AirToxScreen Assessment Methods

The AirToxScreen analysis includes four main steps:

• Compile a National Emissions Inventory (NEI)
• Estimate ambient concentrations of air toxics across the United States
• Estimate population exposures
• Characterize potential health risks from inhalation

Compile a National Emissions Inventory (NEI)

To get the emissionsemissionsThe release of pollutants into the ambient air. data for this assessment, EPA starts with the latest  National Emissions Inventory (NEI). If the AirToxScreen data-year is more recent than the latest NEI, we update the inventory to reflect that year (see our Frequent Questions for more details). This inventory reflects reviews and quality assurance checks made by state and local agencies and other reviewers. We also conducted certain processing steps (for example, placing individual chemicals into groups and converting the emissions into the formats needed for the air quality models). 

AirToxScreen includes these emission source types:

• major stationary sources, also referred to as point sources (including large waste incinerators and factories);
• area and other smaller stationary sources, also referred to as nonpoint sources (including dry cleaners and small manufacturers);
• onroad and nonroad mobile sources (including cars, trucks and boats); 
• fires (including wildfires, prescribed fires and agricultural burning); and
• biogenic sources (emissions from trees, plants and soil microbes).

We also include estimates for:

• background concentrations – air toxics emissions from distant sources, emissions from prior years that persist in the environment, and natural source emissions other than those modeled as biogenic sources; and
• secondarily formed pollutants – chemicals that form in the air through chemical reactions. Please note that biogenic sources also directly emit some pollutants that also form secondarily, including formaldehyde, acetaldehyde and acrolein.

For more information on emission inventories, see the Air Emissions Inventory web page. You can find more detailed information on development of AirToxScreen’s emissions data in Section 2 of the AirToxScreen Technical Support Document (TSD), Emissions. Section 2 also includes more detailed information on how emissions and background concentrations were processed for modeling.

Estimate ambient concentrations of air toxics across the United States

Using two models, EPA estimated ambient concentrationsambient concentrationsA way to describe how much of a pollutant is in the outdoor air. Concentration is usually shown as an amount, or mass, of pollutant per certain volume of air. In AirToxScreen, most concentrations are in micrograms (µg) of air pollutant per cubic meter (m3) of air (a “box” of air one meter on each side). of air toxics across the United States. We used the chemical transport model CMAQ and the dispersion model AERMOD, taking advantage of the strengths of each model.

CMAQ is used to conduct urban- to regional-scale simulations of multiple air-quality issues. CMAQ can model how air pollution disperses and chemically transforms as it travels through the air. CMAQ includes several strengths that make it well suited for AirToxScreen. It conserves mass (that is, if some pollution leaves an area, it is accounted for in the new area); allows long-range pollutant transport; and estimates concentrations of secondarily-formed pollutants such as formaldehyde. We modeled over 50 air toxics plus diesel particulate matter (diesel PM) in CMAQ using a 12-km by 12-km grid resolution for the contiguous-US, a 9-km by 9 km grid resolution for Alaska, and a 3-km by 3-km grid resolution for Hawaii, Puerto Rico and the U.S. Virgin IslandsFires and biogenic emissions were only modeled in CMAQ. Secondary formation of pollutants was also only estimated in CMAQ. CMAQ provides an ambient air concentration for each grid.

AERMOD, EPA’s preferred dispersion model for regulatory purposes, can estimate ambient concentrations at many closely spaced points (called receptors). We based AERMOD receptor locations on grids of receptors within  populated census blocks, monitoring site locations, and evenly distributed points within each CMAQ grid cell for each domain (contiguous US, Alaska, Hawaii and Puerto Rico/Virgin Islands) . The spacing of the receptors within each census block depended on the size of the block and varied between 25 meters and 10 km.  The evenly distributed points within the CMAQ grid cells were as follows:

• 144 within each 12 km grid cell in the contiguous US
• 188 within each 9 km grid cell in Alaska
• 9 within each 3 km grid cell in Hawaii and Puerto Rico/U.S. Virgin Islands

We modeled all AirToxScreen pollutants using AERMOD. We included all emission source types except fires, biogenic and secondary sources, and background estimates in the AERMOD simulations. AERMOD provides an ambient air concentration for each receptor.

For the air toxics modeled in both CMAQ and AERMOD, we used a special hybrid modeling method. This method combines the fine receptor spacing allowed by AERMOD with the full treatment of chemistry and transport that CMAQ provides. Using this hybrid approach, we can “anchor” the ambient AERMOD receptor concentrations to the ambient CMAQ grid concentrations. This allows mass to be conserved and avoids double counting.

