AirToxScreen Frequent Questions

Questions on this page:

2020 Assessment Questions

Q1: What changed with the modeling between the 2019 AirToxScreen and the 2020 AirToxScreen?

A1: There have been many changes between the 2019 AirToxScreen and 2020 AirToxScreen. While emissions and meteorology change year to year, there were also modeling improvements for 2020 AirToxScreen. These modeling improvements include a new receptor strategy, a new urban/rural determination for sources, and a new way to calculate concentrations for gridded sources. Please see the Technical Support Document (TSD) for more information on the modeling changes for 2020 AirToxScreen.

Q2. What is the difference between a census block and a census tract?

A2: The 2020 AirToxScreen will be the EPA’s first national screening assessment to report cancer risks and noncancer hazards at the finer census block level. For example, a census block could contain about 40 people while a census tract could contain about 4,000 people. Earlier versions (as well as versions of the National Air Toxics Assessment, or NATA) all reported risks at the coarser census tract level. However, census blocks are not delineated based on population, and many census blocks do not have any population. For more information on census blocks, please see this Census Bureau webpage on census blocks.

The figure below shows an example of how the spatial resolution is finer at the census block level compared to the census tract level. Specifically, the figure shows a single census block (Block 3014) and how that block is part of a block group (Block Group 3), and how that block and block group are part of a census tract (Census Tract 5.02).

Image showing that census blocks are smaller components of a census block

Q3: Why is the EPA making this change to use census blocks now?

A3: As part of the planned improvements on EPA’s air toxics risk assessments, we are now able to present results at a finer spatial scale. The earlier stages of these improvements included the release of the 2017, 2018, and 2019 AirToxScreen assessments in calendar year 2022. Starting with the 2020 AirToxScreen assessment, AirToxScreen will be presenting results at the census block level and the EPA will also update AirToxScreen annually, which meets the EPA’s goal of improving user access to air quality data. AirToxScreen is relatively new and will continue to be developed as the EPA’s technology and capabilities improve over time. We expect further developments in future versions of AirToxScreen as we advance our capabilities.

Q4: Why are there more geographical areas with elevated cancer risk in the 2020 AirToxScreen than the 2019 AirToxScreen?

A4: There will be more census blocks with elevated cancer risks (cancer risks greater than or equal to 100-in-1 million) than there were census tracts with elevated cancer risks. In 2019 AirToxScreen, there were about 25 geographical areas (census tract level) that had estimated cancer risks equal to or greater than 100-in-1 million. In 2020 AirToxScreen, there are about 130 geographical areas (census block level) with estimated cancer risks at these levels. Note that an area can include one census block or numerous census blocks when all blocks within the area have the same source and pollutant as the cancer risk driver.

The main reason there are more elevated cancer risk areas in 2020 AirToxScreen compared to previous AirToxScreen assessments is due to the change from calculating risks at the census tract level to the census block level. In 2019 AirToxScreen, the census tract risk was the average of multiple census blocks, which in some cases, led to a much lower risk at the census tract level when the census blocks within the track had elevated cancer risks. This is in part because the emissions from stationary industrial facilities are spread out over a larger area and air pollutant concentrations are ‘diluted’.

Some areas that did not show elevated cancer risks at the tract level in past assessments will now show elevated risks at the block level due to calculating risks at this higher resolution (smaller spatial scale). This results in more accurate air pollutant concentrations in areas close to the facilities, which in turn results in more accurate risks for people living near those facilities. In some cases, the highest block-level risks will be several times higher than the highest tract-level risks, which can occur when calculating risks in a small census block (in 2020 AirToxScreen) compared to calculating risks in a large census tract (in 2019 AirToxScreen).

Please see other Q&As and the Technical Support Document (TSD) for more information on the differences between 2019 AirToxScreen and 2020 AirToxScreen.

Q5: The emissions did not change at the facility in my neighborhood, so why did my risks go up between the 2019 and 2020 AirToxScreen results?

A5: Year 2020 AirToxScreen results are published at the census block level, a finer spatial scale than in previous editions that reported results at the census tract level. Census block results increase the resolution by approximately 100-fold from census tract results. Census blocks that are close to or downwind of an emissions source will generally have higher air toxics concentrations and related risks than census blocks that are farther from or upwind of an emissions source. Previous editions averaged modeled census block concentrations and risks to estimate census tract risk.

Please see other Q&As and the Technical Support Document (TSD) for more information on the differences between 2019 AirToxScreen and 2020 AirToxScreen.

Q6: Since 2020 AirToxScreen is using 2020 emissions data, how do I know if my risk has changed significantly since 2020?

A6: The 2020 AirToxScreen provides a snapshot in time using emissions from calendar year 2020. Since 2020, it is possible that emissions reductions have occurred, such as facilities closing down, facilities installing controls, or new regulatory actions that have further reduced emissions, and therefore, further reduced risks throughout the country.

The AirToxScreen mapping tool includes notes about census blocks and facilities where the EPA data shows emissions have been reduced since 2020 that may lead to changes in risk for the current calendar year. These messages appear in a pop-up box with red text, corresponding to each census block and facility as applicable. Emissions increases can also occur, such as emissions from new facilities or increased activity at existing facilities.

Q7: : The emissions did not change at this facility, so why did the risks change between the 2019 and 2020 AirToxScreen results?

A7: Year 2020 AirToxScreen results are published at the census block level, a finer spatial scale than in previous editions that reported results at the census tract level. Census block results increase the resolution by approximately 100-fold from census tract results. Census blocks that are close to or downwind of an emissions source will generally have higher air toxics concentrations and related risks than census blocks that are farther from or upwind of an emissions source. Previous editions averaged modeled census block concentrations and risks to estimate census tract risk.

Please see other Q&As and the Technical Support Document (TSD) for more information on the differences between 2019 AirToxScreen and 2020 AirToxScreen.

Q8: If a facility shutdown or installed controls, how can there still be elevated cancer risk from the facility?

A8: This risk assessment is based on 2020 emissions, so if the facility shutdown or installed controls after 2020, it will not be accounted for in the 2020 AirToxScreen. We have notes in the AirToxScreen mapping tool to provide additional information to identify these areas where emissions are currently expected to be different compared to the 2020 emissions. Changes in risks due to decreased or increased emissions will be captured in future AirToxScreen releases with releases occurring each year; for example, 2021 AirToxScreen will be based on 2021 emissions.

Q9: Based on the elevated risks in my community, will you install monitors?

A9: The risks presented in AirToxScreen are considered a starting point for local risk assessments and additional investigation. While we will not rule out monitoring, that decision would be made after further evaluation with state and local air agencies.

Q10: Why does AirToxScreen use modeling data instead of monitoring data?

A10: For calculating risk across every part of a community, our experts believe that computer modeling gives us the best estimate possible of pollutant concentrations in the air and the risks from breathing that air over many decades. Modeling also allows us to quickly examine how risks are expected to change when emission controls are installed, for example. An air monitor can tell us about pollutants only in the area where the monitor is located and for pollutants the monitor is designed to measure. But computer models let us look at all pollutants across an entire community – not just at the monitor location or for pollutants the monitor is designed to measure. In addition, current monitoring methods cannot always detect pollutants down to all risk levels.

General Assessment Questions

Q1: What are air toxics and what health effects can they cause?

A1: Air toxics (also called toxic air pollutants or hazardous air pollutants) are airborne substances that cause or may cause serious health problems. These can include cancer, reproductive problems or birth defects. Air toxics can also cause harmful environmental and ecological effects.

Section 112 of the Clean Air Act lists a set of 188 pollutants as air toxics. This section requires EPA to identify categories of industrial sources for these listed air toxics. It also requires us to take steps to reduce pollution by requiring sources of these air toxics to install controls or change production processes. Learn more at the Overview of the Clean Air Act. Examples of air toxics include benzene, found in gasoline; tetrachloroethylene, emitted from some dry cleaning facilities; and methylene chloride, used as a solvent and paint stripper by several industries.

