Human Exposure Model (HEM)
Description
EPA’s Human Exposure Model (HEM) is a probabilistic exposure assessment tool used to estimate how people are exposed to contaminants (often air pollutants, but not limited to them) through different pathways and microenvironments. It is designed to support human health risk assessments by translating environmental concentrations into population exposure distributions for specific pollutants, locations, and demographic groups. HEM uses information on contaminant levels, population characteristics, and time–activity patterns to produce quantitative estimates of average and high-end exposures suitable for regulatory analyses and comparative policy evaluations.
Questions this resource can help answer
Given ambient or modeled air pollutant concentrations in a region, what are the resulting exposure distributions for different age or demographic groups?
How do population exposures change under alternative emissions or regulatory scenarios;?
Which pollutants or locations contribute most to high-end exposures in the population?
How do I use this resource?
To use HEM, you begin by clearly defining your assessment scenario: the pollutant or pollutants of interest, exposure pathways and media (typically inhalation, sometimes others depending on configuration), the geographic area, the time frame, and the population groups being evaluated. You then assemble and format the required input data: environmental concentration estimates (often from monitoring or air-quality models), population data, and, where applicable, time–activity patterns and microenvironmental parameters. These inputs are loaded into HEM, and you configure the model to reflect your scenario choices (pollutants, populations, exposure duration, and key assumptions). When you run the model, it combines concentrations with exposure factors to generate distributions of exposure for the selected groups and locations, which you can then export for risk calculations or scenario comparisons.
Resource information
Technical specifications
HEM follows a microenvironmental, exposure-factor–based approach. It starts from environmental concentration inputs and uses exposure factors—such as inhalation rates, time spent in different locations or activities, and, where relevant, factors that link outdoor concentrations to indoor or in-vehicle levels—to estimate how much contaminant a typical person in a given subpopulation contacts over a specified period. By applying these calculations across many individuals or representative patterns, HEM generates probabilistic exposure distributions (e.g., means, percentiles) by pollutant, geography, and population group. These exposure estimates can then be combined with toxicity values or dose–response functions in subsequent risk assessment steps, but HEM itself focuses on the exposure side of the risk equation.
