Summary of Results for the 2002 National-Scale Assessment

INTRODUCTION

Of the 181 air toxics included in the 2002 national-scale assessment,
the risk characterization considers the risk of both cancer and
noncancer effects from inhalation of 124 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 in order 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 2002 national-scale risk assessment is based on a 2002 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 2002 or those that will happen in the
near future due to regulations for mobile and industrial sources (see
further details in the   HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/airtoxred.html"  Air Toxics
Reduction  section of the Web site). This risk assessment represents an
update and enhancement to EPA's 1999 national-scale assessment. The next
assessment will focus on emissions for the year 2005.  It will be
released in late 2009 or early 2010.

Note that in this assessment, the potential carcinogenic risk from  
HYPERLINK "javascript:gloss('gloss.html" \l "diesel')"  diesel PM  is
not addressed because there currently is no unit risk estimate
available. There are noncancer results.   HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/diesel.html"  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   HYPERLINK
"http://cfpub.epa.gov/iristrac/index.cfm" 
cfpub.epa.gov/iristrac/index.cfm.  For more details about the
limitations in the risk characterization, refer to the   HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/limitations.html"  limitations 
section on the Web site.

The risk characterization, which was limited to inhalation risk from
outdoor sources, was designed to answer the following 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   HYPERLINK "javascript:gloss('gloss.html" \l
"upperboundcanrisk')"  upper-bound lifetime cancer risk  greater than  
HYPERLINK "javascript:gloss('gloss.html" \l "ninamillion')"  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   HYPERLINK "javascript:gloss('gloss.html" \l "hi')"  target
organ-specific hazard index  greater than 1.0)? 

For general background on risk characterization, see the   HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/riskbg.html"  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 124 air toxics quantified by the 2002 national-scale assessment, it
is possible to determine which air toxics pose the greatest potential
risk in the United States. A summary 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  HYPERLINK
"javascript:gloss('gloss.html" \l "hazardquotient')"   hazard quotient. 
The risk characterization summary below focuses on results at the
national level, where the EPA believes the results are most meaningful.

EPA assessed 80 of the air toxics as carcinogens.

Number of Air Toxics with WOE Classification* and Unit Risk Estimate

Carcinogenic to humans - 9

Likely carcinogenic to humans - 49

Suggestive evidence of carcinogenic potential - 12

Unit Risk Estimate only - 9

Range from Likely to not likely carcinogenic to humans - 1 (includes the
8 Groups of PAH)

* Based on the   HYPERLINK
"http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=116283"  2005
Cancer Guidelines . For ease of presentation, the WOE scheme under the
1986 Cancer Guidelines are combined with the new scheme, i.e., A =
Carcinogenic to humans; B1 and B2 = Likely carcinogenic to humans; C =
Suggestive evidence; D = Inadequate information; E = Not likely
carcinogenic to humans.

To help understand the results, it should be noted that: 

Concentration results (ambient and exposure) are provided for 181 air
toxics 

Cancer results are presented for 80 air toxics that have quantitative
dose-response information 

Noncancer results are presented for 68 air toxics with quantitative
dose-response information 

Only the respiratory and neurological noncancer hazard quotients are
presented (these organ systems were the noncancer drivers in previous
assessments) 

Many noncancer reference concentrations incorporate protective
assumptions designed to provide a margin of safety. A hazard quotient
greater than one does not necessarily suggest a likelihood of adverse
effects. A hazard quotient equal to or 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 cannot be
translated into a probability that an adverse effect will occur, and is
not likely to be proportional to risk 

For more information on the specific organ or organ systems adversely
affected by the air toxics in this assessment, go to the table in  
HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/02pdfs/health_effects.pdf"  Health
Effects Information (PDF)  (12pp, 82k). 

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

The following Table presents the criteria for classifying the NATA 2002
air toxics and will be helpful in understanding the conclusions below.
In general, drivers and contributors are defined as air toxics showing a
particular level of risk or hazard for some number of people exposed.
They are also presented in order of their cancer weight-of-evidence
(WOE) classification, (i.e., beginning with "carcinogenic to humans").