EPA has classified many of these pollutants as “ to humans,” “likely to be carcinogenic to humans,” or “suggestive evidence of carcinogenic potential.” Air toxics are also associated with many noncancer . These include effects on the lungs and other parts of the respiratory system; on the immune, nervous and reproductive systems; and to organs such as the heart, liver and kidneys. These health effects can range from headaches and nausea to respiratory arrest and death. Severity varies with the amount and length of exposure and the nature of the chemical itself (for example, how it interacts with various organs and organ systems). It can even vary due to unique behaviors and sensitivities of individual people. For example, some chemicals pose hazards to people of certain ages or genetic backgrounds.

Q2: What is AirToxScreen?

A2: The Air Toxics Screening Assessment, or AirToxScreen, is EPA's review of air toxics in the United States, based on modeled air quality. We developed AirToxScreen as a tool for state, local and tribal agencies, and we use its results as well. AirToxScreen helps us find out which air toxics, emission sources, and places may need further study to better understand .

AirToxScreen uses the best science and data available to estimate possible health risks from air toxics. But because of its large, national scale, we must simplify some of AirToxScreen’s input data and analytical methods. That's why we call AirToxScreen a “screening tool” — it helps us estimate risks and tells us where to look further.

AirToxScreen provides screening-level estimates of the risk of cancer and other serious health effects from breathing air toxics. This helps show which air toxics and source types may raise health risks in certain places. Air pollution experts can then study these places in more detail, focusing on where the risks to people may be greatest. They may even choose to perform a smaller-scale local assessment, which allows them to use more detailed data than we can in AirToxScreen.

AirToxScreen gives a “snapshot” of the outdoor air quality as it relates to air toxics. It also suggests the long-term risks to human health if air toxic emissions stay the same. A more detailed explanation of AirToxScreen and the methods used can be found in the AirToxScreen Technical Support Document.

Q3: How can I use AirToxScreen results?

A3: You can use AirToxScreen to:

learn which air toxics may be of concern to you;
better understand risks from air toxics;
open a dialogue with your local air agency about air quality in your area.

Your state, local or tribal agency uses AirToxScreen results to:

prioritize pollutants and emission source types;
identify places of interest for further, more detailed study;
get a starting point for local assessments;
focus community efforts;
inform monitoring programs;
prioritize sensitive locations for outdoor air toxics monitoring.

EPA uses AirToxScreen results to:

learn which pollutants and source types may be of concern;
better understand risks from air toxics;
help decide what other data to collect;
improve data in emission inventories;
expand and prioritize EPA's air toxics monitoring network;
work with communities to design their own assessment;
help target risk reduction activities.

Q4: How should I NOT use AirToxScreen results?

A4: AirToxScreen assessments should not be used:

to pinpoint risk or exposure values at a specific place (like a home or school);
to characterize or compare risks or exposures at local levels (such as between neighborhoods);
to characterize or compare risks or exposures between states;
as the sole basis for risk reduction plans or regulations;
to control specific sources or pollutants; or
to quantify benefits of reduced air toxics emissions.

Please keep a few other things in mind when using AirToxScreen results. While results are reported at the census tract level for the 2017-2019 AirToxScreen, and at the census block level for the 2020 AirToxScreen and later, average exposure and risk estimates are far more uncertain at this level than at the county or state level. Also, AirToxScreen is a screening tool, not a refined assessment. It shouldn’t be used as the sole source of information to regulate sources or enforce existing air quality rules.

In addition, while we do allow users to view results from both the current and previous AirToxScreen assessments in our AirToxScreen Mapping Tool, you should keep some things in mind when comparing different assessments. Over the years, we have improved the source inventory, made modeling changes, revised background calculations, and updated some health benchmarks. Also, each assessment uses a particular year of weather data in the air models that we use to calculate risks, which can also influence results. That’s why it can be misleading to compare this version of AirToxScreen with previous assessments.

Q5: Why is AirToxScreen a "screening" tool? Why doesn’t EPA run a more detailed assessment?

A5: Because of AirToxScreen's broad national scale, we must simplify some of its input data and our analytical methods. That's why we call AirToxScreen a “screening tool” — it's designed to help guide where to look in more detail at air toxics risks.

As for the second question, there are several reasons why we can’t run a detailed, or refined, national analysis right now.

For some source types, we don’t have data at a precise enough level. For example, we don’t know the exact location of widespread, numerous sources such as emissions from homes and gas stations, or exactly how auto emissions are spread across an area. That means we can’t calculate the precise impacts from those sources at a specific point, something needed in a refined analysis.

It would also take lot of time to collect source data at the level of detail needed for a refined assessment. Starting with the 2017 AirToxScreen and continuing through this latest release, EPA is updating this assessment every year. Right now, we can’t update that kind of large, refined dataset annually.

We would need several other things, including local weather data to use in modeling, more detailed human activity-pattern data for better exposure estimates, and more. So, while perhaps one day we will have the data and resources needed to do a refined assessment, we don't have that just yet. AirToxScreen results can, however, inform a more refined, local analysis that uses EPA recommendations and guidance.

Q6: Are there any risks from air toxics that aren't covered by AirToxScreen?

A6: Yes. AirToxScreen looks at just one facet of the air toxics picture — potential health effects due to breathing air toxics from outdoor sources over many years. It also just looks at one point in time: air toxics emissions and weather data used are from a single year (year 2020 in this latest 2020 AirToxScreen), and it assumes that they stay the same throughout one's lifetime. Together, these assumptions mean AirToxScreen can’t account for all risks.

Also, AirToxScreen doesn’t include:

potential cancer risks associated with diesel particulate matter (PM), which may be large (see question 3 below in the Mobile Sources section);
acute effects from short-term exposures to air toxics;
non-inhalation exposures, such as ingestion and skin exposures. These pathways are important for pollutants that stay in the environment and bioaccumulate (build up in tissues of organisms) such as mercury and polychlorinated biphenyls;
exposures and risks very close to specific sources or at highly localized “hotspots” (such as some types of workplace-related or near-roadway-related exposures);
individual exposure extremes. We base all risk estimates on exposure estimates for the individual in each census tract (or block for 2020). EPA considers this to be a “typical” exposure for that tract. Some people may have higher or lower exposures based on where they live or spend most of their time within that tract (or block for 2020);
emissions from indoor sources of air toxics. For certain air toxics and for certain indoor situations, exposure to indoor sources can influence and sometimes dominate total long-term human exposures;
risk estimates for chemicals without adequate dose-response information (for example, cancer risk from cobalt emissions);
impacts of nonroutine increases in facility emissions due to things like equipment startups, shutdowns, malfunctions and upsets;
assessment of adverse environmental effects or other welfare effects;
toxic air pollutants that are not included in the National Emissions Inventory (for example, radionucleides).

Q7: Who is responsible for controlling air toxics?

A7: EPA, state, local and Tribal air programs share responsibility. EPA sets national standards for some sources of air toxics emissions. State, local and tribal programs implement these rules. Some state, local and tribal programs also set their own air toxics rules and conduct their own studies.

Q8: What should I do if I'm concerned about air toxics in my area?

A8: Please contact your state, local or Tribal air program. View a list of state and local programs.

You can also view information on Tribal programs and EPA's Regional Tribal Program coordinators.

Q9: How does AirToxScreen differ from the other screening tools used by EPA? How do I know which tool to use?

A9: AirToxScreen is a national assessment that estimates cancer and noncancer risks from breathing air toxics. AirToxScreen is intended as a screening tool to help users prioritize pollutants, types of emission sources, and places of interest for further study. EPA has designed other tools, such as EnviroAtlas and NEPAssist, to meet certain needs.

Which tool to use depends on your main area of interest. For a quick-look comparison, visit the Other Environmental Risk Screening Tools web page.

Q10: How do AirToxScreen results compare with the data in the EJScreen tool?

A10: The EPA’s EJSCREEN, the Environmental Justice Screening and Mapping Tool, serves a different purpose than AirToxScreen, and therefore, EJSCREEN may present a less detailed and lower resolution version of AirToxScreen data. For any questions on EJSCREEN, please refer to the EJSCREEN website and you may use the EJSCREEN contact form to submit your questions.

Q11: Did AirToxScreen replace NATA? How are they different?

A11: The Air Toxics Screening Assessment, or AirToxScreen, is a part of EPA's Air Toxics Strategy, which is focused on improving public access to air toxics information and data. AirToxScreen falls under the umbrella of our Air Toxics Data Update, which also includes the National Emissions Inventory of air toxics sources.

