GUIDANCE FOR SCHOOL 

SITE RISK ASSESSMENT 

PURSUANT TO HEALTH 

AND SAFETY CODE 

SECTION 901(f): 

GUIDANCE FOR ASSESSING 

EXPOSURES AND HEALTH 

RISKS AT EXISTING AND 

PROPOSED SCHOOL SITES

 FINAL REPORT 

 February 2004

 Integrated Risk Assessment Section 

 Office of Environmental Health Hazard Assessment 

 California Environmental Protection Agency 
 

PPaaggeeLLeeffttBBllaannkk
 i 

Guidance for Assessing Exposures and Health Risks 

at Existing and Proposed School Sites Pursuant to 

Health and Safety Code §901(f): 

FINAL REPORT 

February 2004 

Integrated Risk Assessment Section 

Office of Environmental Health Hazard Assessment 

California Environmental Protection Agency 

 1001 I Street, 12th Floor 

 P.O. Box 4010 

 Sacramento, California 95812-4010 
 ii 

LIST OF CONTRIBUTORS 

Primary Author

Jim Carlisle, D.V.M., M.Sc. 

Collaborators

Deborah Oudiz, Ph.D. 

Thomas Booze, Ph.D. 

Gerald Chernoff, Ph.D. 

David Chan, D.Env. 

Susan Knadle, Ph.D., DABT 

Web-site Posting

Laurie Monserrat 
 iii 

Executive Summary 

This guidance document was prepared to comply with California Health and Safety Code Section 

901(f), which requires the Office of Environmental Health Hazard Assessment (OEHHA) to 

develop and publish a guidance document for use by the Department of Toxic Substances Control 

and other state and local environmental and public health agencies to assess exposures and health 

risks at existing and proposed school sites. It presents methodology for estimating exposure of 

school users to toxic chemicals found as contaminants at existing and proposed school sites, and the 

health risks from those exposures. It incorporates exposure factors unique to the school 

environment, and considers the activity patterns of children from birth through age 18, and of adult 

school employees. It discusses uncertainties and steps that can be taken to address these 

uncertainties. 
 iv 

Table Of Contents 

Introduction and Purpose .................................................................................................... 1

Schools Conceptual Site Model........................................................................................... 2

Potentially Exposed Sub-populations at Schools ................................................................ 4

Exposure and Source Media at Schools ............................................................................. 5

Exposure Pathways............................................................................................................. 6

Model Parameters ............................................................................................................. 11

Risk Assessment............................................................................................................... 23

Sensitivity Analysis............................................................................................................ 24

Uncertainty Analysis.......................................................................................................... 26

REFERENCES.................................................................................................................. 31

Appendix 1: Summary and Interpretation of Results of RTI Study .................................A1-1

Appendix 2: Comments and Responses ....................... A2-Error! Bookmark not defined.
2/5/2004 1

Introduction and Purpose 

Section 901(f) of the California Health and Safety Code states that: “On or before December 31, 

2002, the Office (of Environmental Health Hazard Assessment, OEHHA) shall publish a guidance 

document, for use by the California Department of Toxic Substances Control (DTSC) and other 

state and local environmental and public health agencies to assess exposures and health risks at 

existing and proposed school sites. The guidance document shall include, but not be limited to, all 

of the following: 

(A) Appropriate child-specific routes of exposure unique to the school environment, in addition to 

those in existing exposure assessment models. 

(B) Appropriate available child-specific numerical health effects guidance values and plans for the 

development of additional child-specific numerical health effects guidance values. 

(C) The identification of uncertainties in the risk assessment guidance and those actions that should 

be taken to address those uncertainties.” 

Pursuant to HSC§901(f)(A) and (C), OEHHA is proposing these guidelines for multimedia, 

multipathway, risk assessment at existing and proposed school sites. HSC§901(f)(B) is addressed 

in a separate document (OEHHA, 2002a). 

Need for Guidance 

Children differ from adults anatomically, physiologically, and behaviorally in ways that may affect 

their exposure or their response to exposure to environmental contaminants. For example, on a 

body weight basis, children require more oxygen, food, and water, and have a higher skin surface 

area than adults. Children’s activity patterns are different. Children are in a period of continuous 

change as they move from infancy through puberty and into adulthood. Most previous guidance has 

focused on residential or occupational scenarios, and has treated childhood as a homogeneous life 

stage. Recognizing that children are undergoing rapid development, this guidance addresses the 

differences between children and adults, and between the school setting and other settings. 

Scope of Guidance 

As required by HSC§901(f), this guidance is intended to support assessment of chemical exposures 

and health risks at existing and proposed school sites, to characterize uncertainty in assessing 

exposure and risk in the school setting, and to suggest which areas are most in need of further 

research. It is intended to be sufficiently flexible to accommodate a variety of situations: It may be 

used to support the evaluation of the suitability of a site for future school construction or to support 

the assessment of toxicological risk at an existing school site. These contrasting situations present 

different opportunities to measure contaminant concentrations in environmental media. In the first 

scenario, soil, soil gas, air, and ground water may be available for sampling, but concentrations in 

indoor media will have to be estimated. In the second scenario, indoor media such as surface dust 

and air may be available for sampling. By sampling these media, additional sources of 

contaminants, such as chemicals in building materials and furnishings and chemicals used in 

school operations, can be included in the assessment. 

This guidance specifies toxicity criteria that should be used in assessing risk and hazard. It only 

addresses risk assessment for schools; it does not address chemical exposures that students and staff 

may receive outside the school setting. It does not include project-specific guidance such as 

selection of chemicals of potential concern, site characterization, sampling and analyses strategies, 

and determination of appropriate exposure point concentrations. This guidance does not provide 
2/5/2004 2

risk management application or decision-making criteria. For information regarding the 

application of this document to regulatory programs, contact DTSC or other agencies that may 

utilize this guidance as a part of their regulatory program. This guidance assumes that the user is 

familiar with the principles of chemical risk assessment; it is not intended to provide basic 

instruction in risk assessment. In-progress research is expected to result in 

information that may render this guidance out-dated. The guidance will be 

revised as new information becomes available. 

Tiered Approach 

The model is designed to support a tiered approach to assessment of risk. It can be used in screening 

(Tier 1) mode, with conservative default input values and all pathways included (except pathways 9, 

10, and 12 when these are not appropriate). It also accommodates a Tier 2 analysis using usersupplied site-specific input parameters and/or elimination of pathways that are not appropriate for a 

given site. In some cases, it may be appropriate to add in additional sources of chemicals in the 

environment. For example there may be off-site emissions that may impact on-site concentrations. 

Representative measured contaminant concentrations in various environmental 

media may be used in lieu of modeled values. Case-specific approaches may be 

appropriate for these situations in lieu of, or in addition to default methods. Users should document 

and justify all departures from default conditions so that reviewers can duplicate the modeling 

conditions and verify the result. Use of this guidance in Tier 1 or Tier 2 mode should be discussed 

with and approved by DTSC or other regulatory programs for which the risk assessment is being 

conducted. 

Mathematical models 

Mathematical models can be used to predict exposures and risks to specified groups of people from 

chemicals in specified environmental media under defined conditions. This guidance lays out a 

modeling approach to predicting exposures and risks to preschoolers, students, teachers and other 

school personnel, and their offspring, from chemicals in the soil, shallow ground water, and air at 

the school site. A separate document, (OEHHA, 2003) presents a spreadsheet adaptation of this 

model. The use of this spreadsheet (SchoolScreen.xls) is optional, and the user retains the 

responsibility to ensure that the model parameters including toxicity parameters are current and 

correct. The model is applicable to most chemicals, the notable exception being lead. OEHHA 

recommends the use of the DTSC Lead Risk Assessment Spreadsheet (http://www.dtsc.ca.gov/

ScienceTechnology/ledspred.html) for assessment of exposure to lead at school sites. 

Schools Conceptual Site Model 

A conceptual site model includes the contaminated environmental media, the movement of the 

chemicals within and between environmental compartments (intermedia transfers), the 

concentration of the chemical(s) in various personal exposure media, exposure pathways and routes, 

exposed populations, and the amount of the chemical(s) taken into the body. These movements and 

concentrations may be described by a series of mathematical relationships. This guidance proposes 

a series of such mathematical relationships, which are described below. 

As depicted in Figure 1, this model considers contaminated soil, ground water, and unspecified offsite sources as primary source media. Contaminated soil can be an exposure medium (by ingestion 

or dermal contact) and can be a source for transfer into other media. Chemicals can vaporize from 

soil into indoor or outdoor air and can be entrained into the suspended particle phase. As a default, 
2/5/2004 3

soil is treated as the source of outdoor suspended particulate matter, but a measured concentration in 

on-site particulate matter may replace the calculated value. By this means, total particle-bound 

contaminants from off-site and on-site sources can be included. Vapors can be inhaled indoors or 

outdoors. Soil can be transported indoors, where it becomes a component of interior dust. 

CONTAMINANT

SOURCES

ENVIRONMENTAL

MIGRATION

EXPOSURE

MEDIUM

EXPOSURE

MEDIUM ROUTE OF

EXPOSURE

FIGURE 1: SITE CONCEPTUAL MODEL FOR EXISTING OR PROPOSED SCHOOL SITES

ON-SITE

SOIL

GROUND

WATER

VAPORIZATION

VAPORIZATION

PARTICLE

EMISSION

MATERNAL MILK INGESTION

INGESTION

SURFACE SOIL

PARTICLE

EMISSION UPWIND OFFSITE SOURCES

VAPORIZATION

OUTDOOR AIR* INHALATION

School &

preschool

children

Nursing

School infants

Staff

School

Staff

INDOOR AIR* INHALATION Y Y Y

DERMAL

CONTACT

INGESTION

INDOOR SURFACE

DUST*

DERMAL

CONTACT TRACKED-IN &

WIND-BLOWN

MATERNAL DOSE

(sum of all pathways)

POTENTIALLY EXPOSED

POPULATIONS

INGESTION

* Available representative measured concentrations in these exposure media may be substituted for the model-based estimates. 

Exposure to this dust can be by ingestion or dermal contact, or it can be re-suspended and inhaled. 

Ground water is treated as a source of drinking water (if pathway 12 is selected) and as a source of 

chemicals that may vaporize and contribute to soil vapors. However, a measured soil vapor 

concentration may be substituted for the value estimated from soil and ground water concentrations. 

As depicted in Figure 2, hazard quotients and incremental risks are estimated for each chemical; 

then the hazard quotients and incremental risks associated with the individual chemicals are added 

to arrive at the total hazard index and total risk. If the total hazard index does not exceed one, then 

it may be assumed that the non-cancer toxic effects are unlikely and further analysis of non-cancer 

effects is not necessary. If the total hazard index exceeds one, then it may be useful to separate 

chemicals by target organ and/or mode of action and add the hazard quotients of only those 

chemicals that are likely to act in an additive manner. This target organ/mode of action analysis 

should be documented. 
2/5/2004 4

FIGURE 2: RISK ESTIMATION FOR EXISTING OR PROPOSED SCHOOL SITES

INGESTION OF

MOTHER’S MILK

SOIL INGESTION

INHALATION

OF VAPORS

INHALATION

OF PARTICLES

DERMAL CONTACT

WITH SOIL

WATER

INGESTION

DERMAL CONTACT

WITH DUST

INHALATION

DOSE

DERMAL

UPTAKE

AND

INGESTION

DOSE

CANCER

POTENCY

(INHALATION)

CANCER

POTENCY

(INGESTION)

REFERENCE

DOSE

(INHALATION)

REFERENCE

DOSE

(INGESTION)

SINGLECHEMICAL

CANCER

RISK

SINGLECHEMICAL

NONCANCER

HAZARD

QUOTIENT

MULTIPLECHEMICAL

CANCER

RISK

MULTIPLECHEMICAL

NONCANCER

HAZARD

INDEX*

DUST INGESTION

 * Additivity considered as appropriate, depending on target organ and/or mode of action

Potentially Exposed Sub-populations at Schools 

The model addresses the following school sub-populations. With the exception of pregnant or 

nursing women, genders are not separated. 

1) Students from kindergarten through high school

2) Staff

3) Pregnant or nursing women

4) Pre-schoolers aged one through four

5) Nursing infants less than one year of age in day care at the school site whose mothers are 

students or staff. No sources of contaminants other than those associated with the school 

environment are considered in calculating the concentration in breast milk.

Other groups that may use or visit the school facilities, such as parents and members of the general 

community are not explicitly considered. Since their visits would be less frequent than the students 

and staff, their long-term average exposure would be less than that of the groups listed above. Also, 

it would be possible to assess exposure of nursing infants who did not spend time at the school site, 

but whose mothers were students or staff. However, these children would be exposed less than 

infants described above (group #5). 
2/5/2004 5

Exposure and Source Media at Schools 

Potential Source Media at Schools 

Soil

Soil is often the primary environmental medium to be contaminated when toxic materials are spilled 

or dumped. Soil may be a source medium for contamination of other media such as surface dust or 

airborne particulate matter or vapors. The model can estimate the concentrations of contaminants in 

soil gas based on the concentrations in soil matrix and/or ground water, or these concentrations may 

be measured directly. Soil may be directly contaminated by spills or leaks occurring on the site, or 

may be contaminated by wet or dry deposition from off-site sources. The model does not explicitly 

consider deposition; rather it is assumed that this type of contamination will be included in the 

results of the on-site soil sampling. 

Ground water 

Ground water may be a source of volatile contaminants in indoor and outdoor air. Off-site ground 

water plumes may need to be considered if they are likely to move on-site. 

Off-site sources

Atmospheric emission sources within 1⁄2 mile of the site that have the potential to contaminate onsite air may be important in estimating overall toxic exposures. Examples could include fixed 

facilities with known emissions and mobile sources such as highways, heavily traveled streets, or 

vehicle loading areas. Modeling should evaluate atmospheric concentrations during time frames that 

reflect conditions to which school users will be exposed. 

Potential Exposure Media at Schools 

Soil 

Students and others at school sites may be exposed to soil on the campus. Bare dirt may cover a 

portion of the campus area. Playgrounds and athletic fields may have patches of bare dirt. Even 

paved areas may contain a layer of soil. Soil may be ingested or may contaminate the skin. 

Dust 

Interior surfaces including floors, desks, shelves, and windowsills, may accumulate a layer of dust 

between cleanings. This dust may come from multiple sources, including tracked-in or blown-in 

outdoor soil. Dust may be ingested or may contaminate the skin. 

Air 

Air may contain vapor-phase and/or particulate contaminants. The multiple sources of vapors and 

particles may include on-site and off-site sources. The model estimates indoor and outdoor 

concentrations of pollutants in the particulate or vapor phases based on concentrations in on-site soil 

and/or ground water. Representative measured concentrations of vapor-phase or particulate 

contaminants in outdoor air may be substituted for estimated values. At existing schools 

corresponding indoor measurements may also be used. Measurements should be made in an 

appropriate manner to reflect conditions that school users will be exposed to. 

Drinking Water 

Since ground water may be a source of drinking water (RTI, 2003), the model includes an optional 

equation for assessing exposure via the drinking water pathway. This equation does not predict 

concentrations of contaminants in ground water, but relies on measured values. 

Air contamination by vapor- or particle-phase pollutants originating off-site

Depending on program requirements, modeled on-site concentrations of contaminants originating 

from off-site sources may be added to estimated concentrations of contaminants from on-site 
2/5/2004 6

sources. Modeling should evaluate atmospheric concentrations during time frames that reflect 

conditions to which school users will be exposed. If on-site concentrations are measured under 

representative meteorological conditions (as opposed to modeled), contributions from these off-site 

contamination sources generally should not be added to the resulting measured concentrations, 

because the measured concentrations should include the off-site component. 

Other potential exposure media 

In a recent survey, nearly 8 percent of schools reported that produce for human consumption was 

grown at the site (RTI, 2003). OEHHA considered including food grown in the site soil as an 

exposure pathway, however, a variety of simulations using an array of chemicals representing 

various chemical classes (including volatile organic chemicals, lipophilic organic chemicals, and 

heavy metals) showed that the food pathway never contributed as much as 1 percent of the total risk 

or hazard, even assuming up to 5 percent of the diet being site-grown produce. Therefore, food is 

not included as an exposure medium in the schools exposure model. 

Building materials and indoor products may be important sources of indoor exposure to toxic 

constituents at schools. It may be appropriate to include these sources of chemical exposures 

in the overall assessment of overall hazards and risks at existing schools. Typically these 

assessments would be based on measured atmospheric concentrations in classrooms and other 

indoor areas, and estimated risks, using the same exposure parameters, would be added to 

site-related risks. Hazards would be additive among chemicals sharing a common target organ 

and/or mode of action.

Background (non-site-related) Exposures 

A small incremental dose of a toxic constituent that would otherwise be of no concern, may become 

a concern if the exposed person is already receiving a background dose of the constituent and the 

combined exposures may exceed the toxic threshold. For this reason, risk managers may wish to 

take background exposures into account in their decision-making process. This is of primary 

importance for non-carcinogenic toxic effects, which are generally thought to exhibit a toxic 

threshold. For carcinogens, which are generally treated as exhibiting no threshold, the incremental 

risk posed by a given exposure to a carcinogen does not depend on the individual’s background 

exposure to that or any other carcinogen. 

Exposure Pathways 

Exposure pathways can be direct or indirect. A direct exposure pathway consists of a contaminated 

environmental medium and an exposure route by which the contaminated medium contacts and 

enters the body (e.g. ingestion of contaminated soil, pathway 1, below). An indirect exposure 

pathway consists of a contaminated environmental medium, one or more transfers between 

environmental media and ultimately an exposure medium, and an exposure route by which the 

exposure medium contacts and enters the body (e.g. transfer of chemicals from contaminated soil to 

indoor dust and ingestion of indoor dust, pathway 3, below). 

