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Results

INTRODUCTION

Of the 177 air toxics included in the 1999 national-scale assessment, the risk characterization

considers the risk of both cancer and noncancer effects from inhalation of 133 of these air toxics --

the subset of pollutants with health data based on chronic exposure. The purpose of this

national-scale assessment is to understand these cancer risks and noncancer health effects to help

the EPA and others to identify pollutants and source categories of greatest potential concern, and to

set priorities for the collection of additional information to improve future assessments. The

assessment represents a "snapshot" in time for characterizing risks from exposure to air toxics. The

national-scale assessment is not designed to characterize risks sufficiently for regulatory action.

The 1999 national-scale risk assessment is based on a 1999 inventory of air toxics emissions (the

most complete and up-to-date available). It then assumes individuals spend their entire lifetimes

exposed to these air toxics. Therefore, it does not account for the reductions in emissions that have

occurred since 1999 or those that will happen in the near future due to regulations for mobile and

industrial sources (see further details in the Air Toxics Reduction section of the Web site). This risk

assessment represents an update and enhancement to EPA's 1996 national-scale assessment. The

next assessment, due in 2006, will focus on emissions, concentrations, and risks using information

on emissions in 2002.

Note that in this assessment, the potential carcinogenic risk from diesel PM is not addressed in the

same fashion as for other pollutants. This is because data are not sufficient to develop a numerical

estimate of carcinogenic potency for this pollutant. Learn more about EPA's qualitative assessment of

diesel PM.

Given its broad scope, this risk characterization is subject to a number of limitations due to gaps in

data or in the state of the science for assessing risk. For example, the current assessment does not

yet include results for dioxins, compounds that may contribute substantially to risks. In addition, the

EPA is reassessing the health effects of many pollutants considered in this study. A status report for

all EPA health effect assessments is available at cfpub.epa.gov/iristrac/index.cfm. For more details

about the limitations in the risk characterization, refer to the limitations section on the Web site.

The risk characterization, which was limited to inhalation risk from outdoor sources, was designed to

answer 7 questions:

Which air toxics pose the greatest potential risk of cancer or adverse noncancer effects

across the entire United States?

1.

Which air toxics pose the greatest potential risk of cancer or adverse noncancer effects in

some areas of the United States?

2.

Which air toxics pose lesser, but still significant, potential risk of cancer or adverse

noncancer effects across the entire United States?

3.

Technology Transfer Network

1999 National-Scale Air Toxics Assessment

Last updated on Friday, June 26th, 2009.

http://www.epa.gov/ttn/atw/nata1999/risksum.html

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When risks from all air toxics are combined, how many people have the potential for an

upper-bound lifetime cancer risk greater than 10 in a million?

4.

When potential adverse respiratory or neurological effects from all air toxics are combined,

how many people have the potential for exposures that exceed reference levels intended to

protect against adverse effects (i.e., a hazard index greater than 1.0)?

5.

6. What is the cancer risk from mobile sources?

7. What is the cancer risk from background sources?

For general background on risk characterization, see the discussion in questions and answers format

on this topic.

SUMMARY OF RESULTS

Based on a comparison of the cancer and noncancer risks estimated for the 133 air toxics quantified

by the 1999 national-scale assessment, it is possible to determine which air toxics pose the greatest

potential risk in the United States. Some of these findings are reported below. Cancer risks in this

assessment are presented as lifetime risks, meaning the risk of developing cancer as a result of

exposure to each air toxic compound over a normal lifetime of 70 years. Noncancer risks are

presented in terms of the ratio between the exposure and a reference concentration. This ratio is

called the hazard quotient. The risk characterization summary below focuses on results at the

national level, where the EPA believes the results are most meaningful. To understand the results,

five points should be considered:

EPA assessed 78 of the air toxics as carcinogens.

Classification of Carcinogen Number Assessed

Known (A) 9

Probable, based on incomplete human data

(B1) 5

Probable, based on adequate animal data (B2) 50

Possible, based on incomplete animal data (C) 11

On the border between probable and possible

(B2C) 2

Range from probable (B2) to not classifiable

(D)

1 (Polycyclic Organic Matter has 8

Subgroupings)

Separate risk characterizations are provided for air toxics compounds in each of these

classifications. For more information about the classification of each pollutant, see the table

in Health Effects Information Used in Cancer and Noncancer Risk Characterization(PDF 12

pp., 182 KB)

Additionally, EPA considered adverse health effects other than cancer for 105 air toxics. EPA

calculated the hazard quotient for each pollutant and then combined these hazard quotients

for air toxics that affect the same target organ, to account for multi-pollutant effects. Based

on calculated hazard indices, EPA is reporting results for respiratory and neurological effects

(the two organ systems with the largest hazard indices) for 59 of the air toxics in the

assessment. For more information on the specific organ or organ systems adversely affected

by the air toxics in this assessment, see the table in Health Effects Information Used in

Cancer and Noncancer Risk Characterization(PDF 12 pp., 182 KB)

For reporting cancer and noncancer results, EPA focused on pollutants whose risks were

above specified cancer risk and noncancer hazard levels (e.g., cancer risks exceeding 1 in a

million or hazard quotients greater than 1.0). The levels chosen are not regulatory levels.

