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Add "hard rock" mining to the list of occupations associated with high rates of rheumatoid arthritis (RA), a new study indicated, with silica exposure the presumed malefactor.

Based on survey data from men ages 50 and older in portions of three Western states, odds of developing RA were increased from three- to ninefold among those working in hard rock (i.e., copper, iron, or other metal ore extraction) mines both underground and on the land surface, relative to individuals in occupations with no expected silica exposure.

Risks of RA were also substantially increased for miners of coal and other "soft rocks," according to Paul D. Blanc, MD, MSPH, of the University of California San Francisco, and colleagues .

The results add to already that on-the-job silica exposure promotes development of RA, though the . "These findings further suggest that clinicians should consider patients with relevant work exposures as being at higher risk for developing RA," Blanc and colleagues wrote.

Rates of RA across all 1,988 men participating in the new survey were surprisingly high. A total of 232 participants (11.7%) said they had been diagnosed with RA, including 118 treated either with corticosteroids or disease-modifying antirheumatic drugs (DMARDs), or both. In contrast, a put the prevalence of RA among adult men at about 0.3% in a nationally-representative population.

However, Blanc and colleagues had deliberately targeted their survey at areas with "historically high rates of mortality due to silicosis." These were 24 counties in eastern Utah, western Colorado, and northwestern New Mexico, where mineral extraction is a major industry. Thus, if silica exposure does cause or promote RA, then one would expect the resulting sample to show higher-than-average rates of the disease.

Of the 1,988 participants in the telephone survey (response rate 11%), 6% reported working in underground hard rock mining, with median duration of 2 years (interquartile range 1-10). Some 4% had experience in underground soft rock mining, a category that includes coal, shale oil, and gilsonite extraction, with median duration of 6 years (IQR 3-22). About one in eight participants worked in surface mines that are believed to raise silica dust, and some 18% worked in nonmining industries associated with silica, such as construction. (An individual could report more than one type of exposure.)

After adjusting for smoking status, race, and age, Blanc and colleagues calculated the following odds ratios for steroid-treated RA, with no occupational exposure serving as the reference:

• Any underground hard rock mining: 3.21 (95% CI 1.45-7.10)
• Underground soft rock mining only: 9.74 (95% CI 3.89-24.42)
• Surface mining only: 3.74 (95% CI 2.07-6.75)
• Nonmining silica exposure only: 3.40 (95% CI 1.84-6.27)

A similar pattern was seen for DMARD-treated RA, except that the risk from underground hard rock mining fell short of statistical significance (OR 1.91, 95% CI 0.71-5.12).

When all types of mining experience were combined, RA risk was strongly elevated compared with no expected silica exposure, with highly statistically significant odds ratios ranging from 3.30 to 6.08, depending on how RA was classified (i.e., treated with any steroid, long-term steroids, DMARDs, or either DMARDs or steroids). Non-RA arthritis such as osteoarthritis was also significantly increased with soft rock and surface mining but not underground hard rock mining, with odds ratios of 1.43-3.04.

Blanc and colleagues speculated that the extra high risk associated with soft rock mining indicates that silica is not the only culprit. "[C]arbonaceous materials may also be involved etiologically in RA risk in that occupation," they suggested, noting earlier studies linking black-carbon particulates to protein citrullination and other pathologies associated with RA.

Limitations of the study included the geographic and age restrictions, the low survey response rate, reliance on participants' self-reports of RA diagnoses and treatments received, and lack of data on actual silica or other exposures beyond employment duration.

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