BMC Public Health 2008, 8:21 http://www.biomedcentral.com/1471-2458/8/21 smoking habits" [26]. Within the same cohort analyzed in the present study, Mastrangelo et al. [19] found a lung cancer risk among baggers comparable to our estimates, but using a case-control approach, that by definition takes smoking (as other confounders) into account. Moreover, a clear dose-response relationship between length of exposure and lung cancer was detected among PVC bagger workers. In fact, not exposed workers and PVC bagger workers with a length of employment – used as a proxy of length of exposure – of 0–5, 6–10 and 10+ years, showed the following SMRs: 64.79, 108.35, 216.45 and 355.86, respectively. As this trend (p = 0.054) was observed among workers belonging to the same job category, it was likely unaffected by confounding effects, including those gener- ated by smoking status. Finally, applying the Axelson approach to indirect adjustment for a confounder, it was clear that smoking unlikely played a major role when lung cancer RR was greater than 1.66 [27,28]. The two procedures used in this study (i.e. internal com- parison and separating blue collar workers into subgroups with homogenous exposure) revealed the pattern of dis- eases, likely due to exposures to specific occupational risk factors, typical of each subgroup of workers. Conclusion The present study confirmed the elevated risks for specific tumors, namely liver, hemolymphopoietic system and lung cancers, previously observed by the scientific com- munity (above all, the IARC [2-4]). In particular, two job categories, autoclave workers and PVC baggers, were con- firmed to be at risk for liver diseases and lung cancer, respectively, and another category, PVC compound workers, was found to be likely at risk for liver and lung tumors. Moreover, mortality risk from neoplastic and non-neo- plastic diseases was shown to be high among all blue col- lar workers. These findings confirm that differently exposed workers could become ill from different diseases. The simple and rigorous approach applied in this study aimed to offset main biases and distortions that usually affect occupational cohort studies, and the new evidence emerging from this reanalysis confirms the already docu- mented health risks. We believe that this methodology could be used for the analysis of several other cohort stud- ies. Competing interests During the class action lawsuit against the Marghera pet- rochemical plant, VG and PC were appointed as technical consultants by the plaintiffs Greenpeace and Medicina Democratica, respectively. MC and FM have no compet- ing interests to declare. The study was not financially funded. Authors' contributions VG conceived the study and supervised all aspects of its implementation. MC performed the statistical analyses and wrote related sections. FM participated in the design of the study, assisted with the analyses and led the writing. PC revised all aspects of the study and helped to draft the manuscript. All authors helped to conceptualize ideas and interpret findings. All authors read and approved the final manuscript. Acknowledgements We wish to remember here the courageous Gabriele Bortolozzo, a blue collar worker of the petrochemical plant of Porto Marghera (Venezia, Italy), whose perseverance and mission prompted the epidemiological survey of the cohort reanalyzed here. We thank Pietro Comba and Roberta Pirastu for having shared the dataset with us, and Franco Berrino, Piergiorgio Duca, Luigi Mara, Benedetto Terracini and Lorenzo Tomatis for their encourage- ment and helpful comments. Finally, we are grateful to Mr Thomas John Wiley for his proficient copyediting. References 1. Processo: Petrolchimico. [http://www.petrolchimico.it]. 2. 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