OBJECTIVES To monitor the occurrence of stomach and lung cancer in a newly defined cohort of United Kingdom rubber workers and to report findings for other cancers in an early period of follow up.
METHODS A prospective cohort of 9031 male and female workers from 42 United Kingdom rubber factories has been enumerated. All employees had a minimum of 12 months employment and were first employed at one of the participating factories in the period 1982–91. Mortality data were available for the period 1983–98 and cancer registration data for the period 1983–94. The mortality and cancer incidence experienced by the cohort were compared with expected values based on national rates defined by period, age, and sex.
RESULTS Mortality from lung cancer was close to expectation (men: observed (obs) 11, expected (exp) 12.70, standardised mortality ratio (SMR) 87, 95% confidence interval (95% CI) 43 to 155; women: obs 0, exp 1.34, SMR 0, 95% CI 0 to 275). Mortality from stomach cancer was unexceptional (men: obs 1, exp 2.69, SMR 37, 95% CI 1 to 207; women: obs 0, exp 0.24, SMR 0, 95% CI 0 to 1537). Many statistical comparisons were made both for mortality data and for cancer registration data; only one difference between observed and expected numbers was significant (mortality from cancer of the testis: obs 3 exp 0.51, SMR 589, 95% CI 122 to 1722). Corresponding findings for incident cancers of the testis were unexceptional (obs 5, exp 5.13, standardised registration ratio (SRR) 97, 95% CI 32 to 227).
CONCLUSION The findings should be treated with caution as they relate to an early period of follow up. Nevertheless, they hold out the prospect that the increased SMRs for stomach and lung cancers reported for historical cohorts of United Kingdom rubber workers may not be apparent in more recent cohorts.
- rubber workers
- lung cancer
- stomach cancer
- cohort study
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The evidence of a carcinogenic risk in the rubber industry was last reviewed in detail by the International Agency for Research on Cancer (IARC) in 1982.1 More recently published epidemiological studies on the rubber industry have been reviewed by Kogevinas et al,2 and their likely relevance to the current United Kingdom industry has been commented upon.3 4 Rubber industry work was highlighted as a possible cause of prostate cancer in a recent literature review of occupation and risks of prostate cancer.5
The present study was initiated for two main reasons; firstly, to determine whether or not the improvements in working conditions introduced in the United Kingdom rubber industry in recent decades have been accompanied by the elimination of concerns relating to occupational lung and stomach cancers in the United Kingdom rubber industry,6 7 and secondly, to provide an early warning of any new serious health problems.
Materials and methods
A total of 42 rubber factories in the United Kingdom, all members of the British Rubber Manufacturers' Association (BRMA), supplied identifying particulars, work histories, and information on smoking for those 9031 male and female workers first employed in the factory environment of the participating factories during the 10 year period 1982–91. All study subjects were employed for a minimum period of 12 months. Each period of working was classified by one of 27 product sectors—for example, new tyres, retread, footwear—and one of 55 job categories—for example, weighing of compounding ingredients, extruding, painting of finished goods—the 55 job categories relate to 11 broader job types. Information on cigarette smoking at the time of first employment was available for 4821 study subjects, but only in terms of current smoker (n=2108), ex-smoker (n=372), and lifelong non-smoker (n=2341).
The Office for National Statistics (ONS) provided information on the vital status of each study subject up to the current closing date of the survey (31 December 1998): 8329 (96.3%) subjects were alive, 27 (0.3%) subjects had emigrated, 136 (1.6%) were dead, and 159 (1.8%) subjects were untraced. Flagging is not available in Northern Ireland, and 380 workers from Northern Ireland are not included in the present analysis of 8651 study subjects. The underlying cause of death was coded according to the 9th revision of the international classification of diseases (ICD-9). The ONS also supplied information on cancer registrations (incident cancers) for the period 1971–94.
Expected numbers of deaths were calculated by applying sex, age, and period specific mortalities for England and Wales to corresponding person-years at risk (pyr). Each study subject contributed pyr from the end of the first 12 months of employment to the closing date of the study, death, emigration, or date last known alive, whichever was the earliest. Standardised mortality ratios (SMRs) were calculated as the ratio of observed deaths to expected deaths, expressed as a percentage. These procedures were carried out with the PERSONYEARS programme. Standardised registration ratios were calculated in the same way for the cancer morbidity data, except that the closing date of the study was 31 December 1994. The mean age at the start of employment was 28.5 years and the mean period of follow up (mortality analysis) was 11.9 years.
There was evidence of a strong healthy worker effect in the data with mortality from all causes (men and women combined) being particularly low in the period after first employment (1–4 y: SMR 43, obs 23; 5–9 y: SMR 74, obs 63; 10–19 y: SMR 75, obs 50). Observed and expected numbers of deaths and cancer registrations are shown in the table for several different cancers, together with observed and expected numbers of deaths for the larger non-cancer groupings. Marked deficits in mortality are shown in male workers both for diseases of the circulatory system and all causes. The only significant excess is shown for mortality from cancer of the testis (obs 3, exp 0.51, SMR 589, 95% confidence interval (95% CI) 122 to 1722), although findings for incident cancers of the testis were unexceptional (obs 5, exp 5.13, standardised registration ratio (SRR) 97 (95% CI 32 to 227).
There was a significant gradient (p<0.05) of all causes mortality relative to cigarette smoking (non-smokers: SMR 44, obs 20; ex-smokers: SMR 63, obs 7; current smokers: SMR 93, obs 52; men and women combined, not shown in table). This gradient was most marked for diseases of the circulatory system (non-smokers: SMR 8, obs 1; ex-smokers: SMR 26, obs 1; current smokers: SMR 97, obs 18).
Although at a relatively early stage of follow up, the new study of recent entrants to the United Kingdom industry has failed to find a significant excess of cancer morbidity for any type of cancer and only found a significant excess mortality for a single cause of death, cancer of the testis. The absence of any excess, significant or otherwise, in the morbidity findings for cancer of the testis argues strongly against working in the rubber industry being involved. (Interestingly, there was a significant deficit for mortality from testicular cancer in the earlier BRMA study.6) It is clear that having data on cancer incidence as well as cancer mortality makes confident interpretation easier. The unexceptional findings for cancers of the stomach and lung will be welcome news for United Kingdom rubber workers, albeit they are associated with wide 95% CIs; the position needs to be monitored over the next 5–10 years. In the course of time, there will be a sufficient number of events in this new cohort study to make use of the detailed work history information and the information on smoking in a single analysis.
We thank the member companies of the BRMA for supporting this initiative and for abstracting data onto the study proformas. We thank Jackie Rose of the BRMA for computerisation and double checking of data. We thank the Office for National Statistics for supplying follow up information.