Introduction to Toxic Air Pollutants
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3. Where do toxic air pollutants come from? 
Most air toxics originate from human-made sources, including mobile sources
(e.g., cars, trucks, buses) and stationary sources (e.g., factories, refineries,
power plants), as well as indoor sources (e.g., some building materials and
cleaning solvents). Some air toxics are also released from natural sources
such as volcanic eruptions and forest fires.
When released, the air toxics can be carried by the wind, away from their sources, to other locations. Certain factors, such as the weather, terrain, and the physical properties of the pollutants determine how far they can be transported. These factors also impact the concentration of the pollutants at various distances from the source, the kind of physical and chemical changes undergone and whether the pollutants will degrade, remain airborne or deposit to the land or water.
Some air toxics are of a particular concern because they degrade very slowly or not at all, as in the case of metals such as lead and mercury. These persistent air toxics can remain in the environment for a long time (or forever) and can be transported great distances.
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