However, for two reasons, this seems unlikely to have caused major bias: a) Although only four studies controlled for smoking, two were large studies with substantial exposure allowing precise estimates of URRs. The mean URR in smoking-adjusted studies was statistically compatible with but somewhat higher than that for the studies uncontrolled for smoking and was statistically significant, b) Several methodological articles (Axelson and Steenland 1988; Blair et al.
We established that variation in URRs between industries cannot be explained by chance (particularly coke ovens and aluminum production vs.
Two reasons might account for true variation in URRs. a) A factor that modifies the effect is present to varying degrees in different industries.
Some specific studies with URRs quite different from the mean for their industry deserve specific mention:
1972) have high URRs. The estimate of exposure for retort workers in these plants was 3 [micro]g/[m.sup.3] BaP and was heavily influenced by measurements reported in 1965 from mask samples.
Gibbs' finding of ULR 0.3, 4.2, 4.4, 5.8, 6.6, 7.2, 7.8, and 9.5 translates to URRs 1.02, 1.26, 1.27, 1.35, 1.40, 1.44, 1.48, and 1.58, which are somewhat higher on average than the estimates for the same studies in this analysis but not grossly different.
(At these moderate to low relative risks, log-linear interpolation is close to linear interpolation.) Risk estimates calculated this way for a range of URRs and exposure concentrations are given in Table 5.
The URRs overall had significant and substantial heterogeneity.
That the overall mean and broad pattern of URRs under the linear and log-linear models were similar is reassuring, but having model choice forced by statistical tractability is not ideal.
* A priori considerations and data in the meta-analysis studies suggested use of linear rather than log-linear models, but estimates of URRs from linear models proved intractable in meta-analysis, so we worked with log-linear models.
The average estimated unit relative risk (URR) at 100 [micro]g/[m.sup.3] years benzo[a]pyrene was 1.20 [95% confidence interval (CI), 1.11-1.29] and was not sensitive to particular studies or analytic methods.
For each included cohort we systematically extracted general descriptive information, information on potential modifiers of risk associated with PAHs, and information from which we estimated unit relative risk (URR) increments (see next two subsections).