Riitta Riala

Silica occurs naturally in crystalline (quartz, cristobalite and tridymite) and amorphous forms. Quartz is the most common crystalline form of SiO₂. Pure crystalline quartz is used in electronics and optical instruments. Occupational exposure to quartz occurs mainly when rock or sand containing quartz is used or processed. Main industrial areas where sand is used are cement industry, foundries, manufacture of glass, ceramics and porcelain, manufacture of construction products (e.g., roofing felts, man made vitrous fibers), sand blasting, abrasive tool production and construction industry. Workers are also exposed to quartz when rock or minerals containing varying amounts of quartz are processed, e.g., in the mining of coal and minerals, in rock drilling, in stone cutting, in tunnel and house construction work, and sometimes in farming and in the handling of farm products (IARC 1997). Exposure to tridymite and cristobalite happens after quartz materials (refractory tiles or fibers) have been heated to high (appr. 1100°C) temperatures, as is the case in the kilns in the glass and steel industry.

There are several hundred epidemiological studies that have linked occupational quartz exposure with silicosis. Silicosis is a fibrotic lung disease that is caused by the inhalation and deposition of respirable crystalline silica particles. The association between tuberculosis and silicosis has been also firmly established (WHO 2000). In studies of workers without silicosis, there is some limited evidence that long or high quartz exposures may increase the risk of developing tuberculosis. In the comprehensive review by IARC in 1997, crystalline silica in the form of quartz and cristobalite was classified as carcinogenic to humans (Group 1). Elevated cancer risk has been found, e.g., in gold mining, stone and pottery production and diatomaceous earth

granite industry, refractory brick production, pottery production and diatomaceous earth industry. According to IARC carcinogenicity may be dependent on inherent characteristics of crystalline silica or on external factors affecting its biological activity.

Occupational exposure limit (OEL) for quartz is 0.1 mg/m³ and for cristobalite and tridymite 0.05 mg/m³ in most countries and organizational guidelines. WHO recommended a health-based exposure limit of 40 µg/m³ for free crystalline silica in 1986 (Fedorov 1997). Quartz dust levels vary greatly depending on the concentration of quartz in the rock or mineral, on the country, on the type of industry and on the time of exposure. It is vital to know the quartz content of the processed material so that effective dust prevention and other engineering measures can be introduced.

Mining

At present, the mean respirable quartz levels are around or below 0.1 mg/m³ in Europe and in the United States (IARC 1997, Partanen et al 1995). High quartz exposures have been detected e.g. in South African gold mines with up to 30 % silica content and in Finnish copper mines with 18 % silica content in respirable dust. In an old Finnish copper mine, respirable quartz was estimated to be above 2 mg/m³ during dry-drilling operations before 1940s (IARC 1997). The most exposed mining occupations are pneumatic drill, crusher and load-and-haul operators.

The most effective way to decrease quartz exposure was the introduction of wet-drilling, which happened in western countries during the first half of the last century. Other means in dust prevention, partly following technical development, have been rock drilling machines with remote control operation and ventilated control cabins, high-pressure water spraying of the material during hauling operations and improved ventilation and dust filtration techniques in all underground operations. On the other hand, the rapid development of fully mechanized mining technology has increased total dust levels e.g. in Chinese coal mines (Lu Jianzhang 2001). Dust control technology has not always been introduced as fast as high-efficiency mining technology. In such cases, good respiratory protection is still needed to prevent silicosis risk among the workers.

Granite rock (quartz content from 10 to 30 %) is obtained in quarries and further processed to stone or crushed for road gravel. High exposure levels have been measured in the 1980s in granite industry (mean 0.6-1.5 mg/m³) in Finland and in Denmark. Exposures have been lower (mean below 0.1 mg/m³) in the USA (IARC 1997). Operations and jobs with high exposures are, e.g., rock and stone drilling and cutting, dimensional stone cutting and finishing, rock crushing, sieving and transport. In granite quarries in United States, control methods carried out in the 1930s and 1940s resulted in 10-100-fold reductions in dust levels. The dust control methods applied in quarries and stone industry include wet process methods in drilling and in the cutting of granite, water spraying of the material, exhaust ventilation and ventilated control cabins. In addition, e.g., NIOSH (NIOSH 1992) has recommended additional procedures for the prevention of silicosis in rock drillers:

- before mining begins, assess the potential for exposing workers to crystalline silica
- conduct air monitoring to measure workers’ exposure
- practice good personal hygiene
- washable or disposable protective clothing to the workers
- respiratory protection when dust control is not sufficient
- inform workers about health risks and good working procedures
- provide periodic medical examinations and report the silicosis cases.