Exposure Pathway Equations 

Figure 1 depicts the movements of contaminants into and between environmental and exposure 

media. These movements and the resulting exposures may be described by a series of mathematical 

relationships. This model includes up to 12 pathways by which school users could be exposed to 

chemicals at the school site. Each pathway can be represented by an equation which describes a 

concentration in the source medium, up to two transfer factors that relate the concentration in the 

source medium to a concentration in an intermediate or exposure medium, and a contact rate that 
2/5/2004 7

describes the daily intake of, or contact with, the exposure medium. When the exposure pathway is 

direct (i.e. the environmental medium and the exposure medium are the same, such as ingestion of 

outdoor soil), then no transfer factors are required. The annual average daily dose associated with 

each of these pathways is estimated as follows: 

1. Ingestion of outdoor soil: 

D = CS * IS * AI * FS * FO * EF/(BW * 365 (days/year)), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

IS = daily soil/dust ingestion (kgsoil/day) 

AI = route-specific absorption factor for ingestion (unitless) 

FS = fraction of daily soil/dust ingestion and dermal contact that occurs at school (unitless) 

FO = fraction of daily soil/dust ingestion and dermal contact that occurs outdoors (unitless)

BW = age-specific body weight (kg) 

EF = exposure frequency (days/year) 

2. Dermal contact with outdoor soil: 

D = CS * AD * FS * FO * DS * EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

AD = route-specific absorption factor (unitless) 

FS = fraction of daily soil/dust ingestion and dermal contact that occurs at school (unitless) 

FO = fraction of daily soil/dust ingestion and dermal contact that occurs outdoors (unitless)

DS = Daily dermal contact with soil/dust (kgsoil/kgBW/day) = ∑(ABP * LBP), where 

ABP = body-part-specific area (cm2

/kg) 

LBP = body-part-specific skin loading (kgsoil/cm2/day) 

EF = exposure frequency (days/year) 

3. Migration of chemicals from outdoor soil to indoor dust; ingestion of indoor dust: 

D = CS * TFSD * IS * AI * FS * FI * EF/(BW * 365 (days/year)), where: 

D = Pathway-specific annual average daily dose of contaminant (μg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

TFSD = Transfer factor from soil to indoor dust ((mg/kgdust)/(mg/kgsoil)) 

IS = daily soil/dust ingestion (kgsoil/day) 

AI = route-specific absorption factor for ingestion (unitless) 

FS = fraction of daily soil/dust ingestion and dermal contact that occurs at school (unitless) 

FI = fraction of school soil/dust ingestion that occurs indoors (unitless)

BW = age-specific body weight (kg) 

EF = exposure frequency (days/year) 
2/5/2004 8

4. Migration of chemicals from outdoor soil to indoor dust; dermal contact with indoor dust: 

D = CS * TFSD * AD * FS * FI * DS * EF / 365 (days/year), where 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

TFSD = Transfer factor from soil to indoor dust ((mg/kgdust)/(mg/kgsoil)) 

AD = route-specific absorption factor (unitless) 

FS = fraction of daily soil/dust ingestion and dermal contact that occurs at school (unitless) 

FI = fraction of daily soil/dust dermal contact that occurs indoors (unitless)

DS = Daily dermal contact with soil/dust (kgsoil/kgBW/day) = ∑(ABP * LBP), where 

ABP = body-part-specific area (cm2

/kgBW) 

LBP = body-part-specific skin loading (g/cm2) 

EF = exposure frequency (days/year) 

5. Suspension of soil particles in outdoor air; inhalation of suspended particulate matter (PM10) in 

outdoor air: 

D = CS * TFPM/S * PM10 * BO * TO * AIn * EF / 365 (days/year), where 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

PM10 = Respirable particle load for outdoor air due to resuspension of site soil (kgPM10/Lair) 

BO = Body-weight-normalized breathing rate outdoors (L/min/kgBW) 

TO = Time outdoors at school daily (min/day) 

AIn = route-specific absorption factor (unitless) 

EF = exposure frequency (days/year) 

TFPM/S = Ratio of the concentration of contaminant in outdoor PM10 originating from site soils 

to the concentration of contaminant in soil ((mg/kgPM10)/(mg/kgsoil)) 

A representative measured value for concentration of a chemical in outdoor PM10 may replace the 

value estimated from soil data; in that case the equation becomes: 

D = CPM10 * PM10 * BO * TO * AIn* EF / 365 (days/year), where 

CPM10 = Measured concentration in PM10 (μg/g) 

6. Suspension of respirable indoor dust particles (PM10) in indoor air; inhalation of PM10 in indoor 

air: 

D = CS * TFS/D * TFPM/D * SF * BI * TI * AIn* EF / 365 (days/year), where 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

SF = Respirable particle load for indoor air due to resuspension of dust particles (kgPM10/Lair) 

BI = Weight-normalized breathing rate indoors (Lair/min/kg) 

TI = Time indoors at school daily (min/day) 

AIn = route-specific absorption factor (unitless) 

TFPM/D = Ratio of the concentration of contaminant in indoor PM10 to the concentration of 

contaminant in indoor surface dust ((mg/kgPM10)/(mg/kgdust)) 

TFSD = Transfer factor from soil to indoor dust ((mg/kgdust)/(mg/kgsoil)) 

EF = exposure frequency (days/year) 
2/5/2004 9

7a. Vaporization of volatile chemicals from the soil; penetration of vapors into building interior; 

inhalation of vapors mixed with indoor air:

D = CS * α * VCS * CF * BI * TI * AIn* EF / 365 (days) /year, where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

α = Ratio of chemical concentration in indoor air to that in soil vapor (unitless)1

VCS = Volatilization factor from soil (gsoil/Lvapor) 

CF = Conversion factor (0.001 kg/g) 

BI = Weight-normalized breathing rate indoors (L/min/kg) 

TI = Time indoors at school daily (min/day) 

AIn = route-specific absorption factor for inhalation (unitless) 

EF = exposure frequency (days/year) 

A measured soil vapor concentration may be used in place of the value estimated from soil matrix 

data.1

 In that case the equation becomes: 

D = CSV * α * BI * TI * AIn, where: 

CSV = concentration in soil vapor (mg/L) and 7a and 7b collapse into a single pathway 7. 

7b. Vaporization of volatile chemicals from shallow ground water; penetration of vapors into building 

interior; inhalation of vapors mixed with indoor air: 

D = CGW * α * VCGW * CF * BI * TI * AIn* EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CGW = Concentration of contaminant in ground water (mg/L) 

α = Ratio of chemical concentration in indoor air to that in soil vapor (unitless)1

VCGW = Volatilization factor from ground water (mlwater/Lvapor) 

CF = Conversion factor (0.001 L/ml) 

BI = Weight-normalized breathing rate indoors (L/min/kg) 

TI = Time indoors at school daily (min/day) 

AIn = route-specific absorption factor for inhalation (unitless) 

EF = exposure frequency (days/year) 

A measured soil vapor concentration may be used in place of the value estimated from ground water 

data.1

 In that case the equation becomes: 

D = CSV * α * BI * TI * AIn, where: 

CSV = concentration in soil vapor (mg/L), and 7a and 7b collapse into a single pathway 7. 

8. Inhalation of chemicals vaporized from outdoor soil: 

D = CS * 1/VF * CF * (BO * TO + BI * TI) * AIn* EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CS = Concentration of contaminant in soil (mg/kgsoil) 

VF = Volatilization Factor (ratio of concentration in air to concentration in soil)(Lair/gsoil) 

CF = Conversion factor (0.001 kg/g) 

BO = Weight-normalized breathing rate outdoors (L/min/kg) 

TO = Time outdoors at school daily (min/day) 

BI = Weight-normalized breathing rate indoors (L/min/kg)2

TI = Time indoors at school daily (min/day)2

AIn = route-specific absorption factor (unitless) 

EF = exposure frequency (days/year) 

1 The decision as to which one to use should be made in consultation with the lead agency for the project. 

2 Assumes that HVAC system circulates outdoor air to the indoor spaces. 
2/5/2004 10

9. Inhalation of contaminants in vapors that originate off-site**:

D = CA * (BI * TI + BO * TO) * AIn* EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CAV = Concentration of contaminant vapor in site air (mg/L) 

BI = Weight-normalized breathing rate indoors (L/min/kg) 

TI = Time indoors at school daily (min/day) 

BO = Weight-normalized breathing rate outdoors (L/min/kg) 

TO = Time outdoors at school daily (min/day) 

AIn = route-specific absorption factor for inhalation (unitless) 

EF = exposure frequency (days/year) 

This pathway accommodates modeled on-site concentrations from off-site sources. It is independent 

of (and therefore added to) modeled on-site concentrations. However, representative on-site 

concentrations measured under conditions that would capture contaminants originating both off-site 

and on-site, should include the contribution from both sources and therefore would replace modeled 

concentrations based on on- and off-site sources. 

10. Inhalation of contaminants in suspended particles that originate off-site. 

D = CAP * (BI * TI + BO * TO) * AIn* EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CAP = Concentration of particulate contaminant in site air (mg/L) 

BI = Weight-normalized breathing rate indoors (L/min/kg) 

TI = Time indoors at school daily (min/day) 

BO = Weight-normalized breathing rate outdoors (L/min/kg) 

TO = Time outdoors at school daily (min/day) 

AIn = route-specific absorption factor for inhalation (unitless) 

EF = exposure frequency (days/year) 

This pathway accommodates modeled on-site concentrations from off-site sources. It is independent 

of (and therefore added to) modeled on-site concentrations. However, representative on-site 

concentrations measured under conditions that would capture contaminants originating both off-site 

and on-site, should include the contribution from both sources and therefore would replace modeled 

concentrations based on on- and off-site sources. This pathway may be inappropriate for some 

programs. 

11. Ingestion of contaminants in breast milk (only for infants up to one year old)

D = CBM * IBM * AI* EF / 365 (days/year), where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CBM = Contaminant concentration in breast milk (mg/kgmilk), estimated as: 

BBM * BWM * maternal annual average daily dose (mg/kg/day) where: 

 BBM = breast milk biotransfer factor ((mg/kg)/(mg/day)) 

 BWM = Maternal body weight (kg) 

IBM = Age-specific, weight normalized daily breast milk ingestion (kgmilk/kgBW/day) 

AI = route-specific absorption factor for ingestion (unitless) 

EF = exposure frequency (days/year) 
2/5/2004 11

12. Ingestion of contaminated drinking water:

D = CDW * IDW * FS * AI * EF / 365 (days/year); where: 

D = Pathway-specific annual average daily dose of contaminant (mg/kgBW/day) 

CDW = User-supplied contaminant concentration in school drinking water (mg/L). 

IDW = Age-specific, weight normalized daily drinking water ingestion (ml/day/kg) 

FS = fraction of daily water ingestion that occurs at school (unitless) 

AI = Ingestion absorption factor (unitless) 

EF = exposure frequency (days/year) 

** Vapor inhalation pathways may be omitted for chemicals whose boiling point exceeds 600oK

Each of these equations gives a pathway-specific annual average daily dose of the chemical in 

question. Doses via all pathways that involve the same exposure route (e.g. ingestion) are added 

together to determine the route-specific annual average daily dose. The latter is divided by the 

route-specific reference dose (RfD) to arrive at the route-specific hazard quotient (HQ). Dermal 

exposures are usually combined with ingestion exposures. The route-specific HQs are added to 

give the chemical-specific HQ. In a screening analysis, the chemical-specific HQs for each 

chemical are added to give the Hazard Index. In a more detailed (tier 2) analysis, target organs and 

mechanisms of toxic action may be considered in determining the appropriateness of adding the 

HQs for individual chemicals. 

To compute cancer risk, the route-specific annual average daily dose is converted to a route-specific 

lifetime average daily dose by multiplying by the fraction of a lifetime represented by each 

exposure scenario (ED/AT), i.e. 1/70 of a lifetime for each year of exposure. The route-specific 

lifetime average daily dose is multiplied by the route-specific cancer potency factor to obtain the 

risk for that pathway. The route-specific risks for relevant pathways are added to give the chemicalspecific risk. Finally the chemical-specific risks for each chemical are added to give the total cancer 

risk. Annual risks may be added for a series of years to obtain the total risk for that period. 

Model Parameters 

The pathway equations above require numerical values or parameters, which can be divided into 

“intermedia transfer factors” and “exposure factors,” which are described below and summarized in 

a table at the end of each section: 

Intermedia Transfer Factors 

When the environmental medium and the exposure medium are not the same, one or more 

intermedia transfer factors are involved. Transfer factors describe the relationship between the 

concentration of a chemical in one compartment and the concentration of the chemical in another 

compartment, or, in some cases, the concentration of one medium in another, such as the amount of 

suspended particulate matter in the air. Some indirect pathways – such as vaporization of soil 

contaminants and movement of the vapors into indoor spaces – involve two or more intermedia 

transfer factors. Some transfer factors are chemical-specific; others are general. Many of the 

intermedia transfer factors in this guidance have a default value of one. This begs the question, 

”Why include them if the value is one?” The reasons for their inclusion are 1) including the 

parameter facilitates incorporating a site-specific value without altering the equation structure, and 

2) further research may support a default value other than one in the future. 

Transfer factor from soil to indoor dust (TFSD) 

TFSD is the ratio of the concentration of a chemical in the dust on surfaces inside the school 

building(s) to its concentration in outdoor soil from the schoolyard. This is important because dust 
2/5/2004 12

on indoor surfaces may be a significant source of exposure to chemicals originating in soil and 

transported to the building’s interior through open doors and windows, heating, ventilation, and air 

conditioning (HVAC) systems, building infiltration, and on shoes, clothing, and objects carried into 

the rooms. OEHHA recommends a default value of 2 for this parameter (see Appendix 1 for further 

explanation). 

Transfer factor from soil to outdoor particulate matter (TFPM/S) 

TFPM/S is the ratio of the concentration of contaminant in outdoor PM10, resulting from resuspension 

of on-site soil, to the concentration of contaminant in outdoor soil from the schoolyard. This is 

important because students and other school users may inhale suspended respirable particles in the 

outdoor air. OEHHA recommends a conservative default value of one (1). If samples of outdoor 

PM10 are collected and analyzed, this transfer factor is not needed. 

Transfer factor from indoor surface dust to indoor respirable particulate matter (TFPM/D) 

TFPM/D is the ratio of the concentration of a chemical in indoor PM10 to its concentration in indoor 

surface dust. This is important because students and other school users may inhale suspended 

respirable particles in the indoor air. OEHHA recommends a default value of one, implying that 

indoor dust is resuspended in indoor air with no change in its chemical contaminant concentration. 

Soil vapor to Indoor air (α) 

Alpha is the unitless ratio of the concentration of a chemical in indoor air to its concentration in soil 

vapor. It is a dilution factor for vapors moving from relatively confined spaces in soil pores to the 

better-ventilated building interior. OEHHA recommends the use of the EPA adaptation of the 

Johnson and Ettinger (J&E) model to estimate a value for this parameter. OEHHA recommends a 

default air exchange rate of 4.7 per hour, based on the lower confidence limit on the weighted mean 

value from 94 portable and 26 traditional classrooms (see Appendix 1 for further discussion). 

Default values may be used for the remaining parameters including a sandy soil type. Site-specific 

parameters may be used when justified. 

Volatilization factor from soil (VCS) 

VCS is the ratio of the concentration of a chemical in soil vapor to its concentration in the soil 

matrix. This ratio, in gsoil/Lvapor, depends on the physical and chemical properties of the chemical, 

and on the properties of the soil. OEHHA recommends the Johnson and Ettinger screening model 

(EPA, 2003) to estimate this value. 

Volatilization factor from ground water (VCGW) 

VCGW is the ratio of the concentration of a chemical in soil vapor to its concentration in shallow 

ground water. This ratio, in mlwater/Lvapor, depends on the physical and chemical properties of the 

chemical, and on the properties of the soil. OEHHA recommends the Johnson and Ettinger 

screening model (EPA, 2003) to estimate this value. 

Volatilization factor (VF) 

VF is the ratio of the concentration of a chemical in soil to its concentration in outdoor breathing 

zone air. To calculate VF, OEHHA recommends the use of the equations in EPA’s Soil Screening 

Guidance (EPA, 1996) with chemical-specific parameters and one modification: to better represent 

the possible contaminated area on a school site, EPA’s default high-end value of Q/C of 68.8 for a 

0.5-acre contaminated site is adjusted to 41.24, corresponding to a 10-acre contaminated site. 

Breast milk biotransfer factor (BBM) 

BBM (d/kg) of organic chemicals is estimated as 0.0000002 * KOW. The value of 0.0000002 is an 

empirically determined constant (DTSC, 1994). 

Other Constants 
2/5/2004 13

Respirable particle load for outdoor air (PM10) 

OEHHA recommends a default concentration of 1.8 E-12 kgPM/L (1.8 μgPM/m3

) for site related PM10 

(particular matter less than 10 microns in diameter) in outdoor air. This value is based on the EPA 

Soil Screening Levels document (EPA, 1996). 

Respirable particle load for indoor air (SF) 

SF is the concentration in indoor air of particulate material less than 10 microns in diameter (PM10)

originating from on-site soil. OEHHA recommends a default value of 1.8E-12 kgPM/L (1.8 μgPM/m3). 

This assumes that indoor PM levels are the same as outdoor PM levels. 

Table 1: Transfer Factors and Other Constants 

Factor Units Value Discussed on Page 

TFSD Unitless 2 17 

TFPM/S Unitless 1 17 

PM10 kg PM10/Lair 1.8 E-9 18 

TFPM/D Unitless 1 17 

SF kg PM10/Lair 1.8 E-9 18 

α Unitless Chemical-specific 17 

VCS gsoil/Lvapor Chemical-specific 18 

VCGW mlwater/Lvapor Chemical-specific 18 

VF gsoil/Lair Chemical-specific 18 

BBM d/kg Chemical-specific 18 

Exposure Parameters 

Most existing risk assessment guidance is focused on multi-year residential or occupational 

exposure scenarios. Exposure parameters given in existing guidance are generally long-term 

averages. This guidance is specifically aimed at school populations, including students, teachers 

and other staff, and users of on-site day care. Because children are rapidly changing anatomically, 

physiologically and behaviorally, we recommend a set of exposure parameters for each year until 

age 18. We believe that it is useful to evaluate the exposure of growing children on a year-by-year 

basis for several reasons: 

(1) Some chemicals may exhibit age-specific toxicity. OEHHA is currently evaluating this aspect, 

and plans to publish age-specific toxicity criteria in the near future. Age-specific toxicity criteria 

should be paired with corresponding age-specific exposure estimates, to the extent possible. 

(2) If the exposure parameters are given on a year-by-year basis, model users can aggregate the 

years in a manner that best supports the risk management process. Conversely, if OEHHA were to 

recommend exposure parameters that were averaged over a multi-year period, that averaging period 

might not match the existing or proposed school scenario. In that case it would be difficult to 

disaggregate the exposure parameters then re-aggregate them to match the exposure scenario. 

The principal sources of exposure factor data for this guidance were the Technical Support 

Document for Exposure Assessment and Stochastic Analysis (OEHHA, 2000), the Children’s 

Exposure Factors Handbook (EPA, 2002). When more than one value was available for an 

exposure parameter, preference was given to values that were reported in a way that conformed to 

the assessment methodology, such as age-specific or short age intervals, and values reported as a 
2/5/2004 14

function of body weight. This avoided or reduced the need to interpolate or extrapolate data and to 

convert data to appropriate units using uncertain conversion factors. When percentile estimates 

were available, preference was given to the ninetieth percentile to be consistent with the reasonable 

maximum exposure (RME; EPA, 1989). Where data were considered equally appropriate for the 

analysis, preference was given to OEHHA values. Consideration was given to entering the data as 

distributions rather than as point estimates, but distributions were not available for several critical 

parameters. This approach will be considered in the future if sufficient data become available. 

