Tobacco smoke air pollution is a mixture of more than 4,000 chemical by-products of tobacco combustion, 500 of which are in the gas phase (References 1 and 4). Of these byproducts of secondhand smoke, 172 are known toxic substances, many of which are regulated under existing clean air laws (see list in Table 9.1 of Reference 1). In addition to 3 standard criteria air pollutants (carbon monoxide, particulate matter, and lead) and 33 hazardous air pollutants (HAPs) regulated in ambient air under the federal Clean Air Act, secondhand smoke contains 47 pollutants that are classified as hazardous wastes whose disposal in solid or liquid form is regulated by the Resource Conservation and Recovery Act, 67 pollutants known to be human or animal carcinogens, and 3 industrial chemicals regulated under the Occupational Health and Safety Act.¹