BMC Public Health 2008, 8:21 http://www.biomedcentral.com/1471-2458/8/21 Even among other blue collar workers excesses (not statisti- cally significant) were observed for each considered cause of death, except for liver cirrhosis. Fourteen deaths from hemolymphopoietic system tumors were detected among blue collar workers, but none were seen in the referent group (or among autoclave workers), thereby preventing us from estimating related RRs. As an indicative risk for these tumors, univariate SMRs com- pared to the Italian population were estimated. A signifi- cantly increased SMR was observed among PVC baggers (SMR = 3.77; 95% CI: 1.03–9.66) and among the total blue collar workforce (SMR = 2.27; 95% CI: 1.24–2.81), while PVC compound workers and other blue collar workers showed SMRs equal to 2.26 and 2.29, respectively (not statistically significant). Some of the analyzed causes of death showed well increased RR estimates, at a borderline significance at two- tailed 95% significance level. In particular, most worthy of attention were the higher than two-fold increases found for liver cirrhosis (RRpm = 2.8) among autoclave workers, for lung cancer (RR = 3.13) among PVC baggers and for liver cancer (RRpm = 2.46) among PVC compound workers. These increases would have been statistically significant at an one-tailed 95% significance level. As a final point, 81.3 deaths (out of 229 observed) from all causes and 34.9 deaths (out of 118) from all tumors were estimated to be attributable to blue collar workers' activity in this plant. Discussion Following an extensive review and evaluation of previous studies, the IARC, first in 1979 and then in 1987 [3,4], provided sound evidence for a causal association between VC-PVC exposure and liver cancer, as well as tumors of the brain, lung and hemolymphopoietic system. Despite such an authoritative stance on the issue, a good share of epi- demiological literature continues to consider liver angi- osarcoma as the only proven tumor caused by working in this petrochemical sector, mainly because RRs for this rare tumor have been found to be statistically elevated in almost all published studies [5,8,15-18]. Excesses for hepatic tumors other than liver angiosarcoma, when detected, have been considered as misclassifications of angiosarcoma [15,16]. Risks for other neoplastic and non- neoplastic diseases have been often ignored, with the exception of some findings for brain [17] and hemolym- phopoietic system cancer [18]. Nevertheless, excesses for lung cancer have recently been re-detected in specific sub- groups of workers exposed to PVC dust employed in the same Italian plant [8,19], and our previous studies [6] found excesses of liver tumors and liver cirrhosis among autoclave workers and of lung cancer among PVC baggers. In an attempt to clarify these contradictory results, in which the healthy worker effect (HWE) likely played an important role, the same cohort of VC-PVC workers was reanalyzed using an internal comparison group consti- tuted by unexposed (or less exposed) workers, instead of an external population based on general regional or national figures. External comparison is probably one of the major causes of biased results, mainly due to the eval- uation of very different populations (likely very healthy workers vs. the mix of healthy and unhealthy people in the general population). Consequently, the HWE may lead to a relative underestimation of the true mortality risks among exposed workers. Our reanalysis has revealed a significantly elevated risk for all causes of death among exposed blue collar workers compared to unexposed technicians and clerks. Specifically, increased mortality from both liver tumors (RRpm = 9.57; p < 0.05) and liver cirrhosis (RRpm = 2.80; p < 0.10) was observed among autoclave workers, where both an elevated risk of liver angiosarcoma [7] and the exposure to VC and PVC dusts during reactor cleaning operations have already been documented [10]. A case-control study nested in the same cohort, by holding the confounding factors constant in a logistic regression analysis, found that each extra increase of 1,000 ppm × years of VC cumulative exposure increased the risk of hepatocellular carcinoma by 71% (OR = 1.71; 95% CI, 1.28–2.44) and the risk of liver cir- rhosis by 37% (OR = 1.37; 95% CI, 1.13–1.69) [20]. An increased mortality from all causes (RR = 1.72; p < 0.05) and lung cancer (RR = 3.13; p < 0.10) was clearly observable in PVC baggers. Death risk from lung cancer confirmed a recent case-control study nested in the same cohort, which found a 20% increased risk for this cause of death among PVC baggers for each extra year worked (OR = 1.20; 95% CI = 1.08–1.35; p = 0.0010) after adjustment for age and smoking habits [19]. In addition, previous analyses revealed a statistically significant dose-response relationship (p < 0.05) between duration of employment and lung cancer mortality, and a progressive reduction in mortality after work cessation. The risk decreased from the high value observed 5 years after work cessation (SMR = 2.4) to values lower than unity after 15–20 years [6]. This trend reflected what was observed by Doll and Peto among former smokers after smoking cessation [21], and also by Mastrangelo et al., who reported that "In our [PVC] baggers, recent rather than distant exposure to PVC dust had the most profound effects on lung cancer risk" [19]. The main task of PVC compound workers was to mix granu- lar PVC with many other likely toxic and carcinogenic chemical agents (Tab. 1), whose characteristics differed according to the final PVC-based product [22]. Workers performing this task should be considered as a new at risk Page 5 of 8 (page number not for citation purposes)