While AirToxScreen has similarities to previous versions of the National Air Toxics Assessment, or NATA, it is designed to be a more dynamic, flexible, and timely assessment. In addition, the new name distinguishes this new approach and highlights that, like NATA, this is a screening level assessment. The biggest change going forward is that EPA is releasing a new Air Toxics Data Update and new AirToxScreen assessment annually. This started with the 2017 emissions-year release.

	NATA	AirToxScreen
Frequency of Release	Every 3 to 4 years	Annually
Spatial Resolution of Information	Census tract level	Census tract level for 2017, 2018 and 2019 assessments. Census block level for 2020 assessment and future assessments.
Release of Information	Website	Initially available on EPA’s air toxics website and eventually included in the annual Air Trends Report

Q12: How often does EPA update AirToxScreen?

A12: Starting with the 2017 AirToxScreen assessment, EPA plans to release a new AirToxScreen assessment for every data year moving forward. In 2022, we released three updates to AirToxScreen, one for each data-year (2017-2019), which were the 2017, 2018, and 2019 AirToxScreen assessments. Starting with the 2020 data-year, we intend to release one new assessment each year that includes the latest available data. In May 2024, the 2020 AirToxScreen assessment was released. In 2025, we plan on releasing the 2021 AirToxScreen assessment.

Emissions, Modeling, and Methods Questions

Q1: Which air toxics are included in AirToxScreen?

A1: AirToxScreen estimates and for 180 air toxics plus diesel particulate matter (PM), which we assess for noncancer effects only. Using the concentration estimates for the 180 air toxics plus diesel PM, AirToxScreen estimates cancer risks and noncancer hazards for 138 of these. For the other air toxics, AirToxScreen gives concentration estimates, but no health-effects data are available. You can find a list of all air toxics assessed and the types of results generated for each in Appendix B of the AirToxScreen Technical Support Document.

EPA did not include the following air toxics in this assessment because either no data were reported for them in 2020 or we couldn’t reliably make emission estimates useful for modeling from their reported emissions. We also don't include the air toxic chromium III from the assessment because there are no risk values for that specific compound (only the chromium VI portion of chromium emissions were assessed).

2,3,7,8-tetrachlorodibenzo-p-dioxin
Other dioxins/furans
Asbestos
Fine mineral fibers
Radionuclides
DDE
Diazomethane
Beta-propiolactone
Chromium III

Q2: What are the steps in the AirToxScreen assessment?

A2: AirToxScreen includes the following four major steps for assessing air toxics across the United States (including Puerto Rico and the U.S. Virgin Islands):

Compile a 2020 inventory of air toxics emissions from outdoor sources.
EPA compiled measured or estimated emissions data reported by sources, states and others. We also estimated mobile source and other emissions using models, measurements and a quality-control process. This dataset is called the National Emissions Inventory (NEI). We start with the latest NEI (2020 NEI for 2020 AirToxScreen).. The emission source types in the NEI include major stationary sources (such large waste incinerators and factories), area and other sources (such as gas stations and small manufacturers), on-road and nonroad mobile sources (such as cars, trucks and boats), biogenic sources, and fires. These emissions data become a major part of the modeling input data used in the next AirToxScreen step.
Estimate ambient air concentrations based on the 2020 emissions.
We used the emissions data from step 1 as inputs to two air quality models: the American Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) and the Community Multiscale Air Quality model (CMAQ). These models estimate ambient concentrations of the emitted toxics. We modeled 52 plus diesel particulate matter in CMAQ and all AirToxScreen air toxics in AERMOD. We then combined the results using a hybrid approach that takes advantage of the strengths of both models (the AirToxScreen Technical Support Document describes this hybrid approach). To evaluate model performance, we compared estimated ambient concentrations to air toxics monitoring data.
Estimate population exposures.
Next, we used the estimated ambient concentrations as inputs to an exposure model, the Hazardous Air Pollution Exposure Model (HAPEM), version 8. Estimating exposure is a key step in determining potential health risk. People move from one location to another — for example, from outside to inside. Their exposure isn't the same as it would be if they stayed in one place. People also breathe at different rates depending on their activity levels, so the amounts of air they take in vary in time. For these reasons, the average concentration of a pollutant that people breathe, called their exposure concentration, might be higher or lower than the concentration at a fixed location (ambient concentration). We accounted for this when calculating exposures.
Characterize potential public health risks due to breathing air toxics.
We characterized cancer risks and noncancer health effects using available information on air toxics health effects, current EPA risk assessment and risk characterization guidelines, and the exposures estimated in the prior step. The result is the heart of AirToxScreen: a national view of potential long-term risks to public health due to breathing air toxics from outdoor emission sources, assuming a lifelong exposure to 2020 emission levels. Along with presenting these numbers, we discuss the assessment’s uncertainties and limitations.

You can find more detailed information about these steps in the Technical Support Document.

Q3: What are CMAQ and AERMOD, and how did EPA use them in AirToxScreen?

A3: CMAQ, EPA’s Community Multiscale Air Quality Model, is a computer program that estimates air quality. CMAQ simulates how emissions of multiple air pollutants, emitted by numerous sources at the same time, travel and disperse downwind. From this, it calculates the pollutants’ concentrations across the United States. Not only does CMAQ show how pollutants move and disperse, it also models how they can react with other pollutants and gases in the atmosphere. This is a key difference between CMAQ and some other air quality models.

CMAQ conserves mass (that is, if some pollution blows out of one area, the same amount is tracked into the new area); allows long-range pollutant transport; and estimates concentrations of secondarily-formed pollutants such as formaldehyde. In AirToxScreen, EPA used CMAQ for 52 pollutants, with emissions from point and nonpoint sources, mobile sources, biogenics and fires.

In addition to CMAQ, we ran the dispersion model AERMOD for all AirToxScreen pollutants at all U.S. census blocks for point, nonpoint and mobile sources. Like all similar models, AERMOD uses equations to simulate how the atmosphere disperses pollutants. From this, it can calculate pollutant concentrations at many discrete points (called receptors). Modelers can place these receptors closer together in AERMOD than in CMAQ, a strength of the former model.

We then combined the ambient concentrations estimated by both CMAQ and AERMOD in a hybrid approach, taking advantage of each model’s features and strengths. Detailed information on this approach can be found in Section 3 of the AirToxScreen Technical Support Document.

Q4: Why are all the estimates for the year 2020 and not more recent?

A4: We used 2020 data because emission inventories from that year were the most complete and up to date when we began putting this AirToxScreen release together. It takes time to gather, quality check, and model these data. Note that EPA is updating AirToxScreen for each data year starting with the 2017 AirToxScreen, which was released in March 2022.

Q5: Why is EPA using computer modeling instead of actual measurements to estimate concentrations and exposure?

A5: Right now, we can’t monitor ambient air toxics across the entire country. It would be extremely expensive. Instead, we only measure a subset of air toxics concentrations in a few locations. So for large-scale assessments such as AirToxScreen, we need to use computer models to estimate ambient air toxics concentrations and population exposures nationwide.

EPA does use measured data to evaluate the models. This helps us better understand some of the uncertainties in assessments and improve modeling tools. For example, in a supplemental data file available in our results area here on the AirToxScreen website, we explain the results of model-to-monitor comparisons done for the 2019 AirToxScreen. In addition, we provide annual statistics for air toxics monitoring data in the AirToxScreen Mapping Tool. You can obtain air toxics monitoring data from the Air Monitoring Archive in multiple formats, including Microsoft Access, for historical data years through 2016. You can use EPA’s Air Quality System (AQS) Data Mart for more recent data.

Q6: What kind of changes were made in AirToxScreen because of the review by state, local and tribal air agencies?

A6: EPA appreciates the time taken by state, local and tribal air agencies to preview and comment on the preliminary results of this assessment. Thorough reviews such as these help us continually improve our assessments, which benefits all AirToxScreen users.

These comments covered the areas of:

Facility changes:
- Geographic coordinate changes
- Revisions to stack parameters
Emission changes:
- Additions, deletions, and recalculations
- Changes to chromium speciation, hexavalent chromium percentage

Most of the comments were addressed by making the appropriate changes to the 2020 NEI and 2020 AirToxScreen inventories and/or directly to AERMOD input files for AirToxScreen modeling. The final 2020 AirToxScreen now reflects these changes. Additional comments focused on methodological and toxicological questions, many of which are addressed or answered in various sections of the AirToxScreen website.

