BMC Public Health 2008, 8:21 http://www.biomedcentral.com/1471-2458/8/21 group, since they showed an increased risk of death from all causes (RR = 1.71; p < 0.05) and from liver cancer (RRpm = 2.46; p < 0.10), and RRs for all tumors combined and lung cancer were increased too, even though increase was not statistically significant. Other blue collar workers showed excess (not statistically significant) risks of death for nearly all causes considered. This result suggests that high health risks occurred among all workers in this category, in addition to those usually considered at risk (i.e., autoclave workers and PVC baggers). Deaths from hemolymphopoietic system tumors (14 cases) occurred only among exposed workers (PVC bag- gers, PVC compound workers, other blue collar workers), pre- venting any estimate of RR. For this reason, SMRs were calculated and revealed significantly increased values among the total blue collar workforce and specifically among PVC baggers, while PVC compound workers and other blue collar workers showed more than two-fold increased SMRs (not statistically significant). These findings support the causal association already reported in the scientific lit- erature between occupational exposure and hemolym- phopoietic system tumors [3,4]. Attributable number of deaths The total number of deaths attributable to employment as a blue collar worker in a VC-PVC production plant was estimated to be 83.2 (out of 160) and 81.3 (out of 229) when the follow-up was updated to 1995 and 1999, respectively [23]. The consistency of these values is evi- dence of the reliability of results. The longer follow-up did not lead to an increase in the total number of attributable deaths. Actually, while the RR was estimated to be 2.09 after the previous update (1972–1995), it was estimated to be 0.97 for the most recent 5-year period (1995–1999), suggesting that the effect of the exposure to noxious substances – or exposure itself – may have been greater in the past and lower in more recent years. On the other hand, as expected for dis- eases with a long latency (i.e., tumors), attributable deaths from all tumors increased from 28.1 to 34.9 when the duration of follow-up was increased. Limitations, strengths and remarkable results The present epidemiological study, like many other stud- ies on occupational cohorts, lacked information about measures of individual occupational exposures over time during the employment period. Job category and length of employment were used as surrogate indicators of expo- sure type and cumulative exposure, respectively. Two studies on the same cohort already cited [19,20] revealed a drawback in the cohort database stemming from a mis- classification of both exposure and disease (information bias). In any case, such a misclassification could have led to an underestimation of the risk among exposed workers. A lack of fund prevented a more recent follow-up update, nevertheless the study had a power large enough to detect significant risks. Finally, it could be prudent to bear in mind that the study suffered from some limitation con- cerning the control of possible confounders, even if it unlikely could have lead to a significant modification of the results, as it will be made clearer afterward. Despite these limitations, this analysis was, in our opin- ion, grounded on a reasonable approach and achieved some noteworthy results. First of all, the present reanalysis sought to reduce the comparison bias by using as a refer- ence group – instead of the general population – a work- ing population with limited or exposure to occupational carcinogenic agents and with hiring criteria and duration of employment similar to those of the exposed group(s) under study. In addition, we tried to avoid another possible source of underestimation of the true risk, i.e. the inclusion of unexposed (or less exposed) workers in the exposed group (misclassification of expo- sure). In this way, the HWE and the related underestima- tion of the health risks should have been eliminated or at least controlled. Even if it was not possible to assess whether and how technicians and clerks might differ from the exposed workers other then the exposures of interest, the comparison seemed appropriate, and the socio-eco- nomic status of the exposed and unexposed workers should be fairly comparable. For this reason and as it was found to be at low (or null) exposure to toxic and carcino- genic agents [10], this group of workers (i.e., technicians and clerks) was the best available comparison group, while other blue collar workers included workers employed in different tasks not specialized and/or not confined in a specific workplace, such maintenance staff, cleaning oper- ators, and other workers that handled, or were exposed to, most of the toxic substances listed in table 1. A recent study tried to quantify the possible amount of the bias due to the joint effect of the dilution effect (DE) and the comparison bias (CB) in occupational cohort studies. DE and CB respectively occur when exposed and unexposed workers in a cohort are pooled together and an internal referent group is not used. The study confirmed that "results from occupational cohort studies without internal refer- ent groups may be unreliable" [24]. Then, internal compari- son and separating blue collar workers into subgroups with homogenous exposure reduced the possible DE and resulting estimates were likely not biased by confounding due to smoking or to other factors. As suggested by "good epidemiological practice" guidelines [25], RR estimates obtained using an internal reference are more reliable than those based upon both national and local reference, and, as it was observed, "internal analyses of "dose-response" in cohort studies are unlikely to be seriously confounded by Page 6 of 8 (page number not for citation purposes)