relevant to risk management, is essential.

The integration between risk assessment and risk management also has been emphasized by

Presidential/Congressional Commission on Risk Assessment and Risk Management. In their

Reports Framework for Environmental Health Risk Management and Risk Assessment and Risk

Management In Regulatory Decision-Making (the two-volume “White Book”),(3) the

Commission developed a six-stage integrated framework for environmental health risk

management that can be applied to most situations (Exhibit 27-2):

1. Define the problem and put it in context;

2. Analyze the risks associated with the problem in context;

3. Examine options for addressing the risks;

4. Make decisions about which options to implement;

5. Take actions to implement the decisions; and

6. Conduct an evaluation of the action’s results.

Exhibit 27-2. The CRARM Framework for Risk Management
April 2004 Page 27-4

Not All Risk Management

Decisions are Regulatory

Some risk management decisions are made by

EPA or state, local and tribal (S/L/T) regulators

pursuant to specific statutory criteria. However,

government agencies may have limited authority

to impact many other decisions. For example,

some decisions are made by the individuals who

own or operate the facilities that release air

toxics, while others are made by citizens who are

being impacted by emissions. Risk management

decisions may need to consider looking beyond

technological solutions.

The Commission noted that the process of examining risk management options does not have to

wait until the risk analysis is completed, although a risk analysis often will provide important

information for identifying and evaluating risk management options. In some cases, examining

risk management options may help refine a risk analysis. The Commission also recommended

that all of these steps involve stakeholders (see Chapter 28).

When discussing risk management, it is important to consider where and how changes or

interventions may occur in the causal sequence of environmental impacts since interventions may

reduce pollutants a number of ways along the critical path of environmental impacts. For

example, interventions such as changing manufacturing processes, implementing emissions

controls, or influencing worker behaviors that actively reduce exposure may have a positive

mitigating effect on environmental impacts. In the discussion of risk management that follows, it

is critical to keep in mind the range of ways in which environmental risks can be mitigated; it is

up to the risk managers to determine the most feasible and critical “points of entry” along the

path when developing a risk management strategy.

27.3 Types of Risk Management Decisions Related to Air Toxics

Two general categories of risk management

decisions are relevant to air toxics: emissions

control and siting.

• Emissions control. Emissions control

decisions may involve “command-andcontrol” decisions (e.g., emissions limits)

or incentives (e.g., tax credits for reduced

emissions). EPA’s preference is to

encourage pollution prevention whenever

feasible (see Exhibit 27-3). Emissions

control decisions are most likely to

involve formal risk assessments.

• Siting/locating. These decisions involve

where to locate industrial facilities,

businesses, waste disposal facilities, and transportation routes. Siting decisions are typically

made by S/L/T governments through mechanisms such as zoning, deed restrictions and other

property controls, and in some cases regulation. Many of these decision-making processes

include public involvement in which citizens may seek to influence the final decision. Siting

decisions may involve assessment of environmental impacts pursuant to the National

Environmental Policy Act, other federal statutes, or similar state statutes. Siting decisions

may increasingly involve air toxics risk assessments.
April 2004 Page 27-5

Exhibit 27-3. Pollution Prevention Hierarchy

In the Pollution Prevention Act of 1990, Congress established a hierarchy for the handling of pollution

(see graphic). The Act established as United States policy that pollution should be prevented or

reduced at the source whenever feasible, that pollution that cannot be prevented should be recycled in

an environmentally safe manner whenever feasible, and that pollution that cannot be prevented or

recycled should be treated in an environmentally safe manner whenever feasible. Disposal or other

release into the environment should be

employed only as a last resort and

should be conducted in an

environmentally safe manner.

Pollution prevention is the reduction or

elimination of pollutants at the source. 

As defined in the Pollution Prevention

Act, “source reduction” means any

practice which (1) reduces the amount of

any hazardous substance, pollutant, or

contaminant entering any waste stream

or otherwise released into the

environment (including fugitive

emissions) prior to recycling, treatment,

or disposal, and (2) reduces the hazards to public health and the environment associated with the

release of such substances, pollutants, or contaminants. It includes equipment or technology

modifications, process or procedure modifications, reformulation or redesign of products, substitution

of raw materials, and improvements in housekeeping, maintenance, training, or inventory control. 

Examples of the value of pollution prevention for reducing environmental risks at the community level

are demonstrated by EPA’s Environmental Justice through Pollution Prevention (EJP2) grant program. 

EPA encouraged community groups, tribes, and local governments to identify environmental problems

and generate potential pollution prevention solutions for their communities.

Source: U.S. Environmental Protection Agency. 2002. Environmental Justice Through Pollution

Prevention Program. Updated July 9, 2002. Available at: http://www.epa.gov/opptintr/ejp2/. (Last

accessed April, 2004.) 

27.4 Use of Risk Estimates in Decision-Making

Decision-makers have a number of options when deciding what types of risk estimates to

consider as inputs to risk management decisions. Estimates of human health risk generally fall

into two categories, estimated cancer risk and the estimated noncancer hazard magnitude of

exposure concentration or dietary intake greater than a pre-established reference exposure level),

as described in more detail in Chapters 13 and 22. Non-cancer hazard may be considered for

both acute (short-term) and chronic (longer-term) exposures. In some cases, ecological risk may

be a factor in decision-making.

In some situations, risk managers may choose to consider EPA’s approach for assessing an

“ample margin of safety.” For cancer risks, EPA generally considers incremental risk (or

probability) of cancer for an individual potentially exposed to one or more air toxics. In

protecting public health with an ample margin of safety, EPA strives to provide maximum

feasible protection against risks to health from HAPs by (1) protecting the greatest number of
April 2004 Page 27-6

persons possible to an individual lifetime risk level no higher than 1×10-6 (one in one million)

and (2) limiting to no higher than approximately 1×10-4 (one in ten thousand) the estimated risk

that a person living near a source would have if exposed to the maximum pollutant

concentrations for 70 years. These goals are described in the preamble to the benzene National

Emissions Standards for Hazardous Air Pollutants (NESHAP) rulemaking (54 Federal Register

38044, September 14, 1989) and are the goals incorporated by Congress for EPA’s residual risk

program under Clean Air Act (CAA) section 112(f). Exhibit 27-4 describes some of the key

steps in the development of the 1×10-4 to 1×10-6 carcinogenic risk range.

For non-carcinogenic substances, on the other hand, risk managers may consider a reference level

that is developed based on data from laboratory animal or human epidemiology studies (see

Chapter 12), and to which uncertainty factors are applied. The reference level is usually an

exposure level below which there are not likely to be any adverse effects from exposure to the

chemical. Exposures above the reference level may have some potential for causing adverse

effects. This concept may also be applied generally to ecological risks.

Risk estimate options generally revolve around estimates of individual risk, the number of people

at different risk levels (population risk), and occasionally include the expected incidence of

disease in the entire population. Risk estimates can be derived for the current population as

currently distributed in an area or for a population size and geographic distribution that might

occur in the future; similarly, they may focus on risk estimates for persons currently exposed or

possible risks calculated for a hypothetical individual located where exposures are expected to be

relatively high. It is important to note that risk estimates should strive to take into account both

indoor and outdoor exposure to toxics, when possible.

• Risk to a specified individual. Most risk assessments focus on estimating individual risk

rather than the incidence of adverse effects (e.g., numbers of predicted cancer cases per year)

in a population. There are two general estimates of individual risk:

– High-end risk estimates seek to determine a “plausible worst case” situation among all of

the individual risks in the population. This estimate is meant to describe an individual

who, as a result of where they live and what they do, experiences the highest level of

exposure within some reasonable bounds. Reasonable maximum risk estimates are often

defined conceptually as “above the 90th percentile of the population”(4) but not at a higher

exposure level than the person exposed at the highest level in the population. When

calculated using deterministic methods, the high-end individual is calculated by

combining upper-bound and mid-range exposure factors (e.g., an average body weight,

but high-end ingestion rate) so that the result represents an exposure scenario that is both

protective and reasonable, but not higher than the worst possible case.

– Central-tendency risk estimates seek to determine a reasonable “average” or

“mid-range” situation among all of the individual risks in the population. Many risk

management decisions related to exposure to radioactive substances (e.g., in nuclear

power plants) are based on central-tendency risk estimates.
April 2004 Page 27-7

Exhibit 27-4. Development of the 1×10-4 to 1×10-6 Carcinogenic Risk Range

The 1970 CAA established Section 112 to deal with hazardous air pollutants. Once the EPA

Administrator had identified such a pollutant and “listed” it, he/she was directed to set emission

standards for sources emitting it at levels that would “provide an ample margin of safety to protect the

public health.” The regulation of benzene pursuant to Section 112 illustrates the evolution of riskbased decision-making for carcinogens and the consideration of the “ample margin of safety.”

• EPA listed benzene as a HAP in June 1977 and indicated that the “relative risk to the public”

would be considered in judging “the degree of control which can and should be required.”

• In 1980, the first round of benzene standards followed the proposed procedures in EPA’s 1979

draft airborne carcinogen policy, which reflected a technology-based approach to emission

standard development with a limited role for quantitative risk assessment in establishing priorities

and ensuring that the residual risks following the application of “best available technology” (BAT)

were not unreasonable.

• In 1984, after “weighing all factors,” EPA made several changes to the proposed benzene rules,

arguing that the risks were “too small to warrant Federal regulatory action.” These decisions were

promptly challenged by the Natural Resources Defense Council, who argued about the

uncertainties in the risk estimates and the inappropriate consideration of cost in regulatory

decisions made under Section 112. The issues raised were similar to litigation already pending on

amendments to the original vinyl chloride standards.

• On July 28, 1987, Judge Robert Bork, writing for the D.C. Circuit Court of Appeals, remanded the

vinyl chloride amendments to EPA, finding that the Agency had placed too great an emphasis on

technical feasibility and cost rather than the provision of an “ample margin of safety” as required

by the statute. The opinion also laid out a process for making decisions, consistent with the

requirements of the law. The Bork opinion held that EPA must first determine a “safe” or

“acceptable” level considering only the potential health impacts of the pollutant. Once an

acceptable level was identified, the level could be reduced further, as appropriate and in

consideration of other factors, including cost and technical feasibility to provide the required

ample margin of safety. The Court also held, however, that “safe” did not require a finding of

“risk-free” and that EPA should recognize that activities such as “driving a car or breathing city

air” may not be considered “unsafe.”

• In September of 1989, after proposing several options and receiving considerable public comment,

EPA promulgated emission standards for several categories of benzene sources. EPA argued for

the consideration of all relevant health information and established “presumptive benchmarks” for

risks that would be deemed “acceptable.” The goal, which came to be known as the “fuzzy bright

line,” is to protect the greatest number of persons possible to an individual lifetime risk no higher

than one in 1,000,000 and to limit to no higher than approximately one in 10,000 the estimated

maximum individual risk. The selection of even “fuzzy” risk targets placed greater emphasis on

the development and communication of risk characterization results.

Source: National Academy of Sciences’ Science and Judgment in Risk Assessment (The Blue Book).(2)
April 2004 Page 27-8

Note that, when calculating deterministic risk estimates, both a high end and central tendency

estimate of risk give the risk manager some sense of the range of risks in the population. When

risks to a population are developed using probabilistic methods, this becomes a moot point, since

the result is a distribution of risks across the population, which necessarily includes information

about the full variability of risk across the population – including both high and central tendency

risks. See Chapter 31 for more information on probabilistic approaches to risk assessment.

• Risk to the total population. Whether or not risk to the total population is considered by

EPA may depend on the regulatory authority provided by the CAA. For example, Section

112(k) of the CAA requires EPA to develop an Urban Air Toxics Strategy to reduce HAPs

from area sources to achieve a 75 percent reduction in cancer incidences attributable to such

sources. Two general types of descriptors are used for population risk. One, sometimes

termed population at risk is derived by determining the number of people in a population

with a particular individual risk level (e.g., “1,340,000 people are exposed at the 1×10-6 level,

and 320 people are exposed at the 1×10-4 level”). This is a useful estimate of the variability

of risk in a population.

• Incidence, another descriptor used for population risk, is an estimate of the total number

(incidence) of adverse effects in a population over a specified time period (e.g., a period of 70

years). A screening approach to deriving this estimate for a 70-year period involves

multiplying the estimate of individual risk (central tendency and/or reasonable maximum) by

the number of persons for which that risk estimate was predicted. For example, in a

population of 200 million persons, an individual cancer risk of 1×10-4 (i.e., one in ten

thousand) for everyone in the population would translate to an incidence of hundreds or

thousands of excess cancer cases over a 70-year period (depending on the exposure

assumptions). However, in a small population (e.g., a town of 200 persons), the same

individual cancer risk to everyone would translate to an excess incidence of cancer of less

than one over a 70-year period.

• Present versus future scenarios. Risks may be characterized using present or future

scenarios. Use of present scenarios involves predicting risks associated with the current

exposures to individuals (or populations) that currently reside in areas where exposures are

predicted to occur. For example, a current population risk estimate would use the existing

population within some specified area. The resultant risk estimates are associated with the

presumption that the current exposure conditions exist for the current population over the

period of time associated with the assessment (e.g., into the future). Use of future population

scenarios involves estimating risks associated with exposure conditions to individuals that

might reside, at some future point, in areas where potential exposures may occur (e.g., if a

housing development were built on currently vacant land).

• Potential risk. Risks may be sometimes be characterized for hypothetical exposures. For

example, in a screening air toxics modeling application, a potential risk estimate may be

derived using the location where the maximum modeled exposure concentration occurs,

regardless of whether there is a person there or not. This estimate may be considered along

with the predicted individual risk associated with a currently populated area, such as the MIR,

which reflects risk associated with the maximum exposure concentration at an actual

residence or in a census block with a non-zero population (see Chapter 11).
April 2004 Page 27-9

27.5 Process for Making Risk Management Decisions

A number of different authors and organizations have identified key steps or factors to consider

in making risk management decisions. The discussion in this section is taken largely from the

risk management framework developed by the Presidential/Congressional Commission on Risk

Assessment and Risk Management.(3) The Commissions’s framework has six stages, each of

which is briefly described below. The Commission also noted that the framework is conducted:

• In collaboration with stakeholders; and

• Using iterations if new information is developed that changes the need for or nature of risk

management.

27.5.1 Define the Problem and Put it in Context

The problem/context stage is the most important step in the Risk Management Framework. It

involves:

• Identifying and characterizing an environmental health problem, or a potential problem,

caused by chemicals or other hazardous agents or situations;

• Putting the problem into its public health and ecological context;

• Determining risk management goals;

• Identifying risk managers with the authority or responsibility to take the necessary actions;

and

• Implementing a process for engaging stakeholders. 

These steps are all important, but may be conducted in different orders, depending on the

particular situation. For example, when a federal or S/L/T regulatory agency is mandated by law

to take the lead on an air toxics issue, the steps they take often will proceed in the order listed

above, with the identity of the risk managers already clear, since the agency will have assumed

that role from the start. On the other hand, in a community based effort to characterize the

cumulative risk posed by multiple sources of air toxics in a neighborhood, stakeholders might

have to engage in a collaborative stakeholder process first to identify resources as well as risk

managers with the needed authority to act before the other steps can take place.

27.5.2 Analyze the Risks Associated with the Problem in Context

The nature, extent, and focus of a risk assessment should be guided by the risk management

goals. The results of a risk assessment – along with information about public values, statutory

requirements, court decisions, equity considerations, benefits, and costs – all can influence

whether and how to manage the risks.

Risk assessment can be controversial, reflecting the important role that both science and

judgment play in drawing conclusions about the likelihood of effects on human health and the
April 2004 Page 27-10

Some Factors to Consider in Defining the Problem for an Air Toxics Risk Assessment

• Risk. The specific estimates of risk to be used as inputs to the decision should be defined as

explicitly as possible. Are acute risks (e.g., short-term exposures) the primary concern, or are

exposures over the longer-term more important? Are ecological risks a concern? How certain are

we that our risk estimates are an accurate reflection of true exposure and risk?

• Air toxics of concern. What are the primary air toxics of concern? Are they more prevalent in

indoor or outdoor environments? How many individual chemicals contribute to the risks that need

to be managed? Do these chemicals exert their effects independently, or are some acting in a

synergistic (or antagonistic) manner? Are all equally important, or will reducing exposures to a

subset of these air toxics result in adequate risk reduction? How important is it to manage every

chemical of concern versus only those that pose the greatest risk?

• Sources. What are the primary sources of the air toxics that need to be managed? Where are these

sources located? How many are there? Are they all equally important, or will controlling a subset

result in adequate risk reduction?

• Exposure pathway considerations. What exposure pathways/routes are most important? Are all

equally important, or does a subset represent the greatest risk? Does control of each pathway

require controls over all components of the pathway (e.g., emissions, exposure), or can the

pathway be controlled by controlling a subset of these components?

• Amount of emissions reduction desired/achievable. What is the overall target for

emissions/exposure reduction? How does this relate to risk reduction by the estimates identified

above? Will partial reductions result in significant risk reduction, or is it more of an all-or-none

situation? What technologies are available to achieve the desired level of risk reduction? How

much do the various options cost?

• Spatial and temporal factors. Are releases of concern limited to a relatively brief period of time,

or do data support the emissions being relatively continuous over a longer period of time? Are the

released toxics specific to a single location or are there several wide-spread emission points? 

What is the fate and transport of the released chemicals? How does background risk relate to the

risk reduction strategy?

• Data gaps and uncertainties. What are the main sources of uncertainty in the data used in the risk

assessment? How do these uncertainties affect the risk management decision? Will more

information reduce these uncertainties and can the uncertainty be addressed with available time

and resources? Approaches for identifying and managing uncertainties associated with risk

assessment are discussed in Chapters 13 and Part VII.

environment. Often, the controversy arises from what we do not know and from what risk

assessments cannot tell us, because our knowledge of human vulnerability and of environmental

impacts is incomplete, especially at the relatively low levels of chemical exposure commonly

encountered in the general community.
April 2004 Page 27-11

Alternative Solutions to Unique Problems

Project XL, which stands for “eXcellence and Leadership,” is a national

pilot program that allows state and local governments, businesses, and

Federal facilities to develop with EPA innovative strategies to test

better or more cost-effective ways of achieving environmental and

public health protection. In January 2001, EPA signed the 50th XL

Final Project Agreement. Although EPA is no longer accepting

proposals for new XL projects, EPA will continue to fulfill each of its

commitments under Project XL and will track and monitor the progress

of each XL pilot for the duration of the project.

See www.epa.gov/projectxl for more information.

Supplemental Environmental Projects (SEPs) are part of enforcement

settlements connected with violations of an environmental statutory or

regulatory requirement. As part of the enforcement settlement, a violator

voluntarily agrees to undertake an environmentally beneficial project in

exchange for a reduction in the penalty. See 

www.epa.gov/compliance/civil/programs/seps for more information.

27.5.3 Examine Options for Addressing the Risks

This stage of the risk management process involves identifying potential risk management

options and evaluating their effectiveness, feasibility, costs, benefits, unintended consequences,

and cultural or social impacts. This process can begin whenever appropriate after defining the

problem and considering the context. It does not have to wait until the risk analysis is completed,

although a risk analysis often will provide important information for identifying and evaluating

risk management options. In some cases, examining risk management options may help refine a

risk analysis. Risk management goals may be redefined after risk managers and stakeholders

gain some appreciation for what is feasible, what the costs and benefits are, and what

contribution reducing exposures and risks can make toward improving human and ecological

health.

The Commission noted that stakeholders can play an important role in all facets of identifying

and analyzing options. They can help risk managers:

• Develop methods for identifying risk-reduction options;

• Develop and analyze options; and

• Evaluate the ability of each option to reduce or eliminate risk, along with its feasibility, costs,

benefits, and legal, social, and cultural impacts.

Chapter 28 provides an overview of community involvement and its role in risk assessment and

risk management.
April 2004 Page 27-12

27.5.4 Make Decisions about Which Options to Implement

In most risk management situations, decision-makers will have a number of options from which

to choose. Which option is optimal depends on the particular situation (and in some cases may

be driven by statutory requirements). The following seven are fundamental characteristics of

sound risk management decision making:

• Base the decision on the best available scientific, economic, and other technical information;

• Be sure the decision accounts for the problem’s multisource, multimedia, multichemical, and

multirisk contexts;

• Choose risk management options that are feasible, with benefits reasonably related to their

costs;

• Give priority to preventing risks, not just controlling them;

• Use alternatives to command-and-control regulation, where applicable;

• Be sensitive to political, social, legal, and cultural considerations; and

• Include incentives for innovation, evaluation, and research.

Options to be considered for air toxics fall into the following general categories:

• Regulatory approaches. Pursuant to various sections of the CAA, Congress has authorized

EPA to regulate air toxics. Many S/L/T governments have also authorized agencies to

regulate air toxics. Regulatory approaches include enforceable requirements that identified

sources must meet (or else be subject to legal action, such as fines) as well as

emissions-trading type requirements that focus on controls over sources in total while

allowing flexible emissions among individual sources. 

• Voluntary approaches. EPA and other regulatory agencies are looking beyond regulatory

approaches to reduce risks from air toxics. Non-regulatory (voluntary) approaches are

frequently the preferred option in a number of cases. Decision-makers at S/L/T agencies may

not currently have specific regulatory authority to address specific air toxics problems

identified in a risk analysis (particularly in a novel analysis such as a multi-source,

community-based risk assessment). The types of problems identified may not lend

themselves to regulatory solutions (e.g., they may require changes in the behavior of the

exposed population). Voluntary programs may also allow sources to significantly reduce

overall risk at much lower cost than various regulatory options. Various incentives such as

tax reductions or consumer rebates can be used to encourage voluntary responses.

• Permits and related authorities. Permits offer opportunities for both regulatory and

voluntary risk-management strategies. Many sources release air toxics to the atmosphere

pursuant to permits and related authorities. Permits generally need to be renewed

periodically and/or modified if conditions at the source change beyond some specified
April 2004 Page 27-13

Example Factors to Consider When Evaluating Risk Management Options

• Risk reduction benefits to be realized. Risk management decisions often focus on the

incremental risk associated with the chemical or other hazard being regulated in the absence of

background risks. However, background risk may be important in certain situations. For example,

if a monitoring program measures concentrations of air toxics being transported into a given study

area that result in risks above an “acceptable” level, no level of emissions control within the study

area will be able to reduce risk to an “acceptable” level, and the community may wish to address

the incoming air toxics via discussions beyond the local community.

• Level of uncertainty in the analysis. In the face of highly uncertain risks, decision-makers have

to carefully weigh the consequences of two or more options: making a decision to control

emissions or exposures only to find out later that there was little actual risk (e.g., incurring

unnecessary “cost” to the community), or making a decision not to control emissions or exposures

only to find out later that the risks were real and large (e.g., incurring potentially preventable harm

to the community).

• Implementation costs, both for voluntary approaches (e.g., marketing, process changes, tax

incentives) as well as to regulatory agencies, the regulated community, and the general community

(consumers).

• Technical feasibility. Short of shutting down the emission source altogether, is there an available

technology to reduce or eliminate emissions?

• Legal feasibility. Does the decision-making body have legal authority to both establish and

enforce requirements?

• Effectiveness/timing. Will the option provide effective management of the problem within a

reasonable time-frame?

• Political feasibility. Does the option have the necessary political support?

• Community Acceptance. Do the stakeholders buy-in to the proposed risk reduction alternatives?

Each of these factors may be more or less important depending on the context for the risk

management decision. For example, the risk manager may be required by statute to weigh economic

factors less than technical factors.

amount. This may provide an opportunity to re-write permit conditions so as to reduce highrisk emissions. This might be coupled with voluntary measures or other flexible solutions to

result in overall risk reduction (see box). Agencies may also work with emission sources to

incorporate voluntary measures or other flexible solutions into the permit.

27.5.5 Take Actions to Implement the Decisions

Traditionally, implementation has been driven by regulatory agencies’ requirements. Businesses

and governments (e.g., local municipalities) are generally the implementers. However, the

chances of success may be significantly improved when other stakeholders also play key roles. 

Depending on the situation, action-takers may include public health agencies, other public
April 2004 Page 27-14

agencies, community groups, citizens, businesses, industries, unions/workers, and technical

experts. These groups can help:

• Develop and implement a plan for taking action;

• Explain to affected communities what decision was made and why and what actions will be

taken; and

• Monitor progress.

27.5.6 Conduct an Evaluation of the Action’s Results

At this stage of risk management, decision-makers and other stakeholders review what risk

management actions have been implemented and how effective they have been. Evaluating

effectiveness involves monitoring and measuring, as well as comparing the actual benefits and

costs to estimates made in the decision-making stage. The effectiveness of the process leading to

implementation should also be evaluated at this stage. Evaluation provides important

information about:

• Whether the actions were successful, whether they accomplished what was intended, and

whether the predicted benefits and costs were accurate;

• Whether any modifications are needed to the risk management plan to improve success;

• Whether any critical information gaps hindered success;

• Whether any new information has emerged that indicates a decision or a stage of the process

should be revisited;

• Whether the process was effective and how stakeholder involvement contributed to the

outcome; and

• What lessons can be learned to guide future risk management decisions or to improve the

decision-making process.

27.6 Information Dissemination

The Presidential/Congressional Commission on Risk Assessment and Risk Management noted

that effective risk communication is critical to successful implementation of the risk management

framework.(3) Risk communication engages both the communicator and the audience in listening

and in explaining information and opinions about the nature of risk and other topics that express

concerns, opinions, or reactions to risk messages.(5) The Commission made the following

recommendations with respect to risk communication:

• The complex and often confusing process of communicating information about risks to

diverse affected parties must be improved;
April 2004 Page 27-15

1. National Research Council (NRC). 1983. Risk Assessment in the Federal Government:

Managing the Process (The “Red Book”). National Academy Press, Washington, D.C.

2. National Research Council (NRC). 1994. Science and Judgment in Risk Assessment (The

“Blue Book”). National Academy Press, Washington, D.C.

3. Presidential/Congressional Commission on Risk Assessment and Risk Management.1997. 

Framework for Environmental Health Risk Management (Final Report, Volume 1). 

Available at www.riskworld.com/Nreports/1996/risk_rpt.

Presidential/Congressional Commission on Risk Assessment and Risk Management. 1997. 

Risk Assessment and Risk Management In Regulatory Decision-Making (Final Report,

Volume 2). Available at www.riskworld.com/Nreports/1996/risk_rpt.

4. U.S. Environmental Protection Agency. 1995. Guidance for Risk Characterization. Science

Policy Council, Washington, D.C., February 1995. Available at:

epa.gov/osp/spc/rcguide.htm.

5. National Research Council. 1989. Improving Risk Communication. National Academy Press,

Washington, D.C.

• Decisions about how to allocate resources to reduce risks can be made and explained partly

on the basis of risk comparisons;

• The use of “bright lines” which distinguish between contaminant emissions and exposures

associated with negligible risk levels and those associated with unacceptable risk levels,

needs to be clarified;

• Moving from command-and-control regulation to non-regulatory approaches to risk reduction

can increase both efficiency and effectiveness; and

• Criteria for judicial review, a common element in major regulatory actions, should be

reaffirmed.

Chapter 29 provides an overview of risk communication and it’s role in risk assessment and risk

management.

References
Chapter 28 Community Involvement

Table of Contents

28.1 Introduction ............ ............ ............ ............ ............ ...... 1

28.2 Why is Community Involvement Important? ............ ............ ............ .... 1

28.3 When to Involve the Community ............ ............ ............ ............ . 2

28.4 How to Involve the Community ............ ............ ............ ............ . . 2

28.4.1 Understand Goals, Objectives, and Responsibilities for Effective Community

Involvement ............ ............ ............ ............ ........... 4

28.4.2 Identify Community Concerns and Interest ............ ............ ........... 5

28.4.3 Plan Community Involvement Strategy and Activities ............ ............ . . 5

28.4.4 Identify Possible Tools and Implement Community Involvement Activities ......... 6

28.4.5 Provide Opportunity for Continued Public Interaction ............ ............ . . 7

28.4.6 Release of Risk Assessment and Risk Management Documents ............ ...... 8

References ............ ............ ............ ............ ............ ............ 10

April 2004 Page 28-1

28.1 Introduction

Community involvement can be an important aspect of the risk assessment and risk management

process. Participation of local stakeholders, at various levels and in various forms, can help

ensure a better understanding of the risk assessment results and will promote buy-in to the

selected risk reduction strategies. Encouraging and facilitating community involvement also is 

sometimes required by law. 

This chapter provides a broad overview of community involvement in air toxics risk assessment

and risk management and identifies helpful references on this topic. Also included throughout

this chapter are descriptions of successful air toxics projects and programs where community

involvement was a central component of that success.

This chapter describes the key tools, resources, and other considerations for an effective study

area-specific approach. It is not, however, intended to provide all the information about

conducting community involvement activities. If additional information is needed, contact the

community involvement specialist for your agency.

28.2 Why is Community Involvement Important?

When performing an air toxics risk assessment in a particular geographic area, the community is

often thought of as the people who live within the area of impact of air toxic sources. However,

other parties in the area, such as local industry, also may consider themselves part of the

community. 

In addition to the people who actually live and work in an area, a number of other stakeholders 

also may have a stake in the community’s concerns (e.g., local officials, health professionals,

local media). It is often helpful, when dealing with a community, to keep in mind that many

different people (not just the people who live there) may have an interest in the risk assessment

and management work being undertaken.

