Based on a review of the scientific literature and close scrutiny of mesothelioma cases, there is broad consensus that mesothelioma is most likely to result from crocidolite exposure; it has a strong association with amosite exposure; and, a very weak, if any, association with chrysotile exposure.
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1.8 - IS THERE A THRESHOLD LEVEL FOR CHRYSOTILE
ASBESTOS?
There is ample human and animal evidence that at high and prolonged levels of exposure, chrysotile asbestos can cause disease. The question that remains to be answered is whether or not there exists a threshold level of exposure below which there is no risk of disease.
In the case of asbestosis, there is broad consensus that a threshold level of exposure exists for chrysotile. Indeed, the Ontario Royal Commission on Asbestos (ORCA) reached the following conclusion "In our judgement, asbestosis will not occur in workers exposed to the regulated levels of occupational exposure now in force in Ontario (e.g. 1.0 f/cc on an 8-hour time-weighted average)".
In the case of lung cancer, such broad consensus has not yet been reached, although there is now mounting human evidence pointing to the existence of a threshold. For example, a small number of epidemiological studies of workers exposed only to low levels of chrysotile have concluded that there was no statistically significant excess mortality resulting from chrysotile asbestos exposure (see Table 2).
Because of the small number of studies involving exposure to chrysotile only, it is difficult to draw firm conclusions. However, in reviewing this and other data, a group of experts convened by the World Health Organization in April 1989, reached the conclusion that a level of control for chrysotile asbestos can be achieved, at which the lifetime risks of lung cancer and mesothelioma are very small. Subsequently, the Oxford meeting recommended an exposure limit of 1.0 f/cc or below for chrysotile. It was also recommended that the use of crocidolite and amosite be banned as soon as possible.
A major study by McDonald et al. gives further, but again not conclusive, evidence of the possible existence of a threshold for chrysotile asbestos. Based on an updated study of 11,000 workers, the authors found that "In each of the six classes of exposure up to 300 mpcf x years, the lung cancer SMR (Standard Mortality Ratio = observed mortality / expected mortality) was close to 1.3 (a total of 254 cases of lung cancer among 4,384 men, against 190.6 expected); there was no evidence of a trend". 300 mpcf x years is equivalent to about 1,000 fibre years or 50f/cc for 20 years of exposure. According to McDonald, "The significance of this study is that any deaths from asbestosis or lung cancer arising from current occupational exposure levels are most unlikely."
Therefore, despite mounting evidence, there is not as yet broad scientific consensus of a threshold level of exposure for chrysotile asbestos. What is clear is that, at present levels of occupational exposure, if there are risks, they are exceedingly small.
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The risks to workers at recommended exposure levels of 1.0 f/cc have been estimated by a number of scientific bodies. For example, the ORCA Commission estimated that there would be a 1 in 4 chance of 1 premature death amongst the 1200 workers engaged in chrysotile asbestos product manufacturing (excluding textiles) - a risk level equivalent to that faced by workers in general manufacturing in Ontario.
Similar conclusions were reached by a group of experts convened by the WHO at Oxford, England, in April 1989. In short, WHO predicted lifetime risks under a 1.0 f/cc chrysotile standard for non-smokers for both mesothelioma agnd lung cancer would be approximately 2 in 40,000 or 0.05 in 1,000.
The risk estimates contained in both the ORCA and WHO Oxford reports were based on a linear dose-response curve. However, a recent paper by Liddell concludes that at low levels of exposure for chrysotile asbestos, the dose-response curve is more likely to be sub-linear thus rendering risk estimates based on a linear dose-response curve to be seriously overstated.
The risk to workers exposed to chrysotile asbestos at present occupational exposure levels is therefore extremely low, if indeed one exists at all. Vry few industrial sectors can offer such a safe working environment (see Table 3). This underscores the point that a well controlled chrysotile asbestos product industry can be a model for most other industrial sectors.
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In recent years, public attention has shifted from workplace to general environment and population issues and concerns. This was driven by the simplistic view that 1 fibre can kill. And since there is no evidence of a threshold, the general public was believed to be at risk. This has led to pressure on regulatory authorities to ban all types and all applications of asbestos. The surrounding media attention also spurred dynamic growth in the asbestos removal industry, particularly in the United States and some West European countries.
Common sense is slowly taking control of events. This has been due to a number of realizations. First, since asbestos is ubiquitous in the earth's crust, there is little man can do to control sources of exposure from naturally occurring sources. Secondly, studies have shown that airborne concentrations inside buildings containing asbestos products do not differ significantly from levels in air outside buildings. Moreover, these levels are very low (e.g. less than 0.001 f/cc). Therefore,if there are risks to the general population, they are very low (lifetime risks less than 1 in 100,000) and much smaller than the risks we face in everyday life (see Table 4). Thirdly, there is no epidemiological evidence demonstrating a general population risk from environmental asbestos. Indeed, quite the contrary. Several epidemiological studies show no evidence of higher asbestos-related disease amongst the general population of Québec chrysotile asbestos mining communities compared to other North American cities, despite exposure levels 200 - 500 times higher!
Environmental risks of asbestos have been studied by a number of major scientific bodies. For example, at the WHO Conference on Mineral Fibres in the Non-occupational Environment in Lyon in September 1987, it was concluded by a Group of Experts that for the general population, the risks of mesothelioma and lung cancer, attributable to asbestos, are probably undetectably low. The risk of asbestosis is virtually zero.
A meeting on the Environmental Reduction of Asbestos, convened by the WHO in Rome in 1988, concluded that high density products, such as asbestos-cement and friction materials, do not present unacceptable risks to the general population, although care is needed to contain airborne dust during installation and repair. The meeting also recommended that the use of friable insulation materials containing asbestos should be strongly discouraged on a worldwide basis.
Regarding asbestos-cement pipes in potable water distribution systems, the WHO has concluded that the concentrations of asbestos in drinking water resulting from the use of asbestos-cement pipes do not present a hazard to human health. Based on its ongoing evaluation of scientific findings in the field of drinking water quality, the WHO stated in the 1993 edition of its annual Guidelines for Drinking Water Quality, that "asbestos is a substance not of health significance at concentrations normally found in drinking water". The WHO found "no convincing evidence of the carcinogenicity of ingested asbestos in epidemiological studies of populations with drinking water supplies containing high concentrations of asbestos". An extensive review of animal studies which support the epidemiological data, led the WHO to conclude that "there was no need to establish a health-based guideline value for asbestos in drinking water".
The WHO conclusions are in line with a long list of agencies and scientific committees which have concluded that ingested asbestos is not a health hazard. In a 1991 news release, the U.S. EPA noted that "asbestos is not classified as carcinogen in the regulations because EPA has determined it is a carcinogen only when inhaled, not ingested". In 1989, the Canadian Government concluded in its Drinking Water Quality Guidelines that "there is no consistent, convincing evidence that ingested asbestos is hazardous. There is, therefore, no need to establish a maximum acceptable concentration for asbestos in drinking water".
These and other pronouncements by national and international water control agencies should remove any remaining doubts about asbestos in drinking water posing a health hazard.
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