Reanalysis of mortality in a petrochemical plant producing vinyl chloride and polyvinyl chloride
OBJECTIVE: To study cancer mortality among main subgroups of workers employed in VC/PVC production potentially exposed to toxics and carcinogens. MATERIALS AND METHODS: We reanalyzed the mortality of 1,658 males (follow up 1972-1995; 35,626 person years; 170 deaths) by Poisson regression, adjusting for age, age at hiring, calendar period, length of exposure and latency. We calculated the relative risks (RR) and the 95% confidence intervals (CI) for PVC baggers (n. = 197), PVC compound (n. = 403) and autoclave (n. = 209) workers. As an internal reference we used two groups of workers: technicians and employees (main reference: n. = 202 subjects), with the group of other blue collar workers (n. = 639) added for statistical purposes only (minor reference: n. = 841 subjects). RESULTS: Pooling the four subgroups of exposed workers, the comparison with the main reference group allowed us to detect increased mortality from all causes of death (RR = 2.09; CI = 1.09-4.00; 160 deaths), all tumours (RR = 1.53; ns; 81 deaths), lung cancer (RR = 2.05; ns; 29 deaths) and cardiovascular diseases (RR = 3.57; ns; 29 deaths). The analysis for each specific subgroup revealed increased mortality from all causes of death among both PVC baggers (RR = 2.66; CI = 1.27-5.59) and PVC compound workers (RR = 2.68; CI = 1.36-5.27). Comparison of the minor reference group with the overall population of exposed workers showed increased RRs for other diseases, namely, liver tumour (RR = 4.08; ns; 9 deaths), lymphomas and leukaemia (RR = 2.97; ns; 7 deaths) and liver cirrhosis (RR = 3.30; ns; 11 deaths). The analysis for each specific subgroup revealed significantly increased RR for all tumours among PVC compound workers (RR = 1.74; CI = 1.06-2.85), for lung cancer among PVC baggers workers (RR = 3.04; CI = 1.15-7.99) and for liver cancer (RR = 9.57; CI = 1.69-54.1) and liver cirrhosis (RR = 6.32; CI = 1.37-29.07) among autoclave workers. In addition, the two deaths from brain tumour were observed among PVC compound workers. CONCLUSIONS: To highlight the risk previously reported in the literature and to reduce the possible dilution effect (due to comparison bias, healthy worker effect), it proved methodologically pivotal to separate exposed and probably unexposed workers (considered as an internal reference group).