As noted above, many laws recognize and accommodate the idea that government decisions

should be open to citizen input before a decision is finalized. This is realized through the

required public meetings and public comment periods associated with many government actions. 

For example, the Clean Air Act (CAA) has a number of requirements to provide an opportunity

for the public to review and comment on Agency proposals. In some cases, the public is brought

in at an even earlier stage. 

When risk assessors and risk managers have the opportunity to do so, they should consider

including the public as early as possible in the process. Doing so can lead to some very positive

benefits. For example, if the community participates early on and throughout the process, they

will be in a better position to understand what assessors and risk managers are doing, and there is

a better chance that they will believe that the work being done is in their best interest. The

process works best when the community appreciates that assessors and managers are working

with them and respecting their input (keeping them informed and involved). Ultimately, a

community that is involved early on in the process is a community that may be more willing to

support the risk assessment process and results. This may, in turn, foster the development of risk

reduction strategies the community as a whole can live with and have a stake in.
April 2004 Page 28-2

In contrast, excluding the public from the process may result in community resentment and

rejection of even a sound risk assessment and risk management approach. A “guardian-like”

attitude toward the community that treats people as unknowledgeable and incapable of

meaningful participation does not foster trust and can eventually undermine the process.

In addition to fostering the trust and acceptance of the community, there are many other positive

reasons for early and ongoing involvement. For example, important unrecognized sources of

emissions and exposure pathways may be identified through the community involvement

process. Ultimately, it is important to recognize that community members know their

community and understand the types of solutions that will be most accepted – after all, they live

there!

28.3 When to Involve the Community

When appropriate, community involvement should begin at the earliest possible stage and span

the entire risk and assessment and management process. The level of participation that

community members have in some of the more technical phases of the assessment may be

tailored to their background, expertise, and interest; however, this does not mean the community

cannot serve an important role in the technical phase, as well. The approach taken, as well as the

assumptions and limitations of the analysis, should be clearly explained to the community and

their input should be valued in return.

For certain CAA requirements, the question of when to involve the public is established by law. 

For example, in the Title V permitting process the permitting agency must provide a public

notice and an opportunity to comment on a draft new or revised permit when:

• A facility applies for its first Title V permit;

• A Title V permit is renewed (5 years after issuance);

• The permit is reopened because there is a material mistake in the permit or an update to the

permit is needed because of new requirements (review is limited to the part of the permit that

is being revised); and

• The facility makes a significant change in its operations and applies for a revision to its

permit (review is limited to the part of the permit that is being revised).

For a community-level effort that may include non-regulatory aspects, on the other hand, a

community involvement plan will need to be tailored to specific local needs, particularly if the

ultimate risk reduction efforts will likely involve voluntary action on the part of industry and/or

citizens. As noted above, involving the community at the beginning of and throughout the

process will greatly enhance the likelihood that the air toxics risk reduction plan will receive

community support (even if the community does not agree with all aspects of the analysis).

28.4 How to Involve the Community

Many different approaches have been developed for involving the community in a risk analysis

and management strategy. Exhibit 28-1 illustrates the general framework used both by some

programs in EPA and by the Agency for Toxic Substances and Disease Registry (ATSDR). This

framework emphasizes the need for involving the community throughout the process.
April 2004 Page 28-3

Exhibit 28-1. ATSDR’s Components of Effective Community Involvement

Source: Community Involvement in ATSDR’s Public Health Assessment Process (see box of additional

references at the end of this chapter)

In identifying community concerns and interests, it often is useful to develop a “conceptual map”

of the key organizations and decision-making processes in a community. The map would include

information such as who speaks for various parts of the community, who serves in formulating

perspectives, and what is the process for obtaining consensus within the community.
April 2004 Page 28-4

TIP: Identify local associations or groups by asking community members, respected “elders,” or

other associations. This also can go a long way in demonstrating a commitment to involving and

mobilizing all stakeholder groups, which helps to build trust and creates a more successful

community-involvement process. But, in seeking out community members, do not rely solely on

existing community organizations. Very often community members are not well organized or

represented by existing groups. Just because there is not an organization or group in the study area

does not mean that you can bypass that part of the community.

TIP: Local public health providers, such as

county health departments and hospitals can be a

key partner in the risk analysis and management

processes. These organizations often have

resources (staff and funding) that can be used in

community health activities. Because they are

locally based, involving them as key partners in

the process can create strong local leaders to

promote sustainable activities once a study is

complete.

28.4.1 Understand Goals, Objectives, and Responsibilities for Effective Community

Involvement

At a minimum, goals and objectives for community involvement should include the following

items. All study areas are different, however, and this list is just a suggested starting point (and

may need to be expanded).

• Earning trust and credibility through open and respectful communications;

• Including the community in the design and implementation of risk assessment and risk

management;

• Helping community members understand what the process involves;

• Assisting communities in understanding the possible health impact of exposure to air toxics;

• Informing and updating communities about risk management activities; and

• Promoting collaboration between decision-makers, communities, and other agencies and

stakeholders when carrying out risk management activities.

To reach these goals and objectives, the

following key principles are important:

• Be aware of confidentiality and privacy

issues. Any personal information that

analysts or decision-makers receive from

community members should be respected,

as appropriate.

• Be aware of special needs and cultural

differences. When conveying information

about air toxics and the risk management process, agencies should be aware of non-English

speaking community members and other citizens who may need help in understanding

complicated messages. Also, be sure to consider cultural symbolism. There are notable

examples of the use of a symbol that is acceptable in one culture but that has an unacceptable

meaning in another.

• Maintain effective communication. As part of the trust-building process, analysts and risk

managers should keep community members informed of progress, opportunities for

community involvement, how community input will be used, how community members can

help to reduce exposures, and upcoming issues and events.
April 2004 Page 28-5

• Respect community knowledge and values. It is important to recognize that community

knowledge can provide valuable information for the deliberative processes of risk assessment

and risk management and potentially help to address data gaps. It is particularly important to

try to understand people’s interests (what they care about) during the process (more

discussion of this subject is provided in the next section).

28.4.2 Identify Community Concerns and Interest

One important activity that risk assessors and risk managers can do at the outset of any study is

simply to listen to the community. Since their concerns may or may not match those of the

assessors and managers, the initial phase of community involvement often involves a fair amount

of listening and discussion to help both groups develop a common understanding of what will

and will not be studied during the course of the assessment. In those instances where a

community concern is outside the scope of what can be studied (e.g., occasional combined

stormwater/sewer overflows that cause odors), a willingness on the part of the assessment team

to at least help identify resources or connect them to agencies that can address these concerns

will go a long way to building trust and credibility. Not listening and not responding to

community concerns at the outset may make the process of air toxics assessment and risk

reduction more difficult in the long run and may set expectations that are ultimately not met.

28.4.3 Plan Community Involvement Strategy and Activities

Planning a community involvement strategy and activities is one of the most critical components

for effective community involvement. The type and nature of communication and involvement

activities will depend on (1) the needs and interests expressed by the community during the

previous stages, (2) the potential public health issues, and (3) the resources available for

communication and involvement activities. Exhibit 28-2 provides a broad list of issues to be

considered when developing a community involvement strategy. Not all of these issues must

have solutions initially; however, they may need to be addressed eventually.

Community Involvement Example. Southern Baltimore &

Northern Anne Arundel County Community Environmental

Partnership (CEP). In 1996, the residents, businesses, and

organizations of five Baltimore, MD neighborhoods joined with

local, State, and Federal governments in a CEP to begin a new

effort to find ways to improve the local environment and

economy. This CEP conducted a comprehensive screening of the

cumulative concentration of air toxics from all the industrial and

city facilities in and around the neighborhoods and developed a

first-for-Maryland survey of cancer incidence at the

neighborhood level. Based on this work, the CEP began work

with local facilities on pollution prevention. The work of the

Baltimore CEP was a learning experience for all of the people

who participated. The Partnership tried a lot of new things - some

of them worked and some didn’t. Lessons learned from this work

were carefully documented. The risk screening methodology and

lessons learned are being translated into a how-to manual for community use. For more information

on this manual and other CEPs, see http://www.epa.gov/oppt/cahp.
April 2004 Page 28-6

28.4.4 Identify Possible Tools and Implement Community Involvement Activities

An enormous number of tools and activities exist that risk assessors and managers can use to

encourage community involvement – more than can be described here (the additional resources

listed at the end of this chapter, however, should provide most of any team’s needs in this

regard). They range from the simple phone call, to block parties (at which food may be

provided), to the complex mapping of emissions sources and populations. How many and which

tools and activities should be used or initiated for a given situation depends on the phase of the

risk or public health assessment and management process, the level of community interest, and

the degree of hazard a study area poses. The formation of a partnership with stakeholders or

community-based coalitions can be an effective way to involve the community, access technical

expertise, achieve consensus, leverage resources, and obtain results.

Exhibit 28-2. Issues to Consider When Developing Community Involvement Strategies

Community health concerns:

• How many community members are concerned about the study area?

• What is the level of the community’s concern?

• Is the level of community concern higher (or lower) than the actual risk would suggest?

• Are community concerns unknown?

• Would a physician enhance outreach at community meetings?

• Is information/outreach/health education available now or can this wait until reports are

generated?

Demographics:

• How many community members are potentially affected?

• Are there any potentially sensitive populations that may be exposed?

• Do socio-demographic data suggest need for additional resources, such as translation?

• How do the community members receive information (e.g., newspaper, radio, word-of-mouth)?

Community interest in the risk assessment and management process:

• How involved in the process would the community like to be?

• How would the community like to be kept updated and informed (e.g., newsletters, e-mails)?

• How many community groups or activist groups are involved? How active are they?

• Should the risk assessment/management team facilitate the creation of a community group if one

has not been formed?

• Can information be disseminated at cultural centers? Informal gatherings?

Media support:

• What has the community already heard from the media? Are there misconceptions that need to be

dispelled?

• Will media support require more community involvement resources than usual?

Support of the community:

• Are there Native American communities affected by the pollution? Should a relevant agency be

involved?

• Does the pollution involve an environmental justice issue, air toxics “hot spot,” or other type of

special sites?

• What past experiences has the community had with “the government”? Other agencies?

• Is there a higher than average need for resources, such as for more frequent community updates?

• How active will any regional agency representatives or other agencies be in community

involvement efforts?
April 2004 Page 28-7

Non-English Speakers and

Other Special Needs?

To ensure the participation of everyone in the

community, agencies often use one or more

of the following strategies:

• Offer translators and signers at community

meetings, and check for wheelchair

accessibility.

• Provide additional sessions of meetings

that are offered exclusively in the

community’s secondary language(s).

• Seek out advocates for the severely

disabled or others with special needs.

• Provide education and outreach materials

in both English and secondary languages.

• Develop understandable and culturally

appropriate messages and materials.

Exhibit 28-2. Issues to Consider When Developing Community Involvement Strategies

(Continued)

Public health:

• Is the study area a designated public health hazard? Is hazard acute or chronic?

• Are environmental health risks largely unknown?

• Is the study area considered a high priority? By whom?

• Is there already some risk or health outcome results? Are biological data available?

• Is a health connection plausible between contaminant exposures and community health concerns?

• Are data available for review now ? When will they be available?

• Are there toxics reduction steps already in process?

Community culture and setting:

• What are the current community priorities and projects?

• What are the community organizations?

• Who are the community leaders (unelected)?

• What activities constitute community life?

Other:

• How many people on the study area team? Does everyone know their role?

• What is the time-frame for report development and communication?

• Will any special clearances will be required? At what levels?

• Will document or graphics development resources be needed?

• Are there schools or locations where community meetings can be held?

28.4.5 Provide Opportunity for Continued Public Interaction

While a risk assessment is underway, primary communication and involvement goals include

updating the community on the status of the assessment, obtaining ongoing feedback on the

process, obtaining additional information as needed or available from the community for the

assessment, and recommending public health actions, if needed, about how community members

can reduce exposures. Throughout this process,

the risk assessment/management team should

continue to listen to community concerns and

clearly explain how they will respond to these

concerns. The team also should leverage

community outreach resources whenever possible. 

For instance, federal agencies, state health and

environmental agencies, local health departments,

citizens’ advisory groups, and medical advisory

groups may have funds for involving community

members in the risk assessment/management

process. Collaborating with partner organizations

can strengthen community outreach depth and

coverage.

Generally, community involvement strategies are

situation-specific – risk assessment/management

teams should determine which community
April 2004 Page 28-8

involvement strategies are appropriate given the potential seriousness of the risk, the abilities and

involvement of the community, and the resources available for communication, training, and

outreach. If resources for community outreach are limited, the team may wish to consider how

they can best prioritize resources for community involvement. 

When resources are limited, the team should look for community outreach opportunities during

other community activities, if it would be culturally acceptable. For a determination of cultural

acceptability, ask community leaders or “trusted elders.” 

Finally, some community analyses foster highly interactive relationships with community

members and other stakeholders. For example, the risk assessment and risk management teams

may establish ad hoc working groups to work on specific issues. These groups may include

advisory members from the community or their representatives (e.g., community consultants) and

may be more or less formal, as the circumstances require.

28.4.6 Release of Risk Assessment and Risk Management Documents

At the end of the analysis phase, the next stage of community involvement generally begins (i.e.,

after a draft risk assessment is written). Since the process of data gathering, analysis, and risk

assessment preparation can take many months to years, community interest may have decreased

significantly. However, once the risk assessment is ready for release, public interest often peaks

again. To help ensure a fair and balanced release of information, the risk

assessment/management team and their partners may consider using a more formal process to

release the risk assessment. For example, the team may release the draft for a period of time for

people to read and comment. During the review period, meetings may be held to help describe

the results and how the analysis was done. Once the risk assessment document is finalized, there

typically is a need to communicate the key results, limitations, and recommendations through a

variety of materials including fact sheets, press releases, public meetings, and websites. The risk

management strategy may be presented in a similar fashion, with a draft and final document

presented to – if not also partly written by – the community.

If an agency or other parties will be conducting any follow-up activities in the area (such as

additional environmental sampling or emissions monitoring, cost analyses, health education,

health studies), then additional appropriate community involvement may be planned.
April 2004 Page 28-9

Additional References

Public Health Assessment Guidance Manual (2002 Draft Update) describes the process that ATSDR

uses to sort through the many hazardous waste sites in the U.S. and to determine where, and for whom,

public health actions should be undertaken. Chapter 4 addresses community involvement and

communication. See www.atsdr.cdc.gov/HAC/PHAManual/cover.html.

The Annual EPA Community Involvement Conference brings together public participation and

community involvement professionals from across all EPA programs, as well as their local, State,

Federal, and tribal partners. Conference presentations are designed to emphasize the process of public

participation and community involvement by focusing on techniques and approaches used in EPA’s

national and regional community involvement programs. See epancic.org for upcoming conferences

as well as the proceedings of past conferences.

Public Involvement in Environmental Permits: A Reference Guide (2000) at

www.epa.gov/permits/publicguide.htm was developed by EPA to help make it easier for state and

local agencies to facilitate public participation in environmental permitting decisions for businesses

and facilities under your authority. This guide provides basic information about public participation

requirements and gives examples under several major permits issued by EPA’s air, water, and waste

programs. This guide also details what public participation activities are required under these

programs, as a minimum, as well as those suggested activities that serve to augment the regulatory

requirements.

Air Toxics Community Assessment and Risk Reduction Projects Database at

yosemite.epa.gov/oar/CommunityAssessment.nsf/Welcome has been compiled to provide a resource

of planned, completed, and ongoing community level air toxics assessments across the country. By

sharing information about efforts at the local level to measure, understand, and address air toxics

emissions, this database will help ensure that communities designing and implementing their own

assessments will be able to build upon past efforts and lessons learned.

Community Involvement in ATSDR’s Public Health Assessment Process (2002) provides an overview

of how ATSDR works to involve communities in the public health assessment (PHA) process. It

describes how ATSDR develops community involvement strategies and plans community involvement

activities.
April 2004 Page 28-10

1. Agency for Toxic Substances and Disease Registry (ATSDR). 2002. Public Health

Assessment Guidance Manual (Update): Draft for Public Comment.. Available at:

http://www.atsdr.cdc.gov/HAC/PHAManual/cover.html. 

Additional References (continued)

Superfund Community Involvement Web Site provides communities with a range of tools, including

guidance documents and other information to increase their understanding of Superfund and the

services available to them (e.g., the Technical Outreach Services for Communities Program, Technical

Assistance Grants). See www.epa.gov/superfund/action/community/index.htm.

Superfund Community Involvement Handbook (2002) presents legal and policy requirements for

Superfund community involvement and additional suggestions for involving the community in the

Superfund process. This handbook also provides guidance for community involvement outside of

Superfund. See www.epa.gov/superfund/tools/cag/ci_handbook.pdf for more information.

Community Culture and the Environment: A Guide to Understanding a Sense of Place (2002)

addresses the social and cultural aspects of community-based environmental protection. The

document offers a process and set of tools for defining and understanding the human dimension of an

environmental issue. The report, published by EPA’s Office of Water, is available on the web from

EPA’s publication Web site. The report number is EPA/842/B-01/003.

Community Air Screening How To Manual: A Step-by-Step Guide to Using a Risk-based Approach to

Identify Priorities for Improving Outdoor Air Quality (to be published in 2003) is being developed by

the EPA’s Community Assistance Technical Air Team to make air quality assessment tools more

accessible to communities. It will present and explain a step-wise process that a community can

follow to form a partnership, identify and inventory all local sources of air pollutants, review these

sources to identify the hazards and potential risks, and set priorities and develop a plan for making

improvements.

References
Chapter 29 Risk Communication

Table of Contents

29.1 Introduction ............ ............ ............ ............ ............ ...... 1

29.2 Risk Perception ............ ............ ............ ............ ............ ... 2

29.3 Your Risk Communication Strategy ............ ............ ............ ........... 2

29.4 Risk Comparisons ............ ............ ............ ............ ............ . 3

29.5 Implementing Risk Communication Strategies ............ ............ ............ . . 5

29.5.1 Presentation of Risk Results ............ ............ ............ .......... 5

29.5.2 Working with the Media ............ ............ ............ ............ . 8

References ............ ............ ............ ............ ............ ............ 14

April 2004 Page 29-1

What is Risk Communication?

Risk communication is the way in which decision-makers communicate with various interested parties

about the nature and level of risk, and about the risk reduction strategies to reduce the risk.

Effective Risk Communication:

• Can determine and respond to community concerns;

• Can reduce tension between concerned communities and agency staff; and

• Can explain health risk information more effectively to communities.

29.1 Introduction

The purpose of an air toxics risk assessment is to evaluate the magnitude and extent of exposure

to air toxics and the potential effects on humans and the environment. Risk assessments aid the

process of developing risk management alternatives that minimize risk and maximize

environmental benefits.

The purpose of risk communication is to help in the planning of the risk assessment and to

convey the results of the risk assessment in a way that effectively supports risk management

decisions; this is so that the risk management decisions both meet the goals of the project and

provide some comfort level for stakeholders. Good risk communication strategies are a

fundamental aspect of developing trust among various stakeholders and the community and are

often considered an important first step that can begin even before conducting the risk

assessment. Involving the community, establishing and maintaining relationships, and

networking with other partners (e.g., agencies, organizations, officials, the media) are key

elements in a risk communication strategy. Tailoring communications to the cultural diversity of

the community is important because it may help establish the trust necessary to complete a risk

assessment that meets all stakeholder and community needs. Risk management rooted in

voluntary measures requires effective risk communication to get buy-in.

The subject of risk communication overlaps considerably with related topics discussed in

Chapter 13, including EPA’s philosophy of transparency, clarity, consistency, and reasonableness

(TCCR) as described in its Policy For Risk Characterization.

(1)

This chapter provides an overview of information developed by the Agency for Toxic Substances

and Disease Registry (ATSDR) and other authors to assist the risk assessment team in

communicating the context and results of the risk assessment to the public. Readers are

encouraged to consult the references at the end of this chapter for a more complete discussion of

this important topic. ATSDR also has an excellent website on risk communication resources

(See http://www.atsdr.cdc.gov/HEC/primer.html).

ATSDR has published a handbook on risk communication for its staff.(2) Although focused on

agency staff, this handbook clearly and effectively outlines the detailed steps necessary in order

to develop an effective risk communication plan, and is applicable to all risk assessors and risk

management teams. The tools and information in the ATSDR handbook (and discussed in this

Chapter) will help the risk assessment team:
April 2004 Page 29-2

Why is Risk Communication Important?

1. Provides an opportunity to communicate health risks in a caring, concerned, and

well-planned manner

2. Involves the community in the risk management process

3. Helps alleviate fear or anger and establish trust 

Two-way risk communication works best. Non-experts want access to information and to gain

knowledge. Technical experts and officials also want to learn more about non-experts’ interests,

values and concerns. The audience includes government, industry, citizens, and both technical and

non-technical people. They can all be included in the process as partners.

• Develop a communication strategy;

• Conduct community outreach and evaluation;

• Develop communication messages; and

• Interact effectively with the news media.

29.2 Risk Perception

If people perceive themselves to be at risk, their perception is unlikely to change even if they are

not being exposed or harmed. Elements that affect risk perception include experience, culture,

level of education, outrage factors, who is affected/how they are affected (equal treatment), and

the level of control exercised on an event or events. People’s perceptions of the magnitude of

risk also are influenced by factors other than numerical data. According to Covello(3) and other

authors:(4)

• Risks perceived to be voluntary are more accepted than risks perceived to be imposed. 

• Risks perceived to be under an individual’s control are more accepted than risks perceived to

be controlled by others. 

• Risks perceived to have clear benefits are more accepted than risks perceived to have little or

no benefit. 

• Risks perceived to be fairly distributed are more accepted than risks perceived to be unfairly

distributed. 

• Risks perceived to be natural are more accepted than risks perceived to be manmade. 

• Risks perceived to be generated by a trusted source are more accepted than risks perceived to

be generated by an untrusted source. 

• Risks perceived to be familiar are more accepted than risks perceived to be exotic. 

• Risks perceived to affect adults are more accepted than risks perceived to affect children.

29.3 Your Risk Communication Strategy - The Overall Plan

In general, planning a risk communication strategy includes the following steps:

• Determine the goals of the communication effort;

• Identify communication restraints;

• Identify the audience(s);

• Identify audience concerns;
April 2004 Page 29-3

• Identify what the audience(s) knows about the issues, both correct information and

misinformation;

• Design the message(s) to be sent out to the community;

• Design the “channels”/choose the best methods to reach people;

• Prepare to deliver/present the message;

• Anticipate communication problems;

• Evaluate the program; and

• Modify program as needed.

When working through this process, it is important to know and understand the communication

limits and purpose, know your audience, and whenever possible, pretest your message(s). You

also should communicate early, often, and fully and remember that for many of the people in

your audience, perception is reality. 

A good communication strategy also will use tested principles of good presentation, such as the

use of simplified language to present important content and the ability to be objective (not

subjective) and balanced. Presentations also should not be limited to just one form or just one

medium. 

Try to use spokespersons who can communicate knowledgeably, honestly, clearly, and

compassionately and will listen and deal with specific concerns. Finally, it is important to make

sure that the information provided in the risk communication strategy is conveyed to all segments

of the audience at a level that they can understand and that the communication materials are

honest and up-front about uncertainties. It is often better to say “I don’t know” than to hedge.

The ability to establish constructive communication will be determined, in large part, by whether

or not the audiences perceive the speaker to be trustworthy and believable. Public assessment of

how much we can be trusted and believed is based upon four factors:(1)

• Empathy and caring;

• Competence and expertise;

• Honesty and openness; and

• Dedication and commitment.

29.4 Risk Comparisons

Many successful risk communication efforts have had one major thing in common – a portrayal

that puts the calculated exposure risks from an assessment in perspective, with risk ranges the

public can easily relate to and understand. 

Risk comparisons can help to put risks into perspective. However, irrelevant or misleading

comparisons can harm trust and credibility. Thus, while risk comparisons are commonly used,

they should be used with caution, because some kinds of risk comparisons are more likely to be

perceived as pre-conceived judgments about the acceptability of risks.(1) Guidelines for risk

comparisons have been published,(5) and provide rankings of risk comparisons in terms of their

acceptability to the community. The highest-ranking comparisons are those that presume a level

of trust between the risk communicator and the public, and that consider the factors that people

use in their perception of risk. Exhibit 29-1 describes several example risk comparison rankings.
April 2004 Page 29-4

The general rule-of-thumb is to select from the highest-ranking risk comparisons whenever

possible. When there is no choice but to use a low-ranking risk comparison, do so cautiously,

being aware that it could backfire. The fifth rank, which risk assessors rarely use, consists of

comparisons of unrelated risks (e.g., involuntary vs. voluntary risks). These comparisons have

been found to be very problematic. For example, the risk of driving without a seat belt is a

voluntary risk, while exposure to air toxics is generally considered involuntary by community

members. Covello et al. (5) provide specific examples of each of the comparison ranks, as

associated with a manufacturing facility (http://www.psandman.com/articles/cma-4.htm). Risk

comparison charts are also provided in Appendix B of that document

(http://www.psandman.com/articles/cma-appb.htm), although the authors do not recommend

their use in public presentations.

Exhibit 29-1. Relative Acceptability of Risk Comparisons

• First-rank risk comparisons (most acceptable)

– Of the same risk at two different times 

– With a standard 

– With different estimates of the same risk

• Second-rank comparisons (less desirable)

– Of the risk of doing something versus not doing it

– Of alternative solutions to the same problem 

– With the same risk experienced in other places

• Third-rank comparisons (even less desirable)

– Of average risk with peak risk at a particular time or location 

– Of the risk from one source of an adverse effect with the risk from all sources of the same

effect 

• Fourth-rank comparisons (marginally acceptable)

– With cost; or one cost/risk ratio with another 

– Of risk with benefit 

– Of occupational risk with environmental risk 

– With other risks from the same source

– With other specific causes of the same disease, illness, or injury

• Fifth-rank comparisons (rarely acceptable – use with caution)

– Of risks that may seem unrelated to community members (e.g., smoking, driving a car,

lightning)

EPA has included risk comparisons in some air toxics analyses. For example, the results section

of EPA’s National-Scale Air Toxics Assessment (http://www.epa.gov/ttn/atw/nata/) discusses

general U.S. background risks from air toxics, originating from both mobile sources and other

background sources:

• Mobile Sources. For on-road and non-road mobile sources, EPA estimates that more than

100 million people live in areas of the U.S. where the combined upper-bound lifetime cancer

risk from all air toxics compounds exceeds 10 in a million. This risk estimate is dominated

by the emissions of benzene, formaldehyde, acetaldehyde, and 1,3 butadiene. Regarding

effects other than cancer, acrolein emissions are estimated to lead to exposures above the
April 2004 Page 29-5

reference concentration (i.e., a hazard quotient above 1.0) for approximately 200 million

people in the U.S. EPA expects that in 2007, existing standards affecting emissions of air

toxics compounds from new vehicles will reduce exposure from on-road sources by about 50

percent from 1996 levels, and that substantial reductions also will occur for non-road

emissions.

• Background Sources. EPA estimates that combined upper-bound cancer risks associated

with air toxics compounds from background sources are less than 100 in 1 million throughout

the U.S. However, the entire U.S. population is estimated to exceed an upper-bound cancer

risk level of 10 in a million due to background sources alone (note that in this study

background concentrations include both uncontrollable emissions [e.g., persistent historic

emissions, international or global pollutant transport, contributions from natural sources and

emissions that can be controlled such as long-range pollutant transport within the U.S.]).

29.5 Implementing Risk Communication Strategies

In order to implement risk communication strategies, agencies may need to plan approaches to

public presentations and working with the media. The purpose of communication with the public

is to inform, educate, and enhance cooperative problem solving and conflict resolution.

29.5.1 Presentation of Risk Results

Risk communication strategies also consider the meaning of the information (e.g., will the

listener understand how to use the information in forming opinions, making decisions, and taking

actions). When risks are calculated for air toxics and the risk results are presented to the public,

the community may not be familiar with quantitative risk data and what it means for them. In

order to prevent panic and to encourage participation in and buy-in of risk management

decisions, risk communication strategies are developed that not only reassure the community, but

also explain the potential risks and uncertainties in an understandable, clear, and honest way.

Effective communications also provide information in a community-compatible language or

form. For example, if the community speaks Spanish, then the communications could be in

Spanish as well as English. Similarly, if the community includes Native Americans, the

communications could be in the appropriate language and employ appropriate symbolism. The

effective communication of risks will allow stakeholders to better participate in management

decisions that weigh the benefits of different alternatives against the costs of achieving

“acceptable” levels of risks and the costs of disruptions associated with implementation.

When developing messages, it is important to consider the following questions:

• What does the community already know?

• Is this information factual?

• What does the community want to know?

• What does the community need to know?

• Can the information be misunderstood?

When developing a public education campaign, it is generally most effective if the campaign

highlights no more than three primary messages. More than three primary messages may

convolute the focus of the education campaign. Those developing public education campaigns
April 2004 Page 29-6

Audiences may include:

• Environmental groups;

• Civic organizations;

• Professional and trade organizations;

• Educational and academic groups;

• Religious groups;

• Other government agencies;

• Neighborhood/school organizations;

• Industries; and

• Other organizations.

may wish to test their risk communication messages with trusted audience members before

releasing them to the public. This can ensure that the messages are on-target and help avoid

community objections that decision-makers may not have anticipated. It also is important to

ensure that the message is culturally attuned and fits the language needs of the audience. 