In AirToxScreen, we also add background concentrations, which represent the contributions to ambient concentrations of air toxics resulting from:

• natural sources not already accounted for under biogenic emissions;
• emissions of persistent chemicals that occurred in previous years; and
• long-range transport from distant sources.

These background concentrations represent “lingering” pollutant levels found in a place even with no nearby emissions of those pollutants during the year of the AirToxScreen assessment. We included background directly in all CMAQ modeling. For AERMOD, we calculated a background concentration and added it to the modeled concentration. More information about background concentrations can be found in Section 3 and Appendix D of the TSD.

Using the hybrid approach (with background concentrations added where appropriate), we estimated ambient concentrations at every populated census block receptor.

More information about these models and the process for generating ambient concentration data may be found in Section 3 of the TSD.

Estimate population exposures

In AirToxScreen, we don’t directly measure exposure. Neither AERMOD nor CMAQ directly addresses certain important exposure variables (including human activity patterns, etc.). Instead, we use modeled concentrations as surrogates, or stand-ins, for exposure. This step is important, because the average concentration of a pollutant that people actually breathe may be much higher or lower than the concentration at some fixed modeled location.

To estimate exposure, EPA uses the census tract concentrations calculated in the previous step as inputs to the screening-level inhalation exposure model HAPEM8. HAPEM7 uses census data, human-activity-pattern data, ambient air quality levels, climate data, and indoor/outdoor concentration relationships to calculate an expected range of apparent inhalation exposure concentrations for groups of people. It then provides a best estimate of exposure for a hypothetical typical person for a given census tract.

EPA conducted HAPEM simulation modeling for the 1999 National Air Toxics Assessment (NATA) analysis, the predecessor of AirToxScreen, for each combination of pollutant, census tract and source group. Because running HAPEM requires a lot of time and resources, for the 2002 and 2005 NATAs, we relied on the exposure factors derived for the 1999 analysis and applied them to the modeled ambient concentrations. For the 2011 and 2014 NATAs, and for the 2017, 2018, 2019, and 2020 AirToxScreen assessments, EPA ran HAPEM for several surrogate pollutants (coke oven emissions, diesel PM, benzene, 1,3-butadiene, generic PAHs, chromium VI, and nickel). We then applied these exposure factors to other similar pollutants (for example, benzene was the surrogate for all other gas-phase pollutants). 

For more information about the model, see the HAPEM8 User's Guide. For more information about the method, see Section 4 of the TSD, Estimating Exposures for Populations.

Characterize potential health risks from inhalation

In the last step of AirToxScreen, EPA calculates cancer risks and noncancer hazard indexes by pollutant. To do this, we combine the census tract-level exposure concentration estimates (generated in the previous step) for each census block in the census tract with available unit risk estimates and inhalation reference concentrations for each pollutant. 

For this AirToxScreen release, we calculated cancer risks or hazard indexes for about 140 pollutants. You can find more details about unit risk estimates, inhalation reference concentrations, and risk characterization in EPA’s Dose-Response Assessment web page; in Section 5 of the TSD, Characterizing the Effects of Air Toxics; and in Section 6 of the TSD, Characterizing Risks and Hazards in AirToxScreen.

The 2020 AirToxScreen results section presents ambient and exposure concentrations, cancer risks, and noncancer hazard indexes at the block level. For noncancer effects in the 2020 assessment, we estimated the effect of air toxics on the respiratory target system only. Past assessments have shown that the effect on the lungs and other parts of the respiratory system (known as the respiratory endpoint) tends to drive noncancer risks. Future years of AirToxScreen will provide noncancer block level results for the other 13 target organs in addition to the respiratory target organ.  

When using these results, it is important to consider that AirToxScreen is a screening-level assessment and only suitable to answer certain questions. Avoid using AirToxScreen results to compare or rank states or regions of the country. Air toxics emissions data reported to EPA vary in level of detail from state to state, making comparisons difficult. Certainly, air toxics concentrations, exposures and risks do differ across the United States. It is important to have an idea of how these factors vary throughout the country. But comparisons within a state are more appropriate than comparing one state or region to another.

EPA seeks to protect health with reasonable confidence. But it's important to keep in mind AirToxScreen's variability and uncertainty, which are part of any risk assessment. Scientific estimates of air concentrations, exposures and risks always involve assumptions. These assumptions simplify things to make an assessment possible, but they also introduce uncertainties. For example, the smaller the area, the more uncertain the results. A more complete discussion of both variability and uncertainty is found in Section 7 of the TSD, Variability and Uncertainty Associated with AirToxScreen.