NIOSH
HAZARD REVIEW
Health
Effects of Occupational Exposure
to
Respirable Crystalline Silica
|
Table
13. Summary of epidemiologic studies of silicosis with cumulative
dust exposure data and silicosis risk estimates
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Reference,
country, and study design |
Cohort
|
Definition
of silicosis, mean duration of employment, and
mean yr since first quartz exposure |
Silica
(quartz)
content of respirable dust |
Measure
of association
|
Comments
|
| Hnizdo and Sluis-Cremer [1993], South Africa, cohort study | 2,235 white underground gold miners who were aged 45 to 54 at time of medical examination in 1968–1971, started working after 1938, worked >10 yr, and were followed until 1991. | ILO* category > 1/1and rounded opacities (313 cases); 23.5 yr for total cohort and 26.9 yr for cases; 36 yr for cases. | 30% after heat and acid treatment [Beadle and Bradley 1970]. | Cumulative risk | Authors speculated that these silicosis risk estimates were higher than estimates for Canadian miners reported by Muir et al. [1989a,b] and Muir [1991] because (1) dust exposure may have been under-estimated, (2) South African gold mine dust may be more fibrogenic than Canadian mine dust, (3) average proportion of quartz may be >30%, (4) there may have been differences in age at end of radiological follow-up, and (5) exposures for Canadian miners (Hnizdo's [1995] response to Hughes and Weill [1995]) may have been overestimated. |
| Hughes et al. [1998], United States, retro-spective cohort study | 2,342 white male workers employed at least 1 yr between 1942 and 1987 in one diatomaceous mining and processing facility. Exposure-response analy-ses included the 1,809 men with a radiograph taken more than 1 month after hire. | Small
opacities >ILO profusion category 1/0 and/or large opacities
(81 cases); 5.54 yr†; 11.5 yr. |
Natural diatomite, 3%; calcined diatomite, 20%; flux-calcined diatomite, 60% (see comments). | Cumulative risk | 82 workers had radiographs taken after retirement—development of opacities was not recorded for other workers after they left employment. Quantitative air-monitoring data were available after 1948; respirable dust concentrations before 1948 were estimated [Seixas et al. 1997]. Cumulative risk estimates for radiographic opacities were lower for workers who were hired after 1950 and who had lower average exposures to crystalline silica dust (mainly cristobalite). Estimated percentages of respirable crystalline silica reported by Checkoway et al. [1997] in mortality study of same cohort: 10% for calcined diatomaceous earth, and 20% for flux-calcined diatomaceous earth. |
| Kreiss and Zhen [1996], United States, community-based random sample survey | 134 male residents of a hardrock‡ mining town who were aged >40: 100 silica-exposed hardrock miners (included 32 silicosis cases) and 34 community controls without occupational dust exposure. | ILO category >1/0; 27.6 yr for silicotics and 22.9 yr for non-silicotic miners; 41.6 yr for silicotics and 33.5 yr for nonsilicotics. | 12.3% | Prevalence | Possible
overestimation of silicosis risk because of underestimation of pre-1974
dust and silica exposures. Exposures were also estimated for mines
where there were no exposure data (17.1% of the person-yr of followup).
Risk
estimates were presented for models of cumulative silica dust exposure
or cumulative dust exposure—the models of cumulative silica
dust exposure gave higher estimates. Silicosis (i.e., |
| Muir et al. [1989a,b], Verma et al. [1989], Muir [1991]; Canada; retrospective cohort study | 2,109 current Ontario gold and uranium miners who started and worked >5 yr between 1940 and 1959 and were followed to 1982 or to the end of their dust exposure, whichever came first. | ILO category > 1/1 and small, rounded opacities (32 cases); approximately 20 yr; approximately 25 yr (based on interpretation of data in table and graph of Muir et al. [1989b]). | 6.0% for gold mine dust; 8.4% for uranium mine dust. | Cumulative risk | Retired and former workers not included, which may have under-estimated silicosis risk. Disagreement about silicosis classification among the five readers of the chest X-rays may have "complicated the analysis" [Muir et al. 1989b]. |
| Ng and Chan [1994], Hong Kong, cross-sectional study | 338 current and previous granite workers employed > 1 yr between 1967 and 1985. | ILO category > 1/1 (rounded or irregular opacities); 17.4 yr; not reported. | 27% | Prevalence | Cumulative risks not calculated. Exposure data for 1976–1981 in one quarry and for 1971–1975 and 1976–1981 in another quarry were not available and were assumed to be the same concentrations measured in 1982 for the period 1976–1981 and in 1971 for the period 1971–1985 [Ng et al. 1987]. Possible under-estimate of silicosis risk because decedents were not included. |
| Rosenman et al. [1996], United States, cross-sectional study | 549 current, 497 retired, and 26 current salaried workers that were former production workers in a gray iron foundry that produced automotive engine blocks (total workers =1,072). | ILO
category > 1/0 and rounded opacities (28 cases); 19.2 yr; 28.3 yr. |
Not reported. | Prevalence | Prevalence
of silicosis cases increased with (1) years of employment, (2) cigarette smoking, (3) mean silica exposure, and (4) cumulative silica exposure. Exposure estimates were derived from conversions of "early silica exposure data" collected by impingers. Underascertainment of silicosis cases is likely because there was no systematic radiologic followup of retired workers. Results showed that African-American workers had two times the risk of radiographic silicosis compared with white workers but a similar duration of employment; however, African-American workers had greater mean exposure to silica dust. When exposure to silica was controlled for in the analysis, the prevalence of radiographic silicosis was similar for African-American workers and white workers. |
| Steenland and Brown [1995a], United States, cohort study | 3,330 white male underground gold miners employed > 1 yr between 1940 and 1965 and followed through 1990. | Mortality§ and ILO category > 1/1 (1976 radiographic survey) or "small opacities" or "large opacities" (1960 radiographic survey) (170 cases); 9 yr; 37 yr. | 13%
[Zumwalde et al. 1981] |
Cumulative risk | Silicosis risk estimates could have been affected by (1) combining silicosis deaths with silicosis cases detected by cross-sectional radiographic surveys, (2) difference in quartz content of dust in early years, (3) lack of dust measurements before 1937. |
| *International
Labour Organization. †Median [Checkoway et al. 1997]. ‡Molybdenum, lead, zinc, and gold mining. §Underlying or contributing cause of death was silicosis, silico-tuberculosis, respiratory tuberculosis, or pneumoconiosis. |
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