2.1.1. Cohort studies (Table 2.1)

Table 2.1

Summary of cohort studies of populations exposed to [benzidine derivatives and] azo dyes metabolized to benzidine.

IARC Monograph Volume 29 (IARC, 1982) reported the results of three studies of workers exposed to 3,3′-dichlorobenzidine (Gerarde and Gerarde, 1974; Gadian, 1975; MacIntyre, 1975). At the time, the Working Group noted that these studies examined relatively small cohorts of workers, and that the time since first exposure to 3,3′-dichlorobenzidine was 20 or fewer years for over two thirds of the workers. Also, in the study by Gerarde and Gerarde (1974), follow-up of exposed workers was less than 85% complete. [The Working Group agreed that the significance of these findings is uncertain.]

As part of a notification programme in a dye-intermediary production plant, Schulte et al. (1985) identified a cohort of 1385 workers employed from 1940 to 1972. The cohort was potentially exposed to 3,3′-dimethylbenzidine, 2-naphthylamine, benzidine and 1-naphthylamine, with 2-naphthylamine as the major exposure. A questionnaire was used to obtain information on occupational history, particularly relating to working in the 2-naphthylamine-grinding room, use of protective measures, and history of other jobs with potential exposure to bladder carcinogens. Additionally, information was obtained on alcohol, tobacco, coffee and artificial sweetener use, and personal health history. Follow-up was conducted from date of first employment until 1982. Incidence rates in the cohort were compared to estimated incidence rates for bladder cancer in the United States. For the entire cohort, 13 cases of bladder cancer were observed (overall RR, 3.9; 95% CI, 2.2–6.8). Stratified analyses by race and length of employment also showed statistically significant associations. The most remarkable was among black workers with invasive cancer and more than 10 years of employment (RR, 111.1; 95% CI, 35.0–352.5). The authors reported that, in general, black workers had jobs involving greater exposure to 2-naphthylamine.

In a nested case–control study, Schulte et al. (1986) evaluated the effect of potential risk factors on the risk for bladder cancer. The analysis included the aforementioned 13 bladder-cancer cases and the remaining 1372 workers in the cohort as controls. Workers were considered exposed if they had more than one year of employment or any employment in two departments with potential 2-naphthylamine exposure. The crude odds ratio between exposure and bladder cancer was 7.0 (95% CI, 3.9–12.4). The odds ratio for the association between bladder cancer and duration of employment, controlling for smoking and source of drinking water, was 4.3 (95% CI, 1.8–10.3).

Sinks et al. (1992) evaluated cancer risk at a paperboard-printing manufacturing plant in Georgia, USA. The cohort consisted of 2050 workers employed for more than one day, with duration of employment obtained through company records. Company material safety-data sheets were reviewed and potential carcinogens were identified. From supplier information, the authors determined that pigments were manufactured from 3,3′-dichlorobenzidine and ortho-toluidine, but these substances were not identified in laboratory tests. One bladder-cancer death (SMR 2.6; 95% CI, 0.1–14.5) and one renal cell cancer death (SMR, 1.4; 95% CI, 0.0–7.8) were observed. For the incidence analysis, six cases of renal-cell cancer were seen (SIR, 3.7; 95% CI, 1.4–8.1), but no increase in bladder-cancer risk was observed (3 cases; SIR, 1.1; 95% CI, 0.2–3.1). In a nested case–control study, the risk for renal-cell cancer by duration of employment and department or work process was evaluated. Employment in the finishing department for five or more years was associated with an increased risk for this cancer type (three cases; OR, 16.6; 95% CI, 1.7–453.1). The authors could not determine if workers from the finishing department were exposed to inks.

Naito et al. (1995) conducted a retrospective cohort-mortality study of 442 workers (437 men, 5 women) exposed to one or more substances (mainly benzidine, 2-naphthylamine, 1-naphthylamine, and 3,3′-dimethoxybenzidine) at a benzidine production and dye-manufacturing plant in Japan. No industrial hygiene data for the plant were available; therefore, duration of employment at the facility was used as a surrogate for duration of exposure. Nineteen workers were potentially exposed to 3,3′-dimethoxybenzidine during dye manufacture, and of these only three were exposed solely to 3,3′-dimethoxybenzidine. The authors reported that all workers in the factory wore work clothes, gloves, high rubber boots, and a gas mask. An increased risk for bladder cancer was found among workers engaged in dye manufacture (SMR, 27.0; 95% CI, 8.8–63.0). Increased risks for cancer mortality for other organs were observed, but none were statistically significant [results for lymphohaematopoietic cancers were not reported]. Incidence rate ratios of urothelial cancer increased with duration of exposure (P for trend, 0.04). [Due to the small number of workers exposed to 3,3′-dimethoxybenzidine alone or in combination with other arylamines, it is not possible to attribute the increased risk for bladder cancer to exposure to this substance. Furthermore, reduced exposures are likely due to the reported use of personal protective equipment].