They are provided here simply to help in judging the relative importance of different

pollutants with regard to their potential to cause adverse health effects. For example, those

air toxics compounds producing a cancer risk larger than 1 in a million at one or more

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locations are likely to be of more concern than those producing a cancer risk of less than 1 in

a million at all locations. The determination of what is an acceptable or unacceptable risk

depends on additional factors and more refined information; this larger issue is not addressed

in this risk characterization.

Because many reference concentrations incorporate protective assumptions designed to

provide a margin of safety, a hazard quotient or hazard index greater than one does not

necessarily suggest a likelihood of adverse effects. A hazard quotient/index less than one,

however, suggests that exposures are likely to be without an appreciable risk of noncancer

effects during a lifetime. Furthermore, the hazard quotient/index cannot be translated into a

probability that an adverse effects will occur, and is not likely to be proportional to risk. A

hazard quotient/index greater than one can be best described as only indicating that a

potential may exist for adverse health effects.

Model-to-monitor comparisons for 14 air toxics suggested a general tendency for the model

(ASPEN) to underestimate measured ambient levels. On average, modeled concentrations

ranged from 95% of monitored levels for benzene to 14% for 1,4-dichlorobenzene. Only one

of the 14 pollutants (methylene chloride) had average model-estimated ambient levels above

the monitored levels (143%). Thus, the model to monitor comparison results suggest that

the ASPEN model may systematically underestimate ambient concentrations for 13 of the 14

pollutants that were evaluated. Given that air monitoring data are usually more reflective of

actual ambient conditions and given the apparent tendency of the ASPEN model to

underestimate ambient concentrations, it is possible that ambient concentrations for other

pollutants are underestimated as well. As a result, risk estimates based on the ASPEN model

may be underestimated. Further, the relative importance of these compounds with respect to

cumulative risk (classification as national cancer risk driver or regional cancer risk driver)

may be underestimated.

Because the EPA estimated central tendency exposures for each census tract, the risk

estimates can be best interpreted as upper estimates of risk to typical individuals (if we

assume that exposures are not underestimated). That is because most UREs used in this

assessment (typically, those based on animal data) were based on the statistical upper

confidence limit (UCL) of the fitted dose-response curve. That means true risk would probably

be less, but could be greater. However, a few (typically, those based on human data) were

based on the statistical best fit ("maximum likelihood estimate," or MLE). An example of such

a compound is benzene. This difference between UCL- and MLE-based assessments results in

some UREs that are less conservative than the rest.

The following conclusions on individual air toxics compounds were drawn from the risk

characterization:

National cancer risk driver: The EPA considered the distinction as a "national cancer risk driver" to

apply to any air toxics compounds posing an estimated upper-bound lifetime cancer risk exceeding

10 in a million to more than 25 million people (i.e., more than 8% of the 285 million people counted

by the 2000 Census live in tracts where upper-bound lifetime cancer risks from these compounds

exceed 10 in a million). Only benzene, a known carcinogen, met this criterion (i.e., benzene is the

only national cancer risk driver).

Regional cancer risk drivers: The EPA identified air toxics compounds posing an estimated

upper-bound lifetime cancer risk exceeding either (a) 10 in a million to more than 1 million people

or (b) 100 in a million to more than 10,000 people. Pollutants already identified as national risk

drivers above were not re-listed as regional risk drivers. For known carcinogens this comparison

shows such risks for arsenic, benzidine, 1,3-butadiene, chromium 6, and coke oven emissions. For

probable carcinogens this comparison shows such risks for carbon tetrachloride, cadmium, ethylene

oxide and hydrazine. For possible carcinogens, this comparison shows such risks for naphthalene.

Perchloroethylene, which is classified on the border between probable and possible, and polycyclic

organic matter (POM) which includes compounds which range from probable (B2) to not classifiable

(D), also meet the criteria for regional cancer risk drivers.