“Outrage reducers” are outlined by risk communication specialist Peter Sandman

(www.petersandman.com).

When developing risk-communication messages, decision-makers should (1) review the concerns

and worries of their audience; (2) cover WHO, WHAT, HOW, WHEN, WHERE and WHY; and

(3) develop messages that are consistent with their actions.

Different messages and channels may be needed for different audiences. To communicate

effectively, the risk communicator should try to understand the audience’s values, concerns, and

perceptions. Credibility is enhanced by the degree to which the risk communicator correctly

identifies, anticipates, and empathizes with the specific concerns of his or her audience(s), which

may include:

• Health concerns;

• Safety concerns;

• Environmental concerns;

• Economic concerns;

• Aesthetic concerns;

• Lifestyle/cultural concerns;

• Data and information concerns;

• Fairness/Equity concerns;

• Trust and credibility concerns;

• Process/value concerns (e.g., who makes

decisions and how); and

• Risk management concerns.

It may be worthwhile to develop audience profiles for key audiences. Profiles describe the

members of the audience, whom they trust and go to for information (decision-makers can seek

these people out for advice on communicating with the community), what their prevailing

attitudes and perceptions are, and what concerns and worries motivate their actions. 

It is important to clearly communicate scientific information and uncertainty:

• Provide all information possible, as soon as possible;

• Communicate when there is progress being made;

• Maintain your relationship with the community;

• Be honest about what you do not know;

• Explain how you will work together to find the answers;

• Help the audience understand the process behind your findings;

• Avoid acronyms and jargon;

• Carefully consider what information is necessary; and

• Use familiar frames of reference to which the audience can relate.
April 2004 Page 29-7

Public interactions may also include availability sessions, informal discussions, or poster

sessions. Presentations can occur in a variety of venues some of which are better suited than

others to different situations. Determining the best channels for your message depends on

understanding when to use which tool and knowing how the community prefers to receive

information. Message delivery channels include:

1. Presentations: Speeches to public groups. Benefit: offers the audience a chance to ask

questions; reaches many people at one time. Limitations: if poorly presented, can distort

community perception; cannot sufficiently address individual concerns; can become

argumentative or confrontational.

2. Open Houses/Availability Sessions: Informal meeting where public can talk to staff on a

one-to-one basis. Benefit: allows for one-to-one conversation; helps build trust and rapport.

3. Small Group Meetings: Sharing information with interested community members and

government officials. Benefit: allows two-way interaction with the community. Limitations: 

may require more time to reach only a few people; may be perceived by community groups as

an effort to limit attendance; be sure your information is identical or you may be accused of

telling different stories to different groups.

4. Briefings: Can be held with key officials, media representatives, and community leaders;

generally not open to the public. Benefit: allows key individuals to question risk assessment

staff before release of public information. Limitations: should not be the only form of

community communication; bad feelings may arise if someone feels that they were left off

the invite list.

5. Community mailings: Sends information by mail to key contacts and concerned/involved

members of the community. Benefit: delivery of information quickly; may require less

planning than a meeting. Limitation: no opportunity for feedback.

6. Exhibits: Visual displays to illustrate health issues and proposed actions. Benefits: creates

visual impact. Limitations: one-way communication tool, no opportunity for community

feedback.

7. Fact Sheets: To introduce new information. Benefit: brief summary of facts and issues;

provides background for information discussed during a meeting. Limitations: one-way

communication tool; needs to be well-written and understandable.

8. Newsletters: To inform community of ongoing activities and findings. Benefit: explains

findings; provides background information. Limitations: can backfire if community

members do not understand or misinterpret contents.

9. News Release: Statement for the news media to disseminate information to large numbers of

community members. Benefit: reaches large audience quickly and inexpensively. 

Limitations: may exclude details of possible interest to the public; can focus unneeded

attention on a subject.
April 2004 Page 29-8

10. Public Meetings: Large meeting open to the public where experts present information and

answer questions and community members ask questions and offer comments. Benefit: 

allows community to express concerns and agency to present information. Limitations: can

intensify conflicts, rather than resolve controversies.

Presentations require a careful balancing act between effectively conveying key messages and

avoiding a range of pitfalls. Important “Dos” and “Don’ts” to avoid presentation pitfalls, are

outlined in Exhibit 29-2.

29.5.2 Working with the Media

The media can be a primary source of information on risks to the public. Effective news media

relations have many benefits, complementing other communication efforts. What people read,

see, or hear in news coverage can lend credibility to agencies associated with air toxics risk

assessment, and can help to make it a familiar topic for public discussion. News coverage can

inform people about air toxics issues and help them ask appropriate questions. Skill in media

relations can help risk communications avoid or dispel rumors, respond to criticism, defuse

controversy, and even turn adversity to advantage. 

News coverage is crucial to engaging the attention of decision-makers and earning the support of

opinion leaders. Also, because the news media pay distribution costs, helping journalists cover

the issues is a cost-effective way to communicate.

The best approach to the media, as with the public, is to be open and honest, provide information

tailored to the needs of each type of media, such as graphics and other visual aids, and provide

background material. Journalists also should welcome such materials as fact sheets, press kits,

and lists of experts. Establishing an information center also can be an effective way to make

materials available to the news media (and to the general public). It also is very important that

the material and discussions you have with the media clearly articulate the messages that you

want to find their way into print or onto the TV or radio.

Like other communication efforts, working with the news media is done best when it is based on

a strategy and follows a systematic process. A good strategy seeks opportunities to match the

goals and objectives of the organization with the interests of journalists. As in other

communication strategies, assessing the needs of the audience – journalists – is important to

reaching them effectively.

After you determine that the rules of your organization concerning contacts with the media have

been met, here are a few suggestions on how to deal with news reporters:

• When a reporter calls, be sure to get a name and media affiliation; if what the reporter wants

is not clear to you, ask for a clear explanation; if you are uneasy with a reporter’s query,

decline in a friendly way to continue the conversation.

• Reporters are often under deadline pressure, but you can take enough time to respond

effectively; don’t get pressured into hasty comments that might backfire.
April 2004 Page 29-9

• Do not hesitate to ask for more information about a story before responding to a request for

an interview.

In working with journalists, it is vital to develop good interpersonal relationships. How can you

do that? One rule of thumb followed by experienced practitioners is to adhere to the “Five Fs” –

Fast, Factual, Frank, Fair, and Friendly (Exhibit 29-3).(6)

• Interviews. Frequently, the best way to get a message out is through an in-person interview. 

You should generally assume that all statements you make are “on the record.” Exhibit 29-4

outlines some techniques to prevent poor transmittal of your message.

• Press Releases. Press releases may not be an effective way to transmit a message. However,

in some cases, releases that are targeted to particular media outlets and purposes can be

useful. For example, the publication of a report on air toxics risk might be newsworthy and

of concern to the community, and thus would be sent to local community newspapers. 

Remember that your press release should emphasize, upfront, the messages that you want to

get out to the public.

• Other Platforms. You may have the opportunity to communicate your message through

other platforms such as:

– Letters to the Editor. Keep them short, to the point, and prompt. 

– Commentaries. Radio broadcasts and newspapers print a number of opinion pieces each

day. Bear in mind that submissions are numerous, acceptances rare.

– Talk Radio (and TV). Talk shows may request experts to address various environmental

issues.
April 2004 Page 29-10

Exhibit 29-2. Presentation Dos and Don’ts

• Pitfall: Jargon 

Do: Define all technical terms and acronyms.

Don’t: Use language that may not be understood by even a portion of your audience. 

• Pitfall: Humor

Do: Direct it at yourself, if used.

Don’t: Use it in relation to safety, health, or environmental issues. 

• Pitfall: Negative Allegations

Do: Refute the allegation without repeating it.

Don’t: Repeat or refer to them. 

• Pitfall: Negative Words and Phrases

Do: Use positive or neutral terms.

Don’t: Refer to national problems (problems unrelated to the issue at hand), i.e., “This is not Love Canal.” 

• Pitfall: Reliance on Words

Do: Use visuals to emphasize key points, but be culturally correct for the audience.

Don’t: Rely entirely on words. 

• Pitfall: Temper

Do: Remain calm. Use a question or allegation as a springboard to say something positive.

Don’t: Let your feelings interfere with your ability to communicate positively. 

• Pitfall: Clarity

Do: Ask whether you have made yourself clear.

Don’t: Assume you have been understood. 

• Pitfall: Abstractions

Do: Use examples, stories, and analogies to establish a common understanding, but test them out first to make

sure they are clear, make your point, and are culturally acceptable.

• Pitfall: Nonverbal Messages

Do: Be sensitive to nonverbal messages you are communicating. Make them consistent with what you are

saying.

Don’t: Allow your body language, your position in the room, or your dress to be inconsistent with your

message.

• Pitfall: Attacks

Do: Attack the issue.

Don’t: Attack the person or organization. 

• Pitfall: Promises

Do: Promise only what you can deliver. Set and follow strict orders.

Don’t: Make promises you can’t keep or fail to follow up.

• Pitfall: Numbers

Do: Emphasize performance, trends, and achievements.

Don’t: Focus on or emphasize large negative numbers.
April 2004 Page 29-11

Exhibit 29-2. Presentation Dos and Don’ts (continued)

Pitfall: Guarantees

Do: Emphasize achievements made and ongoing efforts.

Don’t: Say there are no guarantees. 

Pitfall: Speculation

Do: Provide information on what is being done.

Don’t: Speculate about worst cases. 

Pitfall: Money

Do: Refer to the importance you attach to health, safety, and environmental issues; your first obligation is to

public health.

Don’t: Refer to the amount of money spent as a representation of your concern. 

Pitfall: Organizational Identity

Do: Use personal pronouns (“I,” “we”).

Don’t: Take on the identity of a large organization. 

Pitfall: Blame

Do: Take responsibility for your share of the problem.

Don’t: Try to shift blame or responsibility to others. 

Pitfall: “Off the Record”

Do: Assume everything you say and do is part of the public record.

Don’t: Make side comments or “confidential” remarks. 

Pitfall: Risk/Benefit/Cost Comparisons

Do: Discuss risks and benefits carefully (consider putting them in separate communications).

Pitfall: Risk Comparison

Do: Use them to help put risks in perspective.

Don’t: Compare unrelated risks.

Pitfall: Health Risk Numbers

Do: Stress that true risk is between zero and the worst-case estimate. Base actions on federal and state

standards, when possible, rather than risk numbers.

Don’t: State absolutes or expect the lay public to understand risk numbers. 

Pitfall: Technical Details and Debates

Do: Focus your remarks on empathy, competence, honesty, and dedication.

Don’t: Provide too much detail or take part in protracted technical debates. 

Pitfall: Length of Presentations

Do: Limit presentations to 15 minutes.

Don’t: Ramble or fail to plan the time well.

Source: ATSDR Risk Communication Primer(2)
April 2004 Page 29-12

Exhibit 29-3. The “Five Fs” of Media Relations

Fast. Respect journalists’ deadlines. If a journalist telephones for information, return the call

immediately, even if it is past normal office hours. A phone message returned the next day is often too

late. By then, the story already may have been aired or printed.

Factual. Be factual, and make the facts interesting. Stories are to be based on facts. Journalists also

appreciate a dramatic statement, creative slogan, or personal anecdote to help illustrate your point. 

Give the source of any facts and statistics provided.

Frank. Be candid. Never mislead journalists. Be as open as possible and respond frankly to their

questions. As long as there is an explanation of the reason, most journalists will understand and

respect a source even if he or she is not able to answer a question completely or at all.

Fair. Organizations should be fair to journalists if they expect journalists to be fair to them. Favoring

one news outlet consistently, for example, will lose the confidence of the others.

Friendly. Like everyone else, journalists appreciate courtesy. Remember their names; read what they

write; listen to what they say; know their interests; thank them when they cover the issues in a factual,

unbiased way.
April 2004 Page 29-13

Additional Suggested References

Calow, P. 1997. Handbook of Environmental Risk Assessment and Management. Blackwell

Publishers.

Crawford-Brown, D. 1999. Risk-based Environmental Decisions: Culture and Methods. Kluver

Academic Publishers.

Johnson, B.B., Sandman, P.M., and Miller, P. 1992. Testing the Role of Technical Information in

Public Risk Perception by RISK. Issues in Health and Safety, Fall 1992:341-364.

Lundgren, R.E. 1994. Risk Communication: A Handbook for Communicating Environmental, Safety,

and Health Risks. Battelle Press, Columbus, OH.

Langford, Ian. 2002. An existential approach to risk perception. Risk Analysis 22(1):101-120.

U.S. Environmental Protection Agency. 1992. Air Pollution and the Public: A Risk Communication

Guide for State and Local Agencies. EPA 450/3-90/025.

For an additional list of risk communication references, see 

http://www.psandman.com/articles/cma-bibl.htm.

Exhibit 29-4. Interviewing Techniques

• Always think carefully before you answer a question. People often ramble - and say something

they wish they hadn’t if they answer too quickly. Take a moment to consider what you want to say. 

If you need more time, ask for the question to be repeated.

• Don’t talk just to keep a conversation going with a reporter. Experienced reporters will be silent

because often people they interview will talk to fill awkward voids and then say something they

don’t mean to say.

• Ask the reporter to make your affiliation clear in the story.

• Listen carefully to questions and respond clearly. Avoid jargon. If you have a key idea that you

want to get across, repeat it several times, perhaps using different words. This is especially useful

for broadcast: no matter how the tape is edited, you will make your point.

• Don’t hurry: speak slowly, and in short, concise sentences. State your position in simple,

easy-to-understand language. Use everyday examples and analogies, when possible.

• Never talk down to a reporter. You are partners in getting your message across. Arrogance will

come across negatively to an audience. An “attitude” can turn an interview into a confrontation.

• Don’t lose your temper! No matter how antagonized you feel, recognize that this can be a tactic to

get you to say something you do not wish to say. 

• If you don’t know the answer to a reporter’s question, or cannot answer, just refrain from

answering. A lie or bad guess will return to haunt you. You will lose credibility.

• Some reporters may ask to tape an interview over the telephone. This is a common practice for

radio reporters to obtain “sound bites” and to get accurate quotes. The reporter should inform you

of the taping before it begins. Do not repeat an allegation – it could be taken out of context.
April 2004 Page 29-14

1. U.S. Environmental Protection Agency. 1995. Policy for Risk Characterization (“Browner

Memorandum”). Science Policy Council, Washington, DC., March 1995. Available at:

http://64.2.134.196/committees/aqph/rcpolicy.pdf

2. Agency for Toxic Substances and Diseases Registry (ATSDR). 1994. Tools and Techniques

for Effective Health Risk Communication. This is an update of the ATSDR Primer on Health

Risk Communication Principles and Practices, October 1994. Available at:

http://www.atsdr.cdc.gov/HEC/primer.html#EARNING

3. Covello, V.T., Sandman, P. 2001. Risk communication: Evolution and Revolution, in

Wolbarst A. (ed.). Solutions to an Environment in Peril. John Hopkins University Press,

Baltimore, MD: pp. 164-178. Available at:

http://www.phli.org/riskcommunication/article.htm

4. Fischhoff B, Lichtenstein S, Slovic P, Keeney D. 1981. Acceptable Risk. Cambridge

University Press, Cambridge, Massachusetts.

5. Covello, V.T., Sandman, P.M., Slovic, P. 1988. Risk Communication, Risk Statistics and

Risk Comparisons: A Manual for Plant Managers. Chemical Manufacturers Association,

Washington, D.C., 1988. Available at: http://www.psandman.com/articles/cma-0.htm

6. Cutlip, S.M., Center, A.H., and Broom, G.M. 1985. Effective Public Relations.

Prentice-Hall, Englewood Cliffs, New Jersey.

References
PART VI

SPECIAL TOPICS

Introduction to Part VI

Part VI of this Reference Manual provides an overview of three special topics related to air

toxics risk assessment.

• Public Health Assessment (Chapter 30) provides an overview of the process by which public

health agencies may evaluate the public health implications posed by the emissions from air

toxic sources in a community. The public health assessment, if performed, is a

complementary process to risk assessment.

• Probabilistic Risk Assessment (Chapter 31) discusses the process by which probability

distributions are used to characterize variability or uncertainty in risk estimates, a process

aimed at describing risks as a distribution (or range) of potential outcomes.

• Use of Geographical Information Systems (GIS) in Risk Assessment (Chapter 32) provides

an overview of the software and geographic data that allow efficient storage, analysis, and

presentaiton of spatially explicit and geographically referenced information that can help in

the process of conducting risk assessments and reporting results

Chapter 30 Public Health Assessment

Table of Contents

30.1 Introduction ............ ............ ............ ............ ............ ...... 1

30.2 History of Public Health Assessment ............ ............ ............ .......... 2

30.3 Relationship of Public Health Assessment to Risk Assessment ............ ............ . . 3

30.4 What Is Public Health Assessment? ............ ............ ............ ........... 4

30.5 How Is a Public Health Assessment Conducted? ............ ............ ............ . 6

30.5.1 Conduct Scoping ............ ............ ............ ............ ....... 6

30.5.2 Obtain Study Area Information ............ ............ ............ ........ 7

30.5.3 Community Involvement/Outreach/Response to Community Concerns ............ . 7

30.5.4 Exposure Evaluation ............ ............ ............ ............ .... 8

30.5.5 Health Effects Evaluation ............ ............ ............ ............ 9

30.5.6 Draw Public Health Conclusions ............ ............ ............ ...... 12

30.5.7 Recommend Public Health Actions ............ ............ ............ .... 13

30.5.8 Prepare PHA Documents ............ ............ ............ ............ 13

References ............ ............ ............ ............ ............ ............ 14

April 2004 Page 30-1

30.1 Introduction

An adjunct to conducting air toxic risk assessments is public health assessments, which uses

public health tools (e.g., health questionnaires, epidemiology) to investigate the incidence and

prevalence of disease and to find out the current or past health of individuals. While public

health methods are not always used for air toxics risk assessments, they can provide useful

information to answer the question of whether there is evidence that there is a public health

concern, particularly if disease rates are elevated in the assessment area.

Air toxics risk assessment, the main topic of this manual, focuses on assessing the potential risk

that people have for experiencing adverse health effects from exposure to air toxics. The

outcome of a risk assessment is a statement about the likelihood that exposure may result in

disease (e.g., the probability of people developing cancer). The risk assessment process links the

potential exposures to emissions from (often) specific sources to the likelihood of disease

occurring. 

However, in any community, concerns about more than just estimates of the likelihood of risk

often come up. For example, communities where risk assessments are being performed often

express concern about current health effects that may have resulted from past exposures. 

Questions like “was my cancer caused by air pollution” are often on the minds of people who

live where an air toxics risk assessment is being performed. 

The risk assessment process, while a powerful predictive tool for evaluating public health

impacts from air pollution, is not amenable to answering these types of questions. Nevertheless,

questions about disease and past exposures will inevitably come up as the air toxics risk

assessment study moves forward. The risk assessment and risk management team will almost

always have to explain that their assessment tool (risk assessment) is not being used to answer

questions about existing cases of disease.

To help risk assessors and other stakeholders respond to these types of questions, this chapter

provides information on a complementary process to risk assessment called Public Health

Assessment or PHA. It is taken largely from the ATSDR Public Health Assessment Guidance

Manual.

(1) A PHA for air toxics is an analysis and statement of the public health implications

posed by a source or group of sources of air toxics on a given geographic area. It usually is

conducted by a public health agency such as the Agency for Toxics Substances and Disease

Registry or ATSDR (a federal Agency within the Centers for Disease Control and Prevention) or

one of their partner state or local public health agencies. PHAs are not generally performed by

EPA or state, local, or tribal air agencies since PHAs often rely on specialized medical and

epidemiological expertise and due to the difficulty facing these agencies in obtaining and

reviewing medical information for individuals. PHAs are normally performed:

• In response to a request by concerned community members or physicians;

• In response to a real or perceived increase in a health problem noted during routine disease

surveillance systems; and/or

• As part of a broader program such as a proactive analysis of region-specific air quality.
April 2004 Page 30-2

PHAs are performed by ATSDR at each

Superfund site on the National Priorities List. 

ATSDR also performs PHAs when petitioned. 

The term public health assessment (PHA) as

used here, refers to a broad range of

assessment types – from screening-level health

consultations to comprehensive

epidemiological assessments – that are

commonly performed by ATSDR in its work. 

The PHA process, while commonly thought of

as a Superfund-related activity, is amendable

to a wide range of exposure scenarios,

including the evaluation of air toxics impacts

at the community level.

The earliest “bad air”?

The types of air toxics assessments most likely

to include a PHA are those where the pollutants

have a clearly identifiable effect, where the

exposure is relatively widespread, or where there

is a high level of public concern. A PHA will

not necessarily be needed every place an air

toxics risk assessment is performed. However,

the use of the PHA process, in conjunction with

the risk assessment process, is becoming a more

common practice for the purpose of providing

holistic evaluations of air toxics impacts on

communities. 

A PHA may involve an assessment of relevant

environmental data, health outcome data

(e.g., cancer statistics), and community concerns generally associated with a study area where

air toxics are or have been released. A PHA identifies populations living or working on or near

areas for which more extensive public health actions or studies are indicated and is generally

more qualitative, more focused on actual, measurable harm, and past and current exposures.

This chapter describes the history of PHAs, what they are, how they compare to and work in

concert with risk assessments, and how they are conducted. Several case studies are included to

help illustrate the diversity of PHAs and how they compare with and are used with risk

assessments.

30.2 History of Public Health Assessment

PHA as a tool for characterizing and protecting the health of a society can be traced back

thousands of years. The ancient Babylonians, Egyptians, Greeks, and Romans were among the

first known civilizations to describe associations between diseases and sources such as place,

water conditions, climate, eating habits, and housing. One of the

first documented public health “assessments” (though later proven

incorrect) connected the presence of “bad air” around swamps and

marshes with the prevalence of malaria, one of the world’s most

devastating diseases. (It was determined later that the prevalence

of malaria was associated not with air, but with mosquitos, the

transmission vector for the disease, which breed in standing water

associated with those places.) Infectious diseases continued to

dominate public health concerns until the industrial revolution,

although the problems of poor urban air quality from the use of

coal were well documented as early as the end of the 16th century.

The modern use of PHA for air toxics in the U.S. probably began in the mid-1900s in response to

events such as the incapacitating smog episodes in Los Angeles in the 1940s, the polluted air

inversion that killed 20 people in Donora, Pennsylvania in 1948, and the atmospheric nuclear

weapons tests in Nevada in the 1950s. Myriad state and local public health agencies shouldered

much of the burden of air pollutant health assessment at first. Then, at the federal level, the

Federal Air Pollution Control Act of 1955 authorized the Public Health Service (PHS) to conduct
April 2004 Page 30-3

research and technical assistance and work towards a better understanding of the causes and

effects of air pollution.

In 1980, ATSDR was created specifically to conduct PHAs at hazardous waste (Superfund) sites. 

That role has expanded over time to address additional pollution sources, including air toxics. 

ATSDR is not a regulatory agency like EPA, but rather is a public health agency that conducts

assessments and makes recommendations to EPA and others when specific actions at study areas

in question are needed to protect the public’s health. ATSDR conducts PHAs when petitioned by

concerned community members, physicians, state or federal agencies, or tribal governments. 

State and local public health agencies also play an important role with regard to PHAs for air

toxics and other hazards.

30.3 Relationship of Public Health Assessment to Risk Assessment

Both the PHA and the quantitative risk assessment address the potential human health effects of

environmental exposures, but they use different approaches and have different purposes. As

illustrated in Exhibit 30-1, the PHA tends to be less quantitative than the risk assessment and to

focus more on actual past and current exposures. The PHA evaluates observed health outcome

and related data (e.g., cancer clusters, breathing problems, toxics residues in biologic samples) to

determine whether rates of disease or death are or could be elevated in a community and, if so,

whether these outcomes are due to a specific source. The risk assessment, on the other hand,

starts with a specific source and evaluates estimated potential health outcomes, or risks. The

PHA’s subsequent conclusions generally complement the risk assessment process and help

inform the decisions that the state, tribal, or local agency is reaching about a given study area. 

Similarly, the risk assessment provides considerable data to the PHA.

In addition to its focus on health outcome data, such as cancer or asthma incidence, the PHA also

helps put community-provided data and information and community concerns into perspective,

which in turns helps both (1) the community better understand whether they have been exposed

to hazardous substances and, if so, what that means in terms of possible health outcomes, and (2)

the decision-maker better determine what needs to be done to prevent or further study these

exposures (e.g., emissions reductions, health education, biologic monitoring).

The PHA may use similar techniques to those of the quantitative risk assessment, but primarily

as tools either to clearly rule out the existence of public health hazards, to determine that a

clinical disease is really likely in the community, or to identify areas for additional study. At a

minimum, the PHA helps to identify a baseline in the level of disease in a community so that

later studies will have a basis for comparison.
April 2004 Page 30-4

ATSDR Definition of PHA

The evaluation of data and information on

the release of hazardous substances into the

environment in order to assess any [past],

current, or future impact on public health,

develop health advisories or other

recommendations, and identify studies or

actions needed to evaluate and mitigate or

prevent human health effects (42 Code of

Federal Regulations, Part 90, published in

55 Federal Register 5136, February 13,

1990).

Exhibit 30-1. PHAs and Risk Assessments: Differences and Similarities

30.4 What Is Public Health Assessment?

A PHA is an evaluation of relevant

environmental data, health outcome data, and

community concerns associated with a study

area where hazardous substances have been

released. A PHA identifies populations living or

working on or near areas for which more

extensive public health actions or studies are

indicated.

PHAs can range from simple to complex, with the

former activity often termed a health

consultation rather than PHA. This more simple

form generally is conducted in response to a
April 2004 Page 30-5

specific question or request for information pertaining to a hazardous substance or facility. It

often contains a time-critical element that necessitates a rapid response. More complex forms of

a PHA can involve a wide geographical area, many pollution sources, and take months or years

to complete.

Understanding and responding to study area-specific community health concerns is an important

part of the PHA process. These investigations can be conducted to confirm case reports,

determine an unusual disease occurrence, and explore potential risk factors. One frequently cited

concern is the disease cluster – the occurrence of a specific disease or condition above the

expected number for a given geographic location and time period (e.g., the high incidence of

leukemia in a given area). The health agency needs to learn what people in the area know about

a source and source-related exposures and what concerns they may have about its impact on their

health. Therefore, starting early in the assessment process, the health agency generally gathers

information and comments from the people who live or work near the source(s), including area

residents, civic leaders, health professionals, and community groups. Throughout the PHA

process, the health agency should communicate with the public about the purpose, approach, and

results of its public health activities.

The PHA process is iterative and dynamic and may lead to a variety of products or public health

actions. The findings may be communicated in public health assessment or public health

consultation documents, which serve as an aid for developing additional public health actions.

The audience for such products often includes environmental and public health agencies,

communities, and the public health agency itself.

During the course of the PHA process, the public health agency may identify the need to prevent

or better define exposures or illnesses in a particular community. The agency’s response to such

a need might include:

• Issuing a public health advisory (if there is an urgent health threat);

• Initiating an exposure investigation (to better define study area exposures);

• Recommending a health study (to identify elevated illness or disease rates in a community);

and/or

• Conducting health education (for the study area community or health professionals within

the community).

The PHA process also can serve as a triage mechanism, enabling the public health agency to

prioritize and identify additional steps needed to answer public health questions. The science of

environmental health is still developing, and sometimes information on the health effects of

certain substances is not available. When this occurs, rendering certain questions unanswerable

by the available literature, the public health agency will suggest what further research studies

and/or health education services are needed.
April 2004 Page 30-6

30.5 How Is a Public Health Assessment Conducted?

PHAs generally are conducted by public health agency assessors, often supported by a

multi-disciplinary team of scientists, health communication specialists, health educators, and/or

medical professionals. The health agency solicits and evaluates information from other local,

state, tribal, and/or federal agencies; parties responsible for operating sources at a particular study

area; and the community. All of these stakeholders play an integral role in the PHA process. 

The public health agency promotes a team approach to ensures that information used in the

assessment is accurate and up-to-date, ensure that community concerns are identified and

addressed, and fosters cooperative efforts in implementing recommendations and public health

activities. 

Many technical resources exist that provide details about conducting a PHA (see Exhibit 30-2),

and, thus, only a broad overview is provided here. One of the most comprehensive resources is

the ATSDR Public Health Assessment Guidance Manual.

(1) The ATSDR manual focuses on

site-specific PHAs such as Superfund sites; nevertheless, it also can be used to assess air

emissions within a limited geographical area. As described in detail in the ATSDR manual, the

steps of a PHA — whether conducted by ATSDR or a state or local public health agency, and

whether comprehensive or limited to a screening assessment – can be multifaceted and

interactive. Exhibit 30-3 illustrates this by providing an overview of a typical PHA process. The

following subsections describe this process in more detail.

30.5.1 Conduct Scoping

The first step is to establish an overall understanding of the study area and begin to identify the

most pertinent issues. The objective is to quickly gain some baseline information about the study

area and start developing a strategy for conducting the PHA. To help ensure a consistent

approach across study areas, the following steps are followed during this initial phase:

• Initiate study area scoping by performing an initial review of permits and other sources of

study area information, identifying any past health agency or partner activities, identifying

and communicating with study area contacts, and determining the need for a study area visit

to observe actual conditions and speak with study area representatives.