2002 NATA Health Effects Drivers and Contributors Risk Characterization

Risk Characterization

Category	Risk Exceeds (in a million)1	HI > 1.02	Number of People or

Greater Exposed

(in millions)

National Cancer Driver	10	 	25

Regional Cancer Driver	10	 	1

Regional Cancer Driver	100	 	0.01

Nationl Cancer Contributer	1	 	25

Regional Cancer Contributer	1	 	1

 	 	 	 

National Noncancer Driver	 	1.0	25

Regional Noncancer Driver	 	1.0	0.01

1Cancer risks are upper-bound lifetime cancer risks, i.e., a plausible
upper limit to the true probability that an individual will contract
cancer over a 70 year lifetime as a result of a given hazard (such as
exposure to a toxic chemical). This risk can be measured or estimated in
numerical terms (e.g., one chance in a hundred).

2HI = The sum of hazard quotients for substances that affect the same
target organ or organ system. Because different pollutants may cause
similar adverse health effects, it is often appropriate to combine
hazard quotients associated with different substances to understand the
potential health risks associated with aggregate exposures to multiple
pollutants.

National cancer risk driver: 

Benzene: "carcinogenic to humans". 

Regional cancer risk drivers: 

1,3-butadiene, arsenic compounds, chromium 6, coke oven emissions: All
"carcinogenic to human". 

hydrazine, tetrachloroethylene, PAHs: "likely carcinogenic to humans"
(Note that the WOE for the PAHs in the 8 groups range from "likely" to
"not likely carcinogenic to humans"). 

Naphthalene: "Suggestive evidence of carcinogenicity". 

National cancer risk contributors: 

1,4-dichlorobenzene, acetaldehyde, acryonitrile, carbon tetrachloride,
ethylene oxide : All considered "likely carcinogenic to humans". 

Regional cancer risk contributors: 

nickel compounds: "carcinogenic to humans" 

1,3-dichloropropene, beryllium compounds, cadmium compounds, methylene
chloride: all "likely carcinogenic to humans" 

1,1,2,2-tetrachloroethane: "suggestive evidence of human
carcinogencicity" 

N-nitrosomorpholine, methyl tert-butyl ether: No EPA WOE
classifications. 

National noncancer hazard drivers: 

acrolein . 

Regional noncancer hazard drivers: 

2,4-toluene diisocyanate, chlorine, chromium compounds, diesel engine
emissions, formaldehyde, hexamethylene diisocyanate, hydrochloric acid,
manganese compounds, nickel compounds. (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). 

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 carcinogenic or likely carcinogenic to
humans. More than 284 million people live in census tracts where the
combined upper bound lifetime cancer risk from these compounds exceeded
10 in one million risk and more than 2 million people live in census
tracts where the combined upper bound lifetime cancer risk from these
compounds exceeded 100 in one million risk. The overall national average
risk in the U.S. is 36 in a million. 

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 22 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 350,000 people in the U.S. 

Summary Risk Maps (Note: Hawaii, Alaska, and the Virgin Islands are not
included on these maps although they were included in this 2002 NATA.)

  HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/02pdfs/2002NATACancerRiskMonoColor.
pdf"  Estimated County Level Carcinogenic Risk (PDF)  (1pg, 2.1 MB) -
PDF version of map below.

 

  HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/02pdfs/2002NATANonCancerNeuroRiskmo
nocolor.pdf"  Estimated County Level Noncancer (Neurological) Risk (PDF)
 (1pg, 2.1 MB) - PDF version of map below. 

 

  HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/02pdfs/2002NATANonCancerrespRiskMon
oColor.pdf"  Estimated County Level Noncancer (Respiratory) Risk (PDF) 
(1pg, 2.1 MB) - PDF version of map below. 

 

  HYPERLINK
"http://www.epa.gov/ttn/atw/nata2002/02pdfs/NATARisks100inaMil.pdf" 
Census Tracts with 2002 NATA Estimated Cancer Risk Greater than 100 in a
Million (PDF)  (1pg, 104k) - PDF version of map below. 

 

