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. Other releases may be routine but intermittent, such as when a plant's production is done in batches.
Accidental releases can occur during an explosion, equipment failure, or a transportation accident.
The timing and, often, the amount released during accidental releases are difficult to predict. To estimate the amount of a routine release engineers sometimes use a monitor to sample the pollutant as it is released. The amount collected in a given time period is measured in a laboratory. Alternatively, engineers can use an emission model to estimate the amount of pollutant released by a particular facility. An emission model is a set of mathematical equations that represent the processes that occur when a facility generates a pollutant.