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Risk Assessment and Modeling - Human Exposure Model (HEM)

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 HEM-3 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 and 2000 Census data (Note: the model and its datasets have been populated for the 50 states, DC, PR, and the VI only). Each source in HEM-3 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 HEM-3 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 HEM-3, these exposure estimates are combined with pollutant health reference values to estimate cancer risks and noncancer hazards, cancer incidence, and other risk measures.

The HEM-3, version 1.3.1 is available in two versions: a single-facility version intended to model a single facility (or several facilities in close proximity), and a multi-facility version which can be used to model any number of facilities located anywhere in the US. The multi-facility version basically runs the single-facility version in batch mode using detailed input files instead of obtaining inputs through user interfaces as in the single-facility version. The multi-facility version also creates several files that summarize the results of all facilities modeled.

There is post-processing software for the multi-facility HEM-3 which may be used to produce additional outputs of combined and summarized results for the group of facilities modeled. This software is called the Risk and Technology Review (RTR) summary programs because it is used by the USEPA in summarizing the risks from entire source categories that are assessed under the Clean Air Act RTR program.

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User's Guides - Those planning to use HEM-3 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.