Silicosis Mortality, Prevention, and Control --- United States, 1968--2002

Silicosis is a preventable occupational lung disease caused by inhaling dust containing crystalline silica (1); no effective treatment for silicosis is available. Deaths from inhalation of silica-containing dust can occur after a few months' exposure (1). Crystalline silica exposure and silicosis have been associated with work in mining, quarrying, tunneling, sandblasting, masonry, foundry work, glass manufacture, ceramic and pottery production, cement and concrete production, and work with certain materials in dental laboratories. To describe patterns of silicosis mortality in the United States, CDC analyzed data from the National Institute for Occupational Safety and Health (NIOSH) National Occupational Respiratory Mortality System (NORMS) for 1968--2002. This report summarizes the results of that analysis, which indicated a decline in silicosis mortality during 1968--2002 and suggested that progress has been made in reducing the incidence of silicosis in the United States. However, silicosis deaths and new cases still occur, even in young workers. Because no effective treatment for silicosis is available, effective control of exposure to crystalline silica in the workplace is crucial.

NORMS contains national mortality data obtained annually since 1968 from the National Center for Health Statistics (NCHS) for asthma, chronic obstructive pulmonary disease, silicosis, pneumoconiosis, tuberculosis, and other respiratory diseases and conditions (2). For this report, silicosis deaths were identified during 1968--2002, the most recent years for which complete data were available, and include any death certificates for which an International Classification of Diseases (ICD) code* for silicosis was listed as either the underlying or contributing cause of death. Death rates (per million persons aged >15 years) were age-adjusted to the 2000 U.S. standard population. The age-adjusted mortality rates were mapped by county by using geographic information system software. Codes for usual occupation or industry were available for 1985--1999 from 26 states^†. The proportionate mortality ratio (PMR^§) for silicosis was calculated by occupation and industry. For this analysis, PMR was adjusted by age, sex, and race, and 95% confidence intervals (CIs) were calculated by using Poisson regression analysis.

During 1968--2002, of approximately 74 million death certificates, silicosis was recorded as the underlying or contributing cause of death on 16,305; a total of 15,944 (98%) deaths occurred in males. From 1968 to 2002, the number of silicosis deaths decreased from 1,157 (8.91 per million persons aged >15 years) to 148 (0.66) (Figure 1), corresponding to a 93%

decline in the overall mortality rate. The racial distribution of persons who died from silicosis was approximately 88% (14,310 decedents) white, 12% (1,925) black, and <1% (70) other race. Since 1995, on average, 30 deaths per year have been recorded among persons aged 15--64 years.

During 1968--2002, the age-adjusted silicosis mortality rate was elevated in several counties in western states (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Washington), eastern states (Georgia, Kentucky, New York, North Carolina, Maine, Ohio, Pennsylvania, Tennessee, Vermont, Virginia, and West Virginia), and central states (Illinois, Kansas, Michigan, Minnesota, Missouri, Oklahoma, South Dakota, Vermont, and Wisconsin). San Juan County, Colorado, had the highest age-adjusted mortality rate (524 per million persons aged >15 years) (Figure 2).

For usual industry and occupation codes, a total of 1,400 silicosis deaths were reported in the 26 states for one or more selected years during 1985--1999. Metal mining had the highest PMR (39.2; CI = 32.9--46.8) among industries (Table). Among occupations, miscellaneous metal and plastic-processing machine operator had the highest PMR (90.1; CI = 51.5--146.3).

Reported by: KM Bang, PhD, JM Mazurek, MD, MD Attfield, PhD, Div of Respiratory Disease Studies, National Institute for Occupational Safety and Health, CDC.

Editorial Note:

The greatest death toll from silicosis in the United States occurred with excavation of Hawk's Nest Tunnel, critical to the construction of a hydroelectric plant in West Virginia during 1930--1931. Approximately 5,000 workers bored through Gauley Mountain to create the tunnel; an estimated 2,500 worked inside. A subsequent study determined that silicosis claimed the lives of at least 764 workers at Hawk's Nest Tunnel (4). By the end of 1937, a total of 46 states had passed laws relevant to workers with silicosis.

The findings in this report indicate a decline in silicosis mortality during 1968--2002. Two main factors are likely responsible for this trend. First, many of the deaths in the early part of the study period occurred among persons whose main exposure to crystalline silica dust probably occurred before introduction of national compliance standards for silica dust exposure (the Mine Safety and Health Administration [MSHA] and the Occupational Safety and Health Administration use permissible exposure limits [PELs] based on the American Conference of Governmental Industrial Hygienists threshold limit value). These limits began to be applied in the early 1970s and included indirect control through regulation of mixed mine dust in underground coal mines by using the MSHA formula^¶ (5). These regulatory limits, coupled with other recommendations such as that by NIOSH in 1974 (i.e., recommended exposure limit [REL] to respirable crystalline silica shall not exceed 0.05 mg/m³) (6), likely has led to reduced silica dust exposures since the 1970s. Ancillary preventive measures (e.g., respiratory protection, posting warning signs, and recordkeeping or reporting occupational illnesses) might also have reduced personal exposures. The second major factor relates to declining employment in heavy industries (e.g., mining industry from 989,400 employees in 1980 to 512,200 in 2002), where silica exposures were prevalent.