This guidance includes the following parameters. Recommended parameter values are summarized 

in Table 8, at the end of this section:

Soil Ingestion (IS) 

IS is the estimated total daily inadvertent soil and dust ingestion. Geophagia or soil pica is not 

addressed in this document. EPA (2002, Table 5-19) estimated total daily soil and dust ingestion by 

children 1-6 years of age as 100 mg/day mean, with 400 mg/day as an upper end, adding that 200 

mg/day may be taken as a conservative estimate of the mean. EPA (1997) recommended a value of 

50 mg/day for adults. OEHHA (2000, page 4-15) recommends default values of 200 mg/day for 

children 1 - 6, and 100 mg/day for everyone over the age of 6. The estimated daily soil ingestion 

rates at school, shown in the last column of Table 2, are based on OEHHA recommendations. Soil 

ingestion is not normalized to body weight because a) it is not related to any physiologic process 

that would be a function of weight and b) it is not reported in that way in any of the references cited. 

Table 2: Soil Ingestion 

Age EPA estimates (mg/day)

(years) Mean Conservative mean Upper end 

Recommended 

value (mg/day) 

<1 0 

1-6 100 200 400 200 

>6 50 100 

Fraction at school (FS) 

FS is the estimated fraction of total daily soil and dust ingestion and dermal contact that occurs at 

school on school days. It is calculated as the total time at school (indoors plus outdoors) divided by 

16 hours per day. This is based on the assumptions that soil and dust ingestion and dermal contact 

are proportional to time spent at a given locale, and that soil and dust ingestion occur only during 

waking hours, which comprise 16 hours per day. 

Body-part-specific skin loading rate (LBP)

EPA (2002, chapter 8) recommends the data of Kissel et al. (1996, 1998) and Holmes et al. (1996) 

as a basis for estimating body-part- and activity-specific soil skin loading (LBP, kg/cm2/day). 

Geometric mean body-part-specific loadings ranged from 0.02 to 0.09 mg/cm2 for the day-care kids 

(see table below). Although this reference does not provide values for the head and trunk, these 

body parts are likely to be contaminated by soil at rates less than or similar to the legs. Therefore, 

OEHHA recommends a value of 0.02 mg/cm2 for the head and trunk. A “fraction exposed” term is 

not used, since the studies were based on entire body parts irrespective of whether they were 

partially clothed or not. 

OEHHA considers these data to be the best available because they are based on real-world 

exposures to young children in day-care centers, (daycare kids #1a, 1b, 2, and 3) an exposure setting 

similar to that being assessed. The children ranged in age from 1 to 6.5 years and included 17 boys 
2/5/2004 15

and 4 girls (groups 1a and 1b were the same children, measured in the morning and afternoon). 

They wore long pants (16) or shorts (5), long sleeves (7) or short sleeves (14). Most children wore 

low socks and shoes, but 5 were barefoot. Exposure times ranged from 3.5 to 8 hours, with no 

obvious correlation between time and dermal loading. These data are limited by low numbers of 

children, high inter-individual variability, limited age range and the need to match their activities 

with those being assessed. The daycare data are preferred for young children because the setting 

was most similar to the school setting. Data from other groups of children are available: The 

indoor kids (3 to 13 years of age) and tae-kwon-do participants (8 to 42 years) playing on a carpeted 

surface for 1.5 to 2 hours generally had lower dermal exposures to soil than the daycare kids. Nine 

to 14-year-old kids playing in mud for 10 to 20 minutes had much higher dermal exposures (2 – 3 

logs). Thirteen to 15-year-olds playing soccer on grass and bare earth for 40 minutes had a soil 

exposure that was generally similar to the daycare kids. 

Skin Surface area (ABP)

As stated above, EPA (2002) recommends using body-part- and activity-specific soil skin loading 

rates. In order to do this, skin surface area needs to be calculated on a body-part-specific basis. 

Data on fractional area of various body parts are found in Table 8-3 (EPA, 2002). Age-specific 

body surface area data are found in tables 8-1 and 8-2 (EPA, 2002). Table 8-4 (EPA, 2002) 

supplies surface-area to body weight ratios, but these are pooled for ages 2.1 to 19 years. Since it is 

apparent from analyzing the data in Tables 8-1, 8-2 and 11-1 (EPA, 2002) that surface-area-tobody-weight ratios change markedly with age, OEHHA recommends using the age-specific data in 

Tables 8-1, 8-2, and 8-3 to calculate these ratios for children 2 years and older. A sample 

calculation (for a 1-year-old child) is shown below. 

 

 Body 

part 

Fraction 

of 

Bodya

Total 

skin 

Areab

cm2/kg

Fractional

Area 

cm2

/kg 

Skin 

Loadingd

g/cm2

Skin 

Loadinge

g/kg

Head 16.5% X 641 = 105.8 X 0.000020f = 0.0021 

Trunk 35.5% X 641 = 227.6 X 0.000020f = 0.0044 

Arms 13.0% X 641 = 83.3 X 0.000023 = 0.0019 

Hands 5.7% X 641 = 36.4 X 0.000092 = 0.0034 

Legs 23.1% X 641 = 148.1 X 0.000020 = 0.0029 

Feet 6.3% X 641 = 40.2 X 0.000065 = 0.0026 

Total 100% 641 0.0173 

a EPA, 2002, Table 8-3 

b Estimated from EPA, 2002, Tables 8-4. 

d EPA, 2002, Table 8-13 

e Assumes that the school children will be clothed similarly to those in the study (see EPA, 2002, Table 8-12). 

f There are no data for trunk and head. OEHHA suggests that the value for the legs, i.e. 0.02, be adopted for the head and trunk. 

Fraction outdoors (FO) 

FO is the estimated fraction of the daily school-related dermal and ingested soil/dust exposure that is 

acquired outdoors. This is calculated as the time spent outdoors divided by the total time spent 

outdoors and indoors (see below). The implicit assumption is that indoor and outdoor exposure are 

proportional to time spent in those environments. 
2/5/2004 16

Fraction indoors (FI) 

FI is the estimated fraction of the daily school-related dermal and ingested soil/dust exposure that is 

acquired indoors. It is calculated as 1- FO. 

Body weight (BW) 

BW for children up to 3 years old is from EPA, 2002, Table 11-1. The 50th percentile values for 

boys and girls within each year of age were averaged to obtain a representative value. E.g. the body 

weight for one-year-olds is the average of male and female 50th percentile values at 12, 18, and 24 

months. Body weights for children older than 3 years are the means for boys and girls at the 

beginning and end of each age interval from Table 11-2 (EPA, 2002). E.g. the body weight 

estimate for four-year-olds is the average of male and female means (including clothing) at 4 and 5 

years. Mid-range values for body weight are recommended because this parameter appears only in 

the denominator of the soil ingestion and dermal contact equations, and since the numerators are 

thought to be conservative estimates of these parameters, it would be excessively conservative to 

use a low-end body weight. Estimated body weights for various ages are in Tables 3 and 7. 

Table 3: Age Related Body Weights 

Age (years) Weight (kg) Age (years) Weight (kg) Age (years) Weight (kg) 

 6-7 23.75 13-14 53.20 

0-1 7.04 7-8 26.50 14-15 57.05 

1-2 11.08 8-9 29.80 15-16 60.35 

2-3 13.29 9-10 33.90 16-17 62.90 

3-4 16.35 10-11 38.70 17-18 64.15 

4-5 18.55 11-12 43.20 

5-6 21.15 12-13 47.85 

Nursing 

moms 

63.2 

Exposure time, outdoors (TO) 

Estimates of daily outdoor exposure time (TO), shown in Table 4, are from EPA (2002) Table 9-40. 

The data are based on national activity pattern survey data, and are weighted according to gender, 

age, race, employment status, region, season, etc, to represent the U.S. population (Klepeis et al., 

2001). OEHHA recommends the 75th percentile values (in bold below) because when 75th

percentile values for time indoors at school and for time outdoors at school are added, the combined 

time at school ranks at the 95th to 99th percentile for total time spent at school. Data for infants <1 

are not available, so the values for 1-year-olds are recommended as a surrogate. 

Table 4: Minutes Spent Outdoors At School Per School Day 

Age 50th percentile 75th percentile 95th percentile 

1-4 65 140 175 

5-11 60 120 220 

12-18 55 105 225 
2/5/2004 17

Exposure time, indoors (TI) 

Estimates of daily outdoor exposure time (TI), shown in Table ,5 are based on EPA (2002) Table 9-

39). OEHHA recommends 75th percentile values (in bold below) because when 75th percentile 

values for time indoors at school and for time outdoors at school are added, the combined time at 

school ranks at the 95th to 99th percentile for total time spent at school (EPA, 2002, Table 9-34). 

Data for infants less than 1 year of age are not available, so the values for 1-year-olds are 

recommended as a surrogate. 

Table 5: Minutes Spent Indoors At School Per School Day 

Age 50th percentile 75th percentile 95th percentile 

1-4 269 500 595 

5-11 403 445 565 

12-18 420 450 565 

Breathing rate, outdoors (BO) 

BO is the estimated breathing rate for outdoor school activities like walking and running, estimated 

from the data of Wiley, et al. in OEHHA, 2000, p. 3-27. This Guidance recommends using a value 

of 0.75 L/min-kg for all ages. This value assumes that 50 percent of outdoor time is spent in 

moderate activity like outdoor play, outdoor leisure, and golf with a ventilation rate of 0.6 L/min-kg 

and 50 percent in is spent in heavy activity like walking and active sports with a ventilation rate of 

0.9 L/min-kg. Both sets of descriptors in the Wiley, et al. report (moderate activity and heavy 

activity) were deemed consistent with outdoor activities at school. 

Breathing rate, indoors (BI) 

The estimated breathing rate for indoor school activities, were estimated from the data of Wiley, et 

al. (in OEHHA, 2000, p. 3-25 to 3-26). The light activity category (0.3 L/min-kg) contained 

activity descriptions compatible with indoor activities at school, such as eating, talking, reading, and 

homework. The moderate activity category (0.6 L/min-kg) also contained some activity 

descriptions compatible with indoor school activities for younger children, e.g. indoor play. 

Therefore we recommend using an average of the ventilation rates for light and moderate activity, 

i.e. 0.45 L/min-kg, for children up through age 5. For older children the light activity ventilation 

rate of 0.3 L/min-kg is recommended for indoor activities, since their more vigorous activities 

typically take place outdoors. 

Exposure frequency (EF) 

The estimated number of days students or other school users attend school annually. Survey data 

show that the distribution of days of school per year is bimodal, with 94 percent reporting a school 

year of 161 to 187 days and another 6 percent reporting a school year of 228 to 238 days (RTI, 

2003). Based on these results, the recommended default value for a 9-month school year is 180 

days, the modal value for a standard 9-month school year. For year-round schooling, a value of 233 

days per year, the midpoint of the upper range is recommended. 

Breast milk intake (IBM) 

This Guidance recommends a daily breast milk ingestion of 130 g/kg/day for the first 12 months of 

life (OEHHA, 2000, Table 5.13, 90th percentile). 
2/5/2004 18

Daily Water Intake (IDW)

Daily water intake at school was estimated from EPA, 2000, Table 4-12 and EPA, 1989, Table 3-

30. Since OEHHA did not identify any data concerning the proportion of daily water intake that 

occurs at school, we recommend a value of 1⁄2 the 90th percentile daily water intake, based on the 

proportion of waking hours spent at school on school days (EPA, 2000, Table 9-34). The 

recommended water intake rates at school in the last column below are 1⁄2 the 90th percentile value 

from EPA, 2000. 

Table 6: Water Consumption 

Age EPA Mean EPA Median EPA 90th %ile EPA 95th %ile Water intake at 

<1 46 19 127 156 63.5 

1-3 23 17 51 67 25.5 

1-10 19 15 42 56 21 

11-19 12 9 26 33 13 

Pregnant women 18 16 35 40 17.5 

Lactating women 21 21 35 37 17.5 

Adults 21 19 34 17 

Fraction absorbed, inhalation (AIn) 

The chemical-specific ratio of the total dose of a chemical absorbed through the respiratory tract to 

the total amount of the chemical inhaled. In the absence of data to support an alternative value, a 

default value of one should be used. 

Fraction absorbed, ingestion (AI) 

The chemical-specific ratio of the total dose of a chemical absorbed through the gastro-intestinal 

tract to the total amount of the chemical ingested. In the absence of data to support an alternative 

value, a default value of one should be used. 
2/5/2004 19

Fraction absorbed, dermal (AD) 

The chemical-specific ratio of the total dose of a chemical absorbed through the skin to the total 

amount of the chemical that is adsorbed onto the skin. Suggested values in Table 7 are from 

(DTSC, 1994) Table 2. page A-6): 

Table 7: Dermal Absorption Fractions for Compound Classes 

Compound Absorption Source

Arsenic 0.04 OEHHA, 2000

Beryllium 0.01 OEHHA, 2000

Cadmium 0.001 OEHHA, 2000

Hexavalent chromium 0.01 OEHHA, 2000

Lead 0.01 OEHHA, 2000

Mercury 0.1 OEHHA, 2000

Nickel 0.04 OEHHA, 2000

Polychlorinated biphenyls 0.14 OEHHA, 2000

Polychlorinated dibenzo-p-dioxins and dibenzofurans 0.02 OEHHA, 2000

Hexachlorocyclohexanes 0.1 OEHHA, 2000

Polynuclear aromatic hydrocarbons, 0.13 OEHHA, 2000

DEHP 0.1 OEHHA, 2000

4,4’ methylene dianiline 0.1 OEHHA, 2000

Organophosphates, pentachlorophenol 0.25 DTSC, 1994

Chlorinated insecticides 0.05 DTSC, 1994

Other organic chemicals 0.1 DTSC, 1994

Other metals and complexed cyanides 0.01 DTSC, 1994

Free cyanide 0.1 DTSC, 1994

Lifetime Exposure Fraction (ED/AT) 

The fraction of a lifetime represented by each exposure scenario. It enters into the calculation of 

cancer risk but not the calculation of the hazard index. Exposures need to be adjusted according to 

the lifetime exposure fraction because while cancer potency factors are based on lifetime exposure, 

this model estimates school-related exposure and risk for a series of one-year intervals beginning at 

birth. Since exposures differ from year to year, risks for each year are unique. For single-year 

scenarios, ED/AT is 1/70 or 0.014. For staff, the exposure duration is 40 years, a standard 

occupational exposure duration. Annual risks may be added to obtain the aggregate risk for any 

multi-year period. Risks are treated on an individual risk basis. An alternative would have been to 

consider population cancer burden. The latter is unaffected by individual exposure duration, as long 

as the source and the number of exposed people do not change. Treating children’s risk on an 

annual basis has a somewhat similar effect. 
2/5/2004 20

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2/5/2004 21

Table 8: Summary of Recommended Exposure Par

Recommende

Parameter 

Abbreviation See page Units <1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-1

Soil Ingestiona IS 19 mg/day 0 200 200 200 200 200 100 100 100 100 100 Fraction at schoolb FS 19 unitless 0.67 0.67 0.67 0.67 0.59 0.59 0.59 0.59 0.59 0.59 0.59 Fraction outdoorsc FO 21 unitless 0.22 0.22 0.22 0.22 0.22 0.21 0.21 0.21 0.21 0.21 0.21 Body weightd BW 22 Kg 7.04 11.1 13.3 16.4 18.6 21.2 23.8 26.5 29.8 33.9 38.7 Surface area, heade SA 20 Cm2/kg 117 106 63 55 54 50 47 45 42 38 33 Loading rate, headf LBP 19-20 g/cm2/day 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 Surface area, trunke ABP 20 Cm2/kg 229 228 171 128 122 124 126 122 116 107 104 Loading rate, trunkf LBP 19-20 g/cm2 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 Surface area, armse ABP 20 Cm2/kg 88 83 53 58 54 50 47 45 42 39 39 Loading rate, armsf LBP 19-20 g/cm2 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 Surface area, handse ABP 20 Cm2/kg 34 36 24 24 22 19 17 17 17 17 16 Loading rate, handsf LBP 19-20 g/cm2 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 9e-5 Surface area, legse ABP 20 Cm2/kg 132 148 103 108 108 102 98 97 95 90 89 Loading rate, legsf LBP 19-20 g/cm2 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 2e-5 Surface area, feete ABP 20 Cm2/kg 42 40 32 29 28 26 25 25 25 24 23 Loading rate, feetf LBP 19-20 g/cm2 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 7e-5 Breathing rate, outdoorsg BO 23 L/min-kg 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 Exposure time, outdoorsh TO 22 min/day 140 140 140 140 140 120 120 120 120 120 120 Breathing rate, indoorsi BI 23 L/min-kg 0.45 0.45 0.45 0.45 0.45 0.45 0.3 0.3 0.3 0.3 0.3 Exposure time, indoorsi TI 23 min/day 500 500 500 500 500 445 445 445 445 445 445 Exposure frequencyn EF 23 days/yr 180 180 180 180 180 180 180 180 180 180 180 Exp. freq. (year-round)o EF 23 days/yr 232 232 232 232 232 232 232 232 232 232 232 Breast milk intakep IBM 23 g/kg/day 130 0 0 0 0 0 0 0 0 0 0 Water Intake IDW ml/kg/da 63.5 25.5 25.5 25.5 21 21 21 21 21 21 13 Exposure durationq ED 24 Years 1 1 1 1 1 1 1 1 1 1 1 Averaging timer AT 24 Years 70 70 70 70 70 70 70 70 70 70 70 
2/5/2004 22

a The estimated total daily inadvertent soil and dust ingestion, based on OEHHA 2000, page 4-15. Geophagia or soil pica is not addressed in this document. b Fraction of daily soil and dust ingestion and dermal loading that occurs at school, based on the number of hours at school daily divided by 16. OEHHA recommends c Fraction of daily site-related dermal and ingested soil/dust load that is acquired outdoors. Calculated as the time spent outdoors divided by the time spent outdoors plud Body weight data for children up to 3 years old were taken from EPA, 2002: Table 11-1 (50th percentile; mean of boys and girls). For older children, values were take EPA, 2002, Table 8-13 (Assumes that the school children will be clothed similarly to those in the study; see EPA, 2002, Table 8-12). f Estimated from EPA, 2002, Tables 8-1, 8-2 and 8-3 OEHHA suggests that the value for the legs, i.e. 0.02, be adopted for the head and trunk g Based on (OEHHA, 2000, p. 3-27). h Based on 75th percentile values from EPA, 2002, Tables 9-39 and 9-40. Since data for adult staff are not available, OEHHA recommends a default value of 60 minui OEHHA recommends a value of 0.45 L/min-kg for children up through age 5, and a value of 0.3 L/min-kg for older children and adults based on (OEHHA, 2000, p. j Based on 75th percentile values from EPA, 2002, Tables 9-39 and 9-40. Since data for adult staff are not available, OEHHA recommends a default value of 480 minu n The recommended default value for a 9-month school year is 180 days, the standard school-year length in California. o For year-round schooling, a value of 223 days may be used. p Based on (OEHHA, 2000, Table 5.13, 90th percentile). q Exposure Duration is the number of consecutive years of exposure represented by the exposure scenario under evaluation.  S OEHHA recommends a default value of 70 years. 
2/5/2004 23

Risk Assessment 

Chemicals of Concern 

Chemicals of concern should be determined in consultation with the lead regulatory agency on 

the project. Suggested guidance includes DTSC, 1994, section 2.4.6.7. 

