In the 1920s and 1930s, reports from the United Kingdom showed that rubber workers had higher death rates than did the general population, and that the excess deaths were from cancers. Thousands of different materials are used in manufacturing rubber products and which if any of these might be associated with the excess deaths in the industry was not known. Continued concern for the health of rubber workers led to joint company-union occupational health research programmes within the US rubber industry at Harvard University and at the University of North Carolina. The research programmes continued through the decade of the 1970s, after which they were supplanted by jointly sponsored company-union health surveillance and health maintenance programmes based, at least in part, on findings of the research effort.
Work in the Harvard research programme focused generally on mortality in the rubber industry (Monson and Nakano 1976a, 1976b; Delzell and Monson 1981a, 1981b; Monson and Fine 1978) and on respiratory morbidity among rubber workers (Fine and Peters 1976a, 1976b, 1976c; Fine et al. 1976). An overview of the Harvard research has been published (Peters et al. 1976).
The University of North Carolina group engaged in a combination of epidemiological and environmental research. The early efforts were primarily descriptive studies of rubber workers’ mortality experience and investigations of conditions of work (McMichael, Spirtas and Kupper 1974; McMichael et al. 1975; Andjelkovich, Taulbee and Symons 1976; Gamble and Spirtas 1976; Williams et al. 1980; Van Ert et al. 1980). The major focus, however, was in analytic studies on associations between work-related exposures and disease (McMichael et al. 1976a; McMichael et al. 1976b; McMichael, Andjelkovich and Tyroler 1976; Lednar et al. 1977; Blum et al. 1979; Goldsmith, Smith and McMichael 1980; Wolf et al. 1981; Checkoway et al. 1981; Symons et al. 1982; Delzell, Andjelkovich and Tyroler 1982; Arp, Wolf and Checkoway 1983; Checkoway et al. 1984; Andjelkovich et al. 1988). Noteworthy were findings regarding associations between exposures to hydrocarbon solvent vapours and cancers (McMichael et al. 1975; McMichael et al. 1976b; Wolf et al. 1981; Arp, Wolf and Checkoway 1983; Checkoway et al. 1984) and associations between exposures to airborne particulate materials and pulmonary disability (McMichael, Andjelkovich and Tyroler 1976; Lednar et al. 1977).
At the University of North Carolina, the initial analytic studies of leukaemia among rubber workers showed excess cases among workers who had a history of working in jobs in which solvents were used (McMichael et al. 1975). Exposure to benzene, a common solvent in the rubber industry many years ago, and a recognized cause of leukaemia, was immediately suspected. More detailed analyses, however, showed that the excess leukaemias were generally lymphocytic, while exposures to benzene had commonly been associated with the myeloblastic type (Wolf et al. 1981). It was surmised that some agent other than benzene could be involved. A very painstaking review of records of solvent use and solvent sources of supply for one large company showed that use of coal-based solvents, including both benzene and xylene, had a much stronger association with lymphocytic leukaemia than did use of petroleum-based solvents (Arp, Wolf and Checkoway 1983). Coal-based solvents are generally contaminated with polynuclear aromatic hydrocarbons, including compounds which have been shown to cause lymphocytic leukaemia in experimental animals. Further analyses in this study showed an even stronger association of lymphocytic leukaemia with exposures to carbon disulphide and carbon tetrachloride than with exposures to benzene (Checkoway et al. 1984). Exposures to benzene are hazardous, and exposures to benzene in workplaces should be eliminated or minimized to the extent possible. A conclusion, however, that eliminating benzene from use in rubber processes will eliminate future excesses of leukaemia, particularly of lymphocytic leukaemia, among rubber workers may be incorrect.
Special studies at the University of North Carolina of rubber workers who had taken disability retirement showed that disabling pulmonary disease, such as emphysema, was more likely to have occurred among people with a history of work in curing, curing preparation, finishing and inspection than among workers in other jobs (Lednar et al. 1977). All of these work areas involve exposures to dusts and fumes which can be inhaled. In these studies it was found that a history of smoking generally more than doubled the risk of pulmonary disability retirement, even in the dusty jobs which themselves were associated with disability.
Epidemiological studies were under way in the European and Asian rubber industries (Fox, Lindars and Owen 1974; Fox and Collier 1976; Nutt 1976; Parkes et al. 1982; Sorahan et al. 1986; Sorahan et al. 1989; Kilpikari et al. 1982; Kilpikari 1982; Bernardinelli, Marco and Tinelli 1987; Negri et al. 1989; Norseth, Anderson and Giltvedt 1983; Szeszenia-Daborowaska et al. 1991; Solionova and Smulevich 1991; Gustavsson, Hogstedt and Holmberg 1986; Wang et al. 1984; Zhang et al. 1989) at about the same time and continued after those of Harvard and the University of North Carolina in the United States. Findings of excess cancers at various sites were commonly reported. Several studies showed an excess of lung cancer (Fox, Lindars and Owen 1974; Fox and Collier 1976; Sorahan et al. 1989; Szeszenia-Daborowaska et al. 1991; Solionova and Smulevich 1991; Gustavsson, Hogstedt and Holmberg 1986; Wang et al. 1984), associated, in some cases, with a history of work in curing. This finding was duplicated in some studies in the United States (Monson and Nakano 1976a; Monson and Fine 1978) but not in others (Delzell, Andjelkovich and Tyroler 1982; Andjelkovich et al. 1988).
The mortality experience among a cohort of workers in the German rubber industry has been reported (Weiland et al. 1996). Mortality from all causes and from all cancers was significantly elevated in the cohort. Statistically significant excesses in mortality from lung cancer and from pleural cancer were identified. The excess of mortality from leukaemia among German rubber workers barely failed to reach statistical significance.
A case-control study of lymphatic and haematopoietic cancers in eight styrene-butadiene rubber (SBR) facilities identified a strong association between leukaemia mortality and exposure to butadiene. The IARC has concluded that 1,3-butadiene is probably carcinogenic to humans (IARC 1992). A more recent epidemiological study has provided data that confirm the excess in leukaemia mortality among SBR workers exposed to butadiene (Delzell et al. 1996).
Over the years, epidemiological studies among rubber workers have led to the identification of workplace hazards and to improvements in their control. The area of occupational epidemiological research in greatest need of improvement at this time is assessment of past exposures of study subjects. Progress is being made in both research techniques and in databases in this area. Although questions regarding causal associations remain, continued epidemiological progress will surely lead to continued improvements in control of exposures in the rubber industry and, consequently, to continued improvement in the health of rubber workers.
Acknowledgement: I would like to recognize the pioneering efforts of Peter Bommarito, former president of the United Rubber Workers Union, who was primarily responsible for causing research to be done in the US rubber industry in the 1970s and 1980s on the health of rubber workers.