General Information

The Human Exposure Model (HEM) is used primarily for performing risk assessments for sources emitting air toxics to ambient air. The HEM only addresses the inhalation pathway of exposure, and is designed to predict risks associated with chemicals emitted into the ambient air (i.e., in the vicinity of an emitting facility but beyond the facility's property boundary). The HEM provides ambient air concentrations, as surrogates for lifetime exposure, for use with unit risk estimates and inhalation reference concentrations to produce estimates of cancer risk and noncancer hazard, respectively, for the air toxics modeled. The current version of HEM is HEM4, but the previous version (HEM-3) is still available for download.

The HEM4 contains (1) an atmospheric dispersion model, AERMOD , with included meteorological data, and (2) U.S. Census Bureau population data at the Census block level. The model utilizes 2010 Census data (Note: the model and its datasets have been populated for the 50 states, DC, PR, and the VI only), although alternate receptor data can be used to model facilities within and outside the U.S. Each source in HEM4 must be accurately located by latitude and longitude (or Universal Transverse Mercator) coordinates, and its release parameters must be specified. These include stack height, exit velocity, emission rate, etc. Based on the inputs for source parameters and the meteorological data, AERMOD estimates the magnitude and distribution of ambient air concentrations in the vicinity of each source. The model is generally used to estimate these concentrations within a radial distance of 50 kilometers (about 30 miles) from the source. Exposure estimates generated in HEM4 are the ambient air concentrations predicted by AERMOD, in micrograms per cubic meter. These exposure estimates are actually surrogates because exposure variables (e.g., duration, human activity patterns, residential occupancy period, etc.) are not explicitly addressed. In HEM4, these exposure estimates are combined with pollutant health reference values to estimate cancer risks and noncancer hazards, cancer incidence, and other risk measures.

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User's Guides - Those planning to use HEM are encouraged to carefully review the User's Guides. They describe technical information about the models and the steps involved in running them.

Peer Review and Publications

The AERMOD model has undergone review and evaluation as part of the regulatory models process.

Appendix A (labeled Appendix W in CFR) of Guideline on Air Quality Models provides a summary description of the AERMOD model. The SCRAM web site provides documentation of AERMOD.