1 3-Butadiene is a product of incomplete combustion resulting from natural processes and human activity. It is also an industrial chemical used primarily in the production of polymers. It enters the environment from exhaust emissions from gasoline and diesel-powered vehicles, from non-transportation fuel combustion, from biomass combustion and from industrial on-site uses. The highest atmospheric concentrations have been measured in air in cities and close to industrial sources. In this volume the risks to human health and the environment of this chemical are assessed. The general population is exposed to 1 3-Butadiene primarily through ambient and indoor air. Tobacco smoke may contribute significant amounts of 1 3-Butadiene. The available epidemiological and toxicological data provide evidence that 1 3-Butadiene is carcinogenic in humans and may also be genotoxic in humans.

This report considers the biological and behavioral mechanisms that may underlie the pathogenicity of tobacco smoke. Many Surgeon General's reports have considered research findings on mechanisms in assessing the biological plausibility of associations observed in epidemiologic studies. Mechanisms of disease are important because they may provide plausibility, which is one of the guideline criteria for assessing evidence on causation. This report specifically reviews the evidence on the potential mechanisms by which smoking causes diseases and considers whether a mechanism is likely to be operative in the production of human disease by tobacco smoke. This evidence is relevant to understanding how smoking causes disease, to identifying those who may be particularly susceptible, and to assessing the potential risks of tobacco products.