I. Silica dust
Sources and health implications
Silica refers to naturally occurring materials composed of silicon dioxide (Si O2) (50). Silicate minerals, are the largest group of natural minerals with a varying composition but all contain silicon-oxygen tetrahedra as structural components. They are ubiquitous in the earth’s crust in both crystalline and amorphous forms. Amphibole, pyroxene, serpentine, feldspar, mica, garnet and zeolite are generic terms for some common rock-forming silicates. The presence of silica in the environment results from natural processes and from human activity. Some of the uses of silicate minerals associated with construction include: manufacture of both glass and ceramics; incorporation into abrasives, such as sandpaper, and sandblasting materials; in filtering equipment in water and sewage-treatment systems; furnace linings and beds (silica bricks and tiles); filler in paints, plastics, asphalt and cements (finely ground sand); construction purposes (such as roads, earth dams, concrete works).
Occupational exposure to silicate materials leads to a form of scarring of the lungs called silicosis, which interferes with breathing and obstructs the circulation through the lungs (11). According to IARC evaluation (28), crystalline silica is probably carcinogenic. Crystalline silica is a generic term for several crystalline minerals composed of silica, such as quartz, cristobalite, tridymite, coesite and stishovite (50).
Factors influencing exposure
Occupational exposures to silica dust occur in many different industries and under a wide range of circumstances which include mining and quarrying operations; production of glass, cement and ceramics; granite and stone industries; and construction activities generally (e.g. earth based construction activities, production of concrete and bricks).
Acceptable exposure levels
Occupational exposure limits in selected countries are listed in table 13.
Table 13. Occupational exposure limits for respirable quartz, cristobalite and tridymite (mg/m3)
Source: IARC (1987). IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemical to Humans, Silica and some silicates, Volume 42, Lyon, France.
In recent years, the seriousness of the problems of dust-induced diseases has become increasingly apparent. In Egypt it has been reported that the numbers of silicotic cases in 1988 reached 492 as compared to 852 total occupational cases in mining, tunnelling and quarrying operations (51). In Finland, a study (52) on the possibility of a direct association between silica dust exposure and lung cancer, which involved a follow-up of the mortality and morbidity among 1026 granite workers for the period 1940-1989, indicated excess lung cancer mortality during several follow-up periods. The rate ratios were 1.6-3.8 for different latency periods. Lung cancer risk increased with the length of exposure and latency.
The prevention of silica dust-induced diseases is important considering its harmful effect to human health. It is important: to prevent the formation of dust as far as possible; suppressing at its source whatever dust is inevitably formed, and by providing protective equipment to workers to prevent inhalation of dust.
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