Silica exposure, an ancient hazard, remains a serious threat to many U.S. workers, including sandblasters, foundry workers, stonecutters, and those involved in drilling, quarrying, and tunneling through the earth's crust. Diseases associated with the inhalation of silica-containing dusts include silicosis, chronic airways obstruction and bronchitis, tuberculosis and lung cancer. Although OSHA currently has a PEL for silica, there is evidence that it is too high. Also, the absence of a comprehensive standard, including provisions for product substitution, engineering controls, respiratory protection, and medical screening and surveillance, has contributed to inadequate protection of workers. In order to improve worker protection, OSHA is designating silica as a priority for comprehensive rulemaking to reduce the burden of silica-related diseases.

Hazard Description

Nearly 2 million workers, including over 100,000 in high-risk settings, are exposed to crystalline silica (1). High risk settings include sandblasters, painters who do sandblasting, rock drillers and roof bolters, and high risk foundry work.

• Silica exposure is known to cause silicosis, a disabling, progressive and sometimes fatal disease (2-4) involving scarring of the lungs with resulting cough and shortness of breath. Other diseases associated with the inhalation of silica-containing dusts include chronic airways obstruction and bronchitis (5-9), tuberculosis (10-12), several extrapulmonary diseases (13, 14). The International Agency for Research on Cancer (IARC) has identified silica as a potential human carcinogen (15).
• Deaths from accelerated silicosis and silicotuberculosis continue to occur in sandblasters, rock drillers, and workers in other dusty trades (2-4, 16). Deaths with silicosis currently number about 300 cases annually (1). Given the current plateauing of annual silicosis death counts in the United States, current prevention approaches appear to be limited.
• A recent study of gold miners concluded that a 45-year exposure under the current OSHA standard would lead to a lifetime risk of silicosis of 35% to 47%. The authors suggested "that the current OSHA silica exposure level is unacceptably high." (18)

Current Status

While OSHA currently has a permissible exposure limit (PEL) for crystalline silica, over 30% of OSHA-collected silica samples from 1982 through 1991 exceeded the current PEL (16). The current OSHA PELs for crystalline silica are 10 mg/m3 divided by the percent of silica in the dust +2 (respirable) and 30mg/m3 divided by the percent of silica in the dust +2 (total dust). Furthermore, for cristobalite and tridymite, the same formula should be used for determining the PELs, divided by one-half.

Recent studies suggest that the current OSHA standard is insufficient to protect against silicosis (17, 18). Data from these studies project a cumulative risk for silicoses of approximately 77% for a working lifetime of 45 years at a current PEL of 0.10 mg/m3.

Recent studies suggest that the current OSHA standard is insufficient to protect against silicosis (17, 18). Data from these studies project a cumulative risk for silicoses of approximately 77% for a working lifetime of 45 years at a current PEL of 0.10 mg/m3.

NIOSH and the American Conference of Governmental Industrial Hygienists (ACGIH) have recommended exposure limits. The NIOSH recommendation is .05mg/m3, and NIOSH's position is that it is a carcinogen; ACGIH's recommendations are as follows*:

• .05 mg/m3 for cristobalite
• .1 mg/m3 for quartz
• .05 mg/m3 for tridymite
• .1 mg/m3 of contained tripoli respirable quartz

*respirable fraction of the dust

The United Kingdom adopted regulations severly restricting the use of abrasives containing free silica in 1949. NIOSH recommended that the use of sand in abrasive blasting be eliminated in 1974. In 1992, because of continuing observation and investigation of disease and death from abrasve blasting, NIOSH issued an alert reiterating this recommendation. (3)

Rationale

Silica meets the criteria of the Priority Planning Process. Crystalline silica represents a very serious health hazard, as indicated by continuing deaths from accelerated silicosis in sandblasters and rock drillers and by recent studies which demonstrate a statistically significant increase in lung cancer among silica-exposed workers, particularly among those with silicosis. Additionally, exposure studies indicate that some workers are still exposed to very high levels of silica (1, 2-4), and recent studies suggest that the current OSHA standard is insufficient to protect against silicosis (17, 18).

Current prevention approaches have contributed to the stabilization of the annual number of deaths with silicosis in the United States. However, as in the case of lead, there will be no significant progress in the prevention of silica-related diseases without the adoption of a full and comprehensive silica standard, including product substitution, engineering controls, training and education, respiratory protection, and medical screening and surveillance. A full standard will improve worker protection, ensure adequate prevention programs, and further reduce silica-related diseases.

References

1. CDC/NIOSH. Work-Related Lung Disease Surveillance Report, 1994. DHHS (NIOSH) Number 94-120, August 1994.
2. CDC. Silicosis: Cluster in Sandblasters - Texas, and Occupational Surveillance for Silicosis. MMWR 1990; 39:433-37.
3. CDC/NIOSH. Preventing Silicosis and Deaths from Sandblasting. NIOSH Alert, Publication No. 92-102, August 1992.
4. CDC/NIOSH. Preventing Silicosis and Deaths in Rock Drillers. NIOSH Alert, Publication No. 92-107, August 1992.
5. Grahan WGB, Weaver S, Ashikaga T, O'Grady RV. Longitudinal pulmonary function losses in Vermont granite workers. Chest 106:125-130, 1994.
6. Cowie RL, Mabena SK. Silicosis, chronic airflow limitation, and chronic bronchitis in South African gold miners. Am Rev Respir Dis 143:80-84, 1991.
7. Cowie RL, Hay M, Thomas RG. Association of silicosis, lung dysfunction, and emphysema in gold miners. Thorax 48:746-749, 1993.
8. Cowie RL. The influence of silicosis on deteriorating lung function in gold miners. Am J Respir Crit Care Med 149:A406, 1994.
9. Kinsella M, Muller N, Vedal S, Staples C, et. al. Emphysema in silicosis. A comparison of smokers with nonsmokers using pulmonary function testing and computed tomography. Am Rev Respir Dis 141:1497-1500, 1990.
10. Snider DE. The relationship between tuberculosis and silicosis. Am Rev Respir Dis 118:455-460, 1978.
11. Cowie RL, Langton ME, Becklake MR. Pulmonary tuberculosis in South African gold miners. Am Rev Respir Dis 139:1086-1089, 1989.
12. Morgan EJ. Silicosis and tuberculosis. Chest 75:202-203, 1979.
13. Sluis-Cremer GK, Hessel PA, Hnizdo E, Churchill AR, et. al. Silica, silicosis, and progressive systemic sclerosis. Br J Ind Med 42:838-843, 1985.
14. Cowie RL. Silica-dust-exposed mine workers with scleroderma (systemic sclerosis). Chest 92:260-262, 1987.
15. IARC. IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans: Silica and some silicates. Vol. 42. Lyon, France: World Health Organization, International Agency for Research on Cancer, pp 49, 51, 73-11, 1987.
16. Banks DE, Bauer MA, Castellan RM, Lapp NL. Silicosis in surface coalmine drillers. Thorax 38:275-278, 1983.
17. Hnizdo E, Sluis-Cremer GK. Risk of silicosis in a cohort of white South African gold miners. Am J Ind Med 1993, 24:447-457.
18. Steenland NK, Brown D. Silicosis Among Gold Miners: Exposure Response Analysis and Risk Assessment. AJPH, October 1995: 1372-1377.