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 the health effects associated with CCP. In these cross-sectional studies, workers who reacted to CCP might have left the workforce and thus would not have been included. In addition, since many of the researchers did not classify their subjects by exposure level, the overall effect could have been diluted by the mix of workers with high and low potential for CCP exposure.

The strongest evidence for an association between symptoms and CCP exposure comes from the studies of indoor air quality [Skov et al. 1989; Mendell 1991; Zweers 1992; Jaakola and Jaakola 1999]. These studies report a positive (and in several cases a statistically significant) association between CCP exposure and symptoms of skin, eye, and upper respiratory tract irritation (Table 5–2). These are the least susceptible to recall bias because they were not conducted in workplaces where concerns about CCP or other indoor pollutants played a role in their selection for study. Also, none of the indoor air studies were designed primarily to address the CCP question; hence investigator bias is also less likely. These studies used the most rigorous epidemiologic study designs, and the investigators were able to control for a number of potentially confounding exposures when examining the association between symptoms and CCP exposure. Determining whether associations observed in epidemiologic studies are causal is frequently difficult given the observational nature of these studies and the possible influence of confounders and other sources of bias. Such is certainly the case with the epidemiologic CCP literature. Hill [1977] has developed useful criteria for evaluating causality using all of the available data. Epidemiologists have widely adopted these criteria for evaluating the evidence of causality in the epidemiologic literature. The criteria include (1) the strength of the association, (2) the consistency of the association,