National cancer risk contributors: To identify lower but still important risks at the national level, the

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EPA identified air toxics compounds posing an estimated upper-bound lifetime cancer risk exceeding

1 in a million to more than 25 million people (i.e., more than 8% of the 285 million people counted

by the 2000 Census live in tracts where upper-bound lifetime cancer risks from these compounds

exceed 1 in a million). Pollutants already identified as national or regional risk drivers were not

included here. This comparison shows such risks for 4 pollutants that are probable carcinogens;

bis(2-ethylhexyl)phthalate, acetaldehyde, ethylene dibromide, and ethylene dichloride. Two possible

carcinogens also meet this criterion: 1,1,2,2-tetrachloroethane and p-dichlorobenzene.

Regional cancer risk contributors: To identify lower but still important relative risks at the regional or

urban level, the EPA identified air toxics compounds posing an estimated upper-bound lifetime

cancer risk exceeding 1 in a million to more than 1 million people. Pollutants already identified

above as risk drivers or contributors were not included here. For known carcinogens, this

comparison shows such risks for vinyl chloride and nickel. For probable carcinogens, this comparison

shows such risks for acrylonitrile, beryllium, 1,3-dichloropropene, methylene chloride, and quinoline.

Trichloroethylene, which is classified on the border between probable and possible, also meets the

criterion for regional cancer risk contributor.

National noncancer hazard drivers: The EPA identified air toxics compounds for which the hazard

quotient exceeded 1.0 for more than 25 million people (i.e., more than 8% of the 285 million people

counted by the 2000 Census live in tracts where the typical exposure exceeded the reference

concentration for these compounds). The EPA found only one substance, acrolein, to pose the

greatest relative hazard for effects other than cancer. Acrolein's reference concentration is based on

irritation of the lining of the respiratory system.

Regional noncancer hazard drivers: The EPA identified air toxics compounds for which the hazard

quotient exceeds 1.0 for more than 10,000 people (i.e., more than 10,000 people living in census

tracts where the typical exposure exceeded the reference concentration for this compound). The

assessment shows that the following 16 pollutants meet this criteria: antimony, arsenic compounds,

1,3-butadiene, cadmium, chlorine, chromium VI, diesel PM, formaldehyde, hexamethylene

1-6-diisocyanate, hydrazine, hydrochloric acid, maleic anhydride, manganese compounds, nickel

compounds, 2,4-toluene diisocyanate, and triethylamine. Note that the capability of the study to find

potential hotspots in small regions of the country is limited by the tools used in the study, making it

possible that some regional hazard drivers may have been overlooked because they occur in small

regions.

The following conclusions on simultaneous exposure to all air toxics compounds were

drawn from the risk characterization:

Cumulative Cancer Risks: The EPA added the cancer risks from all air toxics compounds listed as

known, probable or possible carcinogens based on human data. More than 270 million people live in

census tracts where the combined upper bound lifetime cancer risk from these compounds exceeded

10 in one million risk. More than 190 million people live in census tracts where risk greater than 10

in a million resulted from known human carcinogens (Class A) alone.

Cumulative Noncancer Hazards: Ideally, hazard quotients should be combined for pollutants that

cause the same adverse effects by the same toxic mechanism. However, because detailed

information on mechanisms was unavailable for most of the substances considered in this

assessment, the EPA used a simpler and more conservative method. Many of the pollutants in this

assessment cause adverse effects in humans or animals by irritating the lining of the respiratory

system or by causing various effects to the nervous system. Although it is not clear that these

respiratory and neurological effects occur by the same mechanisms for all such air toxics

compounds, the EPA protectively assumed that these effects could be added. These additive effects

were represented by a "hazard index," which is the sum of the hazard quotients of the air toxics

compounds that affect the respiratory or nervous system. The respiratory hazard index was

dominated by a single substance, acrolein. The respiratory hazard index exceeded 1.0 for nearly the

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entire U.S. population, and exceeded 10 for more than 48 million people. The neurological hazard

index was similarly dominated by manganese compounds, with minor contributions by cyanide

compounds, ethylene oxide, and mercury compounds. The neurological hazard index exceeded 1.0

for fewer than 800,000 people in the U.S.

The following conclusions on exposure to air toxics compounds from mobile sources and

background were drawn from the risk characterization:

Mobile Sources: For onroad and nonroad mobile sources, the EPA estimates that approximately 110

million people live in areas of the U.S. where the combined upper-bound lifetime cancer risk from all

air toxics compounds exceeds 10 in a million. This risk estimate is dominated by the emissions of

benzene, acetaldehyde, and 1,3-butadiene. Regarding effects other than cancer, acrolein emissions

are estimated to lead to exposures above the reference concentration (i.e., a hazard quotient above

1.0 for respiratory effects) for approximately 220 million people in the U.S.

Background Sources: The EPA estimates that combined upper-bound cancer risks associated with air

toxics compounds from background sources are less than 100 in 1 million throughout the U.S.