Exposure Point Concentration 

Exposure point concentration should be determined in consultation with the lead regulatory 

agency on the project. Suggested guidance includes DTSC, 1992, Chapter 2. 

Toxicity Criteria 

OEHHA cancer potency values and reference exposure levels, which are available at 

(http://www.oehha.ca.gov/risk/ChemicalDB/index.asp) should be preferentially used. Child-specific 

reference doses (chRD) should be used when available. When OEHHA criteria are not available, 

U.S. EPA criteria found in the Integrated Risk Information System (IRIS) database 

(http://www.epa.gov/iriswebp/iris/index.html) should be used when available. If criteria for a given 

chemical are not available either from OEHHA or in IRIS, criteria from other published sources 

may be used, subject to approval by the reviewing agency. 

Risk and Hazard Calculation 

Hazard quotients and incremental risks from all exposure routes are estimated and summed for 

each chemical. The hazard quotients and incremental risks for the individual chemicals are then 

added to calculate the total hazard index and total risk. For screening assessment, the default 

assumption is that hazards posed by individual chemicals are additive. Some non-cancer toxic 

effects of individual chemicals are unlikely to be additive. In those cases, a statement to that 

effect, with documentation based on target organ and/or mode of action, should be included. 

Risk Characterization 

Following the methodology described herein will produce age-specific estimates of hazard and 

risk. At a minimum, the risk characterization should present risk and hazard for each year or 

group evaluated. In order to calculate the risk for a multi-year period, the risks for individual 

years must be added. Hazards are not usually considered to be additive from year to year (i.e. 

the chemical exerts its full effect of within one year). It may be useful to show contributions of 

individual chemicals and/or individual pathways to total risk and hazard. 

Dose (route a) /RfD (route a) = Hazard Quotient (route a) 

Dose (route b) /RfD (route n) = Hazard Quotient (route n) 

Hazard Quotient (chemical a) = ∑Hazard Quotient (route a...n) 

Hazard Index = ∑Hazard Quotient (chemical a...n)* 

* For chemicals acting by a similar mode of action or affecting the same target organ

Dose (route a) * CPF (route a) = Risk (route a) 

Dose (route b) * CPF (route n) = Risk (route n) 

Risk (chemical a) = ∑Risk (route a...n) 

Total Risk = ∑Risk (chemical a...n) 
2/5/2004 24

Sensitivity Analysis 

The calculated risk or hazard may be relatively sensitive or insensitive to changes in various 

input parameters. Sensitivity analysis is important because it can help direct research or data 

gathering toward those parameters that will have the most effect on the outcome. For example it 

would not be highly productive to measure indoor PM10 levels at a site where the primary 

contaminant of concern is trichloroethylene. Local sensitivity is the percent change in the total 

risk or hazard index corresponding to a small change in the value of a specific parameter divided 

by the percent change in that parameter. It is investigated by changing the input parameter 

values one at a time and measuring the effect on the risk or hazard. The local sensitivity is 

dependent on how the parameter is mathematically related to the result. However, it can change, 

depending on other inputs. For example, the model is very sensitive to changes in soil ingestion 

rate when soil contamination is the primary problem at a site, but relatively insensitive to 

changes in soil ingestion rate when ground water contamination is the primary problem at the 

site. The local sensitivity is also heavily influenced by the properties of the contaminant. For 

example, risk from volatile chemicals is sensitive to changes in breathing rate and hours spent 

indoors daily, while risk from non-volatile chemicals is relatively insensitive to changes in these 

parameters. Because of this variation in sensitivity, we focused on the maximum sensitivity 

observed under the conditions of our simulations. 

For this analysis, representative conditions were selected. The only inputs were 0.15 mg/kg of 

the chemical in soil and 0.1 μg/L in shallow ground water. The following table shows the results 

of the analysis for 1-year-olds. The ratio of change in output/change in input has been converted 

to percentages, i.e. a 1:1 ratio would be shown as 100%. Some parameters (e.g. those that appear 

in the denominator) change the output in the opposite direction; these are shown as negative 

percentages. Four chemicals were selected to represent a range of physical and chemical 

characteristics. They include a volatile chemical, a relatively non-volatile lipophilic organic 

chemical, a metal and a metal that is carcinogenic by inhalation but not by ingestion. Each 

chemical was evaluated based on its most sensitive endpoint: For the first three the most 

sensitive endpoint was carcinogenicity; for the fourth, non-carcinogenic toxicity was limiting. 

Table 9: Local Sensitivity 

Local Sensitivity* 

Parameter Vinyl chloride DDT Cadmium Chromium VI Maximum 

Parameter 

Uncertainty 

Indoor dust/outdoor soil 0.10% 74.00% 77.34% 77.29% 77.34% High 

Outdoor PM10/outdoor soil 0.00% 0.00% 0.00% 0.000% 0.000% High 

Outdoor PM10 0.00% 0.00% 0.00% 0.000% 0.000% High 

Indoor PM10 0.00% 0.00% 0.00% 0.000% 0.000% Moderate 

Indoor vapor/Soil vapor (α) 93.90% 3.00% 0.00% 0.000% 93.900% High 

Kow -0.028% 0.00% 0.00% 0.000% -0.028% Moderate 

Fraction absorbed, resp 97.30% 4.10% 0.00% 0.000% 97.30% Moderate 

Fraction absorbed, ingest 0.20% 94.90% 99.19% 99.40% 99.40% Moderate 

Fraction absorbed, dermal 2.50% 1.20% 1.05% 0.73% 2.500% High 

Soil vapor/soil matrix (VCS) 91.10% 0.00% 0.00% 0.00% 91.10% High 
2/5/2004 25

Local Sensitivity* 

Parameter Vinyl chloride DDT Cadmium Chromium VI Maximum 

Parameter 

Uncertainty 

Soil vapor/groundwater (VCGW) 2.90% 3.00% 0.00% 0.00% 3.00% High 

Volatilization Factor (VF) -5.40% -1.00% 0.00% 0.00% -5.40% High 

Soil Ingestion 0.20% 93.20% 98.50% 98.74% 98.74% High 

Fraction at school 0.20% 94.40% 98.50% 98.74% 98.74% High 

Surface area 0.00% 0.80% 0.81% 0.567% 0.81% Moderate 

Fraction outdoors 0.00% 0.50% 0.54% 0.38% 0.54% Moderate 

Body weight -0.20% -85.80% -89.54% -89.77% -89.77% Low 

Breathing rate, outdoors 3.40% 1.60% 0.00% 0.00% 3.40% Moderate 

Exposure time, outdoors 3.40% 1.50% 0.41% 0.29% 3.40% Moderate 

Exposure time, indoors 93.90% 2.60% -0.39% -0.27% 93.90% Moderate 

Breathing rate, indoors 93.90% 3.00% 0.00% 0.00% 93.90% Moderate 

Exposure frequency 100.00% 100.00% 100.00% 100.00% 100.00% Moderate 

Exposure duration 100.00% 100.00% 100.00% 0.000% 100.00% High 

Averaging time -90.90% -90.90% -90.90% 0.00% 0.00% Low 

Area fraction Head 0.00% 0.30% 0.07% -0.044% 0.30% Low 

area fraction Trunk 0.00% 0.10% 0.15% -0.076% 0.15% Low 

area fraction Arms 0.00% 0.00% 0.06% -0.050% 0.062% Low 

area fraction Hands 0.00% 0.10% 0.11% 0.052% 0.11% Low 

Area fraction Legs 0.00% 0.10% 0.09% -0.019% 0.10% Low 

area fraction Feet 0.00% 0.10% 0.08% 0.027% 0.10% Low 

Loading Head 0.00% 0.30% 0.07% 0.069% 0.30% High 

loading Trunk 0.00% 0.10% 0.15% 0.15% 0.15% High 

loading Arms 0.00% 0.00% 0.06% 0.062% 0.062% High 

loading Hands 0.00% 0.10% 0.11% 0.109% 0.11% High 

Loading Legs 0.00% 0.10% 0.09% 0.094% 0.10% High 

loading Feet 0.00% 0.10% 0.08% 0.084% 0.10% High 

Reference Dose 0.00% 0.00% 0.00% 100.00% 100.00% High 

Cancer potency 100.00% 100.00% 100.00% 0.00% 100.00% High 

* Change in risk or hazard divided by change in the input parameter 

Parameters that are well characterized (i.e. possessing low uncertainty) are not large contributors 

to uncertainty in the outcome, regardless of the sensitivity of the outcome to the parameter. 

Therefore, if either the local sensitivity or the range of uncertainty for any given parameter is 

small, changes in that parameter are unlikely to have appreciable impact on risk or hazard. In 

turn, research to reduce the uncertainty in that parameter will be a lower priority because the 

results will have less effect on the outcome than those with greater local sensitivity or uncertainty. 
2/5/2004 26

For example for a parameter with a local sensitivity of less than 1 percent, a 10-fold error in the 

parameter value would change the hazard or risk by less than 10 percent. A change of less than 

10 percent is not likely to change the result expressed to one significant figure. Risk assessors 

generally acknowledge that their results are good to only one significant figure at best. Therefore, 

the analysis of parameter uncertainty below is focused on those with 1 percent or greater local 

sensitivity and moderate to high uncertainty. 

Uncertainty Analysis 

Model Uncertainty 

In time-dependent models, concentrations, flow rates, and dose rates change with time. Timeindependent models like the one described herein assume that conditions are at equilibrium and 

do not change over time. They do not account for source depletion. This could result in 

overestimating risk, particularly if multi-year exposures are considered. 

This model does not consider all possible transport mechanisms or all possible factors affecting 

environmental fate and transport of environmental contaminants. For example, it does not 

consider transport of soil contaminants to ground water, transfer from soil or air into edible 

plants, or redeposition of particulate matter. However, the authors believe it considers the 

principal determinants of chemical exposures at schools. 

Exposure Pathway Uncertainty

This model does not consider all possible exposure pathways. For example, crops could be 

grown in site soil and contaminated ground water could be used to irrigate site-grown crops, 

thereby transferring contaminants to produce eaten by students and staff. Inhalation of volatile 

chemicals while showering is not included. The contribution of these pathways to the overall 

risk or hazard is minimal. 

Parameter Uncertainty 

In addition to a unique exposure scenario, exposure assessment for schools requires a unique set 

of exposure parameters. For example, building parameters, and age distribution and activity 

patterns of the school users differ from typical residential, recreational, and occupational 

settings. As discussed above, under the heading “Sensitivity Analysis,” parameters with a local 

sensitivity of 1 percent or greater and those that have a high level of uncertainty are the primary 

focus of this discussion. 

Transfer factor from soil to indoor dust (TFSD)

Interior dust is an important exposure medium in school site exposure assessment because 

students typically spend much of their time at school in classrooms and other indoor areas. The 

fraction of dust that comes from site soil is poorly characterized, but significant, inasmuch as 

other sources of interior dust are less affected by site selection. This parameter was considered a 

good candidate for further study because it has high local sensitivity (77 percent) and there are 

no published values for this parameter in the school setting. The recommended default dust/soil 

transfer factor (2) is based on relative concentrations of several elements in outdoor soil and 

interior dust at California schools (RTI. 2003b, see Appendix 1). 

Soil vapor to Indoor air (α) 

The ratio of chemical concentration in indoor air to that in soil vapor parameter (alpha) is a good 

candidate for further study because it has a high local sensitivity for some chemicals (up to 94 

percent) and because there are limited data for ventilation rates at schools. Site-specific factors 

such as operation of the HVAC system (positive or negative pressure, ventilation rates, etc.), 
2/5/2004 27

type of foundation, and use of doors and windows will substantially affect alpha. The 

recommended default ventilation rate for use in the Johnson and Ettinger model (4.7 changes/hr) 

is based on ventilation rate data from California schools (RTI, 2003a, see Appendix 1). 

Volatilization factor from soil (VCS) 

The ratio of the contaminant concentration in soil vapor to that in soil matrix depends on the 

physical and chemical properties of the chemical, as well as soil properties. This ratio, in 

μg/Lvapor/(μg/gsoil) (or gsoil/Lvapor ), has a high local sensitivity (up to 91 percent for volatile 

chemicals) and is relatively uncertain. However, the uncertainty can be partially offset by 

sampling soil vapors in addition to soil matrix. Since there is no reason to believe that VCS

would be different in a school environment than in other environments, OEHHA recommends 

the Johnson and Ettinger model (EPA, 2000 (2)) to estimate this value. 

Volatilization factor from ground water (VCGW) 

The ratio of the contaminant concentration in soil vapor to that in shallow groundwater depends 

on the physical and chemical properties of the chemical, as well as soil properties. This ratio, in 

μg/Lvapor/(μg/mlwater) (or mlwater/Lvapor ), has a moderate local sensitivity (up to 3 percent for 

volatile chemicals) and is relatively uncertain. However, the uncertainty can be partially offset 

by sampling soil vapors in addition to ground water. Since there is no reason to believe that this 

factor would be different in a school environment than in other environments, OEHHA 

recommends the Johnson and Ettinger model (EPA, 2000 (2)) to estimate this value. 

Volatilization factor (VF) 

The volatilization factor has a moderate local sensitivity – up to 5.4 percent. It is based on a 

well-reviewed document. However, OEHHA recommends adjusting the contaminated area to 10 

acres (compared to the default value of 0.5 acres) to more closely reflect the size of a school site. 

This reduces VF by approximately 40 percent, which increases the atmospheric concentration by 

about 67 percent, since atmospheric concentration is a function of 1/VF. 

Soil Ingestion (IS) 

Soil and dust ingestion is a good candidate for further study because it has a high local sensitivity 

(up to 99 percent) and high parameter uncertainty. U.S. EPA has estimated soil/dust ingestion by 

children and adults, and these values are widely applied in the residential setting. There are no 

estimates specific to the school environment; however, some of the data, collected in day care 

facilities, may be relevant to a school environment. Research in the area of soil and dust 

ingestion in schools could reduce uncertainty in this parameter. Since the recommended value is 

equivalent to U.S EPA’s conservative estimate of central tendency, the model is unlikely to 

underestimate soil ingestion for most children and adults. However a few children at the upper 

end of the distribution may ingest more soil than the 200 mg/day default. 

Fraction at School (FS)

The fraction of the daily soil ingestion and dermal contact that occurs at school on school days is 

another parameter with a high local sensitivity (up to 99 percent). The recommended values are 

based on the estimated fraction of the waking hours that are spent at school, and the assumption 

that these exposure pathways are proportional to time spent in an environment (i.e., that soil 

ingestion and dermal contact do not occur preferentially at school or at home). The uncertainty 

is in both directions, but the maximum underestimate is less than two-fold, since the 

recommended values range from 58 to 67 percent and the true value could not exceed 100 

percent. 
2/5/2004 28

Body Weight (BW) 

Body weight has a high local sensitivity (up to -90 percent) for chemicals whose exposure is 

primarily by soil ingestion. This is because soil ingestion is not normalized to body weight in 

this model. The negative sign indicates that risk decreases as body weight increases. However, 

body weight is not particularly uncertain. 

Breathing Rate, Outdoors and Indoors (BO, BI)

Outdoor breathing rate has a moderate local sensitivity (up to 3.4 percent for volatile chemicals). 

Indoor breathing rate has a high local sensitivity (up to 94 percent for volatile chemicals). The 

recommended breathing rates are based on studies involving 52 children ranging in age from 3 to 

12 years and another 160 children and adults from age 6 to 77 (OEHHA, 2000, p. 3-8 to 3-13). 

Since activity-specific breathing rates are not available for children in a school environment, we 

assigned average breathing rates for indoor and outdoor activities based on breathing rates for 

similar activities that were reported in those studies. Detailed observations of pre-school and 

school children of various ages could help to reduce the uncertainty in these parameters. 

However, even with more data, variation between schools and between individuals is likely to be 

considerable, and inferences would still have to be made concerning which measured respiration 

rates correspond to the observed activities. 

Exposure Time, Outdoors and Indoors (TO, TI)

The outdoor exposure time has a moderate local sensitivity (up to 3.4 percent for volatile 

chemicals). Indoor exposure time has a high local sensitivity (up to 94 percent for volatile 

chemicals). Data from EPA, 2002, Table 9-40 (used to estimate time spent outdoors on school 

grounds) may overestimate actual time spent outdoors on school grounds since they include time 

spent at playgrounds as well as at school grounds. The sum of the recommended 75th percentile 

exposure times indoors and outdoors is 555 to 620 minutes per day. California law requires a 

minimum of 50,400 minutes of instructional time per year for grades 1-8. Based on a typical 

180-day schedule, this translates to 280 minutes per day. Even allowing another 90 minutes for 

lunch, recesses, and/or between-class time brings the total to 370 minutes, considerably less than 

the recommended 75th percentile estimates. Part of the difference could be explained by other 

time spent at school such as participation in before- or after-school activities. Surveys focused 

specifically on the school environment could help to narrow this range of uncertainty. 

Exposure Frequency (EF)

Exposure frequency has a high local sensitivity (100 percent) because this value enters into every 

calculation of risk and hazard. While it is not particularly uncertain, it is quite variable, ranging 

from the minimum days per year required by law to a maximum for a student, staff member, or 

day-care child who attends the school year-round. Table 10 shows the reported number of 

planned school days for the current school year for the 54 California schools that interpreted the 

question correctly (RTI, 2003). The bimodal distribution suggests that a single value may not 

adequately represent the data. 

Table 10: Frequency Distribution of Annual School Days

Number of Days Number of Schools Percent

Less than 180 days 9 16.7 

180 days 34 63.0 

181 to 187 8 14.8 

188 to 227 0 0

228 to 238 3 5.6 
2/5/2004 29

Lifetime Exposure Fraction (ED/AT) 

Lifetime exposure fraction is the fraction of a lifetime represented by each exposure scenario. It 

has a local sensitivity of 100 percent for carcinogenicity but does not enter into the calculation of 

the hazard index. For single-year scenarios, ED/AT is 1/70 or 0.014. Averaging time (in effect, 

the expected life span) has a relatively low uncertainty and is a widely applied value. Exposures 

need to be adjusted according to the lifetime exposure fraction because while cancer potency 

factors are based on lifetime exposure, this model estimates school-related exposure and risk for 

a series of one-year intervals beginning at birth. This involves interpolation and therefore 

introduces uncertainty. Since exposures differ from year to year, risks for each year are unique. 

Because the risks are calculated on a year-by-year basis, annual risks may be added to obtain the 

aggregate risk for any multi-year period. 

Reference Dose (RfD)

Reference dose has a high local sensitivity (100 percent for non-carcinogenic effects). The 

uncertainty varies from minimal (when the RfD is based on data from sensitive humans) to 

considerable (when multiple uncertainty factors are involved such as when the RfD is based on 

laboratory animals and/or inadequate studies). The need for reference doses reflecting the 

potentially greater sensitivity of children to toxic effects of some chemicals is under evaluation 

by OEHHA.