Ouellet-Hellstrom and Rench (1996) studied workers at the same chemical plant previously studied by Meigs et al. (1986) in Connecticut, USA. The cohort consisted of 704 workers (585 men and 119 women) first employed at a plant during the period 1965–89, after benzidine production was discontinued, and were therefore never exposed to benzidine. Workers were presumably exposed to 3,3′-dichlorobenzidine (predominantly used at the plant), 3,3′-dimethoxybenzidine and 3,3′-dimethylbenzidine. The authors used personnel records at the plant and a questionnaire to obtain occupational history and information on other risk factors such as smoking. Each job title was assigned an exposure score to arylamines based on expert judgment. An annual cumulative exposure score to arylamines was calculated for each worker. Expected numbers of cancers were estimated using cancer-incidence rates for the state of Connecticut. Men had elevated SIRs for cancers of the buccal cavity, bladder, kidney, brain and testis. Only for bladder cancer (SIR, 8.3; 95% CI, 3.3–17.1) and testicular cancer (SIR, 11.4; 95% CI, 1.4–41.1) did this increase reach statistical significance. No excess of lymphohaematopoietic cancers was observed (SIR, 1.1; 95% CI, 0.1–4.1). The two workers with testicular cancer had no exposure to arylamines, and one had worked at the plant for only 15 days. Women had a statistically insignificant increase in breast cancer (SIR, 1.9; 95% CI, 0.4–5.6). The SIR for bladder cancer increased with increasing exposure. This excess bladder cancer occurred among chemical operators (who worked with arylamines over long periods) and mechanics (who had short periods of exposure that was likely intense). The bladder cancer SIRs by cumulative exposure among smokers were higher than for the total cohort. [This study provides strong evidence of the association between exposure to benzidine-based dyes—or benzidine derivatives metabolised to benzidine—and bladder cancer].

Rosenman and Reilly (2004) analysed a cohort of 488 white men employed in a chemical manufacturing facility in Michigan, USA. The facility produced benzidine from 1960 through 1972 and 3,3′-dichlorobenzidine from 1961 to 2001. Workers were identified from social security records. Since no plant records were available, social security data were used to estimate time of first work and years worked. Analyses were conducted for the entire cohort and separately for people who began to work in 1973 or later, after benzidine production was discontinued. For the whole cohort, an excess of bladder-cancer mortality was observed (SMR, 8.3; 96% CI, 1.7–24.4). All cases occurred in those with five or more years of duration of work. There were six deaths from lymphohaematopoietic cancer (SMR, 2.8; 95% CI, 1.04–6.2), including one from non-Hodgkin lymphoma, one from multiple myeloma, two from chronic lymphocytic leukaemia, one from acute leukaemia, and one from chronic myelogenous leukaemia (SMR for leukaemia, 5.1; 95% CI, 1.4–12.9). Only one bladder cancer case (of 22 observed) occurred among workers starting employment after 1972, when the plant was only producing 3,3′-dichlorobenzidine. A statistically significant increase in mortality from lymphohaematopoietic cancer was observed among workers who began work in 1973 or later. [Not enough latency among those exposed only to 3,3′-dichlorobenzidine may have accumulated].

2.2.1. Case reports and proportional mortality studies

Genin (1977) analysed the urine of 22 workers involved in the drying and grinding of azo dyes metabolized to benzidine (e.g. Direct Black 38) and dyes metabolized to 3,3′-dimethoxybenzidine (e.g. Direct Blue 15) (IARC, 1982, 1993). Benzidine was found in the urine of eight workers, and 3,3′-dimethoxybenzidine in the urine of three. A retrospective search of plant records showed five cases of bladder cancer in dryers and grinders.

In a proportional mortality study of 1429 bleachers and dyers presumed to be exposed to dyes metabolized to benzidine in the United Kingdom, Newhouse (1978) showed no excess deaths from cancer of the bladder (14 deaths observed, 13.1 expected) [The Working Group of Volume 29 noted that the study was limited in that no certificates of deaths occurring in the first 20 years after start of exposure were available, and only approximately one third of the workers included in the analysis had actually been exposed to dyes.]

