Man-Made Vitreous Fibres 25 years of epidemiological research on mortality and cancer incidence

An intensive study of the possible adverse health effects of fibre glass and stone/slag insulation wools has taken place over the last 25 years. This review concentrates on the epidemiological research, particularly as it bears on lung cancer mortality and incidence. It also considers how the scientific evidence has affected public and regulatory opinion. There have been two major cohort studies, one in Europe coordinated by The International Agency for Research on Cancer (IARC) and the other in USA, coordinated by the University of Pittsburgh. Each of these populations has been followed up three times, and case-control studies have been nested within the cohort studies. Several other cohort and case-control studies are also reviewed. The trigger for this research programme was that in 1972, two studies were published reporting that implantation of fibres into the pleural or peritoneal cavities of rats produced mesothelial tumours. The production industry responded by funding independent epidemiological, animal experimental, and environmental research, which has continued for the last 25 years. The cohort studies produced evidence suggestive of a possible lung cancer hazard among the insulation wool production workers. The strength of the evidence appeared to increase with continued follow-up. In 1988, IARC considered the evidence on rock and slag production workers to provide limited evidence of carcinogenicity, while the evidence on glass production workers was considered inadequate. When combined with the data on animal studies, especially the studies in which fibres were injected or implanted directly into the animals, IARC classified these insulation wools as possibly carcinogenic in humans. This was followed by increasing public concern and regulatory activity in many countries. The cohort studies have been consistent in showing a raised Standardized Mortality Ratio (SMR), more so for the stone/slag wool producers. Even so, the evidence has never been reported by any of the cohort study authors as strong enough to conclude that a causal effect of fibre exposure existed. Yet, the case-control studies of production workers have produced little evidence of an adverse risk to exposed workers. These include the case-control studies nested within the major cohort studies; the results from the two study designs need resolution. Subsequently, in 2001, IARC reviewed the latest evidence on the hazard of exposure to MMVF and classified the insulation wools produced from glass, rock and slag as Not classifiable as to their carcinogenicity to humans (Category 3). On the epidemiological evidence, the conclusion was that there is inadequate evidence in humans for the carcinogenicity of any of the MMVF. It is postulated in this review that the resolution of these differences lies in large part in the smoking histories of the production workers. The prevalence of ever-smoking tobacco was 10 to 50 per cent higher among the production workers than among the national or regional populations used to calculate the SMRs. These higher smoking rates are enough to account for the higher risks estimated from the cohort studies. This is also the conclusion of the epidemiological experts on the IARC review.