Cancer Potency 

Cancer potency has a high local sensitivity (100 percent for carcinogenic effects). The 

uncertainty varies from moderate (when the potency is based on human cancer incidence data) to 

high (when extrapolated from high-dose rodent data). There is additional uncertainty in 

extrapolating carcinogenic potency determined in a full lifetime study to less-than-lifetime 

exposure scenarios. The typical approach is to assume linearity, i.e. half the exposure is 

equivalent to half the risk. However, there is evidence that less-than-lifetime exposure of some 

carcinogens to children and infants may be more potent in inducing cancer than the same 

exposure later in life. Because exposures at school sites are changing from year to year, and 

because they may be for shorter time periods than residential or occupational exposures, 

OEHHA deems it beneficial to assess risks on a year-by-year basis. Year-by-year exposure 

estimates have the potential to be used in conjunction with future age-specific potency estimates. 

Methodology to evaluate carcinogenic potency of early-in-life exposures is the subject of 

ongoing OEHHA and U.S. EPA projects. EPA’s draft policy for evaluating carcinogenic 

potency of early-in-life exposures has been reviewed by the Science Advisory Board, and EPA is 

responding to the Board’s comments and public comments. 

Fraction Absorbed, Resp (AIn), Fraction Absorbed, Ingest (AI)

The fraction absorbed by the respiratory and ingestion routes has a high local sensitivity (up to 

99 percent). The recommended default value of one implies that absorption is the same in the 

exposure situation as in the study(s) that are the basis for the toxicity criteria, an assumption 

widely accepted in the risk assessment community. In reality, the rats may have been fed or 

dosed with the test chemical mixed into a vehicle that enhances absorption compared to the form 

to which humans will be exposed. Conversely, the rats may have been exposed to a poorly 

absorbed form while humans are exposed to a readily absorbed form, though this seems less 

likely. Route-specific absorption is an important issue for inter-route extrapolation. The 

uncertainty is in both directions but is not likely to exceed a two- or three-fold error, since most 
2/5/2004 30

compounds are readily absorbed the gastro-intestinal or the respiratory mucosa. OEHHA has no 

current plans for research on these parameters. 

Fraction Absorbed, Dermal (AD)

Although the fraction absorbed by the dermal route has a moderate local sensitivity (up to 2.5 

percent), it is potentially an important parameter because data on chronic toxicity or 

carcinogenicity by the dermal route are generally not available and therefore inter-route 

extrapolation is the rule. Current estimates, based on models and experiments using laboratory 

animals and cadaver skin, are relatively uncertain for some chemicals. However, there is no 

reason to believe that dermal uptake would be different in a school environment than in other 

exposure scenarios, and OEHHA has no current plans for dermal uptake research. The 

uncertainty is in both directions. 
2/5/2004 31

REFERENCES 

DTSC, 1994. Preliminary Endangerment Assessment Guidance Manual, State of California, 

Environmental Protection Agency, Department of Toxic Substances Control. 

DTSC, 1992, Supplemental Guidance for Human Health Multimedia Risk Assessments of 

Hazardous Waste Sites and Permitted Facilities (Second printing, 1999), State of California, 

Environmental Protection Agency, Department of Toxic Substances Control. 

EPA, 1989; U.S. Environmental Protection Agency Risk Assessment Guidance for Superfund, 

Human Health Evaluation Manual 

EPA, 1996, Soil Screening Guidance User’s Guide, (EPA540/R-96/018), OSWER, July, 1996 

http://www.epa.gov/superfund/resources/soil/index.htm

U.S. Environmental Protection Agency (EPA), 2002; Child-Specific Exposure Factors 

Handbook, Interim Report, September, 2002, National Center for Environmental Assessment, 

Washington, DC; EPA-600-P-00-002B available at http://www.EPA.gov/NCEA

U.S. Environmental Protection Agency (EPA), 1989; Exposure Factors Handbook, May, 1989 

Update, National Center for Environmental Assessment, Washington, DC; EPA600/8-89/043 

available at http://www.EPA.gov/NCEA

EPA, 2003 Johnson and Ettinger (1991) Model for Vapor intrusion into Buildings (revised) 

http://www.epa.gov/superfund/programs/risk/airmodel/johnson_ettinger.htm 

U.S. Environmental Protection Agency (EPA), 1997; Exposure Factors Handbook, National 

Center for Environmental Assessment, Washington, DC; EPA-600-P-00-002B available at 

http://www.EPA.gov/NCEA

Kissel, J., K. Richter, and R. Fenske, (1996), Field Measurements of Dermal Soil Loading 

attributable to Various Activities: Implications for Exposure Assessment. Risk Anal. 15(1)116-

125. 

Kissel, J., J. Shirai, K. Richter, and R. Fenske, (1998), Investigation of Dermal Contact with Soil 

in Controlled Trials. J. of Soil Contamination 7(6) 737-752 

Klepeis, NE, Nelson WC, Ott WR, Robinson JP, Tsang AM, Switzer P, Behar JV, Hern SC, 

Engelmann WH, The National Human Activity Pattern Survey (NHAPS): A Resource for 

Assessing Exposure to Environmental Pollutants, Journal Of Exposure Analysis and 

Environmental Epidemiology, 11 (3): 231-252 May-Jun 2001.

OEHHA, 2000; California Office of Environmental Health Hazard Assessment; Air Toxics Hot 

Spots Program Risk Assessment Guidelines: Part IV - Technical Support Document for 

Exposure Assessment and Stochastic Analysis 

OEHHA, 2002 Office of Environmental Health Hazard Assessment, California Environmental 

Protection Agency 

OEHHA, 2003; “SCHOOLSCREEN.xls” Draft Schools Risk Screening Model, a Spreadsheet 

Adaptation of the Guidance for Assessment of Exposures and Health Risks at Existing and 

Proposed School Sites, Office of Environmental Health Hazard Assessment, California 

Environmental Protection Agency, February 5, 2004 

RTI 2003a, Portable Classrooms Study 
2/5/2004 32

RTI 2003b, Addendum To The California Portable Classrooms Study Soil Sampling And Metals 

Analyses 

University of California, Davis, 1994, CaltoxTM , a Multimedia Total Exposure Model for 

Hazardous Waste Sites - Spreadsheet Users Guide version 1.5 
A1-1 

Appendix 1: Summary and Interpretation of Results of RTI Study 

OEHHA-recommended values for three exposure parameters in the schools risk assessment 

model are based on RTI (2003): 

1) Composition of interior surface dust with respect to outdoor soil 

2) Classroom ventilation rates 

3) Days of instruction per year 

Composition of interior surface dust with respect to outdoor soil

RTI investigated the relationship of the concentration of nine elements, (As, Cd, Cr, Cu, Pb, Ni, 

Se, Vd, Zn) in soil at 67 school sites to the concentration of those elements in classroom floor 

dust (RTI, 2003). The concentrations of three of the nine elements, chromium, nickel, and 

vanadium, were significantly correlated (r = 0.55–0.64, p<0.001) between these two media. The 

95 percent upper confidence limits (UCL95) on the median dust/soil concentration ratios for these 

three elements were 1.90, 2.54, and 1.53, respectively (Table A-1). OEHHA, therefore, 

recommends a default value of 2 for the “transfer factor from soil to indoor dust” (TFSD), based 

on the mean of the three median UCL95s. 

Table A-1: Indoor Dust to Soil Ratios 

 Correlation Significance Median Ratio 95% C.I. 

Arsenic 0.19 0.10 1.88 1.62-2.05 

Cadmium 0.06 0.58 2.95 2.51-4.00 

Chromium 0.64 <0.001 1.71 1.48-1.90 

Copper 0.03 0.81 2.95 2.39-3.48 

Lead 0.17 0.13 3.07 2.27-3.81 

Nickel 0.58 <0.001 2.18 1.81-2.54 

Selenium 0.19 0.09 0.20 NA1-0.73

Vanadium 0.55 <0.001 1.37 1.26-1.53 

Zinc 0.07 0.55 9.67 7.05-13.64

1 LCL on ratio not calculated due to values below the detection limit. 

Other elements studied had lower correlations, but with the exception of selenium and zinc, 

still had similar median dust/soil ratios in the range of 1.88 to 3.07. The ratio for selenium is 

not reliable because of failure to detect selenium in some samples. The high dust/soil ratio 

for zinc is unclear, but could be the result of some (unknown) indoor source of zinc. 

Classroom Ventilation Rates

RTI (2003a) reported average outdoor airflow into the classrooms of 0.8737 c.f.m. per ft2 of 

floor area (95% C.I. = 0.7894-0.9579)(Table A-2). No data were collected on classroom 

volume. Assuming a ceiling height of 10 feet, this would yield an average classroom 

ventilation rate of 0.087 (95% C.I. = 0.079-0.096) c.f.m. per ft3

 (i.e. changes per minute). 

Multiplying by 60 min/hr yields a mean exchange rate of 5.2 changes per hour (95% C.I. = 
A1-2 

4.7-5.7). OEHHA recommends a default air exchange rate of 4.7/hr, based on the 95 percent 

LCL on the mean. 

Table A-2: Classroom Ventilation Rates

Parameter units mean 95% C.I. 5-95 percentile 

Outdoor air flow/sq.ft. cfm/ft2 0.8737 0.7894-0.9579 0.3179-1.3854 

Days of Instruction per year

Table A-3 shows the reported number of planned school days for the current school year for 

the 54 California schools that interpreted the question correctly (RTI, 2003b). The bimodal 

distribution suggests that a single value may not adequately represent the data. Therefore 

OEHHA recommends 180 days per year for traditional 3-season schools and 232 days per 

year for year-round schools. 

Table A-3: Days of Instruction per year

Number of Days Number of 

Schools

Percent Cumulative 

Percent

Less than 180 days 9 16.7 16.7 

180 days 34 63.0 79.6 

181 to 187 8 14.8 94.4 

188 to 227 0 0 94.4 

228 to 238 3 5.6 100.0 
A2-1 

Appendix 2: Comments and Responses 

UNIVERSITY OF CALIFORNIA PEER REVIEWER COMMENTS 

William W Nazaroff 

Department of Civil and Environmental Engineering 

University of California, Berkeley 

OVERALL SUMMARY COMMENT 

Overall, I found the document clear, generally well organized and easy to follow. The goals, 

methods, and limitations of the approach are well expressed. The document is also concise, 

which is a virtue. I have a concern about the overall scope of the document. Some potentially 

important exposures are not being considered and the justification for the omission is unclear. 

See below for detailed discussion. 

Technically, the equations for assessing exposure and analyzing risk are generally appropriate 

for a screening-level assessment. Some improvement in how the parameters are presented and 

discussed would strengthen the report. I like that a sensitivity analysis was conducted and is 

presented. I think that the identification of parameters that contribute most to uncertainty should 

be expanded. A few concerns and several specific suggestions for improvement are described in 

greater detail in the following section of this review. 

DETAILED COMMENTS 

1. Comment - Scope of Guidance (page 8) 

“Exposures to chemicals in building materials and furnishings and chemicals used in schools are 

beyond the scope of this guidance.” It is not clear that excluding such exposures is responsive to 

the legislation that mandated this document. I find nothing in Health and Safety Code Section 

900-901 that justifies such exclusion. Even if this can be justified, some other exposures could be 

of considerable concern and are neither addressed in the document nor specifically excluded. In 

particular, exposure to emissions from diesel school buses and to herbicides or pesticides used on 

the school grounds should be considered.

Response 

Health and Safety Code Section 901(f) states that OEHHA is to develop guidance for 

assessing exposures and risks at existing and proposed schoolsites. The following text has 

been added to the Guidelines to clarify this issue (see page 6): “Building materials and 

indoor products may be important sources of indoor exposure to toxic constituents at schools. 

It may be appropriate to include these sources of chemical exposures in the overall 

assessment of overall hazards and risks at existing schools. Typically these assessments 

would be based on measured atmospheric concentrations in classrooms and other indoor 

areas, and estimated risks, using the same exposure parameters, would be added to siterelated risks. Hazards would be additive among chemicals sharing a common target organ 
A2-2 

and/or mode of action.” See also added text on page 2: “Representative measured contaminant 

concentrations in various environmental media may be used in lieu of modeled values.” 

Comment 

In the section “Schools Conceptual Site Model” it is stated that “this model considers 

contaminated soil and shallow ground water as primary source media.” In fact, the exposure 

assessment equations include inhalation exposure to contaminants that originate offsite (such 

as at an upwind freeway or industrial facility). The inclusion of these sources should be 

clarified in this section. 

Response 

The wording on page 8 has been revised to include the following sentence: “As depicted in 

Figure 1, this model considers contaminated soil, shallow ground water, and unspecified offsite sources as primary source media.”

Comment 

Table 1 — Exposure Pathways (p. 11) Pathways 9 and 10 can lead to exposures by 

inhalation of indoor air as well as outdoor air. The equations incorporate this pathway. The 

column “exposure medium” should be modified to reflect this.

Response 

Figure 1 is revised to reflect this change: an arrow now indicates that outdoor particulate and 

vapor-phase contaminants can move indoors.

Comment 

3. The term “Fraction of school soil/dust ingestion that occurs indoors” (FI p. 12) appears in 

a few pathway equations, but is not explicitly defined in the later section on Exposure 

Parameters.

Response 

The text on page 21 has been revised to include the following definition: Fraction 

indoors (FI) is the estimated fraction of the daily school-related dermal and ingested 

soil/dust exposure that is acquired indoors. It is calculated as 1- FO. 

Comment 

Concentration of PM10 in outdoor air, PM10 (p. 12). In the pathway equations, this 

parameter needs to be more carefully defined. It is the PM10 concentration on-site 

because of emissions from site soil. In fact, this will be a small fraction of the total PM10 

concentration. Failure to clarify the distinction could cause important confusion. Also, 

below the equation for pathway 5, the CPM10 parameter should have its units 

specified. 
A2-3 

Response 

The definition of PM10 has been revised as follows: 

PM10 = Respirable particle load for outdoor air due to resuspension of site soil (gPM/Lair). 

The units “μg/g” have been added to the definition of CPM10 in equation 5. 

Comment 

5. Respirable particle load for indoor air, SF (p. 12 and p. 15). The definition of this parameter 

should be more carefully delimited. It refers to the indoor air concentration of crustal materials that 

originated on the site.

Response 

The equation has been revised to treat indoor suspended particles as a function of indoor dust, 

which is, in turn, a function of outdoor soil. The default transfer factors are one (1) and two (2), 

respectively (See pathway 8 page 15).

Comment 

6. Pathway 8 should include inhalation of indoor air (p. 13). If a chemical is vaporized from soil 

into outdoor air, then that chemical can enter indoor air with ventilation and be inhaled there. This 

is a distinct pathway from direct intrusion of the vapor into the building from the soil (as addressed 

by the Johnson & Ettinger model). This pathway, therefore, should have a term (BI*TI + BO*TO) 

in place of (BO*TO). 

Response 

The term (BI*TI + BO*TO) has been added to pathway 8 (See page 15).

Comment 

7. Clarify that penultimate paragraph on p. 14 refers to the case of noncarcinogens only.

Response 

We presume that the comment refers to the following paragraph: 

Each of these equations gives a pathway-specific annual average daily dose of the chemical in 

question. The pathway-specific annual average daily dose is divided by the route-specific reference 

dose (RfD) to arrive at the pathway-specific hazard quotient (HQ). The pathway-specific HQs are 

added to give the chemical-specific HQ. In a screening analysis, the chemical-specific HQs for each 

chemical are added to give the Hazard Index. In a more detailed (tier 2) analysis, target organs and 

mechanisms of toxic action may be considered in determining the appropriateness of adding the HQs 

for individual chemicals. 

The paragraph refers to non-carcinogenic effects of both carcinogenic and non-carcinogenic 

chemicals. Carcinogenicity is handled separately.

Comment 

8. Provide a logical ordering of Intermedia Transfer Coefficients and Exposure Parameters 

(p. 15-21). It would be easier to follow the developments if the presentation and discussion 

of the parameters had a transparent logical structure. For example, a table could be provided 
A2-4 

that listed all of the parameters in alphabetical order (according to the symbols), defined 

them in words (1 line), and listed the page number on which the parameter value is discussed.

Response 

A column has been added to tables 1 and 2 to indicate where each parameter is discussed.

Comment 

9. Clarification of transfer factor from soil to outdoor particulate matter, TFPM/S (p. 15). 

The text should make clear that this is the transfer factor that relates the contaminant 

concentration in soil to the contaminant concentration in the outdoor PM that results from 

suspension of site soils. Because of windborne transport, the total PM10 level on site would 

often be much larger than that resulting from releases on site.

Response 

The TFPM/S has been re-defined as the ratio of the concentration of contaminant in outdoor 

PM10 originating from site soils to the concentration of contaminant in soil (see page 13).

Comment 

10. Default parameters for Johnson and Ettinger model (p. 15): I believe that the default 

conditions for the J&E model are appropriate for residential construction but may not be for 

schools. Details like the height of the building, the land area it covers, and the ventilation rate 

may be different between schools and residences. This issue requires some attention, at least 

at the level of further discussion in the document.

Response 

The air exchange rate has been increased to 4.7 changes per hour (6.13e+5 cm3/sec) based on 

(RTI, 2003) 

Comment 

11. Fraction at school, FS (p. 17): Clarify that this parameter represents the fraction of a 

school day (as opposed to an average day) that an exposed individual spends at school.

Response 

Revisions on pages 13 and 20 indicate that FS represents the fraction of a school day.

Comment 

12. Update NHAPS data and reference? (p. 19): Assuming that the appropriate information 

is contained there, the reference to Tsang and Klepeis (not “Klepis”) should be updated to the 

following archival report: Klepeis NE, Nelson WC, Ott WR, Robinson JP, Tsang AM, 

Switzer P, Behar JV, Hern SC, Engelmann WH, The National Human Activity Pattern 

Survey (NHAPS): a resource for assessing exposure to environmental pollutants, JOURNAL 

OF EXPOSURE ANALYSIS AND ENVIRONMENTAL EPIDEMIOLOGY, 11 (3): 231-

252 MAY-JUN 2001. 

Response 
A2-5 

The reference has been changed.

Comment 

13. Table 3 typgraphical error: Change “Cm” to “cm2” in 6 places in the second column.

Response 

The error has been corrected. 

Comment 

14. Strengthen Uncertainty Analysis: The sensitivity and uncertainty analysis is a welcome 

part of this report. Table 4 presents an important, but only partial picture of the degree to 

which different parameters affect the outcome, by indicating the change in intake per unit 

change in each parameter. The other important factor, recognized in the text, is how variable 

and how uncertain the parameters are themselves. Certain parameters vary only over narrow 

ranges, e.g. a factor or two or less. Other parameters can vary over orders of magnitude. The 

parameters that are narrowly variable and well characterized (so possessing low uncertainty) 

are not large contributors to uncertainty in the outcome, regardless of the sensitivity of the 

outcome to the parameter. I would like to see Table 4 complemented by another table that 

provides some indication of the variability of each parameter, and the likely degree of 

uncertainty in its determination. The combination of all of these would provide an overall 

sense of which of the parameters is most important in the overall uncertainty of the risk 

assessment.

