June 2009
Front Matter
Training
29
Air Toxics Emissions
What Are the Sources of Air Toxics?
•Air toxics are both directly emitted by sources and formed in the atmosphere.
•Major sources include chemical plants, steel mills, oil refineries, and hazardous waste incinerators for which there is a specific location provided in the inventory. 
•Area sources are made up of many smaller sources releasing pollutants to the outdoor air in a defined area. 
•Mobile sources include highway vehicles, trains, marine vessels, and non-road equipment (such as construction equipment).
•Natural sources – Some air toxics are also released from natural sources such as volcanoes or fires; typically in the inventory these would be included in area source emissions.
In emission inventory terminology, emissions are grouped as point (major), area, and mobile sources.  The following definitions describe how these terms are used in the emission inventory.
Major sources: 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: Examples include neighborhood dry cleaners, small metal plating operations, gas stations, and woodstoves. These sources may not be identified in the inventory by a specific location.
Mobile sources: 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 performed in batches.  Accidental releases can occur during an explosion, equipment failure, or a transportation accident.  The timing and amount released during accidental releases are difficult to estimate.