Historical cohort study of US man-made vitreous fiber production workers: III. Analysis of exposure-weighted measures of respirable fibers and formaldehyde in the nested case-control study of respiratory system cancer

The most recent findings of our nested case-control study of respiratory sy stem cancer (RSC) among mate fiberglass workers showed some evidence of ele vated RSC risk associated with non-baseline levels of average intensity of exposure (AIE) to respirable fibers (RFib). When adjusted for smoking, this was not statistically significant, and no trend was apparent with increasi ng levels of exposure. Similar findings for PLSC were noted for both cumula tive exposure (Cum) and AIE to formaldehyde (FOR). In this reanalysis of ou r nested case-control study, we explored a possible exposure-response relat ionship between RSC and exposure to RFib or FOR using exposure weighting as an alternative characterization of exposure. Because of the uncertainties in selecting an appropriate exposure-weighting scheme, a range of plausible time lags and unlagged/lagged time windows was considered. As in the initi al analysis of the nested case-control study, RFib and FOR exposures were c ategorized at the deciles of the RSC case distribution. For none of the exp osure weighting schemes considered did we observe an increasing RSC risk wi th increasing Levels of RFib_Cum or RFib_AIE. The exposure-weighted estimat ed risk ratios (RR) for both RFib_Cum and RFb_AIE were generally lower than those obtained from an unweighted model. For FOR Cum, RRs were generally l ower for the time-lagged and unlagged time window models than for the unwei ghted models, although some decile-specific RRs were higher for the lagged time window models. The exposure-weighted RRs for FOR ALE were generally lo wer than the unweighted RRs for all of the weighting schemes considered. Th is reanalysis in terms of categorized exposures reveals no exposure-respons e relationships that were undetected in the original analysis where unweigh ted exposure measures were used. In the schemes considered, exposure weight ing generally reduced the estimated risk of RSC.