Historical cohort study of US man-made vitreous fiber production workers: VI. Respiratory system cancer standardized mortality ratios adjusted for the confounding effect of cigarette smoking

To date, the US cohort study of man-made vitreous fiber workers has provide d no consistent evidence of a relationship between man-made vitreous fiber exposure and mortality from malignant or non-malignant respiratory disease. Nevertheless, there have been small, overall excesses in respiratory syste m cancer (RSC) among workers from the fiberglass and rock/slag wool product ion plants included in the study that were unexplained by estimated worker exposures to respirable fiber or other agents present in the plants. The pr esent investigation was designed to provide a quantitative estimate of the extent to which the overall excess in RSC mortality observed at the total c ohort level among male fiberglass and rock/slag wool workers is a result of the positive confounding effects of cigarette smoking. Because cigarette-s moking data were neither available nor obtainable at the individual level f or all members of the fiberglass and rock/slag wool cohorts, we used the "i ndirect" method to adjust RSC standardized mortality ratios (SMRs) at the g roup (cohort and plant) level. Our adjustment suggested that cigarette smok ing accounts for all of the 7% and 24% excesses in RSC observed, respective ly, for the male fiberglass and rock/slag wool cohorts in the latest mortal ity updates. The same conclusion was reached regardless of which of several alternative formulations were used to adjust local rate-based RSC SMRs. We found that our smoking adjustments were robust with respect to several alt ernative characterizations and (with the exception of one fiberglass plant) produced adjusted RSC SMRs that were lower than their unadjusted counterpa rts. Further, all statistically significantly elevated unadjusted SAMS were reduced to not statistically significant levels. These results reaffirm, t hat RSC SMRs based on US and local rates must take into account the potenti al confounding effects of cigarette smoking. They also suggest that the use of local county mortality rate-based SMRs may not help to adjust for cigar ette smoking to the degree suggested by some investigators.