Q7: How did EPA characterize risk from the modeled exposure estimates?

A7: To evaluate a chemical's potential to cause cancer and other adverse health effects, we:

examine the adverse effects the chemical causes (a “hazard identification”);
determine the exposure to the population (an “exposure assessment”); and
evaluate the specific exposures at which these effects might occur (a “dose-response assessment”).

The chemical’s evaluation is based on studies of humans, animals, and microorganisms, and is usually published in peer-reviewed scientific journals. In this national assessment, we combined information from dose-response assessments with modeled exposure estimates in a “risk characterization” to describe the potential that real-world exposure to air toxics might cause harm. We also looked at the uncertainties of risk characterization.

Q8: How does EPA estimate cancer risk?

A8: EPA typically assumes a linear relationship between the level of exposure and the lifetime probability of cancer from an air toxic (unless research suggests a different relationship). We express this dose-response relationship for cancer in terms of a “unit risk estimate.” The unit risk estimate (URE) is an upper-bound estimate of a person’s chance of contracting cancer over a lifetime of exposure to a particular concentration: one microgram of the pollutant per cubic meter of air. Risks from exposures to concentrations other than one microgram per cubic meter are usually calculated by multiplying the actual concentration to which someone is exposed by the URE.

For example, EPA may determine the URE of an air toxic to be 1 in 10,000 per microgram per cubic meter. This means that a person who breathes air containing an average of 1 microgram per cubic meter for 70 years would have (as an upper bound) a 1 chance in 10,000 chance (or 0.01 percent) of contracting cancer as a result. So, in this example, if the modeled concentration in AirToxScreen for this air toxic is 0.5 micrograms per cubic meter for a particular block, the risk would be 0.5 times the URE, or a 1-in-20,000 cancer risk.

EPA has developed UREs for many substances, and continues to re-examine and update them as knowledge improves. More information on UREs can be found in EPA's Integrated Risk Information System. The UREs used in this assessment are included in the AirToxScreen Supplemental Data folder.

Q9: What are biogenic emissions?

A9: In AirToxScreen, biogenic emissions are those from trees, plants and soil microbes. Biogenic sources emit formaldehyde, acetaldehyde and methanol; formaldehyde and acetaldehyde are key risk drivers in AirToxScreen. Biogenic sources also emit large amounts of other volatile organic compounds. These compounds are not hazardous, but they can react in the air with certain human-caused emissions to form hazardous pollutants.

In AirToxScreen, the biogenic emissions source group only includes primary emissions, or those directly emitted into the air by natural sources. Some compounds emitted by natural sources also react chemically in the air with compounds emitted by humans to make other pollutants; these are included in the Secondary source group; (see question 15 below).

For AirToxScreen, a model computes biogenic emissions using vegetation and land use data for an area. It also uses data such as air temperature and the amount of sunlight received. More information about how we compute and model biogenic emissions in AirToxScreen can be found in Section 2 of the AirToxScreen Technical Support Document.

Q10: Part of the estimated risk is due to "background." What is background?

A10: In AirToxScreen, background risk represents:

the contributions to outdoor air toxics concentrations from natural sources not modeled directly as biogenic emissions;
persistence in the environment of past years’ emissions; and
long-range transport from distant sources.

Background concentrations represent levels of pollutants that would be found in a year even if there had been no recent human-caused emissions. For example, a main contributor to risk from background concentrations is carbon tetrachloride, a common pollutant that now has few emissions sources but persists in the air due to its long half-life.

For AirToxScreen, we estimated background using remote concentration estimates from monitoring and emissions.

Q11: Does AirToxScreen account for background concentrations of ethylene oxide?

A11: AirToxScreen incorporates only known emissions sources of ethylene oxide (EtO). To learn more, please see EPA's Work to Understand Background Levels of Ethylene Oxide.

Q12: Part of the estimated risk is due to "secondary formation," which is widespread across the country. What is secondary formation?

A12: Some hazardous air pollutants form when compounds called VOCs (short for volatile organic compounds) react chemically in the air with other compounds emitted by humans (usually oxides of nitrogen, or NOx). This is known as secondary formation.

During the summer months, most of the VOCs come from actively growing trees and other biogenic sources; in winter, when plant growth slows, wood burning from homes and auto exhaust provide most of the VOCs. Year-round, most NOx comes from the burning of fossil fuels by autos and industry.

Formaldehyde and acetaldehyde are common secondary HAPs (the criteria pollutant ozone also is another secondary pollutant). For AirToxScreen, we estimated secondary formation using the CMAQ model.

Q13: How does EPA update its emissions data to the latest data year?

A13: When putting together the emissions data for AirToxScreen, we start with the annual multipollutant modeling platform, which is based on the latest National Emissions Inventory, or NEI. The NEI is updated every three years, and 2020 is the latest NEI.

Risk Questions

Q1: What does a "1-in-1 million" cancer risk mean?

A1: A lifetime cancer risk of 1-in-1 million means that, for every 1 million people who are continuously exposed to a certain level of a pollutant over 70 years, one person may develop cancer. These risks are on top of any other cancer risks from other sources.

Q2: Why do most of AirToxScreen’s risk estimates seem to be to the nearest ten, like 20- or 30-in-1 million? Weren’t there any 25’s?

A2: In AirToxScreen, as in most of our air toxics risk assessments, we round calculated risks to one significant figure in the last step of the risk calculation. Significant figures are the digits of a number that are meaningful in terms of accuracy or precision. Usually, that's the non-zero digits. So a number with one significant figure has just one non-zero digit.

Let’s look at an example. In AirToxScreen, we round all calculated risks from 25-in-1 million through 34 in-1-million to 30-in-1 million. The “3” is the one significant figure in this number.

Why do we do this? Most of the health benchmarks used in the risk calculation are one significant figure, which results in a risk value that is one significant figure. Our results can only be as precise as the input data we use to calculate risk. We could show risk out to two significant figures or more, but that’s “false precision.” We simply aren’t that certain about things like the precise location and amounts of pollutant releases, the meteorological data, and other inputs. There are also some uncertainties in the air models that we use. Therefore, we aren't that certain about the final calculated risks.

By the way, the larger the risk, the larger the rounding to get to one significant figure. For example, if we calculate a risk of 145-in-1 million, this is rounded down to 100-in-1 million in AirToxScreen. If the risk is 155-in-1 million, it’s rounded up to 200-in-1 million. Again, we always round to one significant figure.

Q3: What does EPA believe constitutes an acceptable level of risk?

A3: Unlike other pollutants that EPA regulates, air toxics have no universal, predefined risk levels that clearly represent acceptable or unacceptable thresholds. However, EPA has made case-specific determinations and made general presumptions that apply to certain regulatory programs. As explained below, we use levels that come from these rulings to guide how we interpret risk in AirToxScreen.

The 1989 Benzene National Emission Standard for Hazardous Air Pollutants (NESHAP) rule set up a two-step, risk-based decision framework for the NESHAP program. This rule and framework are described in more detail in EPA's 1999 Residual Risk Report to Congress.

First, the rule sets an upper limit of acceptable risk at about a 1-in-10,000 (or 100-in-1 million) lifetime cancer risk for the most exposed person. As the rule explains, “The EPA will generally presume that if the risk to that individual [the Maximum Individual Risk] is no higher than approximately 1 in 10 thousand, that risk level is considered acceptable and EPA then considers the other health and risk factors to complete an overall judgment on acceptability.”

Second, the benzene rule set a target of protecting the most people possible to an individual lifetime risk level no higher than about 1-in-1 million. These determinations called for considering other health and risk factors, including risk assessment uncertainty, in making an overall judgment on risk acceptability.

Q4: AirToxScreen results also show a noncancer “hazard index” for my area. What does that mean?

A4: To estimate noncancer health impacts, EPA calculates what’s known as a hazard index. This index accounts for potential noncancer health effects to certain human organs and organ systems due to long-term exposure to air toxics. It accounts for impacts from all air toxics that affect a target organ system in the same (or similar) way, summing a calculated for each air toxic.

