Oxides of carbon, sulfur, and nitrogen; hydrocarbons; particulate matter; and chloro fluoro carbons are The most significant pollutants. In 1980, scientific studies had shown that the air inside homes and office buildings is often more polluted than the air Carbon oxides is the largest single group of pollutants. Sulfur oxides notably sulfur dioxide are among the most dangerous and irritating of all air pollutants. Nitrogen oxides are produced in automobile engines and other devices where combustion takes place. Particulate matter includes smoke, dust, and soot, which may contain such toxic substances as asbestos, fluorides, lead, and mercury. Indoors, tobacco smoke is a major source of particulate matter. Chloro fluoro carbons (CFC's) are chemically stable synthetic substances that have had a variety of industrial uses. Radon is a naturally occurring radioactive gas found in varying concentrations in the ground.