8Air pollution can be described simply as any gas, liquid, or vapor that enters the atmosphere and is breathed in. Air pollution is most commonly described as either primary (mainly chemical) pollutants or secondary pollutants. Primary pollutants are those that are put into the air directly by humans or natural occurrences. Examples of primary air pollutants are carbon dioxide released from vehicles. Or, one may be polluted by the chemicals used to process fossil fuels. Secondary pollutants come in forms of naturally occurring elements, radon gas, and pesticides.