A hazard index (HI) of 1 or lower means air toxics are unlikely to cause adverse noncancer health effects over a lifetime of exposure. However, an HI greater than 1 does not necessarily mean adverse effects are likely. Instead, EPA evaluates this on a case-by-case basis. We consider the confidence level of the underlying health data, the uncertainties, the slope of the (if known), the magnitude of the exceedances, and the numbers or types of people exposed at various levels above the (RfC).

Q5: How were the cancer risk estimates affected by EPA's Guidelines for Carcinogen Risk Assessment (EPA/630/P-03/001F) and Supplemental Guidance for Assessing Susceptibility from Early-Life Exposure to Carcinogens (EPA/630/R-03/003F)?

A5: AirToxScreen is consistent with the methods for estimating cancer risks to children recommended by the 2005 revised Cancer Guidelines and the Supplemental Guidance.

For the following HAPs, we applied a risk factor of 1.6 (see source of factor below) to account for the increase in lifetime cancer risk due to childhood exposures:

Acrylamide
Benzidine
Chloroprene
Coke oven emissions
Ethyl carbamate
Ethylene oxide
Methylene chloride
Nitrosodimethylamine
PAHs

We used this factor because research shows these HAPs have a mutagenic mode of action and because there is no chemical-specific data to show that there are differences between children and adults in the way they respond to exposure to these agents (see explanation below).

In contrast, vinyl chloride does have chemical-specific data available regarding children’s exposure and risk. EPA used these data to derive the , or URE (see the IRIS website for a more thorough explanation). Therefore, the vinyl chloride URE used in AirToxScreen (see Appendix H of the AirToxScreen Technical Support Document) already reflects the risk due to childhood exposures; no further adjustment is necessary.

For trichloroethylene, a carcinogen with a mutagenic mode of action, the age-dependent adjustment factor for the URE only applies to the part of the slope factor reflecting risk of kidney cancer. For lifetime exposure to a constant level of trichloroethylene, we adjusted the URE by a factor of 1.12.

A brief explanation of the adjustments to risk follows: The Supplemental Guidance recommends that risks to children be adjusted for carcinogenic chemicals acting through a mutagenic (cause cancer by damaging genes) and linear (direct-acting) mode of action. Where available data for the chemical are adequate (such as with vinyl chloride), they should be used to develop age-specific potency values. Where available data do not support a chemical-specific evaluation of differences between adults and children, the Supplemental Guidance recommends the use of the following default adjustment factors for early-life exposures: increase the carcinogenic potency by tenfold for children up to 2 years old, and threefold for children from 2 through 15 years old. These adjustments have the aggregate effect of increasing by about 60 percent (a factor of 1.6), the estimated risk for a 70-year (lifetime) constant inhalation exposure.

It’s important to keep in mind that EPA recommends making the default adjustments only for carcinogens:
(1) acting through a mutagenic mode of action;
(2) for which a linear dose response has been assigned; and
(3) for which data to evaluate adult and juvenile differences are not available.

The default adjustments are not recommended for carcinogens whose mode of action is unknown. EPA will determine as part of the IRIS assessment process which substances meet these criteria, and future national assessments will reflect adjustments for those substances.

Q6: Why aren't results for dioxin included?

A6: We did not evaluate exposure and risk related to dioxins in AirToxScreen because we did not evaluate the completeness or accuracy of the state, local and tribal agency data for dioxins. The most significant exposure route for dioxins is ingestion, not inhalation. So dioxins’ relative contribution to AirToxScreen’s risk estimates likely would be small.

Results Questions

Q1: What's the easiest way to review AirToxScreen results?

A1: The AirToxScreen Mapping Tool displays cancer risks, data, and ambient concentrations data on a map and dashboard. The map allows you to examine places of interest anywhere in the country. Key features of the mapping tool include census blocks sorted by total risk, risk by source type, facility-level emissions information, and air toxics monitoring data. You can easily change the displayed data type by toggling the different map layers. You can also download all AirToxScreen data and results and run queries to find the information you want.

The AirToxScreen mapping tool includes the following layers (note: some of these layers are still to be added to the tool):

all emissions sources modeled in AirToxScreen;
cancer risks by pollutant and emissions source type
long-term noncancer hazard indexes
modeled ambient concentrations and exposure concentrations (block level); and
air toxics monitoring sites with monitoring data.

Q2: What does AirToxScreen tell us overall about the risks from air toxics in the United States?

A2: The 2020 AirToxScreen estimates most people’s risks to be between 1-in-1 million and 100-in-1 million. The estimates for a small number of localized areas are higher than 100-in-1 million (see question 2 of the Risk section for more on what this means). People and communities may be concerned about this. It’s important to remember, however, that AirToxScreen wasn’t designed as a final means to pinpoint specific risk values at local levels. The results are best used as a tool to help learn which pollutants, types of emissions sources and places should be studied further.

We should note that AirToxScreen risk estimates do not consider ingestion exposure or skin contact with toxics. They also don’t include indoor sources of air toxics. Because we don’t have health-effects data for some air toxics, out of about 180 toxics modeled, we only assessed 129 for risk. (Diesel particulate matter risk was only assessed for noncancer effects.) Therefore, these risk estimates may represent only some of the total potential risks associated with air toxics.

While air quality continues to improve nationwide, more needs to be done to meet the Clean Air Act's requirements to reduce the potential exposure and risk from these chemicals. EPA will continue to develop air toxics regulations as well as cost-effective pollution prevention and other control options to address indoor and urban pollutant sources that significantly contribute to risk.

Q3: What do these estimates mean for me?

A3: AirToxScreen’s results estimate the amount of air toxics in an area and show general patterns of potential risk for each state and county in the United States. They are best used to help show which pollutants, emissions sources and places should be studied in more detail.

AirToxScreen wasn’t designed to be a final tool for assessing risks. Because of its national scale, it has some limitations in data and methods. Some of these will cause AirToxScreen to report higher risk estimates for an area than may truly exist. At the same time, the most-exposed people in an area may run a higher risk than AirToxScreen calculates.

This means you should avoid using AirToxScreen results as an absolute measure of your risk from air toxics. Instead, perhaps you can discuss the results for your area with your state, local or tribal air agency. These agencies often use AirToxScreen as the starting point for a more refined assessment in an area. The National Association of Clean Air Agencies keeps a list of state and local programs that can help you find your agency. You can get information about tribal programs and EPA's Regional Tribal Program coordinators online as well.

Q4: AirToxScreen shows an elevated risk of cancer in my area. Am I in danger? Am I going to get cancer? What can I do?

A4: First, keep in mind that EPA did not design AirToxScreen to pinpoint specific risk values at any one place. AirToxScreen results show general patterns of potential risk. They shouldn’t be used as an absolute measure of whether any one person’s risk is elevated.

It’s also important to understand that an elevated risk in AirToxScreen does not necessarily mean you are in immediate danger. AirToxScreen risk results are based on breathing air toxics over a very long time — a lifetime, in fact. AirToxScreen can’t tell us much about health risks over shorter periods. We didn’t design it to do that. That said, the amount of a pollutant in the air needed to put a person in any immediate danger is generally much higher than the amount needed to raise long-term risks.

For AirToxScreen, our primary concern is people being exposed to air toxics concentrations associated with elevated cancer risk over many years. Any cancer-causing chemical is potentially dangerous, but just how dangerous depends on:

the toxicity of the chemical;
the concentration to which a person is exposed; and
how long a person is exposed to the chemical.

There are resources available to help answer your questions about air toxics exposure and health.

For adults:
If you think you are being exposed to elevated levels of air toxics in the outdoor air, be sure you keep up with routine health screenings and doctor’s visits. If you have health concerns that you believe are related to air toxics exposure, start by contacting a healthcare provider. In addition, the Agency for Toxic Substances and Disease Registry (ATSDR) can tell you the location of occupational and environmental health clinics. These clinics specialize in recognizing, evaluating, and treating illnesses resulting from exposure to hazardous substances. You can reach ATSDR at: 1-800-CDC-INFO · 888-232-6348 (TTY) or click the website link above.

For children:
If you are concerned about air toxics and your child’s health, contacting your health care provider is a good place to start. If your provider is not familiar with the chemical of concern, they can work with you to contact the Pediatric Environmental Health Specialty Unit (PEHSU) that serves your area. EPA and the TSDR help fund the PEHSUs, which are a source of medical information and advice on environmental conditions that influence reproductive and children’s health. If you don’t have a doctor, you can contact the PEHSU for your area directly.