• Define roles and responsibilities of team members (internal and external).

• Establish communication mechanisms (internal and external) by developing a schedule for

team meetings, thinking about how to present the findings of the assessment, and developing

health communication strategies.

• Develop a study area strategy for completing the various steps in the PHA process and

develop a strategy, identifying the tools and resources that might be needed to evaluate the

study area, communicate the findings, and implement public health actions.

• Based on information obtained during study area scoping, develop an approach that focuses

on the most pertinent public health issues.
April 2004 Page 30-7

Exhibit 30-2. Selected Public Health Assessment Resources

• Agency for Toxic Substances and Disease Registry (ATSDR; www.atsdr.cdc.gov), which publishes

the Public Health Assessment Guidance Manual (current draft is available online; Guidance for

ATSDR Health Studies (1996; available online), Environmental Data Needed for Public Health

Assessments (1994, available online), and other guidance.

• National Institute of Environmental Health Sciences (NIEHS; www.niehs.nih.gov), which

publishes Environmental Health Perspectives and sponsors multidisciplinary biomedical research,

prevention and intervention efforts, and communication strategies that encompass training,

technology transfer, and community outreach.

• American Public Health Association (APHA; www.apha.org), which publishes the American

Journal of Public Health and provides many other resources related to environmental public health.

• National Association of County and City Health Officials (NACCHO; www.naccho.org), which

publishes the Protocol for Assessing Community Excellence in Environmental Health (2000) and

Assessment to Action: Improving the Health of Community Affected by Hazardous Waste (2002).

• National Association of Local Boards of Health (NALBOH) (www.nalboh.org), which maintains

an up-to-date database of contact information for all local boards of health, provides technical

assistance to existing boards of health, and will soon publish the Environmental Health Primer.

30.5.2 Obtain Study Area Information

Throughout the PHA process, various team members will collect information about the study

area, although the initial collection of information is typically the most intensive. Information

sources typically include interviews (in-person or via telephone); study area-specific

investigation reports prepared by federal, state, and local environmental and health departments;

and study area visits. Gathering pertinent study area information requires a series of iterative

steps, including gaining a basic understanding of the study area, identifying data needs and

sources, conducting a study area visit, communicating with community members and other

stakeholders, critically reviewing study area documentation, identifying data gaps, and compiling

and organizing relevant data to support the assessment.

30.5.3 Community Involvement/Outreach/Response to Community Concerns

The community associated with a study area is both an important resource for and a key audience

in the PHA process. Community involvement activities should be developed and implemented

with the following objectives in mind:

• Earning trust and credibility through open, compassionate, and respectful communications.

• Helping community members understand what the PHA process involves and what it can and

cannot do.

• Providing opportunities for communities to become involved in the PHA activities.

• Promoting collaboration between the public health agency, communities, and other agencies.

• Informing and updating communities about the health agency’s work.

• Assisting communities in understanding the possible health impact of exposures to hazardous

substances.
April 2004 Page 30-8

Exhibit 30-3. Overview of a Typical Public Health Assessment Process

Chapter 28 of this reference manual provides a more detailed discussion of community

involvement and outreach.

30.5.4 Exposure Evaluation

For the exposure evaluation, public health assessors review environmental data to determine the

sources of pollutants and exposure pathways/routes. The conceptual model described in Chapter

6 should be a reasonable starting point for the PHA exposure evaluation. Generally, the public

health agency involved does not collect its own environmental sampling data, at least at first, but

rather reviews information provided by federal, state, and local government agencies and/or their

contractors, businesses, and the public. Assessors can indicate what further environmental

sampling may be needed and may collect environmental and biologic samples when appropriate. 

This step involves two key substeps:
April 2004 Page 30-9

Exposure Investigations

When a PHA exposure evaluation concludes

that additional exposure information is

needed, an exposure investigation generally is

conducted. An exposure investigation is the

collection and analysis of study area-specific

information to determine if human

populations have been exposed to air toxics.

This information may include environmental

sampling, exposure-dose reconstruction,

biologic or biomedical testing, and/or

evaluation of medical information.

• Evaluate Environmental Contamination

Data. This step involves determining what

pollutants people may be exposed to and in

what concentrations. This evaluation

involves assessing the quality and

representativeness of available monitoring

data and measurements or modeled estimates

of exposure point concentrations. This is an

important way to ensure that any public health

conclusions and recommendations for the

study area are based on appropriate and

reliable data. Both sampling data and

modeling techniques described in Chapters 9,

10, 18, and 19 are sometimes used to generate

data for PHAs. Evaluation of environmental contamination data typically proceeds

simultaneously with the exposure pathway evaluation.

• Characterize Exposure Pathways. During the exposure pathway characterization, the

assessor evaluates who may be or has been exposed to study area contaminants, for how long,

and under what conditions. This involves identifying and studying the following five

components of a “complete” exposure pathway: a source of air toxics; a mechanism for

release into the air and, in some cases, transfer between media (i.e., the fate and transport of

environmental contamination); an exposure point or area; an exposure route (e.g., ingestion,

dermal contact, inhalation); and a potentially exposed population. The overall purpose of this

evaluation is to understand how people might become exposed to study area contaminants

and to identify and characterize the size and susceptibility of the potentially exposed

populations. If no complete or potentially complete exposure pathways are identified, no

public health hazards exist and there is no need to perform further scientific evaluation. 

When complete environmental or biologic data are lacking for a study area, an exposure

investigation may be recommended to better assess possible impacts to public health. 

30.5.5 Health Effects Evaluation

If the exposure evaluation shows that people have been or could be exposed to pollutants such as

air toxics, the public health assessor will evaluate whether this contact could have resulted in

harmful effects. Assessors use existing scientific information to determine the health effects that

may result from exposures. Public health agencies recognize that children, because of their play

activities and their growing bodies, may be particularly vulnerable to exposures to air toxics. 

Developing fetuses also may be more vulnerable to such exposures. Thus, the impact to children

and developing fetuses is considered first when evaluating the health threat to a community. The

health effects evaluation is composed of two basic substeps: a screening analysis and a more indepth analysis.

• Screening Analysis. Screening is a first step in understanding whether the detected

concentrations to which people may be exposed are harmful. The screening analysis is a

fairly standard process developed to help health assessors sort through the large volumes of

environmental data for a study area. It enables the assessor to safely rule out substances that

are not at levels of health concern and to identify substances and pathways that need to be
April 2004 Page 30-10

examined more closely. For complete or potential exposure pathways identified in the

exposure pathway evaluation, the screening analysis may involve comparing media

concentrations at points of exposure to “screening” values (based on protective default

exposure assumptions) and estimating exposure doses based on study area-specific exposure

conditions. The assessor then compares estimated doses with health-based guidelines to

identify substances requiring further evaluation. Exhibit 30-4 describes several of the

ATSDR-derived comparison values available. See Chapter 12 for how these values are used

in an air toxics risk assessment.

Exhibit 30-4. Definitions of ATSDR-Derived Comparison Values

Environmental Media Evaluation Guides (EMEGs). EMEGs are estimated contaminant

concentrations that are not expected to result in adverse noncarcinogenic health effects based on

ATSDR evaluation. EMEGs are based on ATSDR MRLs and conservative assumptions about

exposure, such as intake rate, exposure frequency and duration, and body weight.

Minimal Risk Levels (MRLs). An MRL is an estimate of daily human exposure to a substance (in

mg/kg/day for oral exposures and parts per million [ppm] for inhalation exposures) that is likely to be

without noncarcinogenic health effects during a specified duration of exposure based on ATSDR

evaluations.

Cancer Risk Evaluation Guides (CREGs). CREGs are estimated contaminant concentrations that

would be expected to cause no more than one excess cancer in a million (10-6) persons exposed during

their lifetime (70 years). ATSDR’s CREGs are calculated from EPA’s cancer slope factors (CSFs) for

oral exposures or unit risk values for inhalation exposures. These values are based on EPA

evaluations and assumptions about hypothetical cancer risks at low levels of exposure.

Reference Media Evaluation Guides (RMEGs). ATSDR derives RMEGs from EPA’s oral reference

doses, which are developed based on EPA evaluations. RMEGs represent the concentration in water

or soil at which daily human exposure is unlikely to result in adverse noncarcinogenic effects.

• In-depth Analysis. For those pathways and substances that were identified in the screening

analysis as requiring more careful consideration, the assessor will examine a host of factors to

help determine whether study area-specific exposures are expected to result in illness. In this

in-depth analysis, exposures are studied in conjunction with substance-specific toxicologic,

medical, and epidemiologic data. Through this analysis, the assessor will be answering the

following question: Based on available exposure, toxicologic, epidemiologic, medical, and

study area-specific health outcome data, are adverse health effects expected in the

community?

Answering this last question can be very challenging. For example, evaluating epidemiological

data involves addressing a number of criteria to assist in judging the causal significance of

associations revealed in studies (epidemiology is described in more detail in Exhibit 30-5). 

Individual criteria, if met, support a causal relationship but do not prove it. The more criteria that

are met, the more likely it is that an observed health effect is causally related to the exposure

under study. The criteria for evaluating causation are:

• Time sequence. Exposure must precede the onset of the disease. A logical sequence of

events must be demonstrated. 
April 2004 Page 30-11

Exhibit 30-5. What Are Epidemiologic Data and How Might They Be Used

in an In-Depth Analysis?

Epidemiologic data are one of the key distinguishing features of PHAs compared to most quantitative

risk assessments. Understanding the strengths and weaknesses of the various types of epidemiologic

studies will help determine the suitability of a particular study in supporting and drawing study area

and substance-specific public health conclusions. Because of the inherent limitations and

uncertainties associated with environmental epidemiologic evaluations (generally due to the lack of

adequate exposure data or sample size), however, epidemiologic data should be used with caution.

The health assessor should call upon an epidemiologist to assist in evaluating the applicability and

usability of literature-based or study area-specific epidemiologic data. The types of epidemiologic

data that may be available and how they may be used are briefly summarized below, in order of

greatest potential utility:

• Analytical studies, such as case-control or cohort studies, evaluate the role of various risk factors

in causing illness or disease by relying on comparisons between groups. Depending on the quality

of the study, it may provide insight to the study area-specific exposure situation under evaluation. 

Study area-specific analytical studies that meet certain design criteria examine study area-specific

exposures and health outcomes in community members. When available, these studies are the most

relevant to the PHA. These data are rarely initially available, but the PHA process may lead to a

recommendation to collect such data. Depending on the individual study design and health

outcome studied, results may provide some insight on the presence or absence of a particular

illness of concern in the community. Unfortunately, establishing a definitive link with a study

area-related exposure is generally difficult if not impossible.

• Descriptive (or ecological) studies examine differences in disease rates among populations over

time or in different geographical locations and may be helpful in identifying plausible associations

between a particular substance and disease. However, descriptive studies provide limited

information on causal relationships (i.e., the degree of exposure or causal agent).

• Case reports that describe an effect in an individual or small group can be considered in the indepth analysis, but may have limited usefulness due to the generally small size of the affected

population and sometimes anecdotal nature of the reports.

• Strength of association. The stronger the association, the more likely it is causal. The

relative magnitude of the incidence of disease in those exposed compared to the incidence in

those who are not exposed can be a valuable measure of the strength of the association.

• Dose-response relationship. The probability and/or severity of the effect should increase

with increasing intensity and duration of exposure.

• Specificity of association. If the effect is unusual or is specific to the studied exposure, a

causal relationship is more easily demonstrated. 

• Consistency. A relationship should be reproducible (i.e., observed in other studies or

analyses).
April 2004 Page 30-12

• Biologic plausibility (or coherent explanation). The link between the “cause” and the

effect should make sense biologically, by what is known about the disease and the exposure

under study. The findings should be validated by what is known about animal models.

Similarly, biologic sampling results (biomarkers) need to be interpreted with caution.

Specifically, issues to consider include: (1) as with environmental sampling data, biologic data

need to be collected by trained professionals and analyzed in a standard way; (2) detected levels

may not be the result of study area-related exposures (e.g., blood lead levels resulting from nonair toxics sources such as flaking paint); (3) results will likely only represent a snapshot of

conditions in time; (4) the association between detected levels and clinical effects may not be

understood based on scientific knowledge; (5) “normal” ranges, particularly for trace elements,

may not be known; and (6) the people tested may not be fully representative of the exposed

population, resulting from a small sample size and variations in exposures across the exposed

population due to different activity patterns.

30.5.6 Draw Public Health Conclusions

Upon completing the exposure and health effects evaluations, the assessor will draw conclusions

regarding the degree of hazard posed by a study area - that is, they will conclude either that the

study area does not pose a public health hazard, that the study area does pose a public health

hazard, or that insufficient data are available to determine whether any public health hazards

exist. The process also involves assigning a hazard conclusion category for the study area or

for an individual exposure pathway (Exhibit 30-6).

Exhibit 30-6. Summary of ATSDR Conclusion Categories

Category Definition

1. Urgent Public

Health Hazard

Applies to study areas that have certain physical hazards or evidence of

short-term (less than 1 year), study area-related exposure to hazardous

substances that could result in adverse health effects and require quick

intervention to stop people from being exposed.

2. Public Health

Hazard 

Applies to study areas that have certain physical hazards or evidence of

chronic, study area-related exposure to hazardous substances that could

result in adverse health effects.

3. Indeterminate

Public Health

Hazard

Applies to study areas where critical information is lacking (missing or has

not yet been gathered) to support a judgment regarding the level of public

health hazard.

4. No Apparent

Public Health

Hazard

Applies to study areas where exposure to study area-related chemicals might

have occurred in the past or is still occurring, but the exposures are not at

levels expected to cause adverse health effects.

5. No Public Health

Hazard

Applies to study areas where no exposure to study area-related hazardous

substances exists.
April 2004 Page 30-13

30.5.7 Recommend Public Health Actions

After drawing conclusions, the public health assessor – usually in cooperation with other team

members and stakeholders – will develop recommendations for actions, if any, to prevent

harmful exposures, obtain more information, or conduct other public health actions. These

actions generally will be detailed in a public health action plan, which will ultimately be part of

the PHA document (or possibly the public health consultation document) developed for the study

area. Note that some public health actions may be recommended earlier in the process. See

Exhibit 30-7 for an overview of the conclusions and recommendations process.

30.5.8 Prepare PHA Documents

The public health assessor may develop various materials during the PHA process to

communicate information about the assessment, including outreach materials, health advisories

that alert the public and appropriate officials to the existence of an imminent public health threat,

and, at the end of the assessment process, a report that summarizes the approach, results,

conclusions, and recommendations. This report generally is either a public health assessment

(PHA) document or a public health consultation (PHC).

Exhibit 30-7. Overview of Typical PHA Conclusion and Recommendation Process
April 2004 Page 30-14

1. Agency for Toxic Substances and Disease Registry (ATSDR). 2002. Public Health

Assessment Guidance Manual (Update): Draft for Public Comment.. Available at:

http://www.atsdr.cdc.gov/HAC/PHAManual/cover.html. 

References
Chapter 31 Probabilistic Risk Assessment

Table of Contents

31.1 Introduction ............ ............ ............ ............ ............ ...... 1

31.2 Tiered Approach for Risk Assessment ............ ............ ............ ......... 2

31.3 Methods for Probabilistic Risk Assessment ............ ............ ............ ..... 4

31.4 Presenting Results for Probabilistic Risk Assessment ............ ............ ......... 6

References ............ ............ ............ ............ ............ ............ 10

April 2004 Page 31-1

A detailed discussion of PRA is beyond the scope of this document. Two documents provide

more detailed introductory information and guidance and should be reviewed if a PRA is

contemplated: 

U.S. EPA. 2001. Risk Assessment Guidance for Superfund (RAGS), Volume III - Part A,

Process for Conducting Probabilistic Risk Assessment. Office of Solid Waste and Emergency

Response. December. EPA 540-R-02-002, OSWER 9285.7-45, PB2002 963302, available

at: http://www.epa.gov/superfund/programs/risk/rags3a/index.htm.

National Council on Radiation Protection and Measurements (NCRP). 1996. A Guide for

Uncertainty Analysis in Dose and Risk Assessments Related to Environmental Contamination. 

NCRP Commentary No. 14, May 1996.

31.1 Introduction

Probabilistic risk assessment (PRA) uses probability distributions to characterize variability or

uncertainty in risk estimates. In a PRA, one or more variables in the risk equation is defined as a

probability distribution rather than a single number. Similarly, the output of a PRA is a range or

probability distribution of risks experienced by the receptors. Note that the ability to perform a

PRA often is limited by the availability of distributional data that adequately describe one or

more of the input parameters. For example, data often are insufficient to assess toxicity in a

probabilistic manner (and therefore, dose-response values such as inhalation unit risks (IURs)

and reference concentrations (RfCs) are included in a PRA analysis as point values). This

general lack of data impacts both human health and ecological receptors.

The primary advantage of PRA is that it can provide a quantitative description of the degree of

variability or uncertainty (or both) in risk estimates for both cancer and noncancer health effects

and ecological hazards. The quantitative analysis of uncertainty and variability can provide a

more comprehensive characterization of risk than is possible in the point estimate approach.

Another significant advantage of PRA is the additional information and potential flexibility it

affords the risk manager. Risk management decisions are often based on an evaluation of highend risk to an individual – for deterministic analyses, this is generally developed by the

combination of a mix of central tendency and high-end point values for various exposure

parameters (see Part II, Chapters 9 and 13). When using PRA, the risk manager can select a

specific upper-bound level from the high-end range of percentiles of risk, generally between the

90th and 99.9th percentiles.

PRA may not be appropriate for every analysis. The primary disadvantages of PRA are that it

generally requires more time, resources, and expertise on the part of the assessor, reviewer, and

risk manager than a point estimate approach. The chief obstacle to using PRA in air toxics risk

assessments is usually the lack of well-documented frequency distributions for many input

variables.
April 2004 Page 31-2

This chapter provides a general overview of PRA as it applies to air toxics risk assessment. It

revisits the tiered approach to risk assessment, introduces calculation algorithms, and identifies

advanced statistical methods currently available to support risk policy decisions.

31.2 Tiered Approach for Risk Assessment

The tiered approach is a process for a systematic, informed progression to increasingly more

complex risk assessment methods including PRA. Exhibit 31-1 presents a schematic

representation of the tiered approach. Higher tiers reflect increasing complexity and, in many

cases, will require more time and resources. Higher tiers also reflect increasing characterization

of variability and/or uncertainty in the risk estimate, which may be important for making risk

management decisions. Central to the concept of a systematic, informed progression is an

iterative process of evaluation, deliberation, data collection, work planning, and communication. 

All of these steps should focus on deciding: (1) whether or not the risk assessment, in its current

state, is sufficient to support risk management decisions (a clear path to exiting the tiered process

is available at each tier), and (2) if the assessment is determined to be insufficient, whether or not

progression to a higher tier of complexity (or refinement of the current tier) would provide a

sufficient benefit to warrant the additional effort.

• The problem formulation step precedes Tier 1 and includes scoping and refinement of the

conceptual site model, including exposure pathways/routes, and identifying chemicals of

potential concern (COPCs).

• In Tier 1, deterministic (point estimate) risk assessment is then performed using the basic

methodology described in Part II (inhalation) and/or Part III (multipathway) of this Reference

Manual. In deciding whether the results of a deterministic risk assessment are sufficient for

decision-making or whether more refined analyses should be implemented, two factors

generally are considered: (1) the magnitude of the estimates of risk (i.e., the value of hazard

indices [HIs] or cancer risks for COPCs), and (2) the level of confidence in these estimates. 

In a Tier I deterministic risk assessment, quantitative risk estimates can be easily calculated,

but the level of confidence associated with these calculations can be difficult to assess. For

example, variability in exposure levels among individual members of the population can

generally only be assessed semi-quantitatively by considering central tendency and high-end

exposure estimates. Uncertainty can often be evaluated only as confidence limits on certain

point estimates (e.g., the concentration term).

In some cases, the results of a Tier 1 risk analysis may be sufficient for decision-making. For

example, a deterministic analysis may indicate very low levels of risk for some air toxics. If

the assessment is considered to be overly conservative (even in light of uncertainties), this

may be sufficient for a “no action” decision for those chemicals. The same analysis may

indicate a very high potential for risk for other air toxics. EPA generally recommends that

the risk manager proceed to higher tiers only when site decision-making would benefit from

additional analysis beyond the point-estimate risk assessment (i.e., when the risk manager

needs more complete or certain information to complete the risk management process). 
April 2004 Page 31-3

Thus, only the combinations of COPC-exposure pathway-receptors of highest potential

concern are generally analyzed using higher level techniques such as PRA.

Exhibit 31-1. Example of a Tiered Approach for Risk Assessment

Adapted from Volume III of EPA’s Risk Assessment Guidance for Superfund(1)

• Tier 2 is represented as an intermediate-level analysis using more realistic exposure

assumptions (e.g., use of actual receptor locations) and more detailed modeling (e.g., a model

that requires additional site-specific inputs). Although not depicted, Tier 2 could incorporate

a sensitivity analysis to identify the most important parameters that are driving the risk

estimate for specific receptors or population groups. Tier 2 also could incorporate limited

(one-dimensional) Monte Carlo techniques.

• Tier 3 is represented as an advanced analysis using probabilistic techniques such as twodimensional Monte Carlo analysis. Results of sensitivity analyses (Tier 2 or Tier 3) could be

used to assess risk distributions for the high-end individuals within the population. The onedimensional Monte-Carlo simulation does not separate variability and uncertainty associated

with the risk estimates. If necessary, separate analyses of uncertainty and variability can be

performed in Tier 3. Techniques such as two-dimensional Monte Carlo simulation can be

used to estimate the relative impact of natural variability and lack of data on the overall

uncertainty in the risk estimate, and can be used to direct additional data gathering or to

support mitigation decisions. 

The deliberation cycle provides an opportunity to evaluate the direction and goals of the

assessment as new information becomes available. It may include evaluations of both scientific

and policy information. (Also note that, while a three-tiered approach was provided in Exhibit
April 2004 Page 31-4

31-1, the tiered approach is really more of a continuum from a point where the analysis is done

with little data and conservative assumptions to a point where there is an extensive data set and

fewer assumptions. In between, there can be a wide variety of tiers of increasing complexity, or,

as discussed in Chapter 3, there may only be a few reasonable choices between screening

methods and highly refined analyses. The three tiered approach is only provided here as an

illustration of the concept, not a prescriptive, fixed methodology.)

31.3 Methods for Probabilistic Risk Assessment

As discussed in previous chapters, there are a number of approaches available for analyzing

uncertainty in risk assessments. For simple screening level analyses, or analyses where there are

only a few major sources of uncertainty, sensitivity analyses may be used to estimate the impacts

of likely variations in the key parameter values. Where scenario uncertainty is important (that is,

there are multiple sequences of events that could contribute to risk), decision tree or Bayesian

statistical analysis are commonly used. The most common numerical technique for PRA

(analyses in which a large number of variables need to be evaluated simultaneously) in largescale air risk assessments is Monte Carlo simulation. Monte Carlo simulation integrates varying

assumptions, usually about exposure, to come up with possible distributions (or ranges) of risk

instead of point estimates. A continuous probability distribution can be displayed in a graph in

the form of either probability density functions (PDFs) or corresponding cumulative

distribution functions (CDFs); however, for clarity, it is recommended that both representations

be presented in adjacent (rather than overlaid) plots.

Exhibit 31-2 illustrates a PDF and CDF for a normal probability distribution for adult body

weight. Both displays represent the same distribution, but are useful for conveying different

information. PDFs are most useful for displaying (1) the relative probability of values; (2) the

most likely values (e.g., modes); and (3) the shape of the distribution (e.g., skewness, kurtosis,

multimodality). CDFs can be used to display (1) percentiles, including the median; (2) high-end

risk range (e.g., 90th to 99th percentiles); (3) confidence intervals for selected percentiles; and (4)

stochastic dominance (i.e., for any percentile, the value for one variable exceeds that of any other

variable). Note that it is helpful to include a text box with summary statistics relevant to the

distribution (e.g., mean, standard deviation).

These results expressed as probability distributions help risk managers decide whether and what

actions are necessary to reduce risk. Monte Carlo simulation has been widely used to explore

problems in many disciplines of science as well as engineering, finance, and insurance.(1) The

process for a Monte Carlo simulation is illustrated in Exhibit 31-3. In its general form, the risk

equation can be expressed as a function of a toxicity term (as a point value) and multiple

exposure variables (Vn) represented as distributions (not point values):

Risk = f(V1, V2, V3, ...Vn) × Toxicity Equation 31-4

The first decision(s) the risk assessor has to make is which of the “Vs” are going to be evaluated

probabilistically. Ideally, every model input that is variable or uncertain should be evaluated to

provide a comprehensive characterization of uncertainty in exposure estimates. In practice, the
April 2004 Page 31-5

number of variables that can be addressed systematically is severely limited by lack of data

related to variability, uncertainty, or both. Sensitivity analyses can often be used to focus the

analysis on the variables that contribute most to the overall uncertainty in risks.

Exhibit 31-2. Examples of Probability Density and Cumulative Distribution Functions

Example of a normal distribution that characterizes variability in adult body weight (males and

females combined). The arithmetic mean = 71.7 kg, and standard deviation = 15.9 kg. Body weight

may be considered a continuous random variable. The left panel shows a bell-shaped curve and

represents the PDF, while the right panel shows an S-shaped curve and represents the CDF. Both

displays represent the same distribution (including summary statistics), but are useful for conveying

different information.

Source: Finley and Paustenbach(2)

Solutions for equations with PDFs are typically too complex for even an expert mathematician to

calculate the risk distribution analytically. However, numerical techniques applied with the aid

of computers can provide very close approximations of the solution. This is illustrated here for

the simplified case in which the assessment variables are statistically independent, that is, the

value of one variable has no relationship to the value of any other variable. In this case, the

computer selects a value for each variable (Vn) at random from a specified PDF and calculates

the corresponding risk. This process is repeated many times (e.g., 10,000), each time saving the

set of input values and corresponding estimate of risk. For example, the first risk estimate might

represent a hypothetical individual who drinks 2 L/day of water and weighs 65 kg, the second

estimate might represent someone who drinks 1 L/day and weighs 72 kg, and so forth. Each

calculation is referred to as an iteration, and a set of iterations is called a simulation.

Each iteration of a Monte Carlo simulation should represent a plausible combination of input

values (i.e., exposure or ecotoxicity variables), which may require using bounded or truncated

probability distributions. However, risk estimates are not intended to correspond to any one

person. The “individuals” represented by Monte Carlo iterations are “virtual,” and the risk

distributions derived from a PRA allow for inferences to be made about the likelihood or

probability of risks occurring within a specified range for an exposed human or ecological
April 2004 Page 31-6

population. A simulation yields a set of risk estimates that can be summarized with selected

statistics (e.g., arithmetic mean, percentiles) and displayed graphically using PDF and CDF for

the estimated risk distribution. 

Exhibit 31-3. Conceptual Model of Monte Carlo Analysis

Random variables (V1, V2, ...Vn) refer to exposure variables (e.g., body weight, exposure frequency,

ingestion rate) that are characterized by probability distributions. A unique risk estimate is calculated

by sampling each set of the random values and calculating a result. Repeated sampling results in a

frequency distribution of risk can be described by a probability density function. In human health risk

assessments, the toxicity term is usually expressed as a point estimate. In ecological risk assessments,

the toxicity term may be expressed as a point estimate or as a probability distribution.

31.4 Presenting Results for Probabilistic Risk Assessment

The complexity of risk evaluation, and particularly of probabilistic methods, may pose a

significant barrier to understanding among the affected and interested parties (and thus to the

utility of the analysis). In the past, regulatory decisions have been evaluated primarily in terms of

point estimates of risk and simple dichotomous decision rules (e.g., “If the point estimate of risk
April 2004 Page 31-7

is above a certain level, take a certain action. If not, take another action.”). In contrast, it may

not be intuitively obvious, even to relatively sophisticated audiences, how to relate the outputs of

quantitative uncertainty evaluation to a particular decision. For example, important aspects of a

regulatory decision may rest on relatively subtle statistical distinctions (e.g., the difference

between a 95th percentile risk estimate and a 95th percent upper confidence limit on a risk

estimate), and the challenges in presenting such information can be formidable. In its recent

guidance, EPA has begun to define concrete approaches to presenting risks and uncertainty

information to decision-makers and stakeholders.(5)

The key factors for successful communication of PRA include early and continuous involvement

of affected and interested parties, a well-developed communication plan, good graphics, a

working knowledge of the factors that may influence perceptions of risk and uncertainty, and a

foundation of trust and credibility. A certain amount of training for interested stakeholders will

likely be necessary to help them understand the complexities of not only risk assessment in

general, but the intricacies of higher levels of analysis. Part III of this Reference Manual

provides guidance on community involvement and risk communication.

When summarizing results of PRA, graphs and tables should generally also include the results of

the point estimates of risk (e.g., central tendency and high-end). 