In foundries technical improvements have decreased quartz dust levels, e.g., in Finnish iron foundries the mean quartz level, 0.5 mg/m³ in the 1970s has halved in the 1980s (Partanen et al 1995, IARC 1997). Exposure to quartz in foundries originates from sand used for molds and cores. Sand may contain 5-100 % quartz. Parting powders and refractory materials containing free crystalline silica are also used.

Jobs with highest quartz exposures are sand preparation, knocking-out (i.e., emptying the mold after casting), cleaning of the castings (fettling, grinding, sand blasting) and furnace and ladle refractory repair. Exposure levels are lower in non-ferrous foundries than in iron and steel foundries (IARC 1997).

Several methods have been introduced to prevent quartz exposure in foundries. Quartz sand and silica flour has been widely substituted by olivine sand and low-silica powders (Landrigan 1986). Improved local ventilation in sand preparation, knocking-out, fettling and grinding has decreased dust exposure. Good housekeeping, i.e. frequent cleaning with effective vacuum cleaner or wet sweeping is needed to keep dust levels down. E.g., in Canada improved control methods applied in foundries in the 1980s decreased the quartz levels from 0.4-21 mg/m³ to 0.6-2.6 mg/m³ in knocking-out, and from 0.95-6.1 mg/m³ to 0.35-3.4 mg/m³ in molding (IARC 1997).

The processes with the heaviest dust generation should be mechanized, e.g. in knockout, or at least ensured that the dusty work is enclosed from other, cleaner work areas (Health and Safety Executive 1992). In molding and in the cleaning of castings, ventilation booths have been successfully applied, e.g., in a model with a turntable easy for manipulating the castings (NIOSH 1997). In fettling, automatic process with good enclosure and local exhaust is effective in dust control.

The best result in decreasing the worker´s exposure during grinding can be received by introducing supply air from the ceiling of the ventilated grinding booth (Enbom1996). As foundry dust is very abrasive, ventilation systems must be well maintained to be effective.

In many countries, the quartz exposures 20-30 years ago were three to five -fold higher than in the early 1990s. The mean quartz concentrations are 0.1-0.2 mg/m³ in potteries and sanitary ware production. Exposure levels have been reduced by enclosures, wetting of the material, use of silica free compounds, better housekeeping and local ventilation.

Quartz exposure occurs also in farming and in the handling of farm products, e.g., in North Carolina in sandy soils (quartz content 29 % in the 4.25 µm fraction) (Stopford 1995). In the USA levels of up to 1 mg/m³ have been reported during certain crop processing operations in the 1980s (IARC 1997). In 2000, in an storage for seed potatoes in Finland, workers in the assorting and weighing of the potatoes were exposed to quartz levels ranging from 0.3 to 1.7 mg/m³ (Junttila 2001). In this case dust control methods in use in the storage were scarce, as the risk of quartz exposure had not been detected before exposure measurements. Local exhaust ventilation and better house keeping should be introduced to decrease the exposure.

Siliceous sand has been used as an abrasive in cleaning metal surfaces before finishing operations. Dust levels are very high in sandblasting, the mean quartz level outside the hood was 4.8 mg/m³ and inside the hood 4-80 times OEL in a study carried out in the USA in the 1970s (IARC 1997). Even when using air-supplied hood the average quartz levels still exceeded occupational exposure limit OEL by three to 34 times. Sandblasting exposes also other workers in a large area (up to 700 m in open areas, e.g., during the sand blasting of a ship wall) to unacceptable quartz levels.