Find PEHSUs in your area: https://www.pehsu.net/findhelp.html
Read more information about the PEHSU program: https://www.pehsu.net/About_PEHSU.html

As a reminder, because AirToxScreen is designed to identify areas for further examination, you may first want to contact your state, local or tribal air agency with any questions or concerns you have about the results. These agencies usually enforce air toxics rules. Often, they also carry out any localized air quality studies needed to get a more precise picture of the air quality in a particular neighborhood.

The National Association of Clean Air Agencies keeps a list of state and local programs that can help you find your agency.

For information about tribal air programs, you may wish to contact the tribal air coordinator for your region. A list is available at EPA's Regional Tribal Program coordinators website.

Q5: How accurate is AirToxScreen?

A5: EPA uses the best available science and data to build AirToxScreen. But because of its large, national scale, we must simplify some of AirToxScreen’s input data and analytical methods. This means that AirToxScreen assessments provide “screening-level" estimates of the risk of cancer and other serious health effects from breathing air toxics. These estimates help inform efforts to learn which air toxics, emission source types, and places may need further study to assess population risk.

Uncertainties in emissions, actual population exposures, and dose-response or health-effects data are common in assessments like AirToxScreen. For example, the smaller the area, the more uncertain the results. Thus, AirToxScreen results are useful to show which pollutants, source types, or places might be associated with higher risks than others. They are not designed to determine exactly how many people are exposed to precise levels of risk or if a certain area is “safe” or not.

Even with these assumptions, we have found good agreement for many pollutants when comparing AirToxScreen concentration results to measured concentrations. You can learn more about these model-to-monitor comparisons in section 3.7 of the AirToxScreen Technical Support Document.

Q6: How does the cancer risk identified in this assessment compare to lifetime cancer risk from all causes?

A6: The 2020 AirToxScreen estimates that, on average, one out of about every 33,000 Americans (or 30-in-1 million) could contract cancer from breathing air toxics if continuously exposed to 2020 emission levels for 70 years. That’s a national average. In some places, the risks are higher; in others, the risks are lower. This estimated risk is on top of any other risks to which a person might be exposed.

Note that AirToxScreen risk estimates are subject to limitations in the data, modeling and assumptions used routinely in any risk assessment. For example, AirToxScreen doesn’t consider ingestion exposures or indoor sources of pollutants. Also, AirToxScreen only estimates long-term cancer risks for air toxics for which EPA has dose-response data. Therefore, these risk estimates may represent only part of the total potential cancer risk associated with air toxics. Use caution when comparing AirToxScreen results to other estimates of risk.

Q7: You show risk data down to the census block level. Are the results accurate enough to draw conclusions at this scale?

A7: EPA recommends that you use the census block data to get patterns of risks within counties rather than to pinpoint specific risk values for each census block. We feel reasonably confident that the patterns — areas of relatively higher and lower levels of exposure and risk within a county — represent actual changes in overall average population risks within the county. We are less confident that the assessment pinpoints the exact locations where higher exposures and risks exists or captures the highest exposures and risks in a county. Keep in mind, we developed AirToxScreen as a screening tool to help identify pollutants and locations that may need further study.

Q8: Based on AirToxScreen, can EPA show which people or places are at greatest risk from air toxics?

A8: Within its limits, yes. Usually, air toxics emissions are higher in areas with more people. Not surprisingly, AirToxScreen results suggest that larger urban areas often carry larger risk burdens than smaller urban and rural areas. This trend is not universal. It can vary from pollutant to pollutant and by source. It may also be affected by exposures and risk from non-inhalation and indoor sources of exposure, which AirToxScreen does not consider.

But keep in mind that AirToxScreen wasn’t designed to pinpoint precise risk values at any one place. AirToxScreen results show general patterns of potential risk. They shouldn’t be used as an absolute measure of whether any one person’s risk is elevated.

Q9: How does this assessment of air toxics data compare to previous national-scale assessments?

A9: Many of the assessment methods we use in AirToxScreen are similar to our previous 2014 and earlier NATA assessments. But we have improved the source inventory, made modeling changes, revised background calculations, and updated some health benchmarks. Also, each assessment uses a particular year of weather data in the air models that we use to calculate risks, which can also influence results. That’s why it can be misleading to compare this version of AirToxScreen with previous assessments.

Q10: Why did results change in my area from the previous national assessment?

A10: As we discuss in the previous answer, there are many reasons why a certain place’s estimated risk might change from one to the next.

In many places, estimated health risks are higher or lower in the this data-year version of AirToxScreen than in previous versions. This may be because of real emissions changes. But it may also be because of changes in our methods or because we use a particular year of weather data in the models that we use to calculate risks (as explained above).

New research can also trigger changes. For example, EPA updated its risk value for the air toxic ethylene oxide in 2016. So when we released the National Air Toxics Assessment (NATA) in 2018, we used the new health-effects data for ethylene oxide. Largely because of this change, more areas showed elevated risks driven by ethylene oxide in that assessment than were shown in previous assessments. This continues to be the case in AirToxScreen. This does not mean there is more of this compound in the air in these places than before. Even if emissions in an area are the same — or possibly even if they are lower — the new stricter health-effects level often means a higher risk estimate.

In the same way, new data can mean lower risk estimates. For example, starting with the 2014 NATA released in August 2018, our assessments have shown that noncancer health hazards are lower across much of the nation than in previous years. This is at least partly due to new health-effects data for acrolein, one of three air toxics that drive most of the noncancer risk in our assessments, that we began using in that 2014 NATA. These data suggest that higher acrolein levels are needed to cause elevated health risks than we once thought. So for the same amount in the air, the health risks from acrolein are lower in AirToxScreen than in past assessments.

These examples help show why you should use caution when comparing one assessment's results to another.

Q11: Has U.S. air quality improved?

A11: Overall, yes. Levels of six key criteria air pollutants continue to fall, and since 1990, EPA has also made great progress in reducing air toxics emissions. We finalized the National Emissions Standards for Hazardous Air Pollutants, or MACT standards, to reduce toxic emissions from over 174 categories of industrial sources. These rules result in nearly two million fewer tons of air toxics released every year.

EPA has also completed all the required emissions standards for smaller sources, known as area sources. Gas stations and dry cleaners are examples of area sources. Individual area source facilities often have low emissions. But there are a lot of them, including many in heavily populated cities. In some urban areas, toxic emissions from area sources can be much greater than those from major sources.

Measured from the 1990 baseline inventory, EPA has set standards for nearly all area sources of urban air toxic pollutants and seven potentially bioaccumulative toxic pollutants (those that can build up in humans and wildlife). We estimate that nearly all emissions from regulated area sources now meet these standards.

Many motor vehicle, nonroad equipment and fuel emission control programs of the past have reduced air toxics and will continue to provide significant emission reductions in the future. Mobile source emissions dropped by about 70 percent — close to 2 million tons of HAPs — between 1990 and 2014. With more fleet turnover, including more electric vehicles on the road, we expect these reductions to increase to 80 percent by the year 2030. In addition, EPA projects that mobile-source diesel particulate matter will decrease by roughly 90 percent from 2005 to 2030 as mobile source rules targeting diesel engines continue to take effect. Also, benzene emissions from mobile sources continue to decrease, as monitoring data confirms.

How much these emissions reductions improve public health will depend on several things, including which chemicals were reduced and where the reductions occurred relative to where people live and work.

Q12: Can I use AirToxScreen to get risks at an exact place, like my home or my child's school?

A12: No. AirToxScreen isn’t designed to predict actual risks at a specific place. It’s a screening tool that simplifies some of its source emissions and other input data and averages risks over a census block. This means AirToxScreen can show general areas where risks may be elevated. But it doesn’t use detailed data or include other information we would need to estimate risks at a specific location.

That said, AirToxScreen can still be useful to you. If AirToxScreen projects low risks for an area, and we are confident that we have modeled the main emission sources nearby, then we can be confident that risks actually are low. There is probably no need to develop a more detailed assessment for that area. But if AirToxScreen shows elevated risks in an area, we know that refined assessments may be needed to accurately show the risks there.

