Identifying AirToxScreen's Risk Drivers

EPA uses several measures to determine which pollutants may contribute most to long-term
cancer risks and noncancer health hazards, both nationally and regionally (for example, in
certain metropolitan areas). This information helps EPA and state, local and tribal air agencies
to better understand which pollutants and sources of pollution that we may need to look at
more closely. For example, if a certain important air toxic comes mostly from vehicles, this may
mean we need to look closer at how to control these vehicle emissions.

To do this, we define "drivers" of risk - pollutants that contribute most to risks and hazards -
and "contributors" - pollutants that may also have an impact but are generally less important
at these scales, nationally and regionally, than drivers. To classify risk drivers and contributors,
we use the criteria shown in the following table.

2018 AirToxScreen Health Effects Drivers and Contributors

Risk Characterization Category

Cancer Risk

Exceeds
(in 1 million)1

Hazard
Index2

Number of People (or
Greater) Exposed
(in millions)

National Cancer Driver

10



25

Regional Cancer Driver

10



1

Regional Cancer Driver

100



0.01

National Cancer Contributor

1



25

Regional Cancer Contributor

1



1









National Noncancer Driver



> 1

25

Regional Noncancer Driver



> 1

0.01

Using these guidelines, we can list the air toxics that show up as drivers and contributors to
cancer risks and noncancer health hazards in the 2018 AirToxScreen:

1	Cancer risks are upper-bound lifetime cancer risks (i.e., a plausible upper limit to the true probability
that a person will contract cancer over a 70-year lifetime from a given hazard (such as exposure to a
toxic chemical).

2	A hazard index of 1 or lower means adverse noncancer effects are unlikely. You can read more about
hazard indexes and how they are calculated on the AirToxScreen website.

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2018 AirToxScreen - Drivers & Contributors

Cancer

National Cancer Driver

Risk >10, Pop >25

Formaldehyde**

National Cancer Contributor

Risk >1, Pop >25

1,3-Butadiene
Acetaldehyde
Benzene

Carbon Tetrachloride
Ethylene Oxide**
Formaldehyde**
Napthalene
PAH POM

Regional Cancer Driver

Risk >10, Pop >1
Risk >100, Pop >0.01

Ethylene Oxide**

Regional Cancer Contributor

Risk >1, Pop >1

1,3-Dichloroprene
Chromium VI
Coke Oven Emissions
Ethylbenzene
Nickel Compounds**

Noncancer

National Noncancer Driver

HI >1, Pop >25

(None)

Regional Noncancer Driver

HI >1, Pop >0.01

Acrolein
Cyanide

Hexamethylene Diisocyanate
Nickel Compounds**

**(Listed in 2 categories)

Note: Risk is "in a millionPopulation is "in millions"

To learn about the health effects of these air toxics, please visit EPA's Health Effects Notebook
for Hazardous Air Pollutants. This website presents fact sheets for these and all other hazardous
air pollutants specified in the Clean Air Act.

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