According to section 39655 of the California Health and Safety Code, a toxic air contaminant (TAC) is “an air pollutant which may cause or contribute to an increase in mortality or an increase in serious illness, or which may pose a present or potential hazard to human health.” In addition, substances which have been listed as federal hazardous air pollutants (HAPs)  pursuant to section 7412 of Title 42 of the United States Code are TACs under the air toxics program pursuant to section 39657 (b) of the California Health and Safety Code.