The US EPA has a list of 187 air pollutants that they consider "hazardous". These toxic pollutants are known or suspected to cause cancer or other serious health conditions. How is it determined whether an air pollutant is hazardous? I'm mainly curious how the list of 187 pollutants was determined. Is there a testing process to determine toxicity that is used? or some other method that distinguishes a pollutant's toxicity? I suspect it could've been done with just literature review, but then I would expect the list to be revised regularly based on health studies.