This screening approach helps EPA and other air agencies focus resources on areas where the potential for health risks is greatest.

Q13: I can search on an address in the AirToxScreen Mapping Tool and get a risk value at that point. How accurate is that value?

A13: The AirToxScreen Mapping Tool does show the risks that AirToxScreen estimates for each census block. But these risks are average risks for the entire block. Since census blocks vary in size, some blocks are large enough for risks to vary across the block. This means that AirToxScreen's block-level risks may not reflect the impacts at any one point. Also, AirToxScreen is a screening level assessment and should not be used to determine an individual person’s risk. You should use the results to find patterns in your general area instead of focusing on the risk values or concentrations at any one point. More focused assessments (for example, air toxics monitoring or local-scale risk assessments) would be needed to better define any concentrations and risks.

Fire Questions

Q1: How does AirToxScreen treat emissions from fires?

A1: We include from prescribed fires, wildfires and agricultural burning in AirToxScreen. EPA worked with the U. S. Forest Service to develop emissions estimates for wildfires and prescribed fires for our data inventory. Some wildfire and prescribed fire data came from EPA estimates based on satellite data, ground-based incident reports, and other national datasets that describe various aspects of fire activity and state activity data. Some states submitted emissions. Emissions estimates for agricultural burning came from state-submitted emissions data or estimates based on satellite detects. For more information on how EPA estimated emissions from wild and prescribed fires, see Section 2 of the AirToxScreen Technical Support Document.

In AirToxScreen, we model fires only with CMAQ. CMAQ can use details specific to the fires included. For example, we used day-specific emissions data for wildfires and fires from prescribed burning; by contrast, EPA only has annual average emissions estimates for other types of emission sources. Also, CMAQ lets us model the extra lift that a very hot fire gives an emission plume (a bit like heating up a hot-air balloon) and how that hot plume spreads as it moves through the air.

Q2. What does AirToxScreen show regarding impacts of wildfires, prescribed fires and agricultural burning?

A2: AirToxScreen looks at combined risk from exposure to smoke from wildfires, prescribed fires and agricultural burns. In nearly all of the country in 2020, that estimated risk was less than 10-in-1 million. (For comparison, the national average risk from air toxics exposure in 2020 was 30-in-1 million from all pollutants examined.)

AirToxScreen's risk estimates are based on a person being exposed to air toxics for 24 hours a day, 365 days a year, for 70 years. The same is done to estimate risk from fires: AirToxScreen takes the level of air toxics that were in smoke in 2020 and assumes people near those fires would be exposed at that same level continuously for 70 years.

Much of western Oregon, along with parts of California, Colorado, and Idaho, saw significant wildfires in 2020. These fires resulted in elevated cancer risks (at or above 100-in-1 million) in these areas in the 2020 AirToxScreen assessment. But because it’s unlikely that a wildfire will burn in these locations for a person’s entire lifetime, the risks AirToxScreen showed in these areas were likely higher than actual risk would be. Thus, we believe that the cancer risks from air toxics in wildfire smoke in these states were likely overestimated in the 2020 AirToxScreen.

(As another example, the Jolly Mountain fire in Washington and the Sonoma Complex fires in California triggered elevated cancer risks in those areas in the 2017 AirToxScreen assessment. But without large fires there in the following years, risks in 2018 through 2020 were much lower in those areas.)

That said, climate change has already led to an increase in wildfire season length, wildfire frequency and burned area. This could affect future risks. Learn more at EPA's Climate Change Indicators: Wildfires website. Also, wildfire smoke can be harmful to health from the fine particles in the smoke. Learn more at How Smoke from Fires Can Affect Your Health.

There are steps you can take to protect yourself when wildfire smoke is in the air near you. The AirNow Fire and Smoke Map can give you information about air quality near you during fires, along with steps to take to reduce your smoke exposure.

Q3. What are the uncertainties in risks from fire emissions?

A3: Unlike many air toxics sources, the size and location of fires often varies from year to year. AirToxScreen only uses one year of data; this includes fire emissions. So the places where fires happened to occur in that one year may bias the risks estimated by AirToxScreen toward those locations.

Further, in risk assessments such as AirToxScreen, we assume a 70-year continuous exposure to air toxics when calculating risk. Because it’s unlikely that a fire will burn in the same location for a person’s entire lifetime, the risks that AirToxScreen calculates due to air toxics found in smoke from fires are likely higher than actual risk would be. This means that cancer risks due to wildfires in are likely overestimated in AirToxScreen.

In addition to these uncertainties, the CMAQ model includes only the 48 contiguous United States, so we approximated concentrations from fires in Alaska, Hawaii and Puerto Rico using CMAQ-calculated concentrations from CONUS states with similar fire emissions. You can read more about this technique in Section 3.6.3 of the AirToxScreen Technical Support Document.

Q4. What is being done to reduce air pollution from wildfires and prescribed fires?

A4: Wildland vegetation management can reduce the threat from wildfires. Prescribed fires are one tool that land managers use to reduce fuel load, unnatural understory and tree density. This helps reduce the risk of catastrophic wildfires, which are frequently of long duration and wide impact and can produce large amounts of air pollutants. Even allowing some wildfires to continue can reduce the chance of a more catastrophic wildfire, which may reduce smoke impacts and related health effects in the long term.

EPA is committed to working with federal land managers, other federal agencies, tribes and states to effectively manage prescribed fire and reduce the impact of wildfire-related emissions. Most prescribed fires are managed using Basic Smoke Management Practices and smoke management programs. USDA and DOI both support efforts to conduct more research into smoke management through the Joint Fire Sciences Program and support broad interagency efforts to address smoke from both wildfires and prescribed fires through the National Wildland Fire Coordinating Group and their Smoke Committee.

EPA is also working to reduce the impacts of climate change. Climate change has already led to an increase in wildfire season length, wildfire frequency and burned area. Learn more about what EPA is doing to tackle the climate crisis at our Climate Change website.

Mobile Source Questions

Q1: What is a "mobile source?" Are there different types?

A1: Mobile sources are air pollution sources that can move from place to place. They are divided into two categories: (1) on-road and (2) nonroad vehicles and engines.

On-road vehicles include:

passenger cars and trucks;
commercial trucks and buses; and
motorcycles.

Nonroad vehicles and engines include

aircraft;
heavy equipment;
locomotives;
marine vessels;
recreational equipment (snowmobiles, all-terrain vehicles, etc.); and
small engines and tools (lawnmowers, etc.).

Q2: How accurate are risk estimates for mobile sources in my area?

A2: As we discuss in question 3 of the Results section above, EPA uses the best available science and data in AirToxScreen. But we also simplify some of these data to make running this national-scale assessment possible. So, like all of AirToxScreen’s risk estimates, you should treat those from mobile sources as screening-level results. We are confident that AirToxScreen does a good job estimating regional risks from mobile sources. As we will explain, though, risks from mobile source emissions at the census block level are prone to uncertainty.

Accurately capturing emissions for sources that move from place to place is challenging, particularly over small areas. Although AirToxScreen presents results at a census block level, concentrations of air toxics emitted from cars, trucks, buses, and motorcycles are averaged across grid cells of either 4-km resolution (for emissions on roadways) or 12-km resolution (for vehicle start and idling emissions). More information about AirToxScreen methodology and limitations for estimating near-road concentrations and associated risks is included in the response to Question 5 below.

Nonroad source emissions — such as those from construction equipment, lawn and garden equipment, and off-road recreational equipment — are more uncertain than on-road emissions. Nonroad equipment population data are not as complete as they are for on-road vehicles, and nonroad source emissions often must be spatially allocated based on land-use type. Even though we’ve improved these estimates over time, nonroad emissions are allocated to grid cells of 12-km resolution and may not always capture differences in local activity. EPA is actively looking for data related to nonroad populations and activity, including geographic allocation data. EPA recognizes that these data can influence AirToxScreen results.

In short, results for mobile sources are very uncertain at the census block level and you should interpret them with caution. In fact, they are only accurate to approximately 4 to 12 km (about 2.5 to 7.5 miles).

Q3: What is EPA doing to reduce emissions of mobile source air toxics?

