Investigation of a cluster of leukaemia in the Illawarra region of New South Wales, 1989-1996

Objectives: To investigate a cluster of leukaemia among young people and as sess the plausibility of a disease-exposure relationship. Design: Descriptive analysis of population-based leukaemia incidence data, review of evidence related to the causation of leukaemia, assessment of env ironmental exposures to known leukaemogens, and resulting risks of leukaemi a. Setting: Illawarra region of New South Wales, Australia, focusing on suburb s between the Port Kembla industrial complex and Lake Illawarra (the Warraw ong area). Main outcome measures: Standardised incidence ratios (SIRs) for leukaemia; current measured and past estimated ambient air benzene concentrations; and expected leukaemia cases attributable to estimates of ambient air benzene concentrations. Results: In 1989-1996, 12 leukaemia cases among Warrawong residents aged le ss than 50 years were observed, more than the 3.49 cases expected from the rate in the rest of the Illawarra region (SIR, 343.8; 99% CI, 141.6-691.7). These people lived in suburbs immediately to the south-southwest of a coke byproducts plant (a major industrial source of benzene, one of the few kno wn leukaemogens). The greatest excess was among 15-24-year-olds (SIR, 1085. 6; 99% CI, 234.1-3072.4). In 1996, ambient air concentrations of benzene av eraged less than 1 part per billion (ppb). Since 1970, ambient air concentr ations of benzene were estimated to have averaged up to 3 ppb, about one-th ousandth of the level at which leukaemia risk has been identified in occupa tional epidemiological studies. Using the risk assessment model developed b y the US Environmental Protection Agency, we estimate that past benzene lev els in the Warrawong area could have resulted in 0.4 additional cases of le ukaemia in 1989-1996. Conclusions: The excess occurrence of leukaemia in the Warrawong area in 19 89-1996 is highly unusual. Current environmental benzene exposure and the r econstructed past environmental benzene exposure level are too low to expla in the large excess of leukaemia. The cause of the cluster is uncertain.