Consistent with EPA’s guidance on risk characterization,(3) the central tendency and high-end

cancer risks and noncancer hazards, along with decision points, should be highlighted on

graphics. The discussions accompanying the graph should emphasize that these values represent

risks to the average and high-end individuals, respectively, and serve as a point of reference to

EPA’s decision point. The distribution of risks should be characterized as representing

variability among the population based on differences in exposure. Similarly, graphics that show

uncertainty in risk estimates can be described using terms such as “confidence interval,”

“credible interval,” or “plausible range,” as appropriate. The graphics need not highlight all

percentiles. Instead, selected percentiles that may inform risk management decisions (such as the

5

th, 50th, 90th, 95th, and 99th percentiles) should be the focus. Exhibit 31-4 presents an example of

a PDF for variability in risk with an associated text box for identifying key risk descriptors.

By understanding the assumptions regarding the inputs and modeling approaches used to derive

point estimates and probabilistic estimates of risk, a risk communicator will be better prepared to

explain the significant differences in risk estimates that have been developed. Special emphasis

should be given to the model and parameter assumptions that have the most influence on the risk

estimates, as determined from the sensitivity analysis.
April 2004 Page 31-8

Exhibit 31-4. Example of Presenting the Results of a Probabilistic Risk Assessment

Hypothetical PRA results showing a PDF (top panel) for cancer risk with selected summary statistics

for central tendency and high-end percentiles. This view of a distribution is useful for illustrating the

shape of the distribution (e.g., slightly right-skewed) and explaining the concept of probability as the

area under a curve (e.g., most of the area is below 1x10-6, but there is a small chance of 2x10-6). 

Although percentiles can also be overlayed on this graphic, a CDF (bottom panel) may be preferable

for explaining the concept of a percentile.
April 2004 Page 31-9

Additional References on Uncertainty Analysis

Burmaster, D.E. and Anderson, P.D. 1994. Principles of good practice for the use of Monte Carlo

techniques in human health and ecological risk assessments. Risk Analysis 14: 477-481.

Cullen, A.C. and Frey, H.C. 1999. Probabilistic Techniques in Exposure Assessment. New York:

Plenum Press.

Fayerweather, W.E., Collins, J.J., Schnatter, A.R., Hearne, F.T., Menning, R.A., and Reyner, D.P.

1999. Quantifying uncertainty in a risk assessment using human data. Risk Analysis 19: 1077-1090

Finkel, A.M. and Evans, J.S. 1987. Evaluating the benefits of uncertainty reduction in environmental

health risk management. Journal of the Air Pollution Control Association. 37: 1164-1171.

Frey, H.C. 1992. Quantitative analysis of uncertainty and variability in environmental policy making. 

Pittsburgh: Carnegie Mellon University.

Hattis, D. and Burmaster, D.E. 1994. Assessment of variability and uncertainty distributions for

practical risk assessments. Risk Analysis 14: 713-730.

Hope, B. K. 1999. Assessment of risk to terrestrial receptors using uncertainty analysis - A case

study. Human and Ecological Risk Assessment 5(1): 145-170. 

Moore, D.R.J., Sample, B.E., Suter, G.W., Parkhurst, B.R., and Teed, R.S. 1999. A probabilistic risk

assessment of the effects of methylmercury and PCBs on mink and kingfishers along East Fork Poplar

Creek, Oak Ridge, Tennessee, USA. Environmental Toxicology and Chemistry 18: 2941-2953.

National Research Council (NRC). 1991. Human Exposure Assessment for Airborne Pollutants. 

Washington DC: National Academy Press.

Roberts, S.M. 1999. Practical issues in the use of probabilistic risk assessment and its applications to

hazardous waste sites. Human and Ecological Risk Assessment. 5(4): 729-868. Special Issue.

Smith, R.L.. 1994. Use of Monte Carlo simulation for human exposure assessment at a Superfund

site. Risk Analysis 14(4): 433-439. 

U.S. Environmental Protection Agency. 1985. Methodology for Characterization of Uncertainty in

Exposure Assessments. Washington DC, EPA-600/8-85-009).
April 2004 Page 31-10

1. Rugen, P. and B. Callahan. 1996. An overview of Monte Carlo: A fifty year perspective. 

Human Health and Ecological Risk Assessment 2(4): 671-680.

2. Finley, B. and D. Paustenbach. 1994. The benefits of probabilistic exposure assessment:

Three case studies involving contaminated air, water, and soil. Risk Analysis 14(1): 53-73.

3. U.S. Environmental Protection Agency. 1992. Guidance on Risk Characterization for Risk

Managers and Risk Assessors. Risk Assessment Council, Washington, DC, February 26,

1992.

References
Chapter 32 Use of Geographic Information Systems

(GIS) in Risk Assessment

Table of Contents

32.1 Introduction ............ ............ ............ ............ ............ ...... 1

32.2 Selecting a GIS ............ ............ ............ ............ ............ ... 2

32.3 Acquiring and Using Demographic Data ............ ............ ............ ....... 4

32.3.1 U.S. Census Data ............ ............ ............ ............ ....... 5

32.3.2 Current and Small-Area Demographic Estimates ............ ............ ...... 5

32.3.3 Public Health Applications ............ ............ ............ ........... 7

32.3.4 Data Access and Distribution ............ ............ ............ ......... 7

32.4 Cartographic Concepts ............ ............ ............ ............ ......... 7

32.4.1 Generalization, Simplification, and Abstraction ............ ............ ...... 10

32.4.2 Map Projections ............ ............ ............ ............ ....... 10

32.5 Using the Internet as a GIS Tool ............ ............ ............ ............ . 10

32.6 Current GIS Applications at EPA ............ ............ ............ ............ 11

32.6.1 ORD/ESD ............ ............ ............ ............ ........... 12

32.6.2 ATtILA ............ ............ ............ ............ ............ . 12

32.6.3 ReVA ............ ............ ............ ............ ............ ... 12

32.7 GPS Technology ............ ............ ............ ............ ............ . 13

References ............ ............ ............ ............ ............ ............ 15

April 2004 Page 32-1

32.1 Introduction

A geographic information system (GIS) can be defined as an organized collection of software

and geographic data that allow efficient storage, analysis, and presentation of spatially explicit

and geographically referenced information. Traditional methods of processing such data have

been extremely labor intensive, such as manually digitizing a map from an aerial photograph and

then adding information about chemical contaminants. A GIS provides a powerful analytical tool

that can be used to create and link spatial and descriptive data for problem solving, spatial

modeling and presentation of results in tables or maps. For air toxics risk assessment, GIS can

be a powerful tool for displaying and analyzing data during the planning, scoping, and problem

formulation phases, during the exposure assessment, and displaying and evaluating the results of

the risk characterization. It is also a very helpful means for communicating information to risk

managers and other stakeholders.

GIS data generally consist of two components: (1) graphical data about geographic features (e.g.,

rivers, land use, political boundaries), and (2) tabular data about features in the geography (e.g.,

population, elevation, modeled ambient concentrations of air toxics). GIS combines these

different types of data using a “layering” technique that references each type of data to a uniform

geographic coordinate system (usually a grid such as latitude and longitude coordinates). 

Layered data can then be analyzed using special software to create new layers of data (see Exhibit

32-1). 

Over the last several years, GIS applications have evolved from very specialized and expensive

analyses that required specialized computers (e.g., supercomputers and workstations) to userfriendly desktop applications utilized by everyday users to do such mundane tasks as print maps

or driving directions. Libraries of geographical information developed for general use (e.g.,

topographical maps, infrastructures, natural resources), and for use by EPA and other regulatory

agencies, can be easily downloaded from different servers and used in air toxics risk assessments. 

One example of a GIS Web-based application is EPA’s Envirofacts system(1) which provides

website access to several EPA databases that provide information about environmental activities

that may affect air, water, and land anywhere in the United States (with much of the data

available in GIS format).

This chapter provides an overview of GIS and its application to air toxics risk assessment. More

detailed information is provided in the Agency for Toxic Substances and Disease Registry

(ATSDR)/Southern Appalachian Assessment GIS (SAAGIS) publication Introduction to

ArcView and Spatial Analysis Techniques for Public Health Professionals.

(2)
April 2004 Page 32-2

Exhibit 32-1. Example Conceptual Model Using GIS

Example of layering within a GIS. The location of monitoring wells, industries, and potential

receptors (homes) are all referenced to the same geographic coordinates. This allows spatial

analysis of the overlap of sources, contaminant plumes, and receptors, as well as a visual

means to communicate complex data sets.

32.2 Selecting a GIS

After risk assessors decide to use a GIS, they must choose a software system. A variety of GIS

software is available from commercial vendors. A key feature in selecting a GIS is identifying a

minimal set of capabilities needed. Important functional capabilities to consider include: data

capture, data storage, data management, data retrieval, data analysis, and data display.(3)

• Data Capture. All data used in a GIS must have a spatial component. This means that all

information brought into the system must be geo-referenced (i.e., correspond to some

physical location). Data capture is the process of incorporating map and attribute data into

the GIS. Geocoding, which is the conversion of analog data to geo-referenced digital format,

is a common way for GIS users to bring map and attribute data into their GIS analyses. Two

common methods of geocoding are scanning and digitizing. Both involve taking non-digital

information (e.g., a hard-copy map), and converting it into a digital format. In addition to

paper files, GIS users often import files from common formats such as AutoCAD DXF. The
April 2004 Page 32-3

newly imported digital information (e.g., the boundary of a state), is geo-referenced by

coordinates so that it corresponds to a physical location. 

In addition to graphical data, GIS incorporates tabular data for objects included in a data

layer. For example, the graphical data associated with a home could consist of its size and

location. The tabular data associated with that home consists of attributes such as who lives

there, when it was built, where its water supply comes from, and what type of heating system

it uses. These attributes would be listed in a table that is linked to the physical location of the

house by the GIS. While obtaining geographical base layers that show boundaries is

essential, data capture also involves attribute data, which necessitates that the GIS software

package have some level of database manager associated with the program. A useful

program will generally have features that allow it to import common database files such as

those from dBASE®, Access

®

, Excel®, and Paradox®. The different software packages will

vary in their ability to check the characteristics of the databases. 

• Data Storage. A GIS can incorporate a tremendous amount of data into a map. Space is a

key issue related to data storage in a GIS. With the decrease in cost of disk storage, the

development of high-density storage media (e.g., CD-ROM), and the incorporation of

compression methods, space is not as critical an issue as it has been in the past. However,

GIS is still relatively memory-intensive. GIS microcomputer software can take up tens of

megabytes of space without data, and a more complete workstation version may use hundreds

of megabytes of space. Add to this the datasets with very high resolution (that can move into

the gigabyte range in size), and there is a the potential for a significant storage problem. 

Some storage problems can be resolved by establishing data sets on a common server,

accessible to multiple users.

• Data Management. A powerful GIS is one which has the ability to manage both map and

attribute data. Every GIS is built around the software capabilities of a database management

system (DBMS). A DBMS is software that is capable of storing, selecting, retrieving, and

reorganizing attribute information. It allows data entry, data editing, and supports several

different types of output. Functions include the ability to select records based on their value. 

Several database functions can work independently of the GIS functions.

• Data Retrieval. A GIS will support the retrieval of features by their attributes or by their

spatial characteristics. A basic retrieval based on spatial characteristics is used to show the

position of a single feature. In addition, a GIS is capable of allowing the operator to use the

map as a query vehicle. A simple way of doing this is to point to a feature and retrieve the

list of attributes for that feature. The database management function also is important for the

data retrieval capacity because it allows for the selection and retrieval based on an attribute. 

Buffering is one retrieval operation that defines a GIS. Buffering allows the user to retrieve

features within a specified distance of a point, line, or area. Overlay is another spatial

retrieval operation in which non-overlapping regions are joined to create a new area. More

sophisticated retrieval operations also are available.

(2)
April 2004 Page 32-4

• Data Analysis. GIS systems vary a great deal in their data analysis capabilities. Basic tasks

that should be included in a GIS are: spreadsheet and database analysis, computing new

attributes, generating summary statistics, creating reports, statistics such as mean and

variance, significance testing, and plotting residuals. In addition, selected geometric tests

should be included (e.g., point-in-polygon analysis, surface partitioning).

• Data Display. GIS software displays information visually as data layers of a map. GIS users

must select the correct map projection to make sure that their maps are not distorted. For

example, large areas, such as continents, must be projected with the earth’s curvature taken

into consideration. Small areas can be projected essentially as flat. GIS software gives users

as wide variety of map projection options to ensure that maps are as accurate as possible. 

Section 32.4.2 discusses map projections in further detail. 

Different data sources and agencies provide digital data that has been processed using

different coordinate systems and map projections. Risk assessors may want to use data layers

from many different sources to create a single map. For example, a topography layer from

the U.S. Geologic Survey might be combined with a layer showing census blocks from the

U.S. Census and a layer showing lead smelters from EPA. Software that can handle a variety

of coordinate systems and map projections is essential to GIS capability to overlay layers

created from many different sources.

32.3 Acquiring and Using Demographic Data

Demography is the study of the size, composition, distribution, and change in population. 

Geographers focused on population studies are also interested in the spatial distribution of

demographic characteristics.(4) Data from the U.S. Bureau of the Census decennial census is the

most common source of residential population information for states, the District of Columbia,

and many U.S. territories (e.g., Puerto Rico, U.S. Virgin Islands, American Samoa, and Guam). 

These data also provide the base for current year population estimates and projections. Risk

assessors are often interested in using demographic data because it allows them to identify

sensitive sub-populations, such as children or the elderly. A GIS lets risk assessors combine

demographic data with data on the location of sources (or estimated ambient air concentrations)

to visualize where human health is potentially at risk (see Exhibit 32-2).

Within a GIS, political and statistical geographic area boundary files are linked to the attribute

data (e.g., age, race, housing value) describing residents and housing units in that area using

Federal Information Processing Standard (FIPS) codes. These codes provide unique identifiers

for various geographic areas. When analyzing census data that is nested within the data hierarchy

(e.g., census blocks within census tracts), it is best to include the FIPS codes for the larger

geographic areas in that hierarchy to ensure that you are using a unique identifier. For example,

connecting the FIPS codes for block 201, census tract 12, Fulton county, state of Georgia, results

in the unique identifier “13089001200201” for that block. Because the codes are nominal

numerals, it is best to treat them as character data (or strings) rather than numbers in the GIS

database (although this may not be consistent across data sources).
April 2004 Page 32-5

Exhibit 32-2. Illustration of the Use of GIS to Identify

Sensitive Receptors Close to Emissions Sources

In this map, the squares represent hazardous waste sites, and the flagged

symbols represent schools. Schools and other locations where sensitive

subpopulations may occur that are close to air toxics emissions sources may

be of particular interest in a risk assessment.

32.3.1 U.S. Census Data

U.S. census data describing the residential population and housing in the U.S. provide the most

complete picture of our nation and its subareas, which makes them very valuable demographic

data. Exhibit 32-3 shows the type of information collected in the 2000 census. Many of the

Census 2000 data files are available for use in GIS.

32.3.2 Current and Small-Area Demographic Estimates

An issue with census data is that the information represents a “snapshot” in time (generally based

on April 1 of the census year). As one moves forward in time, such data may be less reflective of

the actual demographic conditions in the study area. This problem is more pronounced for smallarea data (e.g., census tracts and block groups). While the census data typically are appropriate

for screening-level assessments (e.g., some air quality models include the 2000 census data),

more refined assessments may require more current information, which is available from several

commercial sources.
April 2004 Page 32-6

Exhibit 32-3. Information Collected in the 2000 Census

During census years, households received and were asked to respond to one of two census forms – the

“short form,” which gives the “100-percent component,” or the “long form,” which gives the “sample

component.” Questions on the short form were also found on the long form and thus, were

(theoretically) asked of every household in the nation. Basic population and housing data were

gathered in this way. More detailed population information was obtained from the long form sent to a

sample of households. On average, approximately one in six households received the long form. The

rate varied from one in two households in some smaller areas, to one in eight households for more

densely populated areas.

100 Percent Component from the Short Form

Population

• Name

• Household relationship

• Sex

• Age

• Hispanic or Latino origin

• Race

Housing

• Tenure – owned or rented

Sample Component from the Long Form

Population

Social characteristics

• Marital status

• Place of birth, citizenship, year of entry to the

U.S.

• School enrollment and attainment 

• Ancestry

• Residency five years ago (migration)

• Language spoken at home and ability to speak

English

• Veteran status 

• Disability

• Grandparents as care givers

Economic characteristics

• Labor force status

• Place of work and journey to work

• Occupation, industry, and class of worker

• Work status in 1999

• Income in 1999

Housing

• Units in structure

• Year structure built 

• Number of rooms and number of bedrooms

• Year moved into residence 

• Plumbing and kitchen facilities

• Telephone service

• Vehicles available

• Heating fuel

• Farm residence

Financial Characteristics

• Value of home or monthly rent paid

• Utilities, mortgage, taxes, insurance, and

fuel costs

Source: U.S. Census. Census 2000 Basics. Available at:

http://www.census.gov/mso/www/c2000basics/00Basics.pdf
April 2004 Page 32-7

A number of commercial entities provide annual small-area population and housing estimates

and projections. Estimates are calculated using the most recent decennial census as the

population base and incorporating other, often proprietary, data sources to refine the estimates. 

In addition to providing updated demographics, some vendors have developed segmentation

systems that classify the U.S. population into distinct lifestyle segments or clusters depending on

residential location (“geodemographics”). The idea of clustering is based on the notion that,

more often than not, people will choose to live near others like themselves. This is important to

public health because assessors can be more efficient in identifying and understanding where

potential hazards are concentrated, as well as developing messages that reach people living in

those areas. 

32.3.3 Public Health Applications

The use of census data is central for public health communication planning, program planning,

implementation and information dissemination. For example, the Georgia Division of Public

Health used demographic information to target mammography programs in factory towns

classified as “Mines & Mills” because women in those communities were found to have higher

rates of breast cancer.(5) As another example, the Centers for Disease Control (CDC) Office of

Communication has collaborated with a number of centers on projects that integrate

epidemiological and other data for communication planning including HIV status awareness and

hantavirus prevention.(6) Because exposure to air toxics is often influenced significantly by

proximity to sources, spatial information is essential to identifying areas where human health

might be adversely impacted.

32.3.4 Data Access and Distribution

There are numerous sources for acquiring U.S. census data. In addition to the Census Bureau’s

data access tools, including Factfinder, its Web-based data dissemination system, many public

and private organizations are including census data with GIS or mapping software (e.g., ESRI,

EPA LandView, HUD Community 2020, Geolytics, Claritas, CACI). State governments,

universities, and non-governmental organizations (e.g., CIESIN) are also sources for data. Costs

associated with obtaining the data vary.

32.4 Cartographic Concepts

While spatial information and GIS can be extremely useful, people must have assistance in

observing and studying the great amount and variety of information that is represented on maps. 

Geographic data are extensive and voluminous, so cartography, a technique that is fundamentally

concerned with reducing the spatial characteristics of a large area, makes maps readable and

meaningful. A map is more than a reduction of information to an understandable level. If it is

well made, it is a carefully designed instrument for recording, calculating, analyzing, and in

general, understanding the interrelation of things in their spatial relationship. This section

provides an overview of cartography. A more complete discussion can be found in The

Geographer’s Craft Project.

(7)
April 2004 Page 32-8

One of the most useful approaches to the study of cartography is to view maps as a form of visual

communication – a special purpose language for describing spatial relationships. Cartography is

related to, but different from other forms of visual communication. Cartographers must pay

special attention to coordinate systems, map projections, and issues of scale and direction that are

in most cases of relatively little concern to other graphic designers or artists. But, because

cartography is a type of graphical communication, some insights to the demands of cartography

can be gleaned from the literature of graphical communication and statistical graphics. By

stressing cartography as a form of communication, it is easier to make the point that maps are

really symbolic abstractions – or representations – of real world phenomena. In most cases, this

means that the world represented on a map has been greatly simplified, or generalized, with

symbols being used like words to stand for real things. Some of the most important decisions

cartographers make in the process of cartographic design revolve around: (1) how much to

simplify the situation being depicted; and (2) how to symbolize the relationships being

represented. In order to make good choices, cartographers often ask themselves the following

questions:

• What is the motive, intent, or goal of the map?

• Who will read the map?

• Where will the map be used?

• What data is available for the composition of the map?

• What resources are available in terms of both time and equipment?

By identifying the most important points to be conveyed by the map along with the map’s main

audience, cartographers can prioritize where to direct the audience’s attention with larger

symbols or brighter colors. 
April 2004 Page 32-9

Basic Map Elements

A legend and symbols that inform the viewer of distance, scale, and direction, are basic elements to

any map. The USGS (http://edc.usgs.gov/earthshots/slow/Help-GardenCity/legendstext) provides

examples of common map legends. 

Example U.S. Geological Survey (USGS) Topographic Map Legend

Example Legend for Universal Transverse Mercator (UTM) Projection Zones
April 2004 Page 32-10

Map Making Tips

• Experiment with different layouts

• Think carefully about every element on your

map and whether it has an essential function

• Less is more

32.4.1 Generalization, Simplification, and Abstraction 

As noted above, cartography is a process of

abstraction in which features of the real world

are generalized or simplified to meet the

demands of the theme and audience. Not all

elements or details have a bearing on the

pattern or process being studied and so some

are eliminated to draw the reader’s attention

to those facts that are relevant. Too much

detail can even hide or disguise the message of a map. The amount of detail that can be included

is very much dependent on the scale at which the map will be produced (see Exhibit 32-4).

32.4.2 Map Projections

As section 32.2 notes, the projection used to create a map influences the representation of area,

distance, direction, and shape. This is readily apparent when looking at a flat map of the world

versus looking at a spherical map of the world (i.e., a globe). Maps that ignore the natural shape

of the earth distort the places they are trying to represent. It should be noted when these

characteristics (e.g., area, distance, direction, and shape), are of prime importance to the

interpretation of any map. Some widely used locational reference systems such as the U.S. State

Plane Coordinate system and Universal Transverse Mercator system are based on predefined

projective geometries that are implicit in the use of the coordinate systems themselves. GIS

software packages make it easy for users to choose an appropriate map projection.

32.5 Using the Internet as a GIS Tool

The internet can be a valuable resource for GIS users looking for data. Many federal agencies

provide digital data free for download that can be used with GIS. The Census Bureau, EPA, and

the United States Geological Survey are all good sources of GIS data. For example, in addition

to demographic data, the Census bureau distributes what are called Topologically Integrated

Geographic Encoding and Referencing (TIGER) files. The TIGER/Line files are a digital

database of geographic features, such as roads, lakes, political boundaries, and census statistical

boundaries, available for the entire United States. The database contains tabular information

about these features such as their location in latitude and longitude, the name, the type of feature,

and other important attributes. GIS clearinghouses, universities, and data supply companies are

also good places to look for data. A Web search engine can help users locate sites that contain

the type of data needed for a given project.

Once users locate relevant data, they must then get the data onto their computer. GIS coverages

can take up a lot of computer memory, so choosing the right file transfer method is very

important. Many websites allow direct downloads. This type of transfer involves clicking a link

and specifying a target directory. Other data providers require users to go through a file transfer

protocol (FTP) site. FTP sites allow people to exchange large data files more readily than with

other protocols.
April 2004 Page 32-11

Exhibit 32-4. Effect of Scale on Detail and Abstraction

Finally, the internet can serve as a resource for users looking for technical support or advice. 

Most users will find that GIS software manufacturers offer online support. Some companies 

even have online courses.

32.6 Current GIS Applications at EPA

EPA is an excellent source of GIS data and information for risk assessors. Several offices and

branches can serve as resources for those interested in learning more about GIS and its uses,

especially in the areas of landscape, land cover, and land use. GIS helps EPA integrate geospatial data on a region (e.g., landscape, elevation, climate, slope) with information about

potential exposures to give risk assessors a comprehensive picture of that region’s hazards. 

Because projected land use may be an important input to air models, risk assessors may want

more information on landscape change models. For an overview on this subject, see EPA’s

Projecting Land-Use Change: A Summary of Models for Assessing the Effects of Community

Growth and Change on Land-Use Patterns.(8)
April 2004 Page 32-12

32.6.1 ORD/ESD 

EPA’s Office of Research and Development/Environmental Sciences Division (ORD/ESD)

conducts research, development, and technology transfer programs on environmental exposures

to ecological and human receptors. GIS is an important tool for the type of chemical and

physical stressors characterization conducted, especially with ESD’s emphasis on ecological

exposure. The Division develops landscape and regional assessment capabilities through the use

of advanced spatial monitoring and analysis techniques, such as remote sensing and GIS. For

more information, go to http://www.epa.gov/nerlesd1/.

32.6.2 ATtILA

Another EPA resource is the Landscape Ecology Branch’s ATtILA program, which stands for

Analytical Tools Interface for Landscape Assessments. The Branch uses ATtILA, which is a

GIS, to conducts multiple-stressor regional assessments based largely on geo-spatial landscape

data. As part of these assessments, ATtILA generates complicated landscape metrics, which are

quantitative measurements of the environmental condition or vulnerability of an area (e.g.,

ecological region). ATtILA provides an interface that allows users to easily calculate many

common landscape metrics regardless of their level of GIS knowledge, despite the complexity of

developing the metrics. Four metric groups are currently included in the package (e.g.,

Landscape Characteristics, Riparian Characteristics, Physical Characteristics, and Human

Stresses). ATtILA runs within ArcView®, and is designed to be flexible enough to accommodate

spatial data from a variety of sources. More information is available at:

http://www.epa.gov/nerlesd1/land-sci/northern_california/attila/background.html.

32.6.3 ReVA

Also from EPA’s ORD is the Regional Vulnerability Assessment (ReVA) program. This

program is an approach to regional scale, priority-setting assessment meant to expand

cooperation among the laboratories and centers of ORD, by integrating research on human and

environmental health, ecorestoration, landscape analysis, regional exposure and process

modeling, problem formulation, and ecological risk guidelines. Currently, ReVA is working in

the Mid-Atlantic region to predict future environmental risk. This will help EPA prioritize

efforts to protect and restore environmental quality efficiently and effectively. ReVA is being

developed to identify those ecosystems most vulnerable to being lost or permanently harmed in

the next 5 to 25 years and to determine which stressors are likely to cause the greatest risk. The

goal of ReVA is not exact predictions, but identification of the undesirable environmental

changes expected over the coming years.

Many functions work together to provide ReVA’s regional assessment capability. GIS puts into

a spatial context data on stressors and effects from many sources. Research guides how to apply

this data at the landscape and regional scale and helps EPA understand how socioeconomic

drivers affect environmental condition. The transfer of data and analytical tools to regional

managers is also critical for this tool to be useful. ReVA is considered a GIS because it is

designed to analyze the spatial distribution of sensitive ecosystems by analyzing known
April 2004 Page 32-13

distributions of plant and animal populations or communities within ecosystems. Modern

methods in landscape ecology and characterization help further identify the locations of

ecosystems that are vulnerable to future stress through features such as topography (i.e. increased

erosion potential) and habitat patch configurations. Multimedia assessments across water, air,

terrestrial, and demographic variables are possible at various scales with this tool. For more

information on ReVA, see http://www.epa.gov/reva/approach.htm.

32.7 GPS Technology

Global Positioning System (GPS) technology can be integrated with GIS. GPS technology

allows users with the appropriate technology to obtain almost the exact location of any GPS

receiver. This means that cars can get driving directions while moving, hikers can always know

their exact position for navigating in and out of the wilderness, and the military can track

movements of troops or vehicles. For risk assessments, the location of specific sources (i.e.,

vents) or receptor locations can be accurately determined with GPS. GPS is funded and

controlled by the U.S. Department of Defense (DOD). While there are many thousands of civil

users of GPS world-wide, the system was designed for, and is operated by the U.S. military. 

The system works through specially coded satellite signals that can be processed in a GPS

receiver, enabling the receiver to compute position, velocity, and time (see Exhibit 32-5). Four

GPS satellite signals are used to compute positions in three dimensions and the time offset in the

receiver clock (see Exhibit 32-6).

The GPS provides two levels of service – a Standard Positioning Service (SPS), and a Precise

Positioning Service (PPS). Access to the PPS is restricted to U.S. Armed Forces, U.S. Federal

agencies, and selected allied armed forces and governments. The SPS is available to all users on

a continuous, worldwide basis, free of any direct user charge. A nationwide differential GPS

service (NDGPS) is being established pursuant to the authority of Section 346 of the Department

of Transportation and Related Agencies Appropriation Act. When complete, this service will

provide uniform differential GPS coverage of the continental U.S. and selected portions of

Hawaii and Alaska regardless of terrain, man-made, and other surface obstructions. NDGPS

accuracy is specified to be 10 meters or better. Typical system performance is better than 1 meter

in the vicinity of the broadcast site. Achievable accuracy degrades at an approximate rate of 1

meter for each 150 km distance from the broadcast site.(9)

Receiver costs vary depending on capabilities. Small civil SPS receivers can be purchased for

under $200. Receivers that can store files for post-processing cost more ($2,000 to 5,000).

Receivers that can act as DGPS reference receivers (computing and providing correction data)

and carrier phase tracking receivers (and two are often required) can cost many thousands of

dollars ($5,000 to $40,000).

Receivers are important because they are the intermediary part of the system that connect real

world data to GIS. Satellites send signals to the receiver and users and store the information. 

Sometimes, the user will have to manually record position and time readings and then type those
April 2004 Page 32-14

into a computer later. Other times the user can plug the receiver into a special port on her

computer and download the digital data directly.

