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Mr. Kevin T. Haroff Page 6 silica causes silicosis. Silicosis is a chronic fibrogenic pneumoconiosis caused by the inhalation of free crystalline silica particles. The disease is characterized by discrete nodular pulmonary fibrosis and, in more advance stages, by conglomerate fibrosis and impaired respiratory function. There is no effective specific therapy for the silicosis. The presence of free silica particles in lung parenchyma at autopsy demonstrates that such dusts are respirable. Cooper and Jacobson (1977) demonstrated a clear exposure-response relationship between cumulative exposure to respirable silica and development of the radiographic and functional abnormalities marking silicosis. Diseases caused by exposure to silica have been reviewed by Ziskind, et al. , (1976) , Sargent and Morgan (1980) , Leroy Lapp (1981) , Parkes (1982) , Heppleston (1984) , Landrigan et al. , (1986) , and the World Health Organization (1986) . A number of epidemiological and clinical studies involving silica were reviewed by the National Institute for Occupational Safety and Health (NIOSH) , (1974) and the International Agency for Research on Cancer (IARC) , (1987) , during its investigation of the carcinogenicity of silica and some silicates. The IARC working group concluded that there is sufficient evidence of carcinogenicity of free crystalline silica to experimental animals and limited evidence of carcinogenicity of free crystalline silica to humans. In October 1988, pursuant to the Safe Drinking Water and Toxics Enforcement Act of 1986 (Proposition 65) , free crystalline silica was listed in Section 12000, Title 22, Chapter 3, CCR, as a chemical known to the State of California to cause cancer, and is consequently regulated under the provisions of those regulations. Silica is identified as a hazardous substance by the California Department of Industrial Relations, Division of Occupational Safety and Health (CalOSHA) in "The Director's ' List of Hazardous Substances" (1982) . CalOSHA has established criteria limiting allowable occupational exposures to total and respirable silica dust. (Footnote Continued) tract. When considering silicosis and pneumoconiosis producing dusts, the particles must be sufficiently small to be taken into the alveolar region of the lung, which is commonly the site of toxic action for dusts of this type. According to ACGIH, respirable particles (available for alveolar deposition) are those which are 3.5 ± 1.5 µm in size. Particles in the 10 µm range are considered thoracic particulate mass, and particles less than or equal to 100 Am are considered inspirable.