2.2.2. Cohort studies

Stern et al. (1987) conducted a mortality study in a cohort of 9365 tannery production workers (7085 men, 2280 women), employed at two chrome-leather tanneries in Minnesota and Wisconsin. The processes in both tanneries have remained more or less the same since the end of the 19^th century. The authors reported detectable concentrations of benzidine in bulk dyes in the dye room [but exposure was likely to be due to benzidine-based dyes]. Chemical sampling was conducted at both plants. Detectable concentrations (2 and 55 ppm) of benzidine in two samples of bulk dyes were found in the retan/colour/fat-liquor department. Occupational history was obtained from plant records; duration of employment was used as a surrogate for cumulative exposure, and cancer mortality analyses for select causes also examined workers ever employed at specific departments. Expected mortality rates were calculated using US and State death rates (no major differences were observed in risk estimates using both rates; only the latter are presented here). The authors did not observe an excess in the risk for bladder-cancer mortality (tannery A: one observed death; tannery B: four observed deaths; SMR, 1.0; 95% CI, 0.3–2.5). Risk was not increased for other primary sites of cancer mortality. Analysis by duration of exposure did not show increased mortality risk for those workers with more than 15 years of latency (SMR, 0.5; 95% CI, 0.1–1.3). The authors reported that of the five deaths from bladder cancer, four occurred among workers of tannery B, and two of these deaths occurred in the retan/colour/fat-liquor department (1.0 expected), but each of those had worked for less than two months at the tannery.

A cohort mortality-study of 2926 male workers newly employed in tanneries between 1950 and 1981 and with at least one period of employment of more than six months was conducted by Costantini et al. (1989) in Florence and Pisa, Italy. Known exposures in the plants include dyes metabolized to benzidine, dyes metabolized to 3,3′-dimethylbenzidine and dyes metabolized to 3,3′-dimethoxybenzidine. Employment in the tannery industry was obtained from municipal records, including beginning and end of work. Non-statistically significant increases in mortality were observed for cancers of the kidney, pancreas, lung, bladder (5 deaths; SMR, 1.5; 95% CI 0.5–3.5) and lymphohaematopoietic system. Analyses by latency did not show positive trends, except for bladder cancer, where the SMR increased with latency, although the trend was not statistically significant. All bladder-cancer deaths occurred in workers who entered the cohort between 1950 and 1964.

You et al. (1990) conducted a retrospective cohort study among workers in 17 knitting factories, 10 stocking factories, nine silk printing and dyeing factories and seven printing and dyeing factories in Shanghai, China. The cohort of 1210 workers (1060 men and 150 women) had worked for more than a year in weighing and formulating or dyeing, where they had been exposed to the dust of dyes metabolized to benzidine. Fifteen types of benzidine-derived dyes have been used in these factories, and the dyes used in the largest quantities included Direct Black 38. Cancer mortality and incidence were analysed among exposed workers by dividing them into two groups based on their usage of benzidine-based dyes: high exposure (> 500 kg/month) and low exposure (< 500 kg/month). Only one case of bladder cancer was observed in the exposed group [no risk estimate was provided].

Montanaro et al. (1997) studied mortality in a cohort of 1244 workers employed at a chrome tannery in Genoa, Italy, between 1955 and 1988, where workers were exposed to azo dyes metabolized to benzidine and other chemicals used in the tanning process. Length of employment was obtained from plant records; information on department was available only for 25% of the workers. Mortality for all cancers was 12% higher than expected. An excess of bladder-cancer mortality was observed (10 deaths; SMR, 2.4; 95% CI, 1.2–4.5) as well as an excess of colorectal cancer deaths (SMR, 1.8; 95% CI, 1.1–2.9). No excess deaths due to lymphoma or leukaemia were reported (SMR, 0.4; 95% CI, 0.01–2.0 and SMR, 0; 95% CI, 0.0–0.9, respectively).

2.2.3. Case–control studies

A hospital-based case–control study of 200 male bladder-cancer cases and 148 male controls of the same age range with urinary disorders in Kyoto, Japan, showed that 17 (8.5%) of the cases and 2 (1.4%) of the controls had worked in the silk-dyeing industry. The odds ratio for employment in the silk-dyeing industry was 6.8 (P = 0.002). At least 7 of the 17 patients with bladder cancer who had worked in the dyeing industry were kimono painters, some of whom may have ingested dyes by holding brushes or spatulas in their mouths while working. Reportedly, benzidine-based dyes were used by these kimono painters (Yoshida, 1971). [The Working Group noted that no data on potential confounding factors were provided.]

In a study in a major industrial area of Germany, Myslak et al. (1991) selected 403 malignant and benign bladder-cancer cases from three hospitals, and 426 controls with benign prostate disease from the same hospitals. All of the cases were histologically confirmed. Study participants were mailed a questionnaire to obtain information on complete occupational history and smoking habits. The questionnaires were coded for occupational categories, and study participants were classified as painters if they had been employed in this occupation for at least six months and did not have another occupation known to be associated with bladder cancer. Painters were of interest because benzidine-based dyes were manufactured on a large scale in Germany before 1950, and during that time painters usually prepared the paints themselves. Among the cases, 21 were painters and among the controls eight were painters. The overall relative risk for bladder cancer among painters was 2.8 (95% CI, 1.2–6.3).