Exhibit 32-5. Global Positioning System (GPS) Satellites

GPS satellites orbit the Earth every 12 hours, sending signals to receivers

around the world

Exhibit 32-6. Positioning and Time from Four GPS Satellites

Measurements of code-phase arrival times from at least four satellites are used to estimate four

quantities: position in three dimensions (X, Y, and Z) and GPS time (T).
April 2004 Page 32-15

1. U.S. Environmental Protection Agency. 2004. Envirofacts Data Warehouse. Available at:

http://www.epa.gov/enviro/index.html (Last accessed March 2004).

2. Agency for Toxic Substances and Disease Registry. 2004. Introduction to ArcView and

Spatial Analysis Techniques for Public Health Professionals. An ATSDR/SAAGIS

publication (in press).

3. Clarke, K.C. 1997. Getting Started with Geographic Information Systems. Prentice Hall,

Upper Saddle River, NJ.

4. Plane, D.A., and Rogerson, P.A. 1994. The Geographical Analysis of Population. John Wiley

& Sons, Inc, New York, NY.

5. Weiss, M.J. 2000. The Clustered World. Little, Brown and Company, Boston, MA.

6. Pollard, W., and Kirby, S. 1999. Geographical Information Systems (GIS), Public Health

Data, and Syndicated Market Research Data Bases in Health Communication. Presented at

the National Conference on Health Statistics, Washington, D.C.

7. Kenneth E.F. and Crum, S. The Geographer's Craft Project. Department of Geography,

University of Texas at Austin. 

8. U.S. Environmental Protection Agency. 2000. Projecting Land-Use Change: A Summary of 

Models for Assessing the Effects of Community Growth and Change on Land-Use Patterns. 

Office of Research and Development, Cincinnati, OH. EPA/600/R00/098.

9. U.S. Department of Defense and U.S. Department of Transportation. 2001. 2001 Federal

Radionavigation Plan, DOT/VNTSC/RSPA-01/3; DOD-4650.5. Available at: 

http://www.navcen.uscg.gov/pubs/frp2001/FRP2001.pdf. (Last accessed March 2004.)

References
Glossary

This list of glossary terms was compiled from existing EPA definitions and supplemented, where

necessary, by additional terms and definitions. The wording of selected items may have been

modified from the original in order to assist readers who are new to risk assessment more readily

comprehend the underlying concept of the glossary entry. As such, these glossary definitions

constitute neither official EPA policy nor preempt or in any way replace any existing legal

definition required by statute or regulation.

April 2004 Page 1

A

Absorbed Dose – the amount of a substance that has penetrated the absorption barriers (e.g.,

skin, lung tissue, gastrointestinal tract) of an organism through either physical or biological

processes.

Absorption - The process of taking in, as when a sponge takes up water. Chemicals can be

absorbed through the skin into the bloodstream and then transported to other organs. Chemicals

can also be absorbed into the bloodstream after breathing in or swallowing.

Absorption Barrier - Exchange barriers of the body that allow differential diffusion of various

substances across a boundary. Examples of absorption barriers are the skin, lung tissue, and

gastrointestinal tract wall.

Abiotic Degradation - Degradation via purely physical or chemical mechanisms. Examples

include hydrolysis and photolysis.

Acceptable Risk - The likelihood of suffering disease or injury that will be tolerated by an

individual, group, or society. The level of risk that is determined to be acceptable may depend

on a variety of issues, including scientific data, social, economic, legal, and political factors, and

on the perceived benefits arising from a chemical or process.

Accuracy - The measure of the correctness of data, as given by the difference between the

measured value and the true or standard value.

Active Monitor - A type of personal exposure monitoring device that uses a small air pump to

draw air through a filter, packed tube, or similar device.

Activity Patterns - A series of discrete events of varying time intervals describing information

about an individual’s lifestyle and routine. This information typically includes the locations

visited, the amount of time spent in the locations, and a description of what the individual was

doing in each location.

Acute Effect - Any toxic effect produced with a short period of time following an exposure, for

example, minutes to a few days

Acute Exposure Limits - A variety of short-term exposure limits to hazardous substances,

designed to be protective of human health. Published by different organizations, each limit has a

different purpose and definition.

Acute Exposure - One dose (or exposure) or multiple doses (or exposures) occurring within a

short time relative to the life of a person or other organism (e.g., approximately 24 hours or less

for humans).

Actual Risk - The damage to life, health, property, and/or the environment that may occur as a

result of exposure to a given hazard. Risk assessment attempts to estimate the likelihood of

actual risk.
April 2004 Page 2

Additive Effect - The overall result of exposure to two or more chemicals, in which the resulting

effect is equal to the sum of the independent effects of the chemicals. “Effects” or “Response

Addition” is a method employed in EPA risk assessments of mixtures in which the components

act or are presumed to act independently (without interaction).

Additive Dose - The overall result of exposure to two or more chemicals, when each chemical

behaves as a concentration or dilution of the other chemicals in the mixture. The response of the

combination is the response expected from the equivalent dose of an index chemical. The

equivalent dose is the sum of component doses scaled by their toxic potency relative to the index

chemical.

Adjusted Exposure Concentration - Also called a refined exposure concentration, an estimate

of exposure concentration that has been refined, usually by application of an exposure model, to

better understand how people in a particular location interact with contaminated media.

Administered Dose - The amount of a substance received by a test subject (human or animal) in

determining dose-response relationships, especially through ingestion or inhalation.

Advection - In meteorology, the transfer of a property, such as heat or humidity, by motion

within the atmosphere, usually in a predominantly horizontal direction. Thermal advection, for

example, is the transport of heat by the wind. Advection is most often used to signify horizontal

transport but can also apply to vertical movement. Large-scale horizontal advection of air is a

characteristic of middle-latitude zones and leads to marked changes in temperature and humidity

across boundaries separating air masses of differing origins. 

Adverse Environmental Effect - Defined in the CAA section 112(a)(7) as “any significant and

widespread adverse effect, which may reasonably be anticipated, to wildlife, aquatic life, or

other natural resource, including adverse impacts on populations of endangered or threatened

species or significant degradation of environmental quality over broad areas.”

Adverse Health Effect - A health effect from exposure to air contaminants that may range from

relatively mild and temporary (e.g., eye or throat irritation, shortness of breath, or headaches) to

permanent and serious conditions (e.g., birth defects, cancer or damage to lungs, nerves, liver,

heart, or other organs), and which negatively affects an individual’s health or well-being, or

reduces an individual’s ability to respond to an additional environmental challenge.

Affected (or Interested) Parties - Individuals and organizations potentially acted upon or

affected by chemicals, radiation, or microbes in the environment or influenced favorably or

adversely by proposed risk management actions and decisions.

Agent - A chemical, physical, or biological entity that may cause deleterious, beneficial, or no

effects to an organism after the organism is exposed to it.

Aggregate exposure - The combined exposure of an individual (or defined population) to a

specific agent or stressor via relevant routes, pathways, and sources.

Aggregate risk - The risk resulting from aggregate exposure to a single agent or stressor.
April 2004 Page 3

AirData - An EPA website (http://www.epa.gov/air/data/info.html) that provides access to

yearly summaries of United States air pollution data, taken from EPA’s air pollution databases.

The data include all fifty states plus District of Columbia, Puerto Rico, and the U. S. Virgin

Islands. AirData has information about where air pollution comes from (emissions) and how

much pollution is in the air outside our homes and work places (monitoring).

Air Emissions - The release or discharge of a pollutant into the air.

Air Pressure (Atmospheric Pressure, Barometric Pressure) - The pressure experienced

above the Earth’s surface at a specific point as a result of the weight of the air column, extending

to the outer limit or top of the atmosphere. Consequently, pressure declines exponentially with

height, the rate of decrease being a function of the temperature of the atmosphere. Atmospheric

pressure is generally measured, in meteorology, either in the SI unit hectopascals (hPa) or in the

c.g.s. unit of the same size, the millibar (mb) using a mercury or aneroid barometer, or a

barograph. In the U.S., surface atmosphere pressure is measured in inches of mercury (Hg).

Air Mass - A large volume of air with certain meteorological or polluted characteristics (e.g., a

heat inversion or smogginess) while in one location. The characteristics can change as the air

mass moves away.

Air Toxic - Any air pollutant that causes or may cause cancer, respiratory, cardiovascular, or

developmental effects, reproductive dysfunctions, neurological disorders, heritable gene

mutations, or other serious or irreversible chronic or acute health effects in humans. See

hazardous air pollutant.

Ambient Medium (e.g., Ambient Air) - Material surrounding or contacting an organism (e.g.,

outdoor air, indoor air, water, or soil), through which chemicals can reach an organism.

Ambient Water Quality Criteria (AWQC) - A ecological benchmark level for aquatic

contaminants, published by EPA Office of Water, which is designed to protect 95 percent of all

aquatic species in freshwater or marine environments. Criteria have been developed for both

acute and chronic exposures, although for a limited number of chemicals.

Ample Margin of Safety - This term has regulatory significance in EPA’s air toxics program. It

was interpreted by the Agency in the 1989 notice of final benzene NESHAP (FR54:38044-

38072), and reiterated in the 1990 amendments to the Clean Air Act (sections 112(f) and 112(c)).

AMTIC - Ambient Monitoring Technology Information Center. An EPA website that contains

information and files on ambient air quality monitoring programs, details on monitoring

methods, monitoring-related documents and articles, information on air quality trends and

nonattainment areas, and federal regulations related to ambient air quality monitoring.

[http://www.epa.gov/ttn/amtic/, 2003]
April 2004 Page 4

Analysis - The systematic application of specific theories and methods, including those from

natural science, social science, engineering, decision science, logic, mathematics, and law, for

the purpose of collecting and interpreting data and drawing conclusions about phenomena. It

may be qualitative or quantitative. Its competence is typically judged by criteria developed

within the fields of expertise from which the theories and methods come.

Analysis Plan - A plan that provides all the details of exactly how each part of the risk

assessment will be performed. It usually describes in detail what analyses will be performed,

how they will be performed, who will perform the work, schedules, resources, quality

assurance/quality control requirements, and documentation requirements.

Animal Studies - Toxicity investigations using animals. Such studies may employ animals as

surrogates for humans with the expectation that the results are pertinent to humans or for

investigation of effects pertinent to animals (e.g., for ecological risk assessment).

Antagonistic Effect - The situation where exposure to two chemicals together has less effect

than the sum of their independent effects.

AP-42 - A compilation of air pollutant emission factors. Volume I of the fifth edition addresses

stationary point and area source emission factors. AP-42 is accessible on the Air CHIEF website

(http://www.epa.gov/ttn/chief/ap42/) and is also included on the Air CHIEF CD-ROM.

Applied Dose - The amount of a substance in contact with an absorption boundary of an

organism (e.g., skin, lung, gastrointestinal tract) and is available for absorption.

Area of Impact – The geographic area affected by a facility’s emissions (also known as the zone

of impact).

Area Source (legal sense) - A stationary source that emits less than 10 tons per year of a single

hazardous air pollutant (HAP) or 25 tons per year of all HAPs combined.

Area Source (modeling sense) - An emission source in which releases are modeled as coming

from a 2-dimensional surface. Emissions from the surface of a wastewater pond are, for

example, often modeled as an area source.

Area Use Factor - For an animal, the ratio of its home range, breeding range, or

feeding/foraging range to the area of contamination or the site area under investigation.

Assessment Endpoint - An explicit expression of the environmental value to be protected. An

assessment endpoint includes both an ecological entity and specific attributes of that entity. For

example, salmon are a valued ecological entity; reproduction and population maintenance (i.e.,

the attribute) form an assessment endpoint.

Assessment Questions - The questions asked during the planning/scoping phase of the risk

assessment process to determine what the risk assessment will evaluate.

Atmospheric Stability (Stability) - the degree of resistance of a layer of air to vertical motion.
April 2004 Page 5

ATSDR (Agency for Toxic Substances and Disease Registry) - An Agency of the U.S.

Department of Health and Human Services, whose goal is to serve the public by using the best

science, taking responsive public health actions, and providing health information to prevent

harmful exposures and diseases to toxic substances. Its website (www.atsdr.cdc.gov) includes

information on hazardous substances [e.g., toxicological profiles, minimal risk levels (MRLs)],

emergency response, measuring health effects, hazardous waste sites, education and training,

publications, and special issues (e.g., Children Health).

Averaging Time - The time period over which something is averaged (e.g., exposure, measured

concentration).

B

Background Levels - The concentration of a chemical already present in an environmental

medium due to sources other than those under study. Two types of background levels may exist

for chemical substances: (a) Naturally occurring levels of substances present in the environment,

and (b) Anthropogenic concentrations of substances present in the environment due to human

associated activities (e.g., automobiles, industries).

Background Source - Any source from which pollutants are released and contribute to the

background level of a pollutant, such as volcano eruptions, windblown dust, or manmade source

upwind of the study area.

Benchmark Dose - An exposure due to a dose of a substance associated with a specified low

incidence of risk, generally in the range of 1% to 10%, of a health effect; or the dose associated

with a specified measure or change of a biological effect.

Benthic Burial Rate (kb) - Rate of the deposition of the sediment suspended in a surface water

body column to the benthic sediment surface that becomes no longer available for resuspension

in the water column, effectively becoming part of the sediment “sink.”

Best Available Control Technology (BACT) - An emission limitation based on the maximum

degree of emission reduction (considering energy, environmental, and economic impacts)

achievable through application of production processes and available methods, systems, and

techniques. BACT does not permit emissions in excess of those allowed under any applicable

Clean Air Act provisions. Use of the BACT concept is allowable on a case by case basis for

major new or modified emissions sources in attainment areas and applies to each regulated

pollutant.

Best Professional Judgement - Utilizing knowledge based on education and experience to

determine the best course of action during the course of performing a risk assessment project.

Bias - systematic error introduced into sampling or analysis by selecting or encouraging one

outcome or answer over others.

Binational Toxics Strategy - A Canada-United States jointly-sponsored program that provides a

framework for actions to reduce or eliminate persistent toxic substances, especially those which

bioaccumulate, from the Great Lakes Basin.
April 2004 Page 6

Bioaccumulation - The net accumulation of a substance by an organism as a result of uptake

from and or all routes of exposure (e.g., ingestion of food, intake of drinking water, direct

contact, or inhalation).

Bioavailability - The ability to be absorbed and available to interact with the metabolic

processes of an organism.

Bioaccumulation Factor (BAF) - The concentration of a substance in tissue of an organism

divided by its concentration in an environmental medium in situations where the organism and

its food are exposed (i.e., accounting for food chain exposure as well as direct chemical uptake). 

[EPA, 1999: Residual Risk Report to Congress. EPA453R99001.]

Bioassay - A test conducted in living organisms (in vivo) or with living cells (in vitro) to

determine the hazard or potency of a chemical by its effect on animals, isolated tissues, or

microorganisms. [Based on Air Risk Information Support Center, OAQPS, March 1989:

Glossary of Terms Related to Health, Exposure, and Risk Assessment. EPA/450/3-88/016.]

Bioavailability - A measure of the degree to which a dose of a substance becomes

physiologically available to the body tissues depending upon adsorption, distribution,

metabolism and excretion rates. [Air Risk Information Support Center, OAQPS, March 1989:

Glossary of Terms Related to Health, Exposure, and Risk Assessment. EPA/450/3-88/016.]

Bioconcentration - The net accumulation of a substance by an organism as a result of uptake

directly from an environmental medium (e.g., net accumulation by an aquatic organism as a

result of uptake directly from ambient water, through gill membranes or other external body

surfaces).

Bioconcentration Factor (BCF) - The concentration of a substance in tissue of an organism

divided by the concentration in an environmental medium (e.g., the concentration of a substance

in an aquatic organism divided by the concentration in the ambient water, in situations where the

organism is exposed through the water only).

Biological Medium - Any one of the major categories of material within an organism (blood,

adipose tissue, breath), through which chemicals can move, be stored, or be biologically,

physically, or chemically transformed.

Biological Monitoring - The measurement of chemicals in biological media (e.g., blood, urine,

exhaled breath) to determine whether chemical exposure in humans, animals, or plants has

occurred.

Biologically Effective Dose - The amount of chemical that reaches the cells or target site where

an adverse effect may occur.

Biomagnification or Biological Magnification - The process whereby certain substances, such

as pesticides or heavy metals, transfer up the food chain and increase in concentration. For

example, a biomagnifying chemical deposited in rivers or lakes absorbs to algae, which are

ingested by aquatic organisms, such as small fish, which are in turn eaten by larger fish, fish-
April 2004 Page 7

eating birds, terrestrial wildlife, or humans. The chemical tends to accumulates to higher

concentration levels with each successive food chain level. 

Biotic Degradation (Biodegredation) - Decomposition or metabolism of a substance into more

elementary compounds by the action of organisms (e.g., bacteria, fungi).

Bounding Estimate - An estimate of exposure or risk that is higher or lower than that incurred

by any person in the population. Bounding estimates are useful in developing statements that

exposures or risks are within an estimated range.

Blue Book - The 1994 National Research Council (NRC) report entitled Science and Judgement

in Risk Assessment.

Body Weight (Mass) - The weight or mass of an individual’s body. It can apply to a human or

an ecological receptor.

Breathing Zone - Air in the vicinity of an organism from which respired air is drawn. Personal

monitors are often used to measure pollutants in the breathing zone.

Bright Line - Specific levels of risk or of exposure that are meant to provide a practical

distinction between what is considered “safe” and what is not.

Building Downwash (Plume Downwash) - The interaction of a plume with a structure, such as

a building, which causes the plume to fall to ground.

C

CalEPA (California Environmental Protection Agency) - An Agency within the California

State government whose goal is to protect human health and the environment and to assure the

coordinated deployment of State resources against the most serious environmental risks. There

are six boards that address environmental issues, including air quality, pesticides, toxic

substances, waste management, water control, and the Office of Environmental Health Hazard

Assessment (OEHHA). Note that OEHHA is responsible for developing and providing state and

local government agencies with toxicological and medical information relevant to decisions

involving public health and is a good resource for such information.

Cancer - A group of related diseases characterized by group of diseases characterized by the

uncontrolled growth of abnormal cells.

Cancer Incidence - The number of new cases of a disease diagnosed each year.

Cancer Risk Estimates - The probability of developing cancer from exposure to a chemical

agent or a mixture of chemicals over a specified period of time. In quantitative terms, risk is

expressed in values ranging from zero (representing an estimate that harm certainly will not

occur) to one (representing an estimate that harm certainly will occur). The following are

examples of how risk is commonly expressed: 1.E-04 or 1×10-4 = a risk of 1 additional cancer in

an exposed population of 10,000 people (i.e., 1/10,000); 1.E-5 or 1×10-5 = 1/100,000; 1.E-6 or

1×10-6 = 1/1,000,000.
April 2004 Page 8

Cancer Risk Evaluation Guides (CREGs) - Developed by ATSDR, the concentration of a

chemical in air, soil or water that is expected to cause no more than one excess cancer in a

million persons exposed over a lifetime. The CREG is a comparison value used to select

contaminants of potential health concern and is based on the cancer slope factor (CSF).

Cancer Slope Factor (CSF) - An upper bound (approximating a 95% confidence limit) on the

increased cancer risk from a lifetime exposure to an agent. This estimate, usually expressed in

units of proportion (of a population) affected per mg/kg/day, is generally reserved for use in the

low-dose region of the dose-response relationship; that is, for exposures corresponding to risks

less than 1 in 100. This term is usually used to refer to oral slope factors (i.e., slope factors used

for assessing ingestion exposure).

Carcinogen(ic) - An agent capable of inducing cancer.

Carcinogenesis - The origin or production of a benign or malignant tumor. The carcinogenic

event modifies the genome and/or other molecular control mechanisms of the target cells, giving

rise to a population of altered cells.

Census Bureau (Bureau of the Census) - A Bureau within the Department of Commerce, this

is the country’s preeminent statistical collection and dissemination agency of national

demographic information. It publishes a wide variety of statistical data about people, housing,

and the economy of the nation. The Census Bureau conducts approximately 200 annual surveys

and conducts the decennial census of the United States population and housing and the

quinquennial economic census and census of governments.

Census Block - An area bounded by visible and/or invisible features shown on Census Bureau

maps. A block is the smallest geographic entity for which the Census Bureau collects and

tabulates 100-percent decennial census data.

Census Tract - A small, relatively permanent statistical subdivision of a county or statistically

equivalent entity, delineated for data presentation purposes by a local group of census data users

or the geographic staff of a regional census center in accordance with Census Bureau guidelines.

Designed to be relatively homogeneous units with respect to population characteristics,

economic status, and living conditions at the time they are established, census tracts generally

contain between 1,000 and 8,000 people, with an optimum size of 4,000 people. Census tract

boundaries are delineated with the intention of being stable over many decades, so they generally

follow relatively permanent visible features. However, they may follow governmental unit

boundaries and other invisible features in some instances; the boundary of a state or county (or

statistically equivalent entity) is always a census tract boundary.

Census Tract (or Census Block) Internal Point - A set of geographic coordinates (latitude and

longitude) that is located within a specified geographic entity such as a Census Tract or Census

Block. For many Census Tracts or Blocks, this point represents the approximate center of the

Census Tract or Block; for some, the shape of the entity or the presence of a body of water

causes the central location to fall outside the Census Tract or Block or in water, in which case

the point is relocated to land area within the Census Tract or Block. The geographic coordinates

are shown in degrees to six decimal places in census products.
April 2004 Page 9

Chemical Abstracts Service Registry Number (CASRN) - A unique, chemical-specific

number used in identifying a substance. The registry numbers are assigned by the Chemical

Abstract Service, a division of the American Chemical Society. (Note that some mixtures of

substances, such as mixtures of various forms of xylene, are also given CAS numbers.)

Chemicals of Potential Concern - Chemicals that may pose a threat to the populations within

the study area. These are the chemicals which are carried through the risk assessment process.

Chemical Speciation - Detailed identification of the specific identities and forms of chemicals

in a mixture.

Chemical Transformation - The change of one chemical into another.

Chronic Exposure - Continuous exposure, or multiple exposures, occurring over an extended

period of time or a significant fraction of the animal’s or the individual’s lifetime.

Chronic Health Effects - An effect which occurs as a result of repeated or long term (chronic)

exposures.

Coefficient of Variation (CV) - A dimensionless measure of dispersion, equal to the standard

deviation divided by the mean, often expressed as a percentage.

Cohort - A group of people within a population that can be aggregated because the variation in a

characteristic of interest (e.g., exposure, age, education level) within the group is much less than

the group-to-group variation across the population.

Community - The persons associated with an area who may be directly affected by area

pollution because they currently live in or near the area, or have lived in or near the area in the

past (i.e., current or past residents), members of local action groups, local officials, tribal

governments, health professionals, and local media. Other entities, such as local industry, may

also consider themselves part of the community.

Comparative Risk Assessment - The process of comparing and ranking various types of risks

to identify priorities and influence resource allocations.

Conceptual Model - A written description and/or a visual representation of actual or predicted

relationships between humans or ecological entities and the chemicals or other stressors to which

they may be exposed.

Conductivity (Conductance) - The ability of a material to carry and electrical current.

Confidence Interval - A range of values that has a specified probability (e.g., 95 percent) of

containing the statistical parameter (i.e., a quantity such as a mean or variance that describes a

statistical population) in question. The confidence limit refers to the upper or lower value of the

range.

Coning - In pollution studies, emissions from a chimney stack under atmospheric conditions of

near neutral stability such that concentrations of a pollutant at a given distance downwind from
April 2004 Page 10

the stack may be described by a normal or Gaussian distribution, being the same for both vertical

and horizontal cross-sections perpendicular to the flow.

Consumption Rate - The average quantity of an item consumed or expended during a given

time interval, expressed in quantities by the most appropriate unit of measurement per applicable

stated basis.

Continuous Monitoring - The measurement of the air or water concentration of a specific

contaminant on an uninterrupted, real-time basis by instrumental methods.

Control Technology/Measures - Equipment, processes or actions used to reduce air pollution at

the source.

Convection - The transfer and mixing of heat by mass movement through a fluid (e.g., air or

water). It is one of the major mechanisms for the transfer of heat within the atmosphere, together

with conduction and radiation. The convection process is of major importance in the

troposphere, transferring sensible heat and latent heat from the Earth’s surface into the boundary

layer, and by promoting the vertical exchange of air-mass properties (e.g., heat, water vapor, and

momentum) throughout the depth of the troposphere. Convection is generally accepted to be

vertical circulation, whereas advection is usually horizontal.

Cost-Benefit Analysis - An evaluation of the costs which would be incurred versus the overall

benefits of a proposed action, such as the establishment of an acceptable exposure level of a

pollutant.

Criteria Air Pollutant - One of six common air pollutants determined to be hazardous to human

health and regulated under EPA’s National Ambient Air Quality Standards (NAAQS). The six

criteria air pollutants are carbon monoxide, lead, nitrogen dioxide, ozone, sulfur dioxide, and

particulate matter. The term “criteria pollutants” derives from the requirement that EPA must

describe the characteristics and potential health and welfare effects of these pollutants. It is on

the basis of these criteria that standards are set or revised.

Critical Effect - The first adverse effect, or its known precursor, that occurs to the most

sensitive species as the dose rate of an agent increases.

Cumulative Risk - The combined risk from aggregate exposures to multiple agents or stressors.

Cumulative Risk Assessment - An analysis, characterization, and possible quantification of the

combined risks to health or the environment from multiple agents or stressors.

Cumulative Distribution Function (CDF) - The CDF is alternatively referred to in the

literature as the distribution function, cumulative frequency function, or the cumulative

probability function. The cumulative distribution function, F(x), expresses the probability the

random variable X assumes a value less than or equal to some value x, F(x) = Prob (X # x). For

continuous random variables, the cumulative distribution function is obtained from the

probability density function by integration, or by summation in the case of discrete random

variables.
April 2004 Page 11

Cumulative Risk Assessment - An analysis, characterization, and possible quantification of the

combined risks to health or the environment from multiple agents or stressors.

D

Data Integrity - Refers to security (i.e., the protection of information from unauthorized access

or revision) to ensure that the information is not compromised through corruption or

falsification. Data integrity is one of the constituents of data quality.

Data Objectivity - A characteristic indicating whether information is being presented in an

accurate, clear, complete, and unbiased manner, and as a matter of substance, is accurate,

reliable, and unbiased. Data objectivity is one of the constituents of data quality.

Data Quality - The encompassing term regarding the quality of information used for analysis

and/or dissemination. Utility, objectivity, and integrity are constituents of data quality.

Data Quality Objectives (DQOs) - Qualitative and quantitative statements derived from the

DQO process that clarify study objectives, define the appropriate type of data, and specify

tolerable levels of potential decision errors that will be used as the basis for establishing the

quality and quantity of data needed to support the decisions.

Data Quality Objectives Process - A systematic planning tool to facilitate the planning of

environmental data collection activities. Data quality objectives are the qualitative and

quantitative outputs from the DQO Process.

Data Utility - Refers to the usefulness of the information to the intended users. Data utility is

one of the constituents of data quality.

Delivered Dose - The amount of the chemical available for interaction by any particular organ or

cell.

Deposition (Wet and Dry) - The removal of airborne substances to available surfaces that

occurs as a result of gravitational settling and diffusion, as well as electrophoresis and

thermophoresis in the absence of active precipitation (Dry) or in the presence of active

precipitation (Wet).

Deposition (Flux) - The removal of airborne substances from the air to available surfaces that

occurs as a result of gravitational settling and diffusion, as well as electrophoresis and

thermophoresis.

Dermal - Referring to the skin. Dermal absorption means absorption through the skin.

Dermal Exposure - Contact between a chemical and the skin. [EPA, 1997: Terms of

Environment, http://www.epa.gov/OCEPAterms/.]

Detection Limit - The lowest concentration of a chemical that can reliably with analytical

methods be distinguished from a zero concentration.
April 2004 Page 12

Deterministic - A methodology relying on point (i.e., exact) values as inputs to estimate risk;

this obviates quantitative estimates of uncertainty and variability. Results are also presented as

point values. Uncertainty and variability may be discussed qualitatively, or semi-quantitatively

by multiple deterministic risk estimates.

Developmental Toxicity - The potential of an agent to cause abnormal development.

Developmental toxicity generally occurs in a dose-related manner, may result from short-term

exposure (including single exposure situations) or from longer term low-level exposure, may be

produced by various routes of exposure, and the types of effects may vary depending on the

timing of exposure because of a number of critical periods of development for various organs

and functional systems. The four major manifestations of developmental toxicity are death,

structural abnormality, altered growth, and functional deficit.

Dietary Composition - The fractions of different foods that constitute a given diet.

Differential Heating - The property of different surfaces which causes them to heat and cool at

different rates.

Direct Exposure - Contact between a receptor and a chemical where the chemical is still in the

medium to which it was originally released. For example, direct exposure occurs when a

pollutant is released to the air and a person breathes that air.

Direct-read Monitor - Using a pump to draw the air sample through the detector, this type of air

toxics monitoring device provides a direct reading of the pollutant measurement. The monitor

may be designed as a table-top unit, for example, or it may be rack-mounted such as for use in an

ambient air monitoring station.

Dispersion - Pollutant or concentration mixing due to turbulent physical processes.

