The Hazardous Air Pollutant Exposure Model, Version 4 (HAPEM4) is a computer model that has been designed to estimate inhalation exposure for specified population groups and air toxics.

Through a series of calculation routines, the model makes use of census data, human activity patterns, ambient air quality levels, climate data, and indoor/outdoor concentration relationships to estimate an expected range of inhalation exposure concentrations for groups of individuals.