However, approximately 250 million Americans live in areas where estimated cancer risks exceed 10

in a million due to background sources alone. (Note that in this assessment background

concentrations include both uncontrollable emissions such as persistent historic emissions,

international or global pollutant transport, contributions from natural sources, and emissions that

can be controlled such as long-range pollutant transport within the U.S.)

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site National-Scale Air Toxics Assessments 1999 National-Scale Air Toxics Assessment 1999

Assessment Results > Data Tables

The following may be useful in analyzing the results in the tables:

Document detailing units (PDF 3 pp.,31 KB) for the tables provided on this webpage.

Download table of information (Size: 156 KB; Format: Microsoft Access 2000) which lists

each pollutant in the assessment along with the following: any variations in pollutant name

format used in the State summary and pollutant-specific Microsoft Access files; Chemical

Abstracts Service (CAS) registry number; and indication of whether the pollutant has cancer

and noncancer health effects.

Health Effects Information Used in Cancer and Noncancer Risk Characterization (PDF 12 pp.,

182 KB)

Information about the unit risk estimate (URE) for formaldehyde EPA selected for this

assessment

1999 National Emissions Inventory data page is a resource for source-specific hazardous air

pollutant emissions.

Issues with the National Emissions Inventory

Information about the background concentrations EPA assigned to 28 of the air toxics and

diesel PM. This is the component of the outdoor concentrations of the pollutant estimated to

be attributable to long-range transport, unidentified emission sources, and natural emission

sources.

Approach for modeling Polcyclic Organic Matter (POM) (PDF 6 pp., 541 KB)

County-Level Risk Summaries (Microsoft Excel 2000)

The first three Microsoft Excel 2000 spreadsheets provide percentile distribution of risk across

census tracts, and contribution by source sector to the average risk for each county in the U.S.

plus Puerto Rico and the Virgin Islands. The fourth Microsoft Excel 2000 spreadsheet contains the

pollutant-specific contribution to the total risk.

County-Level Cancer Risk

County-Level Respiratory Risk

County-Level Neurological Risk

County-Level Pollutant-Specific Cancer and Noncancer (Respiratory and Neurological) Risk

County-Level Emission Summaries (Microsoft Access 2000)

Provides pollutant-specific tons/year emissions, including breakdown by source sector, for each

county in the U.S. plus Puerto Rico and the Virgin Islands. These data are presented for 85

pollutants (plus 2 extra subgroupings for arsenic and 8 additional subgroupings of POM) out of the

177 pollutants modeled, and were selected based on the following: pollutant is estimated to

present either a cancer risk greater than 1 in a million or a hazard quotient greater than 0.1 in at

least one census tract.

Note: You must save file before opening.

 

County-Level Ambient Concentration Summaries (Microsoft Access 2000)

Technology Transfer Network

1999 National-Scale Air Toxics Assessment

Last updated on Friday, October 23rd, 2009.

http://www.epa.gov/ttn/atw/nata1999/tables.html

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Provides pollutant-specific ambient concentration statistics (in micrograms per cubic meter)at the

country-level, including breakdown of the average (mean) country-level concentration by source

sector, for each county in the U.S. plus Puerto Rico and the Virgin Islands. These data are

presented for 85 pollutants (plus 2 extra subgroupings for arsenic and 8 additional subgroupings of

POM) out of the 178 pollutants modeled, and were selected based on the following: pollutant is

estimated to present either a cancer risk greater than 1 in a million or a hazard quotient greater

than 0.1 in at least one census tract. To better understand the measurement units presented in

these files please access this detailed explanation.(PDF 3 pp., 31KB)

Note: You must save file before opening.

 

Pollutant-Specific Database (Microsoft Access 2000)

Pollutant-specific, census tract-level estimates for the whole U.S. plus Puerto Rico and the Virgin

Islands including risk, modeled ambient concentration, and exposure estimates (if modeled). Due

to the size of the pollutant list, the hazardous air pollutants are divided alphabetically into two

groups below. To better understand the measurement units presented in these files please access

this detailed explanation.(PDF 3 pp., 31KB)

Note: You must save file before opening.

Group 1 (A-G)

 

Group 2 (H-X)

 

Census Tract-Level State Summary Database of Risk Results (Microsoft Access 2000)

State-specific census tract-level risk files including the contribution of each pollutant to overall risk

(cancer, noncancer-respiratory and noncancer-neurological) and the source sector contribution to

risk. To better understand the measurement units presented in these files please access this

detailed explanation.(PDF 3 pp., 31KB)

Note: You must save file before opening.

 

State Summary Databases (Microsoft Access 2000)

1999 State Summary Data: A description (PDF 1 pp.,10 KB)of the state files is available as well as

an exercise (PDF 10 pp.,24 KB) that one can apply to become familiar with the the use of the

database.

Note: You must save file before opening.

 

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