Disease Cluster - An unusual number, real or perceived, of health events (i.e., reports of cancer)

grouped together in time and location.
April 2004 Page 13

Dose - The amount of substance available for interaction with metabolic processes or

biologically significant receptors after crossing the outer boundary of an organism. The potential

dose is the amount ingested, inhaled, or applied to the skin. The applied dose is the amount of a

substance presented to an absorption barrier and available for absorption (although not

necessarily having yet crossed the outer boundary of the organism). The absorbed does is the

amount crossing a specific absorption barrier (e.g., the exchange boundaries of skin, lung, and

digestive tract) through uptake processes. Internal dose is a more general term denoting the

amount absorbed without respect to specific absorption barriers or exchange boundaries. The

amount of the chemical available for interaction by any particular organ or cell is termed the

delivered dose for that organ or cell.

Dose-Response Assessment - A determination of the relationship between the magnitude of an

administered, applied, or internal dose and a specific biological response. Response can be

expressed as measured or observed incidence, percent response in groups of subjects (or

populations), or as the probability of occurrence within a population.

Dose-Response Curve - A graphical representation of the quantitative relationship between

administered, applied, or internal dose of a chemical or agent, and a specific biological response

to that chemical or agent.

Dust Resuspension - Involves the deposition of dust from the air and its subsequent

resuspension or re-entrainment into the atmosphere.

E

Ecological Risk Assessment - The process that evaluates the likelihood that adverse ecological

effects may occur or are occurring as a result of exposure to one or more stressors.

Eddy - In the atmosphere, a distinct mass within a turbulent fluid that retains its identity and

behaves differently for a short period within the general larger volume flow. An eddy thus

ranges in size from microscale turbulence (1 cm for example) to many hundreds of kilometers in

the form of frontal cyclones and anticyclones. The smallest scale eddies are critical in the

process of, for example, heat and water vapor transfer from the Earth’s surface into the air, while

frontal cyclones transport heat toward the poles.

Emission Factor - The relationship between the amount of pollution produced and the amount

of raw material processed or product produced. For example, an emission factor for a blast

furnace making iron could be the number of pounds of particulates released per ton of raw

materials used.

Emission Inventory - A listing, by source, of the amount of air pollutants discharged into the

atmosphere in a particular place. Two of the more important publicly available emissions

inventories for air toxics studies are the National Emissions Inventory (NEI) and the Toxics

Release Inventory (TRI).

Emission Rate - The amount of a given substance discharged to the air per unit time, expressed

as a fixed ratio (e.g., tons/yr).
April 2004 Page 14

Emissions Inventory Improvement Program (EIIP) - A jointly sponsored effort of the State

and Territorial Air Pollution Program Administrators/Association of Local Air Pollution Control

Officials (STAPPA/ALAPCO) and EPA, and is an outgrowth of the Standing Air Emissions

Work Group (SAEWG). The goal of EIIP is to provide cost-effective, reliable inventories by:

(1) Improving the quality of emissions information, and (2) Developing system(s) for collecting,

calculating, and reporting emissions data. The goal is achieved by developing a set of “preferred

and alternative methods” for all inventory associated tasks. This standardization improves the

consistency of collected data and results in increased usefulness of emissions information.

Emissions Monitoring - The periodic or continuous physical surveillance or testing to

determine the pollutant levels discharged into the atmosphere from sources such as smokestacks

at industrial facilities and exhaust from motor vehicles, locomotives, or aircraft.

Emissions Tracking System (ETS) - This EPA system contains all emissions data submitted

under various clean air market programs. Data from Continuous Emissions Monitoring Systems

at utilities sends the emission data to the utility’s computer system, which then compiles the data

for submission to EPA on a quarterly basis. At the end of each calendar year, EPA compares

tons of emissions emitted with the allowance holdings of the utility unit to ensure that it is in

compliance with the relevant program.

Endocrine Disruptor - Substances which interfere with endocrine system function.

Environmental Data - Any measurements or information that describe environmental

processes, location, or conditions; ecological or health effects and consequences; or the

performance of environmental technology. Environmental data include information collected

directly from measurements, produced from models, and compiled from other sources such as

data bases or the literature.

Environmental Media Evaluation Guides - Environmental Media Evaluation Guides

(EMEGs) are concentrations of a contaminant in water, soil, or air that are unlikely to be

associated with any appreciable risk of deleterious noncancer effects over a specified duration of

exposure. EMEGs are derived from ATSDR minimal risk levels by factoring in default body

weights and ingestion rates. Separate EMEGS are computed for acute (14 days), intermediate

(15-364 days), and chronic (365 days) exposures.

Environmental Medium - Any one of the major categories of material found in the physical

environment (e.g., surface water, ground water, soil, or air), and through which chemicals or

pollutants can move.

Epidemiology - The study of disease patterns in human populations.

Epidemiologic Study, Case Study - A medical or epidemiologic evaluation of one person or a

small group of people to gather information about specific health conditions and past exposures.

Epidemiologic Study, Descriptive - An evaluation of the amount and distribution of a disease

in a specified population by person, place, and time.
April 2004 Page 15

Epidemiologic Study, Analytical - An evaluation of the association between exposure to

hazardous substances and disease by testing scientific hypotheses.

Exposure - Contact made between a chemical, physical, or biological agent and the outer

boundary of an organism.

Exposure Assessment - An identification and evaluation of a population exposed to a toxic

agent, describing its composition and size, as well as the type, magnitude, frequency, route and

duration of exposure.

Exposure Concentration - The concentration of a chemical in its transport or carrier medium

(i.e., an environmental medium or contaminated food) at the point of contact.

Exposure Duration - The total time an individual is exposed to the chemical being evaluated or

the length of time over which contact with the contaminant lasts.

Exposure Factors - Any of a variety of factors that relate to how an organism interacts with or

is otherwise exposed to environmental pollutants (e.g., ingestion rate of contaminated fish). 

Such factors are used in the calculation of exposure to toxic chemicals.

Exposure Frequency - The number of occurrences in a given time frame (e.g., a lifetime) of

contact or co-occurrence of a stressor with a receptor.

Exposure Investigation (in Public Health Assessment) - The collection and analysis of

site-specific information and biologic tests (when appropriate) to determine whether people have

been exposed to hazardous substances.

Exposure Modeling - The mathematical equations simulating how people interact with

chemicals in their environment.

Exposure Pathway - The course a chemical or physical agent takes from a source to an exposed

organism. An exposure pathway includes a source and release from a source, an exposure point,

and an exposure route. If the exposure point differs from the source, a transport/exposure

medium (e.g., air) or media (in cases of intermedia transfer) also is included.

Exposure Profile - The exposure profile (ecological) identifies the receptors and describes the

exposure pathways and intensity and spatial and temporal extent of exposure. It also describes

the impact of variability and uncertainty on exposure estimates and reaches a conclusion about

the likelihood that exposure will occur. The profile may be a written document or a module of a

larger process model.

Exposure Route - The way a chemical enters an organism after contact (e.g., by ingestion,

inhalation, dermal absorption).

Exposure Scenario - A set of conditions or assumptions about sources, exposure pathways,

concentrations of toxic chemicals, and populations (numbers, characteristics and habits) which

aid the investigator in evaluating and quantifying exposure in a given situation.
April 2004 Page 16

Exposure Unit (in Geographical Information System applications) - The geographical area in

which a receptor moves and contacts the contaminated medium during the period of exposure.

F

Factor Information Retrieval System (FIRE) - A database management system containing

EPA’s recommended emission estimation factors for criteria and hazardous air pollutants. FIRE

includes information about industries and their emitting processes, the chemicals emitted, and

the emission factors themselves. FIRE allows easy access to criteria and hazardous air pollutant

emission factors obtained from the Compilation of Air Pollutant Emission Factors (AP-42),

Locating and Estimating (L&E) documents, and the retired AFSEF and XATEF documents.

Fate and Transport - A description of how a chemical is carried through and changes in the

environment.

Fate and Transport Analysis - The general process used to assess and predict the movement

and behavior of chemicals in the environment.

Fate and Transport Modeling - The mathematical equations simulating a physical system

which are used to assess and predict the movement and behavior of chemicals in the

environment.

Fence Line - Delineated property boundary of a facility.

Field Study - Scientific study made in the ambient air to collect information that can not be

obtained in a laboratory.

Food Chain - A sequence of organisms, each of which uses the next lower member of the

sequence as a food source.

Forage - (1) Edible parts of plants, other than separated grain, that can provide feed for grazing

animals or can be harvested for feeding, including browse, and herbage. (2) To search for or to

consume forage (of animals).

Fugitive Release - Emission of a chemical to the air that does not occur from a stack, vent, duct,

pipe or other confined air stream (e.g., leaks from joints).

Fumigation - (1) The use of a chemical compound in a gaseous state, often to kill pests such as

insects, nematodes, arachnids, rodents, weeds, and fungi in confined or inaccessible locations or

in the field. (2) a pattern of plume dispersion produced when a convective boundary layer grows

upward into a plume trapped in a stable layer. The elevated plume is suddenly brought

downward to the ground, producing high surface concentrations.

Future Scenario - A scenario used in risk assessment to anticipate potential future exposures of

individuals (e.g., a housing development could be built on currently vacant land).
April 2004 Page 17

G

Geographic Information Systems (GIS) - A computer program that allows layering of different

types of spatial information (i.e., on a map) to provide a better understanding of the

characteristics of a certain place.

Generally Available Control Technology (GACT) Standard - These standards are less

stringent standards than the Maximum Available Control Technology (MACT) standards, and

are allowed at the Administrator’s discretion for area sources according to the 1990 Clean Air

Act Amendments for area sources.

Grab Sample -A single sample collected at a particular time and place that represents the

composition of the water, air, or soil only at that time and place.

Great Waters Pollutants of Concern - The toxic pollutants of concern to the Great Waters

program are mercury; cadmium and lead (and their compounds); dioxins; furans; polycyclic

organic matter; polychlorinated biphenyls (PCBs); and the pesticides chlordane, DDT/DDE,

dieldrin, hexachlorobenzene, alpha-hexachlorocyclohexane, lindane and toxaphene. Nitrogen

compounds such as nitrogen oxides and ammonia are also pollutants of concern.

Greenhouse Effect - Trapping and build-up of heat in the atmosphere (troposphere) near the

earth’s surface. Some of the heat flowing back toward space from the earth’s surface is absorbed

by water vapor, carbon dioxide, ozone, and several other gases in the atmosphere and then reradiated back toward the earth’s surface. If the atmospheric concentrations of these greenhouse

gases rise, the average temperature of the lower atmosphere will gradually increase.

Greenhouse Gas (GHG) - Any gas that absorbs infrared radiation in the atmosphere. 

Greenhouse gases include, but are not limited to, water vapor, carbon dioxide (CO2), methane

(CH4), nitrous oxide (N2O), hydrochlorofluorocarbons (HCFCs), ozone (O3), hydrofluorocarbons

(HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

Guidelines (human health and ecological risk assessment) - Official documentation stating

current U.S. EPA methodology in assessing risk of harm from environmental pollutants to

human populations and ecological receptors.

H

Half-Life - The time required for a reaction or process to proceed such that half of the original

amount of the substance of interest has reacted or undergone the process. Examples include: (1)

the time required for a pollutant to degrade to one-half of its original concentration; (2) the time

required for half of the atoms of a radioactive element to undergo self-transmutation or decay

(half-life of radium is 1620 years); (3) the time required for elimination from the body to half a

total dose.

Hazard - In a general sense, “hazard” is anything that has a potential to cause harm. In risk

assessment, the likelihood of experiencing a noncancer health effect is called hazard (not risk).
April 2004 Page 18

Hazard Identification - The process of determining whether exposure to an agent can cause a

particular adverse health effect (e.g., cancer, birth defect) and whether the adverse health effect

is likely to occur in humans at environmentally relevant doses.

Hazard Index (HI) -The sum of more than one hazard quotient for multiple substances and/or

multiple exposure pathways. The HI is calculated separately for chronic, subchronic, and

shorter-term duration exposures.

Hazardous Air Pollutants (HAP) - Defined under the Clean Air Act as pollutants that cause or

may cause cancer or other serious health effects, such as reproductive effects or birth defects, or

adverse environmental and ecological effects. Currently, the Clean Air Act regulates 188

chemicals and chemical categories as HAPs.

Hazard Quotient (HQ) - The ratio of a single substance exposure level over a specified time

period (e.g., chronic) to a reference value (e.g., an RfC) for that substance derived from a similar

exposure period.

Health Effects Assessment Tables (HEAST) - An older listing of (usually) interim toxicity

values for chemicals of interest to Superfund, the Resource Conservation and Recovery Act

(RCRA), and the EPA in general. HEAST values are generally placed low on the hierarchy of

Agency recommended toxicity data sources and the compilation will eventually be phased out

altogether.

Health Endpoint - An observable or measurable biological event used as an index to determine

when a deviation in the normal function of the human body occurs.

Health Outcome Data (in Public Health Assessment) - Community-specific health

information such as morbidity and mortality data, birth statistics, medical records, tumor and

disease registries, surveillance data, and previously conducted health studies that may be

collected at the local, state, and national levels by governments, private health care

organizations, and professional institutions and associations.

Health Outcomes Study (in Public Health Assessment) - An investigation of exposed persons

designed to assist in identifying exposure or effects on public health. Health studies also define

the health problems that require further inquiry by means of, for example, a health surveillance

or epidemiologic study.

Health Education (in Public Health Assessment) - Programs designed with a community to

help it know about health risks and how to reduce these risks.

Health Consultation (in Public Health Assessment) - A review of available information or

collection of new data to respond to a specific health question or request for information about a

potential environmental hazard. Health consultations are focused on a specific exposure issue. 

Health consultations are therefore more limited than a public health assessment, which reviews

the exposure potential of each pathway and chemical.

Henry’s Law Constant - The ratio at equilibrium of the gas phase concentration to the liquid

phase concentration of the gas.
April 2004 Page 19

High-End Exposure Estimate - A plausible estimate of individual exposure or dose for those

persons at the upper end of an exposure or dose distribution, conceptually above the 90th

percentile, but not higher than the individual in the population who has the highest exposure or

dose.

Human Exposure Model (HEM) - An EPA model combining the Industrial Source Complex

Short Term air dispersion model (ISCST) with a national set of meteorology files, U.S. census

data, and a risk calculation component that can be used to estimate individual and population

risks.

Hydrolysis - The decomposition of organic compounds by interaction with water.

I

Impervious Surface - A surface that cannot be penetrated by water (e.g., pavement).

Indirect Exposure Pathway - An indirect exposure pathway is one in which a receptor contacts

a chemical in a medium that is different from the one to which the chemical was originally

released (an example occurs with dioxin, which is emitted into the air, deposited on soil and

accumulated in plants and animals which are then consumed by humans).

Individual Risk or Hazard - The risk or hazard to an individual in a population rather than to

the population as a whole.

Indoor Source - Objects or places within buildings or other enclosed spaces that emit air

pollutants.

Industrial Source Complex (ISC) Model - A steady-state Gaussian plume model which can be

used to assess pollutant concentrations from a wide variety of sources associated with an

industrial complex. This model can account for the following: settling and dry deposition of

particles; downwash; point, area, line, and volume sources; plume rise as a function of

downwind distance; separation of point sources; and limited terrain adjustment. ISC3 operates

in both long-term (ISCLT) and short-term (ISCST) modes.

Influential Information - Scientific, financial, or statistical information that will have or does

have a clear and substantial impact on important public policies or important private sector

decisions.

Ingestion - Swallowing (such as eating or drinking).

Ingestion Exposure - Exposure to a chemical by swallowing it (such as eating or drinking).

Inhalation - Breathing.

Inhalation Exposure - Exposure to a chemical by breathing it in.
April 2004 Page 20

Inhalation Unit Risk (IUR) - The upper-bound excess lifetime cancer risk estimated to result

from continuous exposure to an agent at a concentration of 1 μg/m3 in air. The interpretation of

unit risk would be as follows: if unit risk = 2 × 10-6 μg/m3

, 2 excess tumors may develop per

1,000,000 people if exposed daily for a lifetime to a concentration of 1 μg of the chemical in 1

m3

 of air.

Intake - The process by which a substance crosses the outer boundary of an organism without

passing an absorption barrier, e.g., through ingestion or inhalation.

Intake Rate - Rate of inhalation, ingestion, and dermal contact depending on the route of

exposure.

Integrated Risk Information System (IRIS) - An EPA database which contains information on

human health effects that may result from exposure to various chemicals in the environment.

IRIS was initially developed for EPA staff in response to a growing demand for consistent

information on chemical substances for use in risk assessments, decision-making and regulatory

activities. The information in IRIS is intended for those without extensive training in toxicology,

but with some knowledge of health sciences.

Internal Dose - In exposure assessment, the amount of a substance penetrating the absorption

barriers (e.g., skin, lung tissue, gastrointestinal tract) of an organism through either physical or

biological processes.

Inversion - Subsidence Inversion - A temperature inversion that develops aloft as a result of air

gradually sinking over a wide area and being warmed by adiabatic compression, usually

associated with subtropical high pressure areas.

Inversion - Advection Inversion - Associated with the horizontal flow of warm air. Warm air

moves over a cold surface, and the air nearest the surface cools, causing a surface-based

inversion.

Inversion - Radiation Inversion - A thermally produced, surface-based inversion formed by

rapid radiational cooling of the Earth’s surface at night. It does not usually extend above the

lower few hundred feet. Conditions which are favorable for this type of inversion are long

nights, clear skies, dry air, little or no wind, and a cold or snow covered surface. It is also called

a Nocturnal Inversion.

Iterative Process - Replication of a series of actions to produce successively better results, or to

accommodate new and different critical information or scientific inferences.

Isopleths - A delineated line or area on a map that represent equal values of a variable.
April 2004 Page 21

L

Laboratory Studies - Research carried out in a laboratory (e.g., testing chemical substances,

growing tissues in cultures, or performing microbiological, biochemical, hematological,

microscopical, immunological, parasitological tests).

Leaching - The process by which soluble constituents are dissolved and filtered through the soil

by a percolating fluid (usually rainwater).

Life Stage - A phase in the life cycle of an organism.

Line Source - A theoretical one-dimensional source from which releases may occur (e.g.,

roadways are often modeled as a one-dimensional line).

Lofting - In pollution studies, a pattern of flow that occurs when the top of a plume from a

chimney stack disperses into slightly turbulent or neutral airflow conditions, while the lower part

of the plume is prevented from dispersing down toward the surface by a stable boundary layer,

especially at night. [Smith, J. [ed], 2001: The Facts on File Dictionary of Weather and Climate.]

Low-dose Extrapolation - An estimation of the dose-response relationship at doses less than the

lowest dose studied experimentally.

Lowest Observed Adverse Effect Level (LOAEL) - The lowest exposure level in a study or

group of studies at which there are statistically or biologically significant increases in frequency

or severity of adverse effects between the exposed population and its appropriate control group.

Also referred to as lowest-effect level (LEL).

M

Major Source - Under the Clean Air Act, a stationary source that emits more than 10 tons or

more per year of a single hazardous air pollutant (HAP) or 25 or more tons per year of all HAPs.

Margin of exposure (MOE) - The point of departure divided by the actual or projected

environmental exposure of interest.

Mass-Balance Estimate - An estimate of release of a chemical based on, generally, a

comparison of the amount of chemical in raw materials entering a process versus the amount of

chemical going out in products.

Maximum Achievable Control Technology (MACT) - Under the Clean Air Act, a group of

technology based standards, applicable to both major and some area sources of air toxics, that

are aimed at reducing releases of air toxics to the environment. MACT standards are established

on a source category by source category basis.

Maximum Exposed Individual (MEI) - The MEI represents the highest estimated risk to an

exposed individual, regardless of whether people are expected to occupy that area.
April 2004 Page 22

Maximum Individual Risk (MIR) - An MIR represents the highest estimated risk to an

exposed individual in areas that people are believed to occupy.

Metric (or Measure) of Exposure - The quantitative outcome of the exposure assessment. For

air toxics risk assessments, personal air concentration (or adjusted exposure concentration) is

the metric of exposure for the inhalation route of exposure and intake rate is the metric of

exposure for the ingestion route of exposure.

Measurement - In air toxics assessment, a physical assessment (usually of the concentration of a

pollutant) taken in an environmental or biological medium, normally with the intent of relating

the measured value to the exposure of an organism.

Measurement Endpoint - A measurable ecological characteristic that is related to the valued

characteristic chosen as the assessment endpoint. Also known as “measure of effect.”

Mechanical Turbulence - Random irregularities of fluid motion in air caused by buildings or

other nonthermal, processes.

Mechanistic Model - A model that uses information about a chemical or other agent’s

mechanism(s) of action – how it interacts with and harms the target organs – to predict the doseresponse curve or other applications.

Media Concentrations - The amount of a given substance in a specific amount of

environmental medium. For air, the concentration is usually given as micrograms (μg) of

substance per cubic meter (m3

) of air; in water as μg of substance per L of water; and in soil as

mg of substance per kg of soil.

Metabolism - Generally, the biochemical reactions by which energy is made available for the

use of an organism. Metabolism includes all chemical transformations occurring in an organism

from the time a substance enters, until it has been utilized and the waste products eliminated. In

toxicology, metabolism of a toxicant consists of a series of chemical transformations that take

place within an organism. A wide range of enzymes act on toxicants, that may increase water

solubility, and facilitate elimination from the organism. In some cases, however, metabolites

may be more toxic than their parent compound.

Meteorology - The science of the atmosphere, including weather.

Microcosm Studies - Studies of the effects of stressors on multiple species found in multiple

media which are conducted in enclosed experimental systems.

Microscale Assessment - An air monitoring network designed to assess concentrations in air

volumes associated with area dimensions ranging from several meters up to about 100 meters.

Microenvironment - A small 3-dimensional space (e.g., an office, a room in a home) that can be

treated as homogeneous (or well characterized) with regard to exposure concentration of a

chemical.
April 2004 Page 23

Middle Scale Assessment - An air monitoring network designed to assess concentrations typical

of areas up to several city blocks in size with dimensions ranging from about 100 meters to 0.5

kilometer.

Minimal Risk Levels (MRL) - Derived by ATSDR, an MRL is defined as an estimate of daily

human exposure to a substance that is likely to be without an appreciable risk of adverse effects

(noncancer) over a specified duration of exposure. MRLs can be derived for acute, intermediate,

and chronic duration exposures by the inhalation and oral routes.

Mixed (Mixing) Layer - In the atmosphere, that part of the turbulent boundary layer that is

dominated by turbulent diffusion caused by eddies generated by friction with the surface and

thermals arising from surface heat sources. Surface heating during the day and the absence of

temperature inversions allow components of the air within the planetary boundary layer to

exhibit mainly random vertical movements. Such movements may become more organized into

gusts of wind and dust devils during the afternoon. Despite being random, the turbulent

movements allow the transfer of atmospheric properties, such as heat, water vapor, momentum,

and air pollutants, from the near surface up through the planetary boundary layer.

Mixing Height - The depth through which atmospheric pollutants are typically mixed by

dispersive processes.

Mixtures - Any set of multiple chemical substances occurring together in an environmental

medium.

Mobile Source Air Toxics - Air toxics that are emitted from non-stationary objects that release

pollution. Mobile sources include cars, trucks, buses, planes, trains, motorcycles and

gasoline-powered lawn mowers. Another example is a portable generator.

Model - A mathematical representation of a natural system intended to mimic the behavior of the

real system, allowing description of empirical data, and predictions about untested states of the

system.

Model Uncertainty - Uncertainty due to necessary simplification of real-world processes, misspecification of the model structure, model misuse, or use of inappropriate surrogate variables or

inputs.

Modeling - An investigative technique using a mathematical or physical representation of a

system or theory that accounts for all or some of its known properties.

Modeling Node - In air quality modeling, the location where impacts are predicted.

Monitoring - Periodic or continuous physical surveillance or testing to determine pollutant

levels in various environmental media or in humans, plants, and animals.

Monte Carlo Technique- A repeated random sampling from the distribution of values for each

of the parameters in a generic exposure or risk equation to derive an estimate of the distribution

of exposures or risks in the population.
April 2004 Page 24

( ) Pd e q qd q d q dk

k

( ) ...= − − + + ++1 01 2

2

Multipathway Assessment - An assessment that considers more than one exposure pathway. 

For example, evaluation of exposure through both inhalation and ingestion would be a

multipathway assessment. Another example would be evaluation of ingestion of contaminated

soil and ingestion of contaminated food.

Multipathway Exposure - When an organism is exposed to pollutants through more than one

exposure pathway. One example would be exposure through both inhalation and ingestion. 

Another example would be ingestion of contaminated soil and ingestion of contaminated food.

Multipathway Risk - The risk resulting from exposure to pollutants through more than one

pathway.

Multistage Model - A mathematical function used to extrapolate the probability of cancer from

animal bioassay data, using the form:

where:

P(d) = probability of cancer from a continuous, lifetime exposure rate d;

qi = fitted dose coefficients of model; i = 0, 1, . . ., k; and

k = number of stages selected through best fit of the model, no greater than one less

than the number of available dose groups.

Mutagen - A chemical that causes a permanent genetic change in a cell other than that which

occurs during normal growth.

Mutagenicity - The capacity of a chemical or physical agent to cause permanent genetic change

in a cell other than that which occurs during normal growth.

N

National Ambient Air Quality Standards (NAAQS) - Maximum air pollutant standards that

EPA has set under the Clean Air Act for attainment by each state. Standards are set for each of

the criteria pollutants.

National Air Toxics Assessment (NATA) - EPA’s ongoing comprehensive evaluation of air

toxics in the U.S. Activities include expansion of air toxics monitoring, improving and

periodically updating emission inventories, improving national- and local-scale modeling and

risk characterization, continued research on health effects and exposures to both ambient and

indoor air, and improvement of assessment tools.

National Emissions Inventory (NEI) - EPA’s primary emissions inventory of HAPs.

National Emissions Standards for Hazardous Air Pollutants (NESHAPs) - Emissions

standards set by EPA for hazardous air pollutants. Also commonly referred to as the MACT

standards.
April 2004 Page 25

National Emissions Trends (NET) Database - The NET database is an emission inventory that

contains data on stationary and mobile sources that emit criteria air pollutants and their

precursors. The database also includes estimates of annual emissions of these pollutants from

point, area, and mobile sources. The NET is developed every three years (e.g., 1996 and 1999)

by EPA, and includes emission estimates for all 50 States, the District of Columbia, Puerto Rico,

and the Virgin Islands.

Natural Source - Non-manmade emission sources, including biological (biogenic sources such

as plants) and geological sources (such as volcanoes), and windblown dust.

Neighborhood Scale Assessment - An air monitoring network designed to assess concentrations

within some extended area of the city that has relatively uniform land use with dimensions in the

0.5 to 4.0 kilometers range.

Neurotoxicity - Ability to damage nervous system tissue or adversely effect nervous system

function.

New Source Review - A Clean Air Act requirement that State Implementation Plans must

include a permit review that applies to the construction and operation of new and modified

stationary sources in nonattainment areas to ensure attainment of national ambient air quality

standards.

New Source Performance Standards - Uniform national EPA air emission standards which

limit the amount of pollution allowed from new sources or from modified existing sources.

Noncarcinogenic Effect - Any health effect other than cancer. Note that, while not all

noncancer toxicants cause cancer, all carcinogens exhibit noncarcinogenic effects.

No Observable Adverse Effect Level (NOAEL) - An highest exposure level at which there are

no statistically or biologically significant increases in the frequency or severity of adverse effect

between the exposed population and its appropriate control; some effects may be produced at

this level, but they are not considered adverse, nor precursors to adverse effects.

Nonpoint Source (NEI sense) - Diffuse pollution sources that are not assigned a single point of

origin (e.g., multiple dry cleaners in a county which are only described in an inventory in the

aggregate).

Nonroad Mobile Sources - Sources such as farm and construction equipment, gasoline-powered

lawn and garden equipment, and power boats and outdoor motors that emit pollutants.

Non-Threshold Effect - An effect (usually an adverse health effect) for which there is no

exposure level below which the effect is not expected to occur.

Non-Threshold Toxicant - A chemical for which there is no exposure level below which an

adverse health outcome is not expected to occur. Such substances are considered to pose some

risk of harm at any level of exposure.
April 2004 Page 26

Non Steady-state Model - A dynamic model; a mathematical formulation describing and

simulating the physical behavior of a system or a process and its temporal variability.

North American Industry Classification System (NAICS) - NAICS replaced the Standard

Industrial Classification (SIC) beginning in 1997. This industry-wide classification system has

been designed as the index for statistical reporting of all economic activities of the U.S., Canada,

and Mexico. NAICS industries are identified by a 6-digit code. The international NAICS

agreement fixes only the first five digits of the code. The sixth digit, where used, identifies

subdivisions of NAICS industries that accommodate user needs in individual countries.

O

Octanol/Water Partition Coefficient (Kow) - The ratio of a chemical’s solubility in n-octanol to

its solubility in water at equilibrium. This measure is often used as an indication of a chemical’s

ability to bioconcentrate in organisms.

Office of Air and Radiation (OAR) - EPA’s Office responsible for providing information about

air pollution, clean air, air quality and radiation. OAR develops national programs, technical

policies, and regulations for controlling air pollution and radiation exposure. OAR is concerned

with pollution prevention, indoor and outdoor air quality, industrial air pollution, pollution from

vehicles and engines, radon, acid rain, stratospheric ozone depletion, and radiation protection.

