Hazardous Air Pollutant Exposure Model (HAPEM)
Description
EPA’s HAPEM (Hazardous Air Pollutant Exposure Model) is a human exposure model that estimates how people are exposed to outdoor-origin hazardous air pollutants as they move through different microenvironments (home, work, in transit, outdoors). Using ambient concentration inputs, population and activity-pattern data, and microenvironmental factors (e.g., infiltration indoors), HAPEM produces distributions of exposure for various population groups and is used in national? and regional?scale risk assessments such as the National Air Toxics Assessment.
Questions this resource can help answer
Given modeled or monitored ambient levels of specific hazardous air pollutants, what are the resulting exposure distributions for different population groups?
How do exposures differ between subpopulations (e.g., children vs. adults, urban vs. rural) within the same region?
How much do indoor environments and time spent commuting modify exposures relative to outdoor ambient concentrations?
How do changes in ambient concentrations under alternative emissions or policy scenarios translate into changes in population exposure?
Which pollutants and locations contribute most to high?end (e.g., 95th percentile) exposures?
How do I use this resource?
Define your scenario
-Choose pollutants, geographic area (e.g., county, urban area, national), population groups (age, gender, etc.), and time period.
Prepare inputs
-Ambient concentration fields (typically gridded or monitor?based model outputs for the HAPs of interest).
-Population data by area.
-Representative time–activity patterns (people’s movements through microenvironments).
-Microenvironmental parameters (e.g., indoor–outdoor ratios, commuting conditions).
Run HAPEM
-Configure the model with your scenario settings.
-Run simulations for each pollutant and population group; HAPEM combines ambient concentrations with time?activity and microenvironmental factors to generate exposure estimates.
Review outputs
-Examine distributions of exposure (e.g., mean, percentile exposures) by pollutant, group, and geography.
-Export results for use in health?risk calculations or comparisons across scenarios.
Pro tips
Start from an EPA?documented application (e.g., NATA) as a template for your own configuration.
Make sure ambient concentration inputs are consistent in space, time, and units with what HAPEM expects.
Pay close attention to time–activity pattern selection; these strongly influence exposure estimates.
Use default microenvironmental factors unless you have robust local data to justify changes.
Document all scenario assumptions (pollutants, populations, microenvironments, input sources) for transparency and reproducibility.
Treat HAPEM results as population?level exposure distributions, not individual?level predictions.
Resource information
Technical specifications
For each simulated individual or cohort, HAPEM computes exposure as the time?weighted sum of microenvironmental concentrations encountered along their daily pattern. Repeating this across many representative patterns and population groups yields distributions of exposure by pollutant, geography, and demographic group. These exposure distributions can then be combined with toxicity values in subsequent risk assessment steps, but HAPEM itself focuses on the exposure portion of the risk equation.
