Table
13. Summary of epidemiologic studies of silicosis with cumulative
dust exposure data and silicosis risk estimates
|
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 19681971, 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 retirementdevelopment 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 exposurethe models of cumulative silica
dust exposure gave higher estimates. Silicosis (i.e.,
> category 1/1) risk estimates from models of cumulative
dust exposure were similar to estimates for South African gold miners
[Hnizdo and Sluis-Cremer 1993] and U.S. gold miners [Steenland and
Brown 1995a].
|
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 19761981 in one quarry
and for 19711975 and 19761981 in another quarry were not
available and were assumed to be the same concentrations measured
in 1982 for the period 19761981 and in 1971 for the period 19711985
[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. |