Isolation of the sand blasting process, substitution of quartz sand by e.g. corundum, steel grit or non-silicious sand and personal protection are the most important dust control methods. All sand blasting should be carried out in enclosures, which should be regularly checked, as even small leaks can produce high dust emissions (NIOSH 1992, Health and Safety Executive 1992). Whenever possible, operator should work outside the enclosure. Workers must wear supplied air-respirators always when working in the sand blasting area, even during the cleaning.

Construction work is often very dusty, and there are processes in which workers are exposed to high quartz levels. During earth works and tunnel construction, pneumatic rock drilling and excavation work in granite-rich soil exposes workers to high quartz levels, e.g., 0.75 mg/m³ in underground tunneling in the USA (IARC 1997).

Although cement does not contain quartz, concrete dust most often contains quartz due to the quartz content of the stone aggregate. In house construction, finishing of concrete surfaces, cutting of holes and cleaning exposes workers to quartz. Dry grinding, sanding of drywalls and cutting holes in concrete floors or walls are problematic work phases, especially in enclosed areas. E.g., when using hand-held jack-hammers on concrete floor, quartz level of 4.35 mg/m³ was measured (Fairfax 1997). In grinding of concrete surfaces, up to appr. 50 times the permissible quartz level have been measured (IARC 1997). In house renovation new exposures have been detected, e.g. in renovating of concrete element sealants workers are heavily exposed not only to PCBs but also to quartz, up to 6 mg/m³ in very dusty grinding process (Rantio et al 2001). When renovating old brick buildings by tuck pointing, which includes grinding the mortar out by grinders (rotation speed up to 12000 rpm), quartz levels may be 50 times higher than recommended (Heitbrink 2000).

Quartz exposure can be decreased by choosing work methods generating less dust, by wet methods, by using tools with local exhaust ventilation, by good housekeeping and cleaning and with personal protection. Good planning of construction work, so that no changes need to be cut in concrete, and good finishing standard at the concrete element factory eliminates dusty work procedures. When new holes are needed in the concrete, diamond saws (with wetting) should be applied. During demolition work in renovation sites, jackhammers with remote control and ventilated control cabins should be used. The amount of grinding and sanding work should be minimized, and when carried out, grinding tools with good local exhaust and vacuums with high-efficiency particle filter (HEPA) should be applied (NIOSH 1996). When grinding wheels with high rotating speeds are used (tuck pointing, sealant renovation), local exhaust systems should be very carefully designed in order to capture dust with high velocity. Other workers should not enter dusty work areas. Industrial vacuum cleaners with HEPA filters (and other methods not generating dust) should be used in cleaning. Respiratory protection is often necessary.

Refractory bricks have a very high quartz content. Exposure to quartz happens during the manufacture of refractory bricks, and again when kilns with refractory insulation are renovated in foundries, steel plants, glass and cement factories. In factories manufacturing refractory bricks, quartz levels of 0.45 mg/m³ been measured (IARC 1997).^. Refractory ceramic fibers are mainly aluminium silicate fibers, but when heated to above 1100 °C, cristobalite is formed. In the removal of ceramic fibers, cristobalite exposures exceeding occupational exposure limit of by up to 13 times have been measured (Gantner 1986). Recommended dust control methods for the removal of ceramic fibers are the wetting of the material, local exhaust ventilation and personal protection. As in the removal there are also high levels of respirable ceramic fibers present (which are considered as carcinogens), in several publications work methods used in asbestos removal are recommended also for removal of ceramic fibers (Van den Bergen 1994). That includes a.o. an enclosure with negative pressure, an airlock with three compartments and supplied air respirators and disposable overalls for the workers.

When quartz containing materials are used or processed, following procedures should be applied:

Riitta Riala
Uusimaa Regional Institute of
Occupational Health
FIN-00370 Helsinki
Finland
Telephone: +358-9-4747 2936
Telefax: +358-9-4747 2985
E-mail:riitta.riala@occuphealth.fi

References

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Health and Safety Executive: Control of silica dust in foundries. HSE Books. Sudbury 1992.

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