Benzene exposure and hematological effects among offshore workers exposed to crude oil
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Objective: The main objective of the study was to gain more knowledge about the exposure to benzene in the Norway’s offshore petroleum industry, its’ relation to effects on the hematological system and the risk of hematopoietic malignancies. More specifically we wanted to 1) characterize the benzene exposure in Norway’s offshore petroleum industry, 2) investigate the association between exposure to benzene and effects on the immune system, and 3) assess whether workers employed in Norway’s offshore petroleum industry have an increased risk of developing hematopoietic malignancies than the general working population in Norway, focusing on the differences in risk according to the subtypes of leukemia in particular. Methods: In paper I we assessed the workers exposure to benzene in the breathing zone (full work shift) on a crude oil production vessel, including the operation modes ordinary activity, a brief shut down and tank work. In paper II and III we estimated the associations between the benzene exposure and concentrations of benzene in blood and urine among process operators (n=12) and workers maintaining cargo tanks containing crude oil residues (n=13). Referents working in the catering section having the same shift schedule, matched on age and gender, were included. Benzene exposure was measured during three consecutive 12-hour work days. Blood and urine samples were collected prior to the first work shift (baseline), immediately after the third work shift, and prior to the following work shift. In paper IV we investigated the relationship between benzene exposure and alterations of proteins and cells of the immune system, measured by peripheral blood lymphocytes (total lymphocytes, lymphocytes in subpopulations CD3, CD4, CD8, CD19, CD56 and CD4/CD8 ratio), complement factors C3 and C4 and serum concentration of immunoglobulins (IgG, IgA, IgM and IgE). In paper V, we studied the risk of hematopoietic malignancies among offshore workers identified by the Norwegian Registry of Employers and Employees. All subjects registered with offshore-related industrial classification codes or with work location in the North Sea from 1981 to 2003 were included. We drew up to six referents per petroleum worker from the general working population matched by gender, age and community of residence. The cohort comprised 27,919 offshore workers distributed on four job categories offshore and 366,114 referents and was linked to the Cancer Registry of Norway, the Norwegian Education Registry and the Norwegian Cause of Death Registry. Results: Full-shift benzene exposure levels measured in the workers breathing zone during ordinary activity were low for workers on the crude oil production vessel (geometric mean 0.004 ppm, range < 0.001 – 0.22 ppm) (paper I) and also for process operators at a fixed oil- and gas installation (geometric mean 0.005 ppm, range <0.001 - 0.688 ppm ) (paper II). The exposure varied considerably, with some measurements being higher than the Norwegian occupational exposure limit of 0.6 ppm for a 12-hour shift. The process operators at the fixed oil- and gas installation and tank workers on the crude oil production vessel had a mean benzene concentration of benzene in blood post-shift of 1.5 nmol/l and 12.3 nmol/l, respectively (paper II and III). Although only the tank workers’ benzene concentration in blood differed significantly from referents, the biological uptake was significantly related to the exposure levels of benzene in air in both groups. In paper IV we found that the tank workers declined (versus referents) in IgM from baseline to post-shift (t-test, P=0.04), in IgA from baseline to pre–next shift (t-test, P=0.01) and in CD4 T cells from baseline to post-shift (t-test, P=0.04). The suppression correlated with benzene exposure, benzene concentrations in blood and urine and time spent in the tank. The groups did not differ significantly in the change in other immune parameters. In paper V we found that workers in the job category “upstream operator offshore”, who have the potentially highest exposure, had an excess risk of hematopoietic malignancies (rate ratio (RR) 1.90, 95% confidence interval (CI) 1.19–3.02). This was ascribed to increased risk of acute myelogenous leukemia (RR 2.89, 95% CI 1.25– 6.67) and multiple myeloma (RR 2.49, 95% CI 1.21–5.13). Rate ratios were highest for the workers with their first registered engagement in the offshore petroleum industry before 1986. The other job categories had no increased risk, and overall cancer (all sites) did not differ. Conclusion: In spite of relatively high short term peak exposure to benzene during ordinary activity for several job categories on a crude oil production vessel and for process operators on a fixed oil- and gas installation, the full-shift mean exposure is low. Although both process operators on the fixed oil- and gas installation and tank workers on the crude oil production vessel had a low exposure to benzene, the biological uptake was significantly related to the benzene exposure. The internal concentration of benzene among tank workers was higher than expected at the measured exposure levels. This finding is probably due to an extended work schedule and high work load. The same tank workers showed acute alterations in the immune parameters IgM, IgA and CD4 T helper cells that correlated with levels of benzene. The clinical significance of the finding for the tank workers’ health is not known. Given the complexity of the tank atmosphere and in the work environment in general on oil- and gas installations offshore we cannot exclude a possible contribution of other types of specific or combined exposure, but occupational exposure to benzene is the most likely candidate for the reported decline in immune parameters among tank workers and the risk of acute myelogenous leukemia and multiple myeloma found among offshore workers.