Response 

An ‘uncertainty” column has been added and the discussion revised (see page 28).

Comment 

15. Improve reference list: Wherever possible, web links should be provided to government 

reports. 

Response 

Web links have been provided where appropriate. 
A2-6 

Michael T. Kleinman 

Department of Community and Environmental Medicine 

University of California, Irvine

General Comments: 

This guidance is intended to: 

• Support assessment of chemical exposures and health risks at existing and proposed 

school sites, 

• Characterize uncertainty in assessing exposure and risk in the school setting, 

• Suggest which areas are most in need of further research. 

To this end, the guidance addresses the differences between children and adults, and between 

the school setting and other settings. Recognition is given to the concept that children differ 

from adults anatomically, physiologically, and behaviorally in ways that affect their exposure 

to environmental contaminants. A modeling approach is used to predict exposures and risks 

to preschoolers, students, teachers and other school personnel, and their offspring, from 

chemicals in the soil, shallow ground water, and air at the school site. A spreadsheet is 

provided which facilitates the application of the model to estimating exposures. Overall the 

guidance provides and integrated exposure assessment approach, and achieves many of the 

objectives for the guidance. There are, however, some areas in which the guidance could 

have been improved. Some examples of these are provided below. 

Comment

Other sources – the document states “in some cases, it may be appropriate to add in 

additional sources of chemicals in the environment. For example there may be off-site 

emissions that may impact on-site concentrations”. It is not clear what those sources might 

be and some concrete examples would be helpful. 

Response

The following language has been added to the Guidelines to clarify this issue (see page 11):

“Atmospheric emission sources within 1⁄2 mile of the site which have the potential to contaminate onsite air may be important in estimating overall toxic exposures. Examples could include fixed 

facilities with known emissions and mobile sources such as highways, heavily traveled streets, or 

vehicle loading areas.”

Comment 

1. In addition, it is stated that “Exposures to chemicals in building materials and 

furnishings and chemicals used in schools are beyond the scope of this guidance” No 

justification is given as to why these on-site sources would not be considered while some 

unspecified off-site sources would be included. An example that comes to mind would 

be radon from building materials, formaldehyde from floor coverings etc. The guidance 

should provide a generalized approach as to how these factors could be included in a total 

exposure assessment. 
A2-7 

Response

The following text has been added to the Guidelines to clarify this issue (see page 12): 

“Building materials and indoor products may be important sources of indoor exposure to 

toxic constituents at schools. It may be appropriate to include these sources of chemical 

exposures in the overall assessment of overall hazards and risks at existing schools. 

Typically these assessments would be based on measured atmospheric concentrations in 

classrooms and other indoor areas, and estimated risks, using the same exposure parameters, 

would be added to site-related risks. Hazards would be additive among chemicals sharing a 

common target organ and/or mode of action.” 

Comment 

2. Target Organs – The document states “Hazard quotients and incremental risks are 

estimated for each chemical; then the hazard quotients and incremental risks associated 

with the individual chemicals are added to arrive at the total hazard index and total risk. 

If the total hazard index does not exceed one, then it may be assumed that the non-cancer 

toxic effects are unlikely and further analysis of non-cancer effects is not necessary. If 

the total hazard index exceeds one, it may be useful to separate chemicals by target organ 

and/or mode of action and add the hazard quotients of only those chemicals that are likely 

to act in an additive manner. This target organ/mode of action analysis should be 

documented.”. Having said that it would have been useful for the document to specify 

precisely how such a target organ approach might be addressed, at least in general terms. 

Thus it would be very useful if the worksheet of potential contaminants could be indexed 

with respect to target organs to facilitate the computation of target organ specific hazard 

indices as suggested in USEPA OAQPS 2001. 

Response 

We agree that it would be useful if the worksheet of potential contaminants were indexed 

with respect to target organs to facilitate the computation of target organ specific hazard 

indices; this is something we will address as resources permit. 

Comment 

3. The approach taken to distinguish between characteristics of children in different age 

groups is reasonable and appropriate. 

Response 

No response 

Comment 

4. The factor that lowers enthusiasm for this guidance the most is that it treats school 

exposure for the most part in a vacuum, i.e. personal exposure of individuals also 

includes exposures incurred away from school and from other sources. The risks from 

the school exposures, if any, are only a part of the overall risk. While it may be beyond 

the scope of this specific document to estimate these risks, it should be clearly 

expostulated within the document that children in different parts of California have 

different background exposures. It would seem to be important that these be considered 

at least in part before determining that exposures to be incurred in a school setting are 
A2-8 

acceptable. Some suggestions should be included as to how these background risks can 

be estimated – at least on some average level. There are some modeling approaches 

(ASPEN) and emissions inventories (Toxic Release Inventory) that could be used to 

establish and update potential community background levels for specific chemicals found 

on the school site so that the school exposure does not represent the “straw that breaks the 

camel’s back”. 

Response 

A given incremental dose of a toxic constituent that would otherwise be of no concern, may 

be a concern if the receptor is already receiving a background dose of the constituent near the 

toxic threshold. For this reason, risk managers may wish to take background exposures into 

account in their decision-making process. This is suggested in new language added to the 

Guidelines (see page 12). The “camel’s back” phenomenon would impact only those sites 

where risk management decisions are driven by non-carcinogenic effects. Since carcinogens 

are treated as exhibiting no threshold, background exposure levels would not affect the 

incremental risk posed by the contamination at the school site. 

Comment 

5. The document states “Concentration of site-related particulate material less than 10 

microns in diameter in outdoor air (PM10). OEHHA recommends a default value of 1.8 

E-9 g PM/L (1.8 μg/m3

). This value is based on the EPA Soil Screening Levels 

document (EPA, 1996)”. This value seems to be very low when contrasted with 

measurements of resuspended dust in California made by the Air Quality Management 

District. On an annual basis resuspended surface material contributes about 20-30 

percent of PM10. Given the average PM10 concentration in California is higher than that 

in most communities in the US, a more realistic default value would be on the order of 5 

to 10 μg/m3

. The authors should consider increasing this default value and re-estimating 

the sensitivity to this parameter. 

Response 

The proposed value is for respirable particulate matter resuspended from the site. If the 

suggested value of 5 to 10 μg/m3

 represents total resuspended surface material, then the 

proposed value of 1.8 μg/m3

 would represent 18 to 36 percent of the total, which we believe 

is a conservative estimate of the fraction of total resuspended surface material that would be 

from the site. 

Comment 

The use of the spreadsheet was mentioned but it would be extremely helpful to provide the 

documentation and instructions as a appendix to the guidance document. 

Response 

A new sheet “User’s Guide” has been added to the spreadsheet, in order to clarify how the 

spreadsheet should be used. We have avoided making the spreadsheet part of the Guidance 

in order to avoid the implication that the spreadsheet must be used in order to comply with 

the guidance.
A2-9 

Comment 

I attempted to use the spreadsheet, and inserted Soil Cleanup Levels for several toxic 

compounds. I estimated that at a benzene level of 60 μg/kg soil the exposure produced a 

cancer risk of 2.5 x 10-5. Other toxic compounds produced higher values. I presume that I 

did the calculations incorrectly but it would be useful to check the spreadsheet’s output with 

values taken form some set of soil cleanup guidelines. 

Response 

The spreadsheet gives a single-highest-year risk estimate of 1.6e-8 for 1-2 year-olds at a 

benzene level of 60 μg/kg soil. There is reason to expect that the estimated risk at a soil 

concentration corresponding to a Soil Cleanup Levels would not be 10-6 if the Soil Cleanup 

Level is based on a scenario other than the school scenario. The legislature mandated that 

we develop methodology for estimating risk at schools based on the assumption that risk at 

schools would be different from risk associated with residential, commercial, and other 

scenarios. 

COMMENTS RECEIVED DURING THE PUBLIC COMMENT PERIOD

December 20, 2002 – January 30, 2003 

AND PUBLIC WORKSHOP, January 24, 2003 

Dr. Mark C. Rigby, Tetra Tech 

Comment

“The purpose of this [PEA] screening evaluation is to provide the risk manager with an 

estimate of the potential chronic health hazard from contamination at the site. The 

anticipated use of this screening evaluation is to assist the risk manager in deciding whether 

further site characterization, risk assessment, or remediation is necessary." The objective of 

the PEA at a proposed school site is to provide a timely and health-protective screening level 

evaluation, as stated in the quote above from the PEA manual. 

Response

The draft guidance document was prepared to comply with California Health and Safety 

Code Section 901(f), which requires OEHHA to develop and publish a guidance document 

for use by the Department of Toxic Substances Control (DTSC) and other state and local 

environmental and public health agencies to assess exposures and health risks at existing and 

proposed school sites. Although DTSC may choose to use the Guidance within the PEA 

framework, there is no such requirement in the law. Other agencies may use the Guidance 

in other contexts such as CEQA. The law mandates the use of “appropriate child-specific 

routes of exposure unique to the school environment, in addition to those in existing 

exposure assessment models” and the “identification of uncertainties in the risk assessment 

guidance and those actions that should be taken to address those uncertainties.” 
A2-10

Comment 

The exposure equations are not in the standard form given in U.S EPA's RAGS and use a 

different terminology. 

Response

While some minor aspects may differ, the general form of the equations is consistent with 

U.S EPA's RAGS, i.e. the concentration in a contact medium times a contact rate with that 

medium times an absorption rate to give a daily dose, which, when multiplied by an 

exposure frequency (expressed as a fraction of a year) gives an annual average daily dose. 

The annual average daily dose, multiplied by exposure duration divided by averaging time 

gives a lifetime average daily dose. 

Comment 

The dermal exposure equation provided in the draft guidance is more complicated than that 

given in RAGS. 

Response 

The draft guidance follows the methodology found in the EPA Children’s Exposure Factors 

Handbook, which more accurately describes the way soil adheres to skin in a “real-world” 

situation. 

Comment 

Several exposure parameters not in RAGS are used, for which the default factor is assumed 

to be 1. Eliminating these exposure factors would simplify and expedite the use of the 

guidance and would not change the outcome from the default scenario. The exposure 

parameters that with a default of 1 are: AI AD TFSD TFPM/S TFI/O Ain. The indoor dust 

pathway, as provided in the default form given in the draft guidance, does not differ from 

the outdoor dust/soil exposure pathway. As such, it is redundant. 

Response 

A guiding principle in developing this guidance was that implicit assumptions should be 

made explicit. Besides making the methodology more transparent, this allows for replacing 

the value of one (1) with a chemical-specific value other than one, when new data support an 

alternative value. The legislative mandate requires identification of uncertainties in the risk 

assessment guidance and those actions that should be taken to address those uncertainties. 

These transfer factors are uncertain and therefore must be included in order to capture the 

uncertainty. A discussion of the factors mentioned in the comment follows:

TFSD, TFPM/S, and TFI/O are transfer factors, not exposure parameters. Since RAGS does 

not deal with intermedia transport, these parameters would not be expected to be in RAGS. 

TFSD - OEHHA currently recommends a default value of 2 for TFSD, based on the results 

of recent research. 
A2-11

AD - Absorption by the dermal route is found in RAGS and chemical-class-specific values 

are proposed. 

AIn and AI - Absorption by the inhalation and ingestion routes have implicit values of unity 

in RAGS. This guidance makes the value of unity explicit. 

Comment

Exposure parameters are given for 1 year intervals for children. This requires that the 

consultant derive appropriate exposure parameters for each site de novo, demonstrate that 

they are appropriate, and that DTSC approve them. This may lengthen the PEA process for 

each site. To expedite the process, default exposure parameters could be provided for the 

most frequent types of schools, e.g. pre-schools, Kindergartens, Elementary schools, Middle 

schools w/elementary schools, Middle schools, High schools w/middle schools, and High 

Schools. Providing the yearly exposure parameters in an Appendix would allow consultants 

to derive specific exposure parameters for those cases that did not fit into the defaults given 

above.

Response 

Default exposure parameters are provided for each year from birth through age seventeen 

and for adults. The burden of gathering this information is not placed on the user. 

Default parameters for multi-year periods were considered and rejected because this method 

would base the hazard index on an average exposure for a multi-year time period and would 

not capture the single highest year. Furthermore, the use of individual years gives the 

assessor and the reviewing agency more flexibility. While individual years can be 

aggregated into groups to match a proposed exposure scenario, multi-year bins can not be 

easily disaggregated. Disaggregation would be necessary if the assessment period did not 

match the exposure scenario. For example some districts have elementary schools covering 

grades K-6, while others have primary schools from K-3 and middle schools from 4-6 on 

different sites. 

Comment 

The draft guidance provides a simplified equation for determining outdoor air concentrations 

of volatiles that have migrated from subsurface soil or shallow groundwater sources. The 

consultant need only supply the chemical concentration in soil/groundwater and the 

chemical-physical properties. For indoor air, however, the draft guidance states that the 

Johnson and Ettinger model from USEPA should be used, but no defaults are supplied. The 

PEA process would be greatly expedited if default building and soil properties, as well as 

contaminant depths, were supplied. If this were done, a simplified model could be derived 

(akin to the VF emissions model) that only requires the input of chemical concentration in 

soil/groundwater and chemical-physical properties. 

Response 

OEHHA is proposing a default air exchange rate of 4.7 changes per hour (613,426 cm3

/sec in 

the EPA indoor air model with the default building volume – any changes in building volume 

would have to be accompanied by corresponding changes in flow rate to maintain the same 

exchange rate), based on recent studies of California classrooms (RTI, 2003).(see Guidance 
A2-12

page 17). Default parameter values will be considered for other parameters. DTSC’s indoor 

air working group is developing recommended default values for some parameters. 

Comment 

To expedite the PEA process as much as possible, default simplified risk assessment 

equations could be provided. Such equations were provided in the original PEA manual 

(DTSC 1999) and only require the input of chemical concentrations and toxicity values (in 

addition to any modeling necessary to calculate concentrations).

Response 

Default risk assessment equations are provided. They are not simplified, but they only 

require the input of chemical concentrations in selected media and toxicity values (which are 

provided for some chemicals). These risk assessment equations have been incorporated into 

a spreadsheet, which can be recalculated virtually instantaneously by most computers. While 

it would be possible to further simplify the equations by collapsing all exposure parameters 

into a single pathway exposure factor as the PEA does, this would sacrifice transparency and 

the ability to substitute case-specific parameters in a tier 2 assessment when appropriate. 

Comment 

The draft guidance includes the assessment of risks from the migration of offsite dusts and 

vapors to the school site. This is more appropriately addressed in an EIA/EIS. Risk 

assessments normally evaluate the risks from contaminants that originate at the site. 

Response 

This guidance may be used in a variety of contexts including environmental impact analysis. 

For some purposes, some pathways in the model may not be appropriate. Pathways may be 

eliminated with the approval of the reviewing agency.

Comment

If local background is evaluated in the risk assessment, it is to subtract the risks from local 

background from the site-specific risks 

Response 

The guidance addresses methodology for estimation of dose and risk from environmental 

contamination at a proposed or existing school site. Contaminant source allocation, and 

management of contamination are in the domain of risk management, and are outside the 

scope of this guidance. 

Comment

Draft USEPA guidance is cited as the source of some of the information. However, by its 

very nature, draft guidance is rather labile. 

Response

The guidance referred to is now interim final. 
A2-13

Bill Piazza, Los Angeles Unified School District (LAUSD) 

Comment 

In general, the District agrees with the refined methodology recommended in the draft 

assessment protocol which allows consideration of "reasonable" exposures anticipated 

to occur at school sites. Nevertheless, the methodology is unnecessarily specific in its 

attempt to quantify risk. The District contends that until toxicity factors are developed 

for school aged children, the quantification of risk for each grade level will not reveal 

a significant difference over the risk value predicted with average exposure factors for 

a given occupancy. 

Response 

For some chemicals, the estimated dosage in the first year of life is as much as 2.7 times the 

average for birth through age seventeen. OEHHA considers a 2.7-fold difference worth 

considering. And once the algorithms are incorporated into a spreadsheet, the extra 

calculations are little, if any, extra effort. 

Comment 

In addition, the methodology is not consistent with existing assessment methodologies 

utilized for the Safe Drinking Water and Toxic Enforcement Act (Proposition 65), Air Toxic 

"Hot Spots" Information and Assessment Act (AB 2588) and related California Air 

Resources Board (ARB) assessment activities prepared under the auspices of the Toxic Air 

Contaminant Identification and Control Act (AB 1807). The District's concern is 

exemplified with the pending adoption of ARB's Regulation Order to limit school bus idling 

and idling at schools. The purpose of the air toxic control measure is to "reduce public 

exposure, especially school aged children's exposure" to pollutants by "limiting unnecessary 

idling" of specified vehicular sources "at and around schools and while riding school buses 

and other types of school transportation." Please note that the ARB utilized the assessment 

methodology outlined in OEHHA's Air Toxic Hot Spot Program Risk Assessment 

Guidelines, Part IV, Technical Support Document for Exposure Assessment and Stochastic 

Analysis to establish a set of defined control measures. The specific exposure assumptions 

are presented in ARB's Staff Report: Initial Statement of Reasons (ISOR), Appendix C: 

Idling Diesel School Bus Health Risk Assessment Methodology. The District believes that 

due to the regulatory nature of the assessment, which identifies operational controls for 

school bus owner/operators to actually reduce school-based exposures, justifies its use as an 

appropriate methodology. Nevertheless, the District is aware that one may argue that the 

various State agencies and their associated regulatory programs require different 

methodologies to assess risk. As such, assumptions such as exposure frequency and duration 

may differ producing varying risk values for a given exposed population. However, the 

District believes that a school is a school regardless of the specific regulatory program. To 

argue that one agency should assess school exposures with one set of assumptions while 

another consider different exposure variates for the same occupancy, promotes a lack of 

consistency between the various State boards and departments and does little to encourage 

the harmonization in the practice of risk assessment within Cal/EPA. As a result, the 

assessment of a school-based occupancy must be consistent with all programs which quantify 

risk for this sensitive subpopulation. 
A2-14

Response 

This Guidance also utilizes some of the assessment methodology and parameter values 

outlined in OEHHA's Air Toxic Hot Spot Program Risk Assessment Guidelines, Part IV, 

Technical Support Document for Exposure Assessment and Stochastic Analysis. To the 

extent that there are differences, these reflect the different mandate for this program. 

