Silica and lung cancer: a continuing controversy.

The question whether silica is carcinogenic is not new, but there has been a resurgence of research over the last two decades with the use of more powerful epidemiological methodologies. There is sufficient evidence for the carcinogenicity of crystalline silica in animals. A large number of cohort and case-control studies consistently suggest a modest excess of lung cancer in workers with occupational silica exposure (relative risk less than 2). However, in many studies, the association is confounded by exposures to cigarette smoke, and environmental cocarcinogens like radon daughters, polyaromatic hydrocarbons and asbestos. The excess risk of lung cancer is more pronounced in workers with silicosis (relative risk of 2 to 4). Silica may act as a direct carcinogen or indirectly by the adsorption of cocarcinogens such as polyaromatic hydrocarbons from cigarette smoke or industrial pyrolysis products, and/or by impairing pulmonary clearance, thereby increasing the effective dose and duration of exposure to these carcinogens. Pulmonary fibrosis itself may be a precursor to the development of lung cancer.