Any airborne pollutant that is not addressed in the National Ambient Air Quality Standards (NAAQS) but may reasonably be expected to cause one of several health problems such as cancer, respiratory, cardiovascular, or developmental difficulties, reproductive dysfunctions, neurological disorders, heritable gene mutations, or other serious, irreversible, chronic, or acute health effects in humans.
Source: U.S. Environmental Protection Agency. 2013. Terminology Service Terms of Environment. Accessed on 9 September 2013. Available at http://iaspub.epa.gov