Letters to the Editor Re: "dose-response Associations of Siuca with Nonmaugnant Respiratory Disease and Lung Cancer Mortality in the Diatomaceous Earth Industry"

In an update by Checkoway et al. (1) of mortality among diatomaceous earth workers, asbestos exposure misclassification and asbestos-silica interactions may have influenced study results and interpretation. In a reanalysis (2) of the original cohort mortality study to examine confounding by asbestos exposure, workers employed before 1930 were excluded because asbestos was definitely used in the facility, but specific jobs and exposure levels could not be identified. In this updated study, pre-1930 workers were included, and Checkoway et al. reported that they "extrapolated the 1930 job-specific intensity estimates to earlier years of plant operation" (1, p. 682). In fact, no asbestos was used in die facility in 1930, and measures of asbestos exposure were assigned to maintenance workers only (3). Thus, production workers employed before 1930 were considered "nonexposed" when some may have been exposed to high levels of asbestos. High levels of exposure to asbestos are well established as carrying their own risk of lung cancer. In addition, confounding by asbestos exposure for the pre-1930 workers cannot be dismissed, because confounding was investigated only in workers whose main asbestos exposure occurred after 1950 (2), when levels were considerably lower than would have been anticipated during the pre-1930 period.