Despite the reductions in mortality associated with silicosis, silica overexposure remains widespread, indicating a need for hazard surveillance and developing work place--specific interventions. An estimated 121,100 workers were exposed to equal to or greater than the NIOSH REL in 1993 (7). The industries with the largest number of workers exposed include research and testing services (46,200 workers), masonry (20,400), heavy construction (12,200 [excluding highway construction]), and iron and steel foundries (8,600). Industries with fewer workers include painting and paper hanging (5,100), structural clay products (2,100), metal services (1,300), and cut stone (700) (7). By industry, metal mining had the highest PMR for 1985--1999. At least 12% of metal mining exposures exceeded the MSHA PEL during 1990--1999 (8). Findings from the Sentinel Event Notification Systems for Occupational Risks indicated that 58% of reported cases of silicosis from Michigan, New Jersey, and Ohio occurred in workers in primary metal industries (9). Additional industries with elevated PMRs involved miscellaneous nonmetallic and stone products and pottery and related products. The pottery industry deals with silica-containing clay, which is the raw material for manufacturing crockery, pottery, and flint.

The geographic patterns of silicosis offer some guidance for intervention. By county, the greatest age-adjusted mortality rates were clustered in western states, northeastern states, and north Atlantic states. The mortality rates in counties in these states were often associated with mining or construction industries. For example, the high age-adjusted death rate in Lawrence County, South Dakota (38.3 per million persons aged >15 years), might be associated with gold mining in that area. A study of 3,328 gold miners exposed to silica in South Dakota reported an increased risk for silicosis (9).

The findings in this report are subject to at least five limitations. First, accuracy of the coding of usual industry and occupation on death certificates was not verifiable because individual work histories are not listed on death certificates. Second, codes for usual industry and occupation were available only for the period 1985--1999 for 26 states; thus, these data might not be nationally representative. Twenty-four states do not provide decedents' employment data to NCHS. Third, the state of residence at death is not always the state in which decedents' exposures occurred. Fourth, no exposure information is listed on death certificates. Therefore, no silica exposure-response relationship was evaluated. Finally, physicians might have misclassified or underreported silicosis deaths.

Despite these limitations, NORMS can provide useful information on silicosis by location, industry, and occupation, suggesting ways to target preventive intervention. The decreased silicosis mortality trends suggest that considerable progress has been made toward elimination of silicosis since 1968. However, silicosis deaths and new cases are still occurring, even in young workers in the United States. Because no effective treatment for silicosis is available, primary prevention (i.e., engineering or other control of exposure) should be maintained or improved to reduce worker morbidity and mortality.

Acknowledgments

The findings in this report are based, in part, on contributions by JM Wood, MS, G Syamlal, MPH, Div of Respiratory Disease Studies, National Institute for Occupational Safety and Health, CDC.

References

1. Elmes PC. Inorganic dusts. In: Raffle PAB, Adams PH, Baxter PJ, Lee WR, eds. 8th ed. Hunter's diseases of occupations. Boston, MA: Little, Brown, and Co.; 1994.
2. National Institute for Occupational Safety and Health. National Occupational Respiratory Mortality System (NORMS). Available at http://webappa.cdc.gov/ords/norms.html.
3. World Health Organization. International classification of diseases and related health problems. 10th revision, Geneva, Switzerland: World Health Organization; 1992.
4. Cherniack M. The Hawks' Nest incident: America's worst industrial disaster. New Haven, CT: Yale University Press; 1986.
5. National Institute for Occupational Safety and Health. NIOSH criteria for a recommended standard: occupational exposure to respirable coal mine dust. Cincinnati, OH: US Department of Health and Human Services, CDC, National Institute for Occupational Safety and Health; 1995. (NIOSH) publication no. 95-106.
6. National Institute for Occupational Safety and Health. NIOSH criteria for a recommended standard: occupational exposure to crystalline silica. Cincinnati, OH: US Department of Health, Education, and Welfare, Public Health Service, CDC, National Institute for Occupational Safety and Health; 1974. (NIOSH) publication no. 75-120.
7. Linch KD, Miller WE, Althouse RB, et al. Surveillance of respirable crystalline silica dust using OSHA compliance data (1979--1995). Am J Ind Med 1998;34:547--58.
8. National Institute for Occupational Safety and Health. Work-related lung disease surveillance report. Cincinnati, OH: US Department of Health and Human Services, CDC, National Institute for Occupational Safety and Health; 2003. (NIOSH) publication no. 2003-111.
9. Steenland K, Brown D. Mortality study of gold miners exposed to silica and monasbestiform amphibole minerals: an update with 14 more years of follow-up. Am J Ind Med 1995;27:217--29.

* ICD-8 code 515 (1968--1978), ICD-9 code 502 (1979--1998), and ICD-10 code J62 (1999--2002) (3).

^† Alaska, Colorado, Georgia, Hawaii, Idaho, Indiana, Kansas, Kentucky, Maine, Missouri, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, North Carolina, Ohio, Oklahoma, Rhode Island, South Carolina, Tennessee, Utah, Vermont, Washington, West Virginia, and Wisconsin.

^§ Defined as the observed number of deaths from silicosis in a specified occupation or industry divided by the expected number of silicosis deaths. The expected number of deaths was calculated by using the total number of deaths in the Bureau of Census Industry Code or Census Occupation Code of interest multiplied by a proportion defined as the number of cause-specific deaths for the condition of interest in all industries/occupations, divided by the total number of deaths in all industries/occupations.

^¶ (10 mg/m³) / (% quartz) and direct control of crystalline silica as respirable quartz in metal/non-metal mining using the formulas: (10 mg/m³) / (% quartz + 2) for general industry or (250 mppcf) / (% quartz + 5) for the construction industry.

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Table


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Date last reviewed: 4/28/2005