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Current Trends Exposure Trends in Silica Flour Plants -- United States, 1975-1986

A 1979 National Institute for Occupational Safety and Health (NIOSH) investigation of excessive free silica exposures identified 23 cases of acute silicosis in employees at two Illinois silica flour plants (1). This led to a NIOSH report (2) emphasizing the hazards of silica exposure in the silica flour industry. NIOSH subsequently issued a description (3) of engineering controls designed to reduce exposures, and has followed this in 1988 by an analysis of the exposure levels and exposure trends in all U.S. silica flour producers for 1975-1986.

The data used for the analysis were collected by the Mine Safety and Health Administration (MSHA). MSHA measured respirable quartz exposures at 28 plants while conducting routine inspections for compliance with safety and health regulations promulgated under the 1977 Federal Mine Safety and Health Act. Quartz is a form of crystalline free silica, the principal agent responsible for silicosis. The dust samples were collected using personal breathing-zone air samplers. The quartz content in each respirable dust sample is used in computing the permissible exposure limit (PEL) for that sample (4). For samples with a high percentage of respirable quartz, as is typically the case in the silica flour industry, this computation results in an effective PEL of approximately 0.1 mg/m3.

Free silica levels in 52% of the samples tested exceeded the corresponding MSHA PEL. Although the percentage of samples exceeding the PEL decreased from 1982 to 1986, 32% still exceeded the PEL in 1986 (Figure 1). The proportion of the samples exceeding twice the PEL followed a similar pattern; the highest concentration recorded in 1986 was 11.3 times the PEL.

At one of the two Illinois plants investigated by NIOSH (1,5), 14% of environmental samples exceeded the PEL in 1984, 29% in 1985, and 30% in 1986. Overexposures in the other plant (1,6) were 60% in 1984, 50% in 1985, and 30% in 1986. Reported by: Div of Respiratory Disease Studies, National Institute for Occupational Safety and Health, CDC.

Editorial Note

Editorial Note: Silicosis is a debilitating fibrotic disease of the lungs that is caused by inhalation, retention, and pulmonary reaction to respirable particles of crystalline free silica. Chronic silicosis is pathologically and radiologically characterized by the silicotic nodule. In early stages, the nodules remain isolated, but as the disease progresses the nodules coalesce to form mass lesions, or progressive massive fibrosis. Acute and accelerated forms of silicosis may develop after shorter and more intense exposures to crystalline silica. Silicosis may be associated with pulmonary infections (particularly tuberculosis), restrictive ventilatory impairment, cor pulmonale, respiratory failure, and premature death.

Despite long recognition of the cause of silicosis and the means to prevent it, this disease remains an important source of occupational morbidity and mortality. Reliable morbidity statistics are not available, but NIOSH has used death certificate data to estimate that 2152 silicosis-attributable deaths among men greater than or equal to 25 years of age occurred in the United States during 1975-1986 (7,8).

"Silica flour" is produced by the drying and milling of mined quartz and consists of fine particles, a large percentage of which are respirable. The very small particle size makes this one of the most hazardous forms of silica. Despite some exposure reduction since 1982, the continued overexposures to respirable free silica in silica flour plants indicate a continued need for control measures in the silica flour industry. When compared with all metal and nonmetal mines regulated by MSHA, silica flour plants had a frequency of overexposure to free silica more than three times that of the other facilities during 1975-1986.

The data on which these analyses were based have limitations. First, the data do not represent a randomized or systematic sample of workers' exposures and are not subject to rigorous statistical treatment. Second, the data set does not provide information on the level of plant activity at the time of sampling. Third, exposures to individual workers may actually be less than those reported here because of the use of respirators. Despite these limitations, the data confirm the continued existence of overexposure to free silica at levels associated with adverse health effects.

Prevention of silicosis was targeted as a 1990 health objective for the United States (9). NIOSH has recommended a 10-hour, time-weighted average level of 0.05 mg/m3 (free silica) as the level required to prevent silicosis (10). Silicosis is reportable under the Sentinel Event Notification System for Occupational Risks (SENSOR) program. As a cooperative program between NIOSH and 10 state health departments*, SENSOR is designed to improve state and local capacity to conduct surveillance of selected occupational illnesses. Unless efforts to achieve a work environment within the NIOSH-recommended level are increased, the 1990 objective will not be met, and respirable free silica exposures will continue to constitute a health hazard in the silica flour industry.


  1. CDC. Silicosis--Illinois. MMWR 1980;29:205-6.

  2. CDC. Silica flour: silicosis (crystalline silica). Cincinnati,

Ohio: US Department of Health and Human Services, Public Health Service, 1981; DHHS document no. (NIOSH)81-137. (NIOSH current intelligence bulletin no. 36).

3. CDC. Health hazard control technology assessment of the silica flour milling industry. Cincinnati, Ohio: US Department of Health and Human Services, Public Health Service, 1984; DHHS publication no. (NIOSH)84-110.

4. Office of the Federal Register. Code of federal regulations: mineral resources--exposure limits for airborne contaminants. Washington, DC: Office of the Federal Register, National Archives and Records Administration, 1988. (30 CFR ***56.5001).

5. CDC. Hazard evaluation and technical assistance report no. 79-104-107. Cincinnati, Ohio: US Department of Health and Human Services, Public Health Service, 1979.

6. CDC. Hazard evaluation and technical assistance report no. 79-103-108. Cincinnati, Ohio: US Department of Health and Human Services, Public Health Service, 1979.

7. CDC. Health, United States, 1986. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, 1987; DHHS publication no. (PHS)87-1232.

8. CDC. Health, United States, 1988. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, 1989; DHHS publication no. (PHS)89-1232.

9. Public Health Service. Promoting health/preventing disease: objectives for the nation. Washington, DC: US Department of Health and Human Services, Public Health Service, 1980:41. 10. CDC. Criteria for a recommended standard: occupational exposure to

. . crystalline silica. Cincinnati, Ohio: US Department of Health, Education, and Welfare, Health Services and Mental Health Administration, 1974; document no. (NIOSH)75-120.

*California, Colorado, Massachusetts, Michigan, New Jersey, New York, Ohio, Oregon, Texas, and Wisconsin.

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