Comment 

1. Units of measure are not consistent with industry standard. This may present an 

unnecessary source of error upon unit conversion and present some difficulty in reviewing 

empirical data and related workbook calculations. For example: 

• Soil concentration (e.g., ug/g to mg/kg) 

• Particulate airborne concentrations (e.g., ug/l to ug/m3) 

• Volatile airborne concentrations (e.g., ug/l to ug/m3)) 

• Cancer Potency Factors (e.g., ug/kg/day to mg/kg/day) 

Response 

• Soil concentration units have been changed to mg/kg 

• Particulate and volatile airborne concentrations have been changed to mg/l 

• Cancer Potency Factors units have been changed to (mg/kg/day)-1 

Comment 

2. Calculation of the hazard index does not consider toxicological endpoints. This is not 

consistent with existing guidance (OEHHA, 2000). The inclusion of this refinement is most 

relevant as many removal actions currently undertaken by the District are based upon 

screening values. As such, unity may be exceeded necessitating an unwarranted response 

action. The ability to readily identify and quantify the hazard index should be included in 

the proposed methodology. The following excerpt from U. S. EPA's Risk Assessment 

Guidance for Superfund Volume I - Human Health Evaluation Manual (RAGS) underscores 

the viability of the District's concern regarding dose additivity. Another limitation with the 

hazard index approach is that the assumption of dose additivity is most properly applied to 

compounds that induce the same effect by the same mechanism of action. Consequently, 

application of the hazard index equation to a number of compounds that are not expected to 

induce the same type of effects or that do not act by the same mechanism could overestimate 

the potential for effects, although such an approach is appropriate at a screening level. This 

possibility is generally not of concern if only one or two substances are responsible for 

driving the HI above unity. If the HI is greater than unity as a consequence of summing 

several hazard quotients of similar value, it would be appropriate to segregate the 

compounds by effect and by mechanism of action and to derive separate hazard indices for 

each group. 

Response 

Calculation of the hazard index does consider toxicological endpoints. See pages 8 and 25. 

Comment 

3. Several exposure variates differ from existing guidance (OEHHA, 2000). Many are taken 

from a draft guidance document (U.S. EPA, 2000). If these values are more appropriate, 

then OEHHA should revise current guidance for consistency. For example: 
A2-15

• Skin surface area (U-S- EPA, 2000) 

• Breathing Rates (not consistent with OEHHA recommended values) 

• Body Weights (U.S. EPA, 2000) 

Response 

The Child-Specific Exposure Factors Handbook is now a citable Interim Report 

The skin surface area data were generated in a setting similar to a school environment (day 

care) and are therefore relevant for school exposure estimation. 

The OEHHA-recommended breathing rates (Table 3.22, in Technical Support Document for 

Exposure Assessment and Stochastic Analysis, OEHHA, September 2000) are for assessment 

of long-term average exposures. They are not activity-specific as required. 

The recommended body weights have been revised to agree with Tables 10.1 (staff) and 10.3 

in (Technical Support Document for Exposure Assessment and Stochastic Analysis, 

OEHHA, September 2000). 

Comment 

4. Exposure times should be reviewed and revised, as appropriate, following input from 

school district personnel. Stakeholder input, rather that U.S. EPA's draft documentation, 

should be utilized to develop viable exposure times. For example, OEHHA assumes most 

kindergarten students spend over nine hours per day at school. To the contrary, most 

kindergarten students spend no more than 4 hours per day at school. 

Response 

The guidance has been revised to reflect recent survey data (RTI, 2003) with respect to 

exposure frequency. Children 0-6 may be in day care for a full school or work day.

Comment 

5. Uncertainty with the use of the Johnson and Ettinger Model should be discussed. OEHHA 

should address concerns raised regarding the model's accuracy before recommending its use. 

Response 

Uncertainty associated with the use of the Johnson and Ettinger Model is discussed in the 

Uncerainty section. 

Comment 

If utilized, the model must be programmed to account for vapor intrusion into institutional 

(e.g., Department of Education approved) buildings and not a single-family residence. For 

example, a default air exchange rate of 0.45 per hour is inappropriate for institutional 

buildings with markedly higher ventilation rates which range from 4 to 7 air changes per 

hour (e.g., 50 CFM per person). 

Response 

The guidance has been revised to reflect recent survey data (RTI, 2003) with respect to 

default air exchange rates. 
A2-16

Comment 

6. The school's conceptual site model must be further defined. School-based exposures must 

be plausible and likely to occur for a given occupancy. A discussion similar to U.S. EPA 

guidance must be included to further define "reasonable" exposure pathways. The District's 

recommendation is exemplified by the following excerpt from RAGS. There are two steps 

required to determine whether risks or hazard indices for two or more pathways should be 

combined for a single exposed individual or group of individuals. The first is to identify 

reasonable exposure pathway combinations. The second is to examine whether it is likely 

that the same individuals would consistently face the "reasonable maximum exposure" 

(RME) by more than one pathway. Identify exposure pathways that have the potential to 

expose the same individual or subpopulation at the key exposure areas evaluated in the 

exposure assessment, making sure to consider areas of highest exposure for each pathway for 

both current and future land uses (e.g., nearest downgradient well, nearest downwind 

receptor). For each pathway, the risk estimates and hazard indices have been developed for a 

particular exposure area and time period; they do not necessarily apply to other locations or 

time periods. Hence, if two pathways do not affect the same individual or subpopulation, 

neither pathway's individual risk estimate or hazard index affects the other, and risks should 

not be combined. Once reasonable exposure pathway combinations have been identified, it is 

necessary to examine whether it is likely that the same individuals would consistently face 

the RME as estimated by the methods described in Chapter 6. Remember that the RME 

estimate for each exposure pathway includes many conservative and upper-bound parameter 

values and assumptions (e.g., upper 95th confidence limit on amount of water ingested, 

upper-bound duration of occupancy of a single residence). Also, some of the exposure 

parameters are not predictable in either space or time (e.g., maximum downwind 

concentration may shift compass direction, maximum ground-water plume concentration 

may move past a well). For real world situations in which contaminant concentrations vary 

over time and space, the same individual may or may not experience the RME for more than 

one pathway over the same period of time. One individual might face the RME through one 

pathway, and a different individual face the RME through a different pathway. Only if you 

can explain why the key RME assumptions for more than one pathway apply to the same 

individual or subpopulation should the RME risks for more than one pathway be combined. 

Response 

All pathways included in the guidance may affect the same child with two exceptions: The 

breast milk pathway does not affect children above one year of age, and the soil ingestion 

pathway does not affect children less than one year of age. Thus, these pathways are not 

additive. A column has been added to Table 2 indicating for each exposure parameter 

whether it is mid-range or upper end, and some discussion of upper end versus mid-range has 

been added to the uncertainty section on page 30. 

Comment 

Some discussion on acceptable level of risk should be introduced. OEHHA administers 

Proposition 65 which supports the State's "level posing no significant risk" of one in one 

hundred thousand (I.0E-05), not the value of one in one million (I 0E-06) as used in 

Environmental Assessment screening guidance and adopted as the Department of Toxic 

Substances Control's acceptable level of risk. As noted above, removal actions currently 

undertaken by the District are based upon these screening evaluations. The District contends 
A2-17

that the "no significant risk levels" established by the State are relevant and appropriate and 

should be considered when refined assessment activities are conducted. 

Response 

Acceptable or target risk levels are in the risk management domain. The guidelines cover 

risk assessment and exclude risk management. 

Comment 

Please note that the District considers all relevant and appropriate exposures to assess risk for 

its existing school occupancies. As such, the District has developed a guidance document 

and associated Excel spreadsheet to quantify school-based risk. The methodology is based 

upon the above referenced technical support document (OEHHA, 2000) with exposure 

parameters assigned by occupancy (e.g., kindergarten through the 6 grade). The District 

believes it is consistent with OEHHA's existing assessment methodology. The program's 

format allows for quick data entry and is robust in its computational ability to quantify risk 

for a suite of identified compounds. The guidance document is included for your review and 

consideration. Staff is currently finalizing the Excel spreadsheet for distribution and will 

forward an electronic copy for your review the week of February 3rd.

Response 

OEHHA has received and reviewed LAUSD’s guidelines. 

Lee Shull and Mark Bowland, Montgomery Watson Harza (MWH) 

General Comment 

As a follow up to recent discussions, this letter presents MWH's review comments on 

OEHHA's draft (File date 10/3/02, header date August 20, 2002) Schools Risk Screen 

Model (Model). The Model we reviewed was provided by OEHHA to Mr. Ernest Silva of the 

Coalition for Adequate School Housing (CASH). Our review has been performed on behalf 

of CASH. Both CASH and MWH greatly appreciate the opportunity to review the Model 

and provide these comments, and look forward to assisting OEHHA however we can as 

the agency continues its development of the schools program. Whereas this letter provides 

substantively technical comments, we will provide our comments on the Model as it relates 

to policy and implementation issues in a separate letter. 

For practical purposes, we have organized our comments on the Model and the associated 

guidance document into four basic areas: (1) identification of potentially fatal 

flaws/errors/omissions in the Model, (2) critical flaws/errors/omissions in the Model, (3) 

suggested user interface improvements, and (4) general/editorial comments. 

Comment 

1. Input-output sheet. Results include a "0-18 + staff' endpoint. This endpoint assumes that 

a child spends the entirety of his/her education (preschool through high school) at a single 

campus, and then post college teaches an entire career at the same campus. This 
A2-18

essentially assumes 43-year exposure duration (ED), which is greater than the current 

residential default assumption. We believe this endpoint is an unreasonable point of 

departure for decision making. We encourage OEHHA to develop a more reasonable ED 

value for inclusion in the model. 

Response 

Which years are aggregated is a case-specific, user/reviewer decision. We have removed the cells 

labeled "0-18 + staff” to avoid the implication that any particular exposure duration is "approved" 

by OEHHA. 

Comment 

2. Groundwater and soil vapor input cell. If a value of "0" is placed in both the groundwater 

and soil vapor concentration input cells, calculation errors (#value!) Prevent production of 

useful risk or hazard values. 

Response 

This error has been corrected. 

Comment 

3. J&E model database. If a chemical that is not part of the J&E model database is selected, 

calculation errors occur due to J&E malfunction. Documentation instructs the user to add the 

chemical to the database, but gives no procedure for performing this function (the sheet is 

password protected). 

Additionally, no guidance is given to direct the user when it is essential to add a volatile 

chemical to the database, or what data must be entered into all the relevant "vlookups" 

sheets for all three J&E model components (i.e., soil, soil gas, groundwater). Additional 

text should be added to the model documentation and model spreadsheets outlining the 

procedures necessary for adding these features. Also, we suggest a single "vlookups" sheet 

for all three model components to reduce the potential for user entry error. 

Response 

The duplicate "vlookup" and "chemprops" sheets have been eliminated, so that the relevant 

information needs to be added in only one place. Guidance has been added as to when and 

how to add chemicals to the database. 

Comment 

4. Route-to-route extrapolation. In cases where route-to-route extrapolation is rejected 

in Input-Output (cells B 13 and/or B 18 given values of "0"), all exposure sheets do not 

reference the user supplied RfD or CSF values for soil direct contact. 

Response 

This error has been corrected. 

Comment 

5. Input-Output Sheet. If a user allows the model to perform route-to-route extrapolation of 

oral and dermal toxicity criteria from inhalation toxicity criteria, the model may use an 
A2-19

inappropriate toxicity metric for the oral and dermal pathways. For example, the inhalation 

cancer slope factor for 1,4-dichlorobenzene is 4 x 10-2. The oral cancer slope factor is 5.4 x 

10-3, or 7 times lower. Similarly, the inhalation cancer slope factor for 1,3-butadiene is 0.6, 

whereas the oral cancer slope factor is 3.4, or six times higher. An additional issue with this 

procedure is the implication that use of inhalation toxicity factors as surrogates for oral and 

dermal exposures is appropriate. For numerous inorganic chemicals (nickel, chromium 6+, 

cadmium) this implication has potentially enormous ramifications, as these inorganics are 

not currently considered carcinogens by the oral routes. We suggest adding a toxicity 

criteria table containing both oral/dermal and inhalation toxicity criteria, or defaulting to a 

user input toxicity criteria for each route and for each chemical for the oral and dermal 

pathways. 

Response 

The intent is not to automatically default to inter-route extrapolation. In order to make this 

intent clearer, the spreadsheet now uses an exclusive user-supplied CPF and RfD as an 

alternative when these parameters are not in the database. The reviewer should ensure that 

the assessor documents any user-supplied CPFs or RfDs.

Comment 

6. Input-Output Sheet. Results for cancer risk for scenario 0-4 sums 0-5 results, 5-10 

scenarios sums 0-6 results, not 5-10 results. 

Response 

The error for 5-10 year olds has been corrected. The sum for ages 0-4 should include 4-5, 

since these are four-year-olds.

Comment 

7. Volatilization assumption It is doubtful that DDT (and similar semi-volatile compounds) 

would actually volatilize in appreciable concentrations. It is our opinion that semi-volatiles such as 

DDT should not be modeled to indoor air. 

Response 

We agree that DDT and similar semi-volatile compounds will not volatilize in appreciable 

concentrations, because of the low volatility of these compounds. For example, the indoor air 

pathway contributes 0.04 percent of total risk for DDT. 

Comment 

8. Fate. Cell C22 contains a reference error that prevents a breast milk pathway calculation. 

Response 

This error has been corrected. 

Comment 

9. Input-Output sheet + Fate. Units for chemical concentration in PM10 are different between 

these sheets (ug/L versus ug/g). 
A2-20

Response 

This error has been corrected.

Comment 

10. Fate. Toggles don't function when a chemical is not in the database (e.g., metals) because an 

error message is created. Logic equations rather than simple arithmetic equations would prevent 

this from occurring. Also, see comment #3. 

Response 

I was unable to replicate the problem 

Comment 

11. Fraction at school is used to partition daily soil ingestion. Guidance documents indicate 

that this fraction should also be incorporated into dermal dose calculations as indicated by the 

equation on page 4 of the guidance document. We agree with this conclusion. However, this 

fraction appears only to be included in the age 3-4 exposure spreadsheet and in none of the other 

dermal dose calculations. 

Response 

This error has been corrected. 

Comment 

12. Dermal exposure assessment. The approach for dermal exposure assessment represents a 

departure from current OEHHA, DTSC, and USEPA dermal assessment protocols. Additional 

discussion highlighting the need/utility of this new protocol should be included in the 

guidance document. 

Response 

The selected approach is recommended by EPA, and is based on data relevant to the school 

scenario. The Guidance discusses the salient arguments for selection of this approach. 

Comment 

13. Fate. Cell H7. The on-site soil-to-indoor dust Transfer Coefficient (dermal) does not appear to 

be used in the calculation. 

Response 

This error has been corrected. 

Comment 

14. Indoor air modeling. Default indoor air modeling is based on Johnson and Ettinger model 

parameters for residential homes, which clearly do not apply to schools. The model and model 

documentation do not provide adequate guidance on what model parameters may be modified or 

how such modifications may be incorporated. We suggest, for screening, applying conservative 

but non-residential parameters. 
A2-21

Response 

OEHHA proposes a default air exchange rate of 4.9 per hour, the lower confidence limit on the 

weighted mean value from 94 portable and 26 traditional classrooms (RTI, 2003). This and 

other parameters may be changed from default values when justified and documented. 

Comment 

15. Age-specific exposure sheets. Soil Ingestion. P. 11. Of the OEHHA school model 

guidance document cites OEHHA (2000) guidance for soil ingestion. OEHHA (2000) 

recommends 200 mg/day soil ingestion for children age 1-6 and 100 mg/day for "everyone else." 

This approach is inconsistent with the available data, which indicate adults, and especially adults in 

non-soil intensive exposure work environments, do not consume 100 mg/day. This approach is also 

inconsistent with other Cal/EPA guidance for worker soil and dust exposures, and is also 

inconsistent with USEPA's recommended 50 mg/day for adults. 

Response 

The soil ingestion rate in question (100 mg/day) is partioned such that only 58 percent of that 

amount (58 mg/day) is assumed to occur at school. Since most estimates of occupational exposure 

consider only the fraction of total exposure that occurs in the occupational setting, the assumptions 

are not far apart (50 mg/day versus 58 mg/day). 

Comment 

16. Potential users of the spreadsheet. It is critical that OEHHA state that the spreadsheet and 

guidance document are `expert tools' and not intended to be layperson tools. 

Response 

The follosing sentence has been added on page 8: “ This guidance assumes that the user is familiar 

with the principles of chemical risk assessment; it is not intended to provide basic instruction in 

risk assessment.”

Comment 

17. Dust exposure. The utility of differentiating indoor dust from outdoor dust in the model is 

uncertain. Most schools will go through the assessment process prior to construction and 

performing any measurement of chemical concentrations in indoor dust. Separating soil ingestion, 

dermal and inhalation pathways in this fashion may lead to more confusion than clarity. 

Response 

Recent field studies have demonstrated a concentrating effect for several elements in indoor dust 

compared to school-yard soil (RTI, 2003). OEHHA is now recommending a default value of 2 for 

the soil/dust transfer factor (see page 17). 

Comment 

18. Toxicologic endpoints. No discussion or delineation in the model/model documentation is 

presented that addresses assessment of specific toxicologic endpoints (also referred to as 

target organ toxicity). This is an important subject and should be explicitly addressed in the 

guidance, and where possible, in the model itself. 
A2-22

Response 

The additivity of hazard quotients for different chemicals based on their target organ and/or mode 

of action is discussed on page 9: “If the total hazard index does not exceed one, then it may be 

assumed that the non-cancer toxic effects are unlikely and further analysis of non-cancer effects is 

not necessary. If the total hazard index exceeds one, it may be useful to separate chemicals by 

target organ and/or mode of action and add the hazard quotients of only those chemicals that are 

likely to act in an additive manner. This target organ/mode of action analysis should be 

documented.” Similar language is in the sheet “user’s guide.” Chemical-specific information on 

additivity is available elsewhere. 

Comment 

19. Stakeholder involvement in Model development. No indication is provided in the guidance 

documents that stakeholders have been included in the development of key exposure parameters 

such as exposure time and exposure frequency. This lack of involvement increases uncertainty 

as to whether the values used in the model for these parameters are representative of California 

school conditions. CASH is in a unique position to provide data specific to public schools for these 

parameters. 

Response 

The opportunity for stakeholder involvement was open through May 2003. All recommendations 

for parameter values that were supported by documentation were considered. 

Comment 

20. Annual risk estimations. The assessment of theoretical upper-bound cancer risk and noncancer hazard indices on a year-by-year basis is a new risk assessment approach. Hopefully, 

OEHHA has carefully considered the propriety of this approach. We assume that this approach is 

used in consideration of the new age-specific toxicity criteria (OEHHA, 2002 draft, p.11) under 

development by OEHHA. Until these criteria are developed, we are uncertain of this approach. 

Response 

The language in HSC Section 901 suggests that the legislators were concerned that typical 

risk assessment paradigms (which use longer-term average exposure rates) fail to consider 

the specific exposures and sensitivities of children. OEHHA took this as a mandate to look 

specifically at the exposure parameters of young children. To do this effectively required 

using age-specific exposure parameters. As noted in the comment, this approach is also 

anticipates development of age-specific toxicity criteria by OEHHA. 

