Work in the metal industry and nasopharyngeal cancer mortality among formaldehyde-exposed workers

Abstract

Objective To investigate further the possibility that the large nasopharyngeal cancer (NPC) mortality excess among a cohort of formaldehyde-exposed workers may be related to occupational factors external to the study plant. Methods Subjects were 7345 workers employed at a plastics-producing plant (1941–1984) in Wallingford, Connecticut evaluated independently as part of a National Cancer Institute cohort study. Vital status for 98% of the cohort and cause of death for 95% of 2872 deaths were determined through 2003. Reconstructed worker exposures to formaldehyde were used to compute unlagged and lagged exposure measures. We computed standardized mortality ratios (SMRs) based on US and local county rates. In a nested case–control study we evaluated mortality risks from NPC and from all other pharyngeal cancers combined (AOPC) in relation to formaldehyde exposure while accounting for potential confounding or effect modification by smoking or external (non-Wallingford) employment. Job applications, Connecticut commercial city directories and a previous survey were used to assign subjects to three external job groups. Results We observed no new deaths from NPC and one additional AOPC death (pharynx unspecified) yielding, respectively, SMRs of 4.43 (7 deaths, 95% CI = 1.78–9.13) and 1.71 (16 deaths, 95% CI = 1.01–2.72). Five of seven NPC cases worked in silver smithing (including brass plating and other jobs related to silver or brass) or other metal work (including steel working and welding), and this type of work was relatively rare in the remaining study population (OR = 14.41, 95% CI = 1.08–82.1). For AOPC, we found a moderate increase in risk for other metal work (OR = 1.40, 95% CI = .31–5.1). Interaction models suggested that NPC and AOPC risks were not elevated in subjects exposed only to formaldehyde. Conclusions The results of our nested case–control study suggest that the large nasopharyngeal cancer mortality excess in the Wallingford cohort may not be due to formaldehyde exposure, but rather reflects the influence of external employment in the ferrous and non-ferrous metal industries of the local area that entailed possible exposures to several suspected risk factors for upper respiratory system cancer (e.g., sulfuric acid mists, mineral acid, metal dusts and heat). Our findings may also help to explain why the associations with formaldehyde and nasopharyngeal cancer reported in the 1994 update of the 10-plant NCI formaldehyde cohort study were unique to the Wallingford plant (Plant 1 in NCI study). Further updates of the NCI formaldehyde cohort study should include co-exposure data on silver smithing and other metal work for all study plants to help explain the unique findings for nasopharyngeal cancer in Plant 1 compared with the other nine plants.