AIRBORNE CARCINOGENS

Air pollution has been recognized as a cancer risk for many years. More than 2,800 different chemicals have been identified in the air or emission sources. Only about 10% of these chemicals have been evaluated in bioassays for genetic or carcinogenic effects. Hydrocarbons, nitrogen-containing organics, and halogenated organics account for nearly 60% of the airborne chemicals that have been studied in long term animal cancer bioassays or short-term genetic bioassays. The sources that emit the highest number of these potentially carcinogenic chemicals are sources involving combustion (e.g., tobacco smoke, automobile exhaust, and coal combustion). Quantitative estimates of the risk of airborne carcinogens in outdoor air consistently show that polycyclic organic matter (POM) from products of incomplete combustion (PICs) make the largest single contribution to human cancer risk. Although the POM emissions from various air pollution sources are chemically similar and induce cancer by a similar genotoxic mechanism, the cancer risk per unit of exposure of these emissions may vary by several orders of magnitude. Among these combustion sources motor vehicle emissions account for the greatest cancer risk in outdoor air. Environmental tobacco smoke (ETS) and radon are the major sources of cancer risk from indoor exposures. There are, however, many uncertainties in identifying the important airborne carcinogens and quantitating the human cancer risk of air pollution. One important uncertainty is the role of atmospheric transformation products in human cancer.