European Journal of Epidemiology 12: 659-660, 1996. © 1996 Kluwer Academic Publishers. Printed in the Netherlands.

LETTER TO THE EDITOR

A c l u s t e r of c a n c e r d e a t h s a m o n g w a s t e w a t e r t r e a t m e n t w o r k e r s Susanna Lagorio, Marco De Santis & Pietro Comba Istituto Superiore di Sanit& Rome, Italy Accepted in revised form 29 August 1996

Key words: Cancer death, Infection, Sewage workers

Sewage workers may be exposed to a wide array of infectious agents and chemicals, including possible mutagens [1]. Increased risks of different neoplasms have been reported in few relevant epidemiologic investigations [2-6]. Following the signaling of a cancer cluster at a wastewater treatment plant in Prato (Italy), a retrospective cohort mortality analysis was carried out. The plant processes 100,000 m 3 of municipal (51%) and industrial (49%) sewage per day. Most industrial sewage comes from textile manufacturing plants. Sludges are incinerated in a Nichols-Herreshoff oven. Workers employed during the period 1981-1994 were considered eligible for the study. Company pay rolls were used to enumerate cohort members. Vital status and causes of death were ascertained through the Registry Office of the municipalities of residence and of death. Based on longest held job, workers employed in processing and servicing tasks were considered as 'exposed' (52 men, 534 person-years at risk), in juxtaposition with 'unexposed' administrative and technical workers (8 men and 8 women, 158 person-years of observation). Cause-specific relative risks were estimated by the standardized mortality ratios (SMR) of observed to expected deaths, calculated based upon cause-, sex-, age-, and calendar time-specific mortality rates of the regional population, using the OCMAP/PC software [7]. No deaths occurred among unexposed workers. Among exposed workers, 5 deaths were recorded (1.2 expected). All deaths were due to malignant neoplasms (0.5 expected). Lung cancer mortality showed a twenty-fold increase, based on 3 observed vs 0.14 expected cases. One case of stomach cancer and one case of colon cancer were observed vs 0.05 and 0.03 expected cases, respectively. Four out of five cancer deaths were recorded among workers employed for 6-10 years and followed-up for 6-10 years, and the other case occurred in the in '> 10 years' exposure-latency time window. Beside the inadequate power due to the small sample size, this study has other obvious limitations. Although the contamination of the influent wastewater with mineral oils used in the textile process might represent a carcinogenic hazard [8],

no quantitative exposure estimate was available. A cross-sectional study performed by the Prato Local Health Unit among current and some retired sewage workers, provided the following estimate of smoking habits: current smokers 40%, former smokers 31%, non smokers 29%. It seems very unlikely that relative risks of the observed entity might be entirely attributable to a confounding effect of smoking [9]. No excess of respiratory neoplasms was found in previous sewage worker cohorts [2-5]. In fact, the presence of a sludge incinerator is an uncommon feature of the facility under study, and an increased lung cancer risk was reported among Swedish incinerator workers [10]. The risk of stomach cancer was increased in previous studies of sewage workers [3-5], although its possible determinants are still unknown. An increased risk of gastric neoplasms was also noticed among Swedish waterworks and public cleaning workers [11]. A relevant methodological drawback stems from the 'public concern-driven' nature of the present investigation. The authors were unaware of names and other characteristic of cases included in the reported cluster, and these were enrolled because of the study design chosen. However, time or spatial aggregations of rare diseases may well be chance findings [12-13]. In conclusion, we deem that the possible carcinogenic effects of specific occupational exposures in wastewater treatment plants should be further explored by analytical studies of adequate power.

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Address for correspondence: Susanna Lagorio, MD, Istituto Superiore di Sanith, Viale Regina Elena 299, 1-00161, Rome, Italy Phone: +39 6 49902138