The detailed lung radiographic response to silica exposure has not been described. In estimating the exposure–response relationship in silicosis with statistical models, the absence of baseline (unattributable) risk can disable relative-rate estimation or produce widely varying estimates. This obstructs identification of optimum exposure metrics and invalidates comparisons and meta-analyses, which assume a common background rate.
A cohort of 3,000 Chinese tin miners with more than 1,000 cases of silicosis was analyzed for the period 1961–1994. Regular surveillance documented three stages of silicosis. To examine the exposure–response relationship, the intercept in relative-rate models was fixed to correspond to 1% of the observed silicosis rate. Exposure metrics for contributions in different time-windows were simultaneously evaluated, as were burden and cumulative burden metrics.
Silica exposures that most contributed to silicosis onset occurred in the period 5–10 years prior (excess annual rate per 10 mg-year/m3, ER = 0.158, 95% CI = 0.125–0.192, or 16% per year). During 10–20 year prior, the excess rate contribution was much smaller (ER = 0.048, 95% CI = 0.037–0.060) but larger again during 20–30 year prior to onset (ER = 0.112, 95% CI = 0.098–0.126). For advanced silicosis, all time periods contributed about equally to the rate of onset.
Reliable estimates of parameters were observed, demonstrating exposure contributions over time. Burden metrics with different half-lives suggested some reversibility for silicosis onset with a half-life of 20 years. Advanced silicosis was best predicted with a cumulative burden metric which was consistent with prior observations that previously deposited silica continues to cause pulmonary damage. Am. J. Ind. Med. 56:267–275, 2013. © 2012 Wiley Periodicals, Inc.