- Group A (human carcinogen) : A Group A (human carcinogen) is Compounds for which human data are sufficient to demonstrate a cause and effect relationship between Exposure and cancer incidence (rate of occurrence) in humans. In the national-scale assessment, the 7 air toxics classified as human carcinogens are: arsenic compounds, benzene, 1,3-butadiene, chromium compounds, coke oven emissions, nickel compounds, and vinyl chloride.