Office of Air Quality, Planning, and Standards (OAQPS) - An EPA Office within OAR

whose primary mission is to preserve and improve air quality in the United States. As part of

this goal, OAQPS monitors and reports on air quality, air toxics, and emissions. They also

respond to visibility issues, as they relate to the level of air pollution. In addition, OAQPS is

tasked by the EPA with providing technical information for professionals involved with

monitoring and controlling air pollution, creating governmental policies, rules, and guidance

(especially for stationary sources), and educating the public about air pollution and what can be

done to control and prevent it.

OAQPS Toxicity Table - The EPA Office of Air and Radiation recommended default chronic

toxicity values for hazardous air pollutants. They are generally appropriate for screening-level

risk assessments, including assessments of select contaminants, exposure routes, or emission

sources of potential concern, or to help set priorities for further research. For more complex,

refined risk assessments developed to support regulatory decisions for single sources or

substances, dose-response data may be evaluated in detail for each “risk driver” to incorporate

appropriate new toxicological data. (http://www.epa.gov/ttn/atw/toxsource/summary.html)
April 2004 Page 27

Office of Radiation and Indoor Air (ORIA) - An EPA Office within OAR whose mission is to

protect the public and the environment from the risks of radiation and indoor air pollution. The

Office develops protection criteria, standards, and policies; works with other programs within

EPA and other agencies to control radiation and indoor air pollution exposures; provides

technical assistance to states through EPA’s regional offices, and to other agencies having

radiation and indoor air protection programs; directs an environmental radiation monitoring

program; responds to radiological emergencies; and evaluates and assesses the overall risk and

impact of radiation and indoor air pollution.

Office of Transportation and Air Quality (OTAQ) - An EPA Office within OAR whose

mission is to reconcile the transportation sector with the environment by advancing clean fuels

and technology, and working to promote more liveable communities. OTAQ is responsible for

carrying out laws to control air pollution from motor vehicles, engines, and their fuels. Mobile

sources include: cars and light trucks, large trucks and buses, farm and construction equipment,

lawn and garden equipment, marine engines, aircraft, and locomotives.

Onroad Mobile Source - Any mobile source of air pollution such as cars, trucks, motorcycles,

and buses that travels on roads and highways.

Open Pit Source - Large, open pits, such as surface coal mines and rock quarries.

Operating Permit Program - A program required by the Clean Air Act; requires existing

industrial sources to obtain an"operating permit". The operating permit program is a national

permitting system that consolidates all of the air pollution control requirements into a single,

comprehensive “operating permit” that covers all aspects of a source’s year-to-year air pollution

activities.

P

Particle-bound - Reversibly absorbed or condensed onto the surface of particles.

Particulates/Particulate Matter (PM) - Solid particles or liquid droplets suspended or carried

in the air.

Partitioning - The separation or division of a substance into two or more compartments. 

Environmental partitioning refers to the distribution of a chemical into various media (soil, air,

water, and biota).

Partitioning Model - Models consisting of mathematical equations that estimate how chemicals

will divide (i.e., partition) among abiotic and biotic media in a given environment based on

chemical- and site- specific characteristics.

Passive Monitor - A type of air toxics monitor that collects airborne pollutants by absorption

onto a reactive material (for example, sorbent tube, filter) for subsequent laboratory analysis. No

pump is used to draw the air across the reactive material. This type of monitor is usually used

for personal exposure monitoring or work space monitoring.
April 2004 Page 28

Pathway Specific Risk - The risk associated with exposure to a chemical agent or a mixture of

chemicals via a specific pathway (e.g., inhalation of outdoor air).

Persistent, Bioaccumulative, and Toxic (PBT) Chemicals - Highly toxic, long-lasting

substances that can build up in the food chain to levels that are harmful to human and ecosystem

health. They are associated with a range of adverse health effects, including effects on the

nervous system, reproductive and developmental problems, cancer, and genetic impacts.

Percentile - Any one of the points dividing a distribution of values into parts each of which

contain 1/100 of the values. For example, the 75th percentile is a value such that 75 percent of

the values are less than or equal to it.

Persistence - Refers to the length of time a compound stays in the environment, once introduced. 

A compound may persist for very short amounts of time (e.g., fractions of a second) or for long

periods of time (e.g., hundreds of years).

Persistent Organic Pollutants (POPs) - Highly stable organic compounds used as pesticides or

in industry. They are also generated unintentionally as the byproduct of combustion and

industrial processes. POPs are a special problem because they persist in the environment,

accumulate in the tissues of living organisms, and are toxic to humans and wildlife. POPs with

these characteristics are typically semi-volatile, enabling them to move long distances and

condense over colder regions of the earth. These properties lead to increased concern for the

toxic effects that they can exert on a range of biota, in particular on top-of-the-food chain

species, even at extremely low levels in the ambient environment.

Personal Air Monitoring Device - Unlike a passive air toxics monitor, this device uses a pump

to draw the air sample through to measure exposure in the immediate vicinity of an individual. 

The air sample can be drawn across a reactive material (to be analayzed in a laboratory), or it can

be drawn through a direct-read detector.

Personal Monitoring - A measurement collected from an individual’s immediate environment

using active or passive devices to collect the samples.

Pervious Surface - A surface that can be penetrated (usually in reference to water; e.g., crop

land).

Pharmacodynamics - Process of interaction of pharmacologically active substances with target

sites, and the biochemical and physiological consequences leading to therapeutic or adverse

effects.

Pharmacokinetics - The study of the absorption, distribution, metabolism, and excretion of

chemicals in living organisms and the genetic, nutritional, behavioral, and environmental factors

that modify these parameters.

Photolysis - The breakdown of a material by sunlight; an important mechanism for the

degradation of contaminants in air, surface water, and the terrestrial environment.
April 2004 Page 29

Physical Factors - Manmade and/or natural characteristics or features that influence the

movement of pollutants in the environment (e.g., settling velocity, terrain effects).

Physiologically Based Pharmacokinetic (PBPK) Model - A computer model that describes

what happens to a chemical in the body of a human or laboratory animal. It describes how the

chemical gets into the body, where it goes in the body, how it is changed by the body, and how it

leaves the body.

Piscivorous - A species feeding preferably on fish.

Planning and Scoping - The process of determining the purpose, scope, players, expected

outcomes, analytical approach, schedule, deliverables, QA/QC, resources, and document

requirements for the risk assessment.

Plume - The visible or measurable presence of a contaminant in the atmosphere, once released

from a given point of origin (e.g., a plume of smoke from a forest fire).

Plume Height - The elevation to which a plume travels (i.e., the sum of the release height and

plume rise).

Plume Rise - The height to which a plume rises in the atmosphere from the point of release.

Plume Transport - The movement of a plume through the atmosphere and across land and

water features.

Plume Washout - The removal of a substance from the atmosphere via a precipitation event.

PM-10/PM-2.5. PM-10 or PM10 refers to particles in the atmosphere with a diameter of less

than ten or equal to 10 micrometers. PM-2.5 or PM2.5 refers to smaller particles in the air (i.e.,

less than or equal to 2.5 micrometers in diameter).

Point of Departure (PoD) - The dose-response point that marks the beginning of a low-dose

extrapolation. This point can be the lower bound on dose for an estimated incidence or a change

in response level from a dose-response model (BMD), or a NOAEL or LOAEL for an observed

incidence, or change in level of response.

Point of Exposure - The location of potential contact between an organism and a chemical or

physical agent.

Point of Release - Location of release to the environment.

Point Source (NEI sense) - A source of air pollution which can be physically located on a map.

Point Source (non-NEI sense) - A stack, vent, duct, pipe or other confined air stream from

which chemicals may be released to the air.

Pollutant Release and Transfer Registries (PRTRs) - The international equivalent to the

Toxics Release Inventory (TRI). PRTRs are data banks of recorded information of the releases
April 2004 Page 30

and transfers of toxic chemicals from industries, such as manufacturers, mining facilities,

processors, or government-owned and operated facilities.

Population Risk or Hazard - Population risk refers to an estimate of the extent of harm for the

population or population segment being addressed. It often refers to an analysis of the number of

people living at a particular risk or hazard level.

Potential Risk - Estimated likelihood, or probability, of injury, disease, or death resulting from

exposure to a potential environmental hazard.

Potential Dose - The amount of a compound contained in material swallowed, breathed, or

applied to the skin.

Practical Quantitation Limit - The lowest level of quantitation that can be reliably achieved

within specified limits of precision and accuracy during routine laboratory operating conditions.

Precision - A measure of the reproducibility of a measured value under a given set of

circumstances.

Present Scenario - Risk characterizations using present scenarios to estimate risks to individuals

(or populations) that currently reside in areas where potential exposures may occur (e.g., using

an existing population within some specified area).

Prevailing Wind - Direction from which the wind blows most frequently.

Prevention of Significant Deterioration (PSD) - An EPA program in which state and/or federal

permits are required in order to restrict emissions from new or modified sources in places where

air quality already meets or exceeds primary and secondary ambient air quality standards.

Primary Standard - A pollution limit based on health effects. Primary standards are set for

criteria air pollutants.

Probabilistic - A type of statistical modeling approach used to assess the expected frequency

and magnitude of a parameter by running repetitive simulations using statistically selected inputs

for the determinants of that parameter (e.g., rainfall, pollutants, flows, temperature).

Probabilistic Risk Assessment/Analysis - Calculation and expression of health risks using

multiple risk descriptors to provide the likelihood of various risk levels. Probabilistic risk results

approximate a full range of possible outcomes and the likelihood of each, which often is

presented as a frequency distribution graph, thus allowing uncertainty or variability to be

expressed quantitatively.
April 2004 Page 31

Probability Density Function (PDF) - The PDF is alternatively referred to in the literature as

the probability function or the frequency function. For continuous random variables, that is, the

random variables which can assume any value within some defined range (either finite or

infinite), the probability density function expresses the probability that the random variable falls

within some very small interval. For discrete random variables, that is, random variables which

can only assume certain isolated or fixed values, the term probability mass function (PMF) is

preferred over the term probability density function. PMF expresses the probability that the

random variable takes on a specific value.

Problem Formulation (in Ecological Risk Assessment) - The initial stage of a risk assessment

where the purpose of the assessment is articulated, assessment endpoints and a conceptual model

are developed, and a plan for analyzing and characterizing risk is determined.

Problem Statement - A statement of the perceived problem to be studied by the risk assessment.

Problem statements often also include statements about how the problem is going to be studied.

Public Health Consultation (Public Health Assessment) - See health consultation.

Public Health Assessment (PHA) - An evaluation of hazardous substances, health outcomes,

and community concerns at a hazardous waste site or other potential source of pollutants to

determine whether people could be harmed from coming into contact with those substances. The

PHA also lists actions that need to be taken to protect public health.

Public Health Advisory (in Public Health Assessment) - A statement made by a regulatory

agency that a release of hazardous substances or contamination by microbial pathogens poses an

immediate threat to human health. The advisory includes recommended measures to reduce

exposure and reduce the threat to human health.

Public Health Hazard Category (in Public Health Assessment) - Statements about whether

people could be harmed by conditions present at the site in the past, present, or future. One or

more hazard categories might be appropriate for each site. ATSDR’s five public health hazard

categories are no public health hazard, no apparent public health hazard, indeterminate public

health hazard, public health hazard, and urgent public health hazard.

Q

Qualitative Uncertainty Estimate - A detailed examination, using qualitative information, of

the systematic and random errors of a measurement or estimate.

Quality Assurance Project Plan - A document describing in comprehensive detail the

necessary quality assurance, quality control, and other technical activities that must be

implemented to ensure that the results of the work performed will satisfy the stated performance

criteria.

Quality Assurance - An integrated system of activities involving planning, quality control,

quality assessment, reporting and quality improvement to ensure that a product or service meets

defined standards of quality with a stated level of confidence.
April 2004 Page 32

Quality Control - The overall system of technical activities whose purpose is to measure and

control the quality of a product or service so that it meets the needs of its users. The aim is to

provide data quality that is satisfactory, adequate, and dependable.

R

Random Variable - A quantity which can take on any number of values but whose exact value

cannot be known before a direct observation is made. For example, the outcome of the toss of a

pair of dice is a random variable, as is the height or weight of a person selected at random from a

city phone book.

Receptor (modeling sense) - In fate/transport modeling, the location where impacts are

predicted.

Receptor (non-modeling sense) - The entity which is exposed to an environmental stressor.

Red Book - 1983 NRC publication entitled Risk Assessment in the Federal Government:

Managing the Process.

Reference Concentration (RfC) - An estimate (with uncertainty spanning perhaps an order of

magnitude) of a continuous inhalation exposure to the human population (including sensitive

subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.

Reference Dose (RfD) - An estimate (with uncertainty spanning perhaps an order of magnitude)

of a daily oral exposure to the human population (including sensitive subgroups) that is likely to

be without an appreciable risk of deleterious effects during a lifetime.

Reference Media Evaluation Guides (RMEG) - A type of comparison value derived by

ATSDR to protect the most sensitive populations. They do not consider carcinogenic effects,

chemical interactions, multiple route exposure, or other media-specific routes of exposure, and

are very conservative concentration values designed to protect sensitive members of the

population.

Regional/National Scale Assessment - An air monitoring network designed to assess from tens

to hundreds of kilometers, up to the entire nation.

Relative Potency Factor - The ratio of the toxic potency of a given chemical to that of an index

chemical.

Release Parameters - The specific physical characteristics of the release (e.g., stack diameter,

stack height, release flow rate, temperature).

Representativeness - The degree to which one or a few samples are characteristic of a larger

population about which the analyst is attempting to make an inference.
April 2004 Page 33

Reproductive Toxicity - The occurrence of biologically adverse effects on the reproductive

systems of females or males that may result from exposure to environmental agents. The toxicity

may be expressed as alterations to the female or male reproductive organs, the related endocrine

system, or pregnancy outcomes. The manifestation of such toxicity may include, but not be

limited to, adverse effects on onset of puberty, gamete production and transport, reproductive

cycle normality, sexual behavior, fertility, gestation, parturition, lactation, developmental

toxicity, premature reproductive senescence, or modifications in other functions that are

dependent on the integrity of the reproductive systems.

Residual Risk - The extent of health risk from air pollutants remaining after application of the

Maximum Achievable Control Technology (MACT).

Resources - Money, time, equipment, and personnel available to perform the assessment.

Risk (in the context of human health) - The probability of injury, disease, or death from

exposure to a chemical agent or a mixture of chemicals. In quantitative terms, risk is expressed

in values ranging from zero (representing the certainty that harm will not occur) to one

(representing the certainty that harm will occur). (Compare with hazard.)

Risk Assessor(s) - The person or group of people responsible for conducting a qualitative and

quantitative evaluation of the risk posed to human health and/or the environment by

environmental pollutants.

Risk Assessment - For air toxics, the scientific activity of evaluating the toxic properties of a

chemical and the conditions of human or ecological exposure to it in order both to ascertain the

likelihood that exposed humans or ecological receptors will be adversely affected, and to

characterize the nature of the effects they may experience.

Risk Assessment Forum - A standing committee of senior EPA scientists which was

established to promote Agency-wide consensus on difficult and controversial risk assessment

issues and to ensure that this consensus is incorporated into appropriate Agency risk assessment

guidance.

Risk Assessment Work Plan - A document that outlines the specific methods to be used to

assess risk, and the protocol for presenting risk results. The risk assessment workplan may

consist of one document or the compilation of several workplans that, together, constitute the

overall risk assessment workplan.

Risk Characterization - The last phase of the risk assessment process in which the information

from the toxicity and exposure assessment steps are integrated and an overall conclusion about

risk is synthesized that is complete, informative and useful for decision-makers. In all cases,

major issues and uncertainty and variability associated with determining the nature and extent of

the risk should be identified and discussed. The risk characterization should be prepared in a

manner that is clear, transparent, reasonable and consistent.

Risk Communication - The exchange of information about health or environmental risks among

risk assessors and managers, the general public, news media, and other stakeholders.
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Risk Management - The decision-making process that uses the results of risk assessment to

produce a decision about environmental action. Risk management includes consideration of

technical, scientific, social, economic, and political information.

Risk Manager(s) - The person or group responsible for evaluating and selecting alternative

regulatory and non-regulatory responses to risk.

Root Uptake - The uptake of compounds available in the soil and their transfer to the above

ground portions of the plant.

Route-to-Route Extrapolation - Calculations to estimate the dose-response relationship of an

exposure route for which experimental data do not exist or are inadequate, and which are based

on existing experimental data for other route(s) of exposure.

Runoff - That part of precipitation, snow melt, or irrigation water that runs off the land into

streams or other surface water. It can carry pollutants from the air and land into receiving

waters.

S

Sample - A small portion of something designed to evaluate the nature or quality of the whole

(for example, one or several samples of air used to evaluate air quality generally).

Sampling and Analysis Plan - An established set of procedures specifying how a sample is to

be collected, handled, analyzed, and the data validated and reported.

Sampling Frequency - The time interval between the collection of successive samples.

Science Advisory Board (SAB) - A group of recognized, non-EPA experts who advise EPA on

science and science policy.

Scenario Uncertainty - Uncertainty due to descriptive errors, aggregation errors, errors in

professional judgment, or incomplete analysis.

SCREEN3 - An air dispersion model developed to obtain conservative estimates of air

concentration for use in screening level assessments through the use of conservative algorithms

and meteorology.

Screening-level Risk Assessment - A risk assessment performed with few data and many

conservative assumptions to identify exposures that should be evaluated more carefully for

potential risk.

Secondary Production/Pollutant - Formation of pollutants in the atmosphere by chemical

transformation of precursor compounds.

Secondary Standard - A pollution limit based on environmental effects (e.g., damage to

property, plants, visibility). Secondary standards are set for criteria air pollutants.
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Sensitive Subgroups - Identifiable subsets of the general population that, due to differential

exposure or susceptibility, are at greater risk than the general population to the toxic effects of a

specific air pollutant (e.g., depending on the pollutant and the exposure circumstances, these may

be groups such as subsistence fishers, infants, asthmatics, or the elderly).

Settling Velocity/Rate - The maximum speed at which a particle will fall in still air. It is a

function of its size, density, and shape.

Silage - Stored vegetation used as feed for cattle.

Simulation - A representation of a problem, situation in mathematical terms, especially using a

computer.

Soil Volumetric Water Content - The soil-water content expressed as the volume of water per

unit bulk volume of soil.

Soil Dry Bulk Density - The mass of dry soil per unit bulk volume.

Soil Erosion - Detachment and movement of topsoil or soil material from the upper part of the

soil profile, by the action of wind or running water, especially as a result of changes brought

about by human activity, such as unsuitable or mismanaged agriculture.

Solar Radiation - Energy from the sun. Of importance to the climate system, solar radiation

includes ultra-violet radiation, visible radiation, and infrared radiation.

Solubility - The amount of mass of a compound that will dissolve in a unit volume of solution. 

Aqueous solubility is the maximum concentration of a chemical that will dissolve in pure water

at a reference temperature.

Source - Any place or object from which pollutants are released.

Source Category - A group of similar industrial processes or industries that are contributors to

releases of hazardous air pollutants. The 1990 amendments to the Clean Air Act (CAA) requires

that the EPA publish and regularly update a listing of all categories and subcategories of major

and area sources that emit hazardous air pollutants.

Source Characterization - The detailed description of the source (e.g., location, source of

pollutant releases, pollutants released, release parameters).

Spatial Variability - The magnitude of difference in contaminant concentrations in samples

separated by a known distance.

SPECIATE - EPA’s repository of Total Organic Compound (TOC) and Particulate Matter (PM)

speciated profiles for a variety of sources for use in source apportionment studies. The profiles

in the system are provided as a library of available profiles for source-receptor and source

apportionment type models, such as Chemical Mass Balance 8 (CMB8).
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Stable Conditions (in the Atmosphere) - Air with little or no tendency to rise, that is usually

accompanied by clear dry weather. Stable air holds, instead of dispersing, pollutants. [National

Weather Service, Southern Region Headquarters’ Jetstream Weather School,

http://www.srh.weather.gov/jetstream/append/glossary.htm and EPA, 1997: Terms of

Environment, http://www.epa.gov/OCEPAterms/.]

Stack - A chimney, smokestack, or vertical pipe that discharges used air.

Stack Release - The release of a chemical through a stack.

Stack Testing - The monitoring, by testing, of chemicals released from a stack.

Stakeholder(s) - Any organization, governmental entity, or individual that has a stake in or may

be impacted by a given approach to environmental regulation, pollution prevention, energy

conservation, etc.

Standard Industrial Classification (SIC) - A method of grouping industries with similar

products or services and assigning codes to these groups.

Standard Operating Procedure (SOP) - A established set of written procedures adopted and

used to guide the work of for a specific project. For example, an air monitoring study would

include SOPs on sample collection and handling and SOPs on analytical requirements and data

validation and reporting.

Standing Crop - The quantity of plant biomass in a given area, usually expressed as density (dry

mass per unit area) or energy content per unit area.

Stationary Source - A source of pollution that is fixed in space.

Steady-state Model - Mathematical model of fate and transport that uses constant values of

input variables to predict constant values of receiving media concentrations.

Stochastic - Involving or containing a random variable; involving probability or chance.

Stressor - Any physical, chemical, or biological entity that can induce adverse effects on

ecosystems or human health.

Stressor-response Profile or Relationship (in Ecological Risk Assessment] - The product of

characterization of ecological effects in the analysis phase of ecological risk assessment. The

stressor-response profile/relationship summarizes the data on the effects of a stressor and the

relationship of the data to the assessment endpoint.
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Structure-activity Relationship (SAR) - Mathematical or qualitative expression of the

relationships between biological activity or toxicity of a chemical to its chemical structure or

substructure. Ideally, such relationships can be formulated as Quantitative Structure Activity

Relationships (QSARs), in which some degree of predictive capability is present. [Air Risk

Information Support Center, OAQPS, March 1989: Glossary of Terms Related to Health,

Exposure, and Risk Assessment. EPA/450/3-88/016.]

Support Center for Regulatory Models (SCRAM) - An EPA website that is a source of

information on atmospheric dispersion models (e.g., ISCST3, SCREEN 3, and ASPEN) that

support regulatory programs required by the Clean Air Act. Documentation and guidance for

these computerized models are a major feature of this website. This site also contains computer

code, data, and technical documents that deal with mathematical modeling for the dispersion of

air pollutants.

Synergistic Effect - A situation in which the overall effect of two chemicals acting together is

greater than the simple sum of their individual effects.

T

Target Organ - The biological organ(s) most adversely affected by exposure to a chemical

substance (e.g., the site of the critical effect).

Target Organ Specific Hazard Index (TOSHI) - The sum of hazard quotients for individual

air toxics that affect the same organ/organ system or act by similar toxicologic processes

Temporal Variability - The difference in contaminant concentrations observed in samples taken

at different times.

Teratogenesis - The introduction of nonhereditary birth defects in a developing fetus by

exogenous factors such as physical or chemical agents acting in the womb to interfere with

normal embryonic development.

Terrain Effects - The impact on the airflow as it passes over complex land features such as

mountains.

Terrestrial Radiation - The total infrared radiation emitted by the earth and its atmosphere in

the temperature range of approximately 200 to 300 Kelvin. Terrestrial radiation provides a

major part of the potential energy changes necessary to drive the atmospheric wind system and is

responsible for maintaining the surface air temperature within limits of livability.

Thermal Turbulence - Turbulent vertical motions that result from surface heating and the

subsequent rising and sinking of air.

Threshold Dose/Threshold - The lowest dose of a chemical at which a specified measurable

effect is observed and below which it is not observed.
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Threshold Effect - An effect (usually an adverse health effect) for which there is an exposure

level below which the effect is not expected to occur.

Threshold Toxicant - A chemical for which there is an exposure level below which an adverse

health outcome is not expected to occur.

Tiered Analysis - An analysis arranged in layers/steps. Risk assessments/analyses are often

conducted in consecutive layers/steps that begin with a reliance on conservative assumptions and

little data (resulting in less certain, but generally conservative answers) and move to more studyarea specific data and less reliance on assumptions (resulting in more realistic answers). The

level of effort and resources also increases with the development of more realistic data.

Time-integrated Sample - Samples are collected over a period of time. Only the total pollutant

collected is measured, and so only the average concentration during the sampling period can be

determined.

Time-trend Study - Samples spaced in time to capture systematic temporal trends (e.g., a

facility might change its production methods or products over time).

Time-weighted Sum of Exposures - Used in inhalation exposure modeling. Provides a total

exposure from all different microenvironments in which a person spends time.

Toxic Air Pollutants - see hazardous air pollutant.

Toxicity - The degree to which a substance or mixture of substances can harm humans or

environmental receptors.

Toxicity Assessment - Characterization of the toxicological properties and effects of a chemical,

with special emphasis on establishment of dose-response characteristics.

Toxicity Test - Biological testing (usually with an cell system, invertebrate, fish, or small

mammal) to determine the adverse effects of a compound.

Toxicology - The study of harmful interactions between chemicals and biological systems.

Toxic Release Inventory (TRI) - Annual database of releases to air, land, and water, and

information on waste management in the United States of over 650 chemicals and chemical

compounds. This data is collected under Section 313 of the Emergency Planning and

Community Right to Know Act.

Trajectory - The track taken by a parcel of air as it moves within the atmosphere over a given

period.

Transformation - The change of a chemical from one form to another.

Transparency - Conducting a risk assessment in such a manner that all of the scientific

analyses, uncertainties, assumptions, and science policies which underlie the decisions made

throughout the risk assessment are clearly stated (i.e., made readily apparent).
April 2004 Page 39

Turbulence - Irregular motion of the atmosphere, as indicated by gusts and lulls in the wind.

U

Uncertainty - Uncertainty represents a lack of knowledge about factors affecting

exposure/toxicity assessments and risk characterization and can lead to inaccurate or biased

estimates of risk and hazard. Some of the types of uncertainty include scenario uncertainty,

parameter uncertainty, and model uncertainty.

Uncertainty analysis - A detailed examination of the systematic and random errors of a

measurement or estimate (in this case a risk or hazard estimate); an analytical process to provide

information regarding the uncertainty.

Uncertainty Factor (UF) - One of several, generally 10-fold factors, used in operationally

deriving the RfD and RfC from experimental data. UFs are intended to account for (1) the

variation in sensitivity among the members of the human population; (2) the uncertainty in

extrapolating animal data to humans, i.e., interspecies variability; (3) the uncertainty in

extrapolating from data obtained in a study with less-than-lifetime exposure to lifetime exposure,

i.e., extrapolating from subchronic to chronic exposure; (4) the uncertainty in extrapolating from

a LOAEL rather than from a NOAEL; and (5) the uncertainty associated with extrapolation from

animal data when the data base is incomplete.

Universal Soil Loss Equation - An equation used to predict the average annual soil loss per unit

area per year.

Unit Risk Estimate (URE) - The upper-bound excess lifetime cancer risk estimated to result

from continuous exposure to an agent at a concentration of 1μ g/L in water, or 1 μg/m3 in air.

The interpretation of unit risk would be as follows: if the water unit risk = 2 x 10-6 μg/L, 2 excess

tumors may develop per 1,000,000 people if exposed daily for a lifetime to 1 μg of the chemical

in 1 liter of drinking water.

Unstable Conditions (in the Atmosphere) - An atmospheric state in which warm air is below

cold air. Since warm air naturally rises above cold air (due to warm air being less dense than

cold air), vertical movement and mixing of air layers can occur.

Uptake - The process by which a substance crosses an absorption barrier and is absorbed into

the body.

Urban Scale Assessment - An air monitoring network designed to assess the overall, citywide

conditions with dimensions on the order of 4 to 50 kilometers. This scale would usually require

more than one site for definition.
April 2004 Page 40

V

Vapor - The gas given off by substances that are solids or liquids at ordinary atmospheric

pressure and temperatures.

Variability - Refers to the observed differences attributable to true heterogeneity or diversity in

a population or exposure parameter. Examples include human physiological variation (e.g.,

natural variation in body weight, height, breathing rate, drinking water intake rate), weather

variability, variation in soil types and differences in contaminant concentrations in the

environment. Variability is usually not reducible by further measurement of study, but it can be

better characterized.

Volatilization/Vapor Release - The conversion of a liquid or solid into vapors.

Volume Source - In air dispersion modeling, a three dimensional volume from which a release

may occur (e.g., a gas station modeled as a box from which chemicals are emitted).

W

Watershed - The land area that drains into a stream; the watershed for a major river may

encompass a number of smaller watersheds that ultimately combine at a common point.

Weight-of-Evidence (WOE) - A system for characterizing the extent to which the available data

support the hypothesis that an agent causes an adverse health effect in humans. For example,

under EPA’s 1986 cancer risk assessment guidelines, the WOE was described by categories “A

through E,” Group A for known human carcinogens through Group E for agents with evidence

of noncarcinogenicity. The approach outlined in EPA’s proposed guidelines for carcinogen risk

assessment (1996 and updates) considers all scientific information in determining whether and

under what conditions an agent may cause cancer in humans, and provides a narrative approach

to characterize carcinogenicity rather than categories.

White Book - 1996 Presidential Commission on Risk Assessment and Risk Management

(CRARM) publication entitled Risk Assessment and Risk Management in Regulatory DecisionMaking.

Wind Rose - A graphical display showing the frequency and strength of winds from different

directions over some period of time.