A3: Mobile source air toxics emissions dropped by about 60 percent — close to 1 million tons of HAPs — between 2008 and 2020. Diesel particulate matter emissions have dropped approximately 80 percent between 1996 and 2020. Reductions of these and other mobile source emissions are expected to continue in the future as a result of additional fleet turnover and as EPA rulemakings (such as those described below) continue to take effect.

EPA’s 2007 mobile source air toxics rule controlled the benzene content of gasoline, vehicle emissions at cold temperatures, and emissions from portable fuel containers. EPA’s Tier 3 vehicle and fuel standards, issued in 2014, are expected to cut emissions of air toxics from motor vehicles between 10 and 30 percent by 2030 (depending on the pollutant). In 2024, EPA issued new emission standards for light-, medium- and heavy-duty vehicles. These standards will result in reduced emissions of many mobile source air toxics.

Other programs reducing mobile source air toxics are:

low-sulfur gasoline and diesel requirements;
heavy-duty engine and vehicle standards;
controls for small spark-ignition engines and recreational marine engines;
the locomotive and commercial marine rule;
standards for nonroad diesel engines; and
the North American and Caribbean Emission Control Areas (ECAs), established to reduce emissions from ships.

Several nonregulatory programs are also reducing mobile source air toxics. Examples include EPA's Diesel Emissions Reduction Act (DERA), SmartWay, and Ports Initiative programs. More than 73,700 engines, vehicles, or other pieces of equipment were replaced or retrofitted to run cleaner with DERA funds during fiscal years 2008 to 2018, according to the DERA 5th Report to Congress. Eligible activities include the retrofit or replacement of existing diesel engines, vehicles, and equipment with EPA and California Air Resources Board (CARB) certified engine configurations and verified retrofit and idle reduction technologies. View more information on EPA's DERA Program website.

Another example is the SmartWay Transport Partnership which, through improving freight efficiency and providing alternatives to long-duration truck idling, has avoided an estimated 114,000 short tons of PM¬2.5 since 2010.

EPA’s Ports Initiative is another example of a nonregulatory program to reduce emissions from mobile sources in near-port communities. The Inflation Reduction Act of 2022 provided EPA with $3 billion to fund zero-emissions port equipment and infrastructure as well as climate and air quality planning at U.S. ports. This new funding program will build on EPA’s Ports Initiative to help reduce diesel pollution, including air toxics, in near-port communities and build a foundation for the port sector to transition over time to fully zero-emissions operations.

Learn more about EPA's programs to reduce air toxics from mobile sources.

Q4: Why are only non-cancer risks calculated for diesel PM and not cancer risks?

A4: EPA has not developed or adopted a cancer unit risk estimate (URE) for diesel exhaust. In the 2002 Health Assessment Document for Diesel Engine Exhaust, EPA concluded that diesel exhaust is likely to be carcinogenic to humans at environmental levels of exposure, but found that data from the health studies available at the time were not suitable for estimating cancer potency.

However, EPA has concluded that diesel exhaust is among the substances that the national assessment suggests pose the greatest cancer risk. The 2002 Health Assessment Document evaluated several studies linking increased lung cancer risk with diesel PM. Exposures in several of these studies are in the same range as ambient exposures throughout the United States.

Several large studies have been published that strengthen the weight of evidence that diesel exhaust is carcinogenic to humans. Two of these studies included quantitative estimates of exposure. Partly based on these studies, the International Agency for Research on Cancer (IARC) elevated its classification of diesel exhaust to “carcinogenic to humans” (Class 1) in 2012.

In 2012, EPA and industry asked the Health Effects Institute (HEI) to convene a panel to review recently published epidemiology studies of occupational exposures to diesel engine exhaust. It was hoped that the panel could determine whether new studies could be used in a quantitative risk assessment to calculate a cancer unit risk estimate (URE). In a final report published in late 2015 , the panel concluded that newer studies progressed toward addressing major limitations in previous epidemiologic studies of diesel engine exhaust. Although uncertainties remain, they concluded that the newer studies gave a basis for a QRA of diesel engine exhaust exposures, specifically to diesel engine exhaust from older diesel engines.

Currently there are no plans at the EPA to conduct a quantitative risk assessment for diesel engine exhaust. However, EPA continues to act to reduce diesel emissions through standards for heavy trucks and engines. For example, because of EPA actions, on-road diesel engines manufactured in 2007 and later have much more advanced emission control systems. This results in much lower emissions with different composition than the diesel engines that formed the basis of the currently available epidemiology studies. Thus, a cancer potency based on available epidemiology studies may not be relevant to highway diesel engines that meet these most recent standards.

The studies reviewed by HEI show why it’s important to continue reducing emissions and exposures to diesel exhaust. EPA continues to implement diesel exhaust emission standards reducing exposures. Programs like the Diesel Emissions Reduction Act, the Clean School Bus Program, Smartway and the Ports Initiative also help reduce emissions from the legacy diesel engine fleet.

The Inflation Reduction Act of 2022 provides EPA with $3 billion to fund zero-emissions port equipment and infrastructure as well as climate and air quality planning at U.S. ports. This new funding program will build on EPA’s Ports Initiative that helps our nation’s ports, a critical part of our infrastructure and supply chain, to address public health and environmental impacts on surrounding communities. Further, the Inflation Reduction Act invests $1 billion to replace existing Class 6 and Class 7 non-zero-emission heavy-duty vehicles with zero-emission models.

Also, EPA has designated a chronic (RfC) for diesel PM of 5 micrograms per cubic meter. This level is based on specific noncancer effects found in several animal studies, which showed adverse changes in lungs such as inflammation and lesions. AirToxScreen uses this value to estimate the diesel PM .

You can learn more about this subject on EPA’s Mobile Source Pollution and Related Health Effects website.

Q5: There has been increased concern about the health effects associated with pollution near roads. What can AirToxScreen tell us about health risks from exposure to near-road pollution?

A5: Research consistently finds that people who spend a lot of time near high-traffic roads are at greater risk for several . Motor vehicle pollutant concentrations tend to be higher closer to the road, with the highest levels generally within the first 500 feet (about 150 meters) of a roadway and reaching background levels within approximately 2,000 feet (about 600 meters) of a roadway, depending on the pollutant, time of day, and surrounding terrain.

However, for a given location, AirToxScreen may not be able to accurately capture near road exposures due to limitations in our data (see question 2 above for more information). That’s because AirToxScreen’s air quality modeling can’t pinpoint high concentrations along specific roadways. We do try to partially account for near road exposures using the Hazardous Air Pollutant Exposure Model (HAPEM). One of the factors considered by HAPEM to estimate exposure is proximity to roadways, but that is calculated as the average across a census tract (about 4,000 people). In place of AirToxScreen, refined modeling or monitoring studies can give a more precise picture of air quality in these areas.

EPA has a website focused on near-roadway air pollution and health, as well as a website about ongoing near-source air pollution research.

Q6: AirToxScreen results show impacts associated with a port in my community. How accurate are these estimates, and what can be done to reduce the impacts?

A6: As with other sources, AirToxScreen results for ports highlight places where a more refined analysis may be needed. AirToxScreen’s analysis of ports has some uncertainties and limitations:

Although pollutant concentrations in ports comes from many emission sources, AirToxScreen only includes emissions from commercial marine vessels within ports.
Port emissions for those marine vessels come from state and local agency submittals or, in most cases, EPA’s estimates.
Due to its national scale, AirToxScreen simplifies port boundaries more than a local assessment would (see Section 2 of the AirToxScreen Technical Support Document).
We have limited data on which to base emission estimates for toxics from commercial marine vessels.

Because of these limitations, results are uncertain at the census block level and should be interpreted with caution. Still, AirToxScreen findings suggest that people who live and work near ports may experience elevated risks. EPA has taken many actions that have reduced risks in recent years. These actions include Tier 2 and Tier 3 standards on oceangoing marine vessels, sulfur control on marine fuel oil, and designation of an emission control area (ECA) off our coasts. Finally, EPA's Ports Initiative program helps speed up the use of cleaner technologies, promotes clean air planning practices such as conducting emissions inventories, and spurs community engagement to address diesel emissions at ports across the country. As part of EPA’s Ports Initiative and through funding in the Inflation Reduction Act of 2022, EPA plans to provide $3 billion in Clean Ports Program funds to support zero-emission port equipment and infrastructure as well as climate and air quality planning at U.S. ports.