Step 1 entails identifying pollutants likely to be in the air. In Step 2, the amounts of these pollutants released from different sources are estimated. In Step 3, the concentrations of the pollutants are estimated for the geographic areas of interest. Finally, Step 4 provides estimates of the number of people who breathe air containing the pollutant at different levels or at some selected level, such as a regulatory standard or a health benchmark level. Many chemicals found in factories, consumer goods, sewage treatment plants, and other sources can be released to the air as toxic air pollutants.
Point sources are sources that have a specific location.
Point sources include chemical plants, steel mills, oil refineries, and hazardous waste incinerators. Pollutants can be released when equipment leaks, when material is transferred from one area to another, or when waste is given off from a facility through smoke stacks. Area sources of toxic air pollutants are made up of many smaller sources releasing pollutants to the outdoor air in a defined area. Examples include automobiles, neighborhood dry cleaners, small metal plating operations, gas stations, and woodstoves. Routine releases, such as those from industry, cars, landfills, or incinerators, may follow regular patterns and happen continuously over time.