Concise International Chemical Assessment Document (CICAD) No. 24. Geneva, Switzerland: World Health Organization, International Programme on Chemical Safety (IPCS), 2000 Jan; :1-50
This CICAD on crystalline silica, quartz was based on the following three extensive peer-reviewed documents on the health effects of crystalline silica, including quartz: (1) a review of published human studies and reports on the adverse health effects of quartz exposure (NIOSH, forthcoming), (2) a review of the carcinogenicity studies conducted by the International Agency for Research on Cancer (IARC, 1997), and (3) a review of the non-cancer health effects of ambient quartz (US EPA, 1996). The source documents had different emphases on different end-points, and the CICAD was developed to assess all the adverse health effects identified in these documents. It is to be noted that despite the different emphases, the final conclusions of all source documents were very similar. A comprehensive literature search of several on-line databases was conducted. Data identified as of March 1999 are included in this review. This CICAD considers the most common form of crystalline silica (i.e., quartz). It does not consider experimental studies of the effects of other forms of crystalline silica (e.g., cristobalite, tridymite, stishovite, or coesite), coal dust, diatomaceous earth, or amorphous silica, because their in vitro toxicities differ from that of quartz. Differences in induction of fibrogenicity of quartz, cristobalite, and tridymite were demonstrated in vivo in an early rat study. However, there are virtually no experimental studies that systematically evaluated exactly the same material to which humans are exposed. The IARC Working Group considered the possibility that there may be differences in the carcinogenic potential among polymorphs of crystalline silica. However, some of the epidemiological studies evaluated lung cancer among workers in "mixed environments" where quartz may be heated and varying degrees of conversion to cristobalite or tridymite can occur (e.g., ceramics, pottery, and refractory brick industries), and exposures specifically to quartz or cristobalite were not delineated. Although there were some indications that cancer risks varied by industry and process in a manner suggestive of polymorph-specific risks, the Working Group could reach only a single conclusion for quartz and cristobalite. The CICAD reflects the discussion and conclusion of that source document; therefore, when considering the carcinogenicity of quartz in the occupational setting, it does not distinguish between epidemiological studies of quartz and those of cristobalite.