Comment 

21 Consistency with other OEHHA guidance. The model incorporates assumptions that vary 

from other currently available OEHHA risk assessment guidance (OEHHA, 2000) without 

defining the need for the variation. Variations from published agency guidance should be 

discussed including rationale as to why OEHHA believes the approach/assumptions in the model is 

more applicable to the schools risk assessments. 

Response 

Though it is not further described in the comment letter, we assume that “(OEHHA, 2000)” 
A2-23

refers to the “Technical Support Document for Exposure Assessment and Stochastic 

Analysis” (TSD). The TSD was created for a different purpose than the Schools Guidance. and 

the differences in mission and mandate require some differences between the two. For example, 

the Schools Guidance must include “Appropriate child-specific routes of exposure unique to the 

school environment in addition to those in existing exposure assessment models.” This requires a 

different approach, focusing on the rapidly changing exposure patterns of young children and the 

unique features of the school environment as they affect exposures. Because children are rapidly 

changing anatomically, physiologically and behaviorally, we recommend a set of exposure 

parameters for each year until age 18. In contrast, the TSD presents methodology for estimating 

long-term average exposures in a 24-hour-per-day exposure setting. Nonetheless, internal 

consistency to the extent possible was a consideration in developing this guidance. Exposure 

parameters are discussed below in the context of internal consistency. 

The proposed value for Soil Ingestion, 200 mg/day is the value recommended in the TSD, 

and ie equivalent to EPA’s “conservative estimate of the mean. 

Fraction at school: There is no equivalent parameter in the TSD. 

Body-part-specific skin loading rate and body-part-specific skin surface area differ from 

the methods and parameters recommended in the TSD. These methods and parameters are 

from EPA (2002) guidance which antedates the TSD. OEHHA considers these data to be the 

best available for the Guidance because they are based on real-world exposures to young 

children in day-care centers, an exposure setting similar to the School setting addressed in the 

Guidance 

Fraction outdoors and Fraction indoors also have no equivalent parameter in the TSD. 

However, these parameter values are based on time-activity studies reported in the TSD. 

Body weight data for children 1 to 18 years are from OEHHA, 2000, Table 10.3. For 

pregnant or nursing women, and for staff the data are from OEHHA, 2000, Table 10.1. The 

value for children up to 1 year old is from EPA, 2002, because there is no equivalent value in 

the TSD (OEHHA, 2000).

Exposure time, outdoors and Exposure time, indoors: Although OEHHA (2000) considered 

activity-related breathing rates and time spent at those activities, the focus was to develop an 

amalgamated breathing rate over time. The Guidance differs in that it considers indoor and 

outdoor breathing rates separately, since these environments may have different contaminant 

loadings. Again, OEHHA believes that this approach is consistent with the legislative 

mandate. 

Breathing rate, outdoors and Breathing rate, indoors were estimated from the data of 

Wiley, et al in OEHHA, 2000, pp. 3-25 to 3-27. These ventilation rates are based on the 

activity descriptors in the Wiley, et al. report, that were deemed consistent with outdoor and 

indoor activities at school, respectively 

Exposure frequency, the estimated number of days students or other school users attend 

school annually is based on California DHS and ARB survey data. There is no equivalent 

parameter in the TSD. 

The recommended Breast milk intake of 130 g/kg/day for the first 12 months of life is based 

on (OEHHA, 2000, Table 5.13, 90th percentile). 
A2-24

Lifetime Exposure Fraction (ED/AT) The TSD recommends an exposure duration of 70 

years (ED/AT = 1). This is not relevant for a schools exposure scenario. 

Fraction absorbed, inhalation and Fraction absorbed, ingestion are not found in the TSD. 

The Guidance does not at present suggest values other than unity. Indeed, some commentors 

have suggested that a factor with a value of one is pointless, and should be omitted. HSC 

Section 901 requires consideration of uncertainty in the model. To omit these absorption 

fractions would be to ignore a source of uncertainty. 

Fraction absorbed, dermal: In order to maximize consistency with the TSD, OEHHA now 

recommends the following dermal absorption fractions: 

Compound Absorption 

fraction 

Source 

Arsenic 0.04 OEHHA, 2000 

Beryllium 0.01 OEHHA, 2000 

Cadmium 0.001 OEHHA, 2000 

Hexavalent chromium 0.01 OEHHA, 2000 

Lead 0.01 OEHHA, 2000 

Mercury 0.1 OEHHA, 2000 

Nickel 0.04 OEHHA, 2000 

Polychlorinated biphenyls 0.14 OEHHA, 2000 

Polychlorinated dibenzo-p-dioxins and 

dibenzofurans 

0.02 OEHHA, 2000 

Hexachlorocyclohexanes 0.1 OEHHA, 2000 

Polynuclear aromatic hydrocarbons, 0.13 OEHHA, 2000 

DEHP 0.1 OEHHA, 2000 

4,4’ methylene dianiline 0.1 OEHHA, 2000 

Organophosphates, pentachlorophenol 0.25 DTSC, 1994 

Chlorinated insecticides 0.05 DTSC, 1994 

Other organic chemicals 0.1 DTSC, 1994 

Other metals and complexed cyanides 0.01 DTSC, 1994 

Free cyanide 0.1 DTSC, 1994 
A2-25

Comment 

22. Acceptable risk. No indicators in the guidance or in the model are provided as to what 

metrics OEHHA is applying for judging what is an "acceptable risk." 

Response 

Acceptable or target risk levels are in the risk management domain. The guidelines cover 

risk assessment and not risk management. 

Comment 

23. Model parameters. All model parameters that can be changed should be clearly and plainly 

listed on the input-output sheet. This would include all pathway toggles, dermal, oral, and 

inhalation absorption parameters, chemical properties, exposure frequency, etc. 

Response 

Use of the model in screening mode with all defaults involves entering data only in the 

shaded cells in “Input-output” column B. Changing values outside that range will move 

the user into tier 2 and will require justification of all proposed changes from the default 

condition. 

Comment 

24. Input-Output Sheet. For hazard results, no indication is provided as to which hazard estimate 

to use. We suggest a "=max (range)" formula be inserted for each scenario. 

Response 

This suggestion has been adopted. See Input-output cell H25. 

Comment 

25. Equations. We suggest locking cells containing equations that should not be modified. 

Response 

Sheets other than “Input-output” will be locked when the spreadsheet is released, to 

prevent inadvertent changes to formulas and parameter values. However, reviewers should 

still verify the calculations against their own (unmodified) copy of the workbook. 

Comment 

26. Chemical properties. We suggest adding chemical property input cells on Input 

Output sheets that are activated only when a chemical is not in the database. 

Response 

Chemical properties can be entered in the “Vlookup” sheet. 

Comment 

27. Input-Output Sheet. Currently no dermal absorption fractions are defined in the "database", 

but are presented in the guidance document. We suggest adding a table of values from the guidance 

into the model. See comment #21 above. 
A2-26

Response 

This has been done.

Comment 

28. Fate sheet. Q/C value based on 10-acre site for LA. Meteorological conditions are 

different for Northern and Southern California. We suggest a table containing a range of 

Q/C values, with a toggle to allow the user to define what region of California in which the site is 

located. 

Response 

Regional Q/Cm ratios would make a minor change in a minor pathway. It is not clear that 

the added complexity brings commensurate benefit. 

Comment 

29. Age-specific exposure sheets. Dividing soil ingestion rate by body weight to create a soil 

ingestion per kilogram body weight rate implies that soil ingestion is directly correlated with body 

weight, which is not consistent with the exposure equations listed in the guidance document. These 

two parameters are not directly correlated. 

Response 

Cell H18 is now changed to g/day, then divided by body weight in the “contact rate” cell, in 

order to avoid the appearance that soil ingestion and body weight are treated as correlated. 

Comment 

30. J&E Model parameters. We suggest including pertinent Johnson & Ettinger model 

parameters in the Input-Output sheet. 

Response 

This will be considered for future spreadsheet enhancements. 

Comment 

31. Grammatical errors. Numerous spelling and grammatical errors exist throughout the 

model spreadsheets, and associated documentation. 

Response 

We will correct these as we identify them. 

Comment 

32. Fate. Breast milk BCF reference should be University of California (1994), not 

DTSC (1994). 

Response 

This has been corrected. 
A2-27

Comment 

33. Fate + individual age range calculation spreadsheets. We suggest adding additional 

clarification for each pathway delineated by a number. When printed, there is no 

indication which pathway is soil ingestion or dermal contact (1 vs. 2 vs. 3). 

Response 

Widening column 1 to accommodate the pathway name would make the sheet will be too 

wide to display on a single screen and would duplicate information in column B (exposure 

medium) and column E (exposure route). 

Comment 

34. Units. Input output sheet does not specify units for user defined CSF or RfD. 

Response 

This has been corrected.

Comment 

35. Fate sheet. Units of M8 & M9 should be μg/L. 

Response 

This has been corrected. 

Comment 

36. Age-specific intake spreadsheets. The water intake parameter is not used in any 

calculations. 

Response 

This has been eliminated 

Comment 

37. Guidance document, p.10. Outdoor air PM10 levels. The document states OEHHA 

recommends 1.8E-9, but the spreadsheet uses 5.0E-8. 

Response 

This has been corrected.

REFERENCES (Cited in the reponses) 

1. RTI, 2003, Final Report, OEHHA Soil Sampling Augmentation, RTI International 

Project #RTI/08381-01F, April 2003 

2. OEHHA, 2000 Technical Support Document for Exposure Assessment and Stochastic 

Analysis, Air Toxics Hot Spots Program Risk Assessment Guidelines Part IV, 

September, 2000 
A2-28

COMMENTS RECEIVED DURING THE PUBLIC COMMENT PERIOD

November 7, 2003 - December 8, 2003 

Deborah Oudiz, Department of Toxic Substances Control (DTSC) 

Comment 

1. The title is too restrictive. Please drop “existing and proposed” from the title.

Response 

The title of the document comes directly from the enabling legislation. OEHHA staff feel 

that removing the word “Proposed” from the title actually restricts the sites where this 

guidance should be used, while inclusion of the word covers those sites intended by the 

legislation. Therefore, OEHHA deems it appropriate that the title of the document be a 

direct reflection of the language in the statue: “...the office shall publish a guidance 

document, for use by the Department of Toxic Substances Control and other state and local 

environmental and public health agencies, to assess exposures and health risks at existing and 

proposed schoolsites.”

Melanie Marty, Office of Environmental Health Hazard Assessments (OEHHA) 

Comment 

1. Please add text discussing the possibility of future age-specific cancer potency.

Response 

Text discussing the possibility of future age-specific cancer potency has been added (see 

page33). 

Comment 

2. Please add text discussing about population risk 

Response 

Two sentences discussing population risk have been added (see page 24). 

Comment 

3. Please change the exposure duration for staff to 40 years to be consistent with other 

OEHHA guidance.

Response 

The exposure duration for staff has been changed to 40 years (see pages 24 and 25). 
A2-29

Bill Piazza, Los Angeles Unified School District 

Comment 

The timeline specifted for public comment is inadequate. The District is cognizant of 

OEHHA's reliance on the requirements set forth in Health and Safety Code Section 57003 for 

receiving public comment. However, nothing in the statute precludes OBHHA from 

extending the timeline to ensure that the proposed guidelines are "based upon sound 

scientific methods, knowledge and practice." In fact, the statute does not limit the public 

comment period to 30 days but specifically states that following the public workshop the 

proposed guidance shall be circulated "for a period of at least 30 days." The District believes 

its request to extend the public comment period is appropriate and underscored by the extent 

of time OEHHA has taken to amend and respond to initial comments on the original draft 

guidance. The District did not receive a copy of the revised guidance nor its response to 

comments until November 7, 2003. This is more than 280 days from our initial comment 

submittal of January 31, 2003. Notwithstanding, the District is aware that time is required to 

review and respond to both peer and pubic comments, as well as revise the draft guidance, as 

appropriate. In fact, OEHHA commissioned additional classroom studies, one of which was 

designed to determine the contaminant composition of interior surface dust (i.e., dust mat and 

classroom floor dust) to outdoor soil concentrations. Although the District is supportive of 

this effort, this data is new information which was not previously available for public 

scrutiny.. Clearly, additional time is required to address the adequacy of this study and 

exposure potential of indoor contaminants to a given school occupy. 

Response 

Three substantive changes were introduced in the current draft of the Guidance: Minor 

changes to the default school calendar, inclusion of an outdoor/indoor transfer factor, and 

revised default outdoor/indoor air exchange, all based on the RTI, 2003 report. The relevant 

section of the RTI report (Attachment 1) is 2 pages long. Thirty days appears to be adequate 

time to review and comment on the report and the interpretation thereof. 

Comment 

This is most relevant as OEHHA relies upon the study's findings to recommend a default soil 

to indoor dust transfer factor of two (2). This assumption is significant and may drive the risk 

estimate for a given occupancy. The District believes this default assumption is problematic. 

One major flaw is that OEHHA assumes "day care centers" are representative of student 

exposures in the first through the twelfth grade. To the contrary, the study relied upon by 

OEHHA collected indoor dust from floor surfaces. It is unlikely that students at these grade 

levels will sit, play, and crawl on the floors for the preponderance of their tenure at school. 

This is relevant as it is reported in the Report to the California Legislature, Environmental 

Health Conditions in California's Portable Classrooms (ARB, DHS, June 2003) that floor dust 

contaminants are "especially a concern for young children who spend time on the floor and 

can be exposed to the dust contaminants by hand-to-mouth contact and skin contact." The 

District agrees that special consideration be given to this sensitive occupancy, however, we 

believe most older students generally walk into their classrooms and sit at desks or a similar 

work surface, thereby eliminating a direct exposure pathway to surface floor dust. Please note, 

the OEHHA commissioned study did not identify dust loadings or contaminant concentrations 

at task level (e.g., desk and related working surfaces) where direct contact is likely to occur. 

Nor did the study provide any information to assess the reentrainment potential of floor dust 
A2-30

which may contribute to task level loadings and exposures through the inhalation pathway, 

District staff is cognizant of the U.S. Environmental Protection Agency's (U.S. EPA) guidance 

to assess the reentrainment potential of road surface dust from the movement of vehicular 

sources along a roadway segment. We are unaware of similar guidance associated with the 

inhalation of resuspended floor dust from a student walking across a classroom floor. 

Response 

OEHHA made no assumption that "day care centers" are representative of student exposures 

in the first through the twelfth grade. On the contrary, exposure parameters change year by 

year, with the exposure parameters for high school students closely resembling those for 

adults. 

OEHHA acknowledges that the concentration of contaminants in dust vacuumed from the 

floor may be different from that in dust on table-tops and window sills ond other surfaces, 

and that more data on these potential exposure media would be desireable. Nonetheless, the 

multiple sources of interior dust are essentially the same (though possibly in different 

proportions) whether the dust is suspended in the air as particulate matter, or collected from 

table-tops, window sills, or floors. If a site is to be assessed prior to construction of a school, 

some assumption must be made about the concentration of contaminants in indoor dust on 

various surfaces. The comment does not include a suggested alternative value. OEHHA 

believes that the existing data on the concentration of contaminants in floor dust relative to 

the concentration in outdoor soil provide the best currently available estimate of 

concentration of contaminants in all interior dust relative to the concentration in outdoor soil. 

Comment 

To evaluate the adequacy of the dose algorithms utilized in the draft guidance, District staff 

prepared a comparative analysis of values predicted utilizing identified equations consistent 

with U.S. EPA's Risk Assessment Guidance for Superfund (RAGS). According to OEHHA's 

response to concerns raised regarding the discrepancy between the proposed exposure 

equations and the "standard form given in the U.S. EPA RAGS" guidance, it was reported 

that although "some minor aspects may differ, the general form of the equations is consistent 

with U.S. EPA RAGS." To that end, the values predicted by either approach should be 

consistent. With all exposure variates being equal, the District was unable to predict similar 

values for several exposure pathways. In fact, the District's annual average daily dose values 

were as much as two to three orders of magnitude more 'conservative (i.e., health protective) 

than those identified with the OEHHA predictive model. Due to the limited time to address 

this discrepancy, the District requests clarification and validation of the proposed exposure 

equations relative to the established format identified in regulatory guidance. 

Response 

The general form of the equations is consistent with U.S. EPA RAGS. This does not mean 

that the input values or the actual output results are the same as in RAGS. Indeed, if they 

were, there would be no reason to have schools risk assessment guidance. The equations and 

inputs referred to in the comment are from EPA’s Children’s Exposure Factors Handbook. 

“Validation” in this context usually refers to field testing of theoretical equations to describe 

nature. In this case, the algorithms are based on field measurements. EPA apparently felt 

that they were sufficiently “valid” to adopt in their guidance. 
A2-31

Comment 

In addition to the lack of reproducibility of several dose values, OEHHA's report of only 

"minor" changes to other exposure pathway equations (e.g., inhalation of contaminants in 

vapors that originate off site) may arbitrarily bias the predicted annual average daily dose. 

For example, by scaling the breathing rate for the time spent in school (e.g., 540 minutes per 

day), the modeled concentration must coincide with the identified school-based exposure. 

This is not standard practice and would require post-processing of the model output to 

generate the appropriate value. Existing guidance would simply utilize the model scalar 

option to predict the annual average exposures for a given time period (i.e., ending hours 8 

through 16) averaged over the number of hours specified in the meteorological data set (i,e., 

8760 hours). As such, the dose algorithm would simply require a daily breathing rate value. 

In anticipation of OEHHA's concern that there are differences in indoor versus outdoor 

breathing rates that would preclude the use of a daily breathing rate, the District believes that 

reasonable assumptions can be made to approximate an enhanced rate which accounts for 

variable light, moderate and heavy activity levels representative of a public school setting. 

This approach is discussed in OEHHA's discussion characterizing the proposed ventilation 

rates for the proposed guidance. In addition, OEHHA's Technical Support Document for 

Exposure Assessment and Stochastic Analysis provides a detailed discussion on ventilation 

rates and related distribution profiles to assist in developing high end estimates that reflect 

daily activity patterns associated with student and staff exposures. Notwithstanding, the 

proposed guidelines should either be revised to reflect the form and format of the exposure 

pathway equations that reflect existing regulatory guidance and practice or direct the user to 

make the appropriate adjustments to the dispersion model output. 

Response 

Changes have been made to the Guidance to direct the user to make the appropriate 

adjustments to the dispersion model output (see pages 10 and 11). 

Comment 

Please note, there are additional issues which require further consideration. However, due to 

the time allotted for public comment, the District is limited to its current discourse. However, 

the District trusts that its concerns will be addressed to allow for continued discussion and 

comment on the proposed guidelines. In its current form, the District has reservations about 

the adequacy of the proposed guidance. I can be reached at (213) 241-3926 should you have 

any questions or need additional information. 

Response 

No response