Summary of Results for the 1999 National-Scale Assessment

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

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?
Which air toxics pose the greatest potential risk of cancer or adverse noncancer effects in some areas of the United States?
Which air toxics pose lesser, but still significant, potential risk of cancer or adverse noncancer effects across the entire United States?
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?
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)?
What is the cancer risk from mobile sources?
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 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 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 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.)

Summary of Results for the 1999 National-Scale Assessment

INTRODUCTION

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