When potentially noxious substances are discharged into the atmosphere at a rate that exceeds its capacity to disperse them by dilution and air currents, the resulting accumulation is air pollution. It may take the form of haze, dust, mist (which may be corrosive), or smoke and may contain oxides of sulfur and nitrogen and other gases that may irritate the eyes, respiratory tract, or skin, and other substances that may be harmful to the environment or to human health. Absorption may occur in amounts sufcient to cause acute or chronic systemic toxicity. Air pollution has been greatly underestimated as a cause of illness and death. In May 2000, acting Canadian Environment Commissioner Richard Smith quoted government statistics indicating that smog adversely affected the health of 20,000,000 Canadians and caused 5,000 premature deaths annually in 11 major population centers. This is in comparison to 4936 deaths from breast cancer, 3622 from prostate cancer, 3064 from motor vehicle accidents, and 665 from malignant melanoma. Air pollution obviously is an important health hazard.