Asbestos Exposure and the Carcinogenic Threshold

One question that has puzzled many researches is whether or not there is a definitive threshold regarding levels of asbestos exposure and disease development.  One interesting study that explores this issue is called, "Asbestos - the TLV approach" - Ann N Y Acad Sci. 1976;271:152-69. by Nicholson WJ.  Here is an excerpt: "Abstract - A review of the control of carcinogenic exposures using the TLV approach presents a prospect of limited effectiveness. With asbestos, as with any carcinogen, no threshold is known below which no health effect may be manifest. At best, we have only limited dose-response information at levels much above those of practical concern. In the case of asbestos, current exposures can only be described crudely at any level of exposure, and health effects are only known for past high, but ill-defined, exposures. Limited information exists on the effects of synergistic interactions with other materials.

The current U.S. TLV, based on data concerned with occurrence of asbestosis, has not been evaluated with regard to possible effectiveness in the prevention of asbestos cancer. Yet cancer is the heart of the asbestos-hazard problem. Finally, enforcement of the existing TLV, especially for asbestos has been limited, frequently absent, and often ineffective. Workers are exposed in many situations to levels much above the current standard. As discouraging as this picture may seem, a TLV can be useful for stimulating the development and application of engineering-control procedures. The application of these procedures, however, must be specified and mandated in future standards to lower worker exposures to the minimum commensurate with existing technology. As technology is developed that makes lower exposure levels possible in a large part of the industry, TLVs should be reduced to take advantage of that technology."

Another study is called, "Asbestosis: a study of dose-response relationships in an asbestos textile factory" by G. Berry, J. C. Gilson, S. Holmes, H. C. Lewinsohn, S. A. Roach - British Journal of Industrial Medicine 1979;36:98-112.  Here is an excerpt: "Abstract - A group of 379 men who had worked at an asbestos textile factory for at least 10 years has been followed up. The prevalence of crepitations, 'possible asbestosis', certified asbestosis, small opacities in the chest radiograph and values of lung function have been related to dust levels. The type of asbestos processed was predominantly chrysotile although a substantial amount of crocidolite had also been used in the past. There was a higher prevalence of crepitations than had been observed previously at the same factory. The presence of crepitations is not a specific effect of asbestos exposure and 'possible asbestosis', a combined judgement of two physicians on whether a man had developed signs which might be attributable to early asbestosis, was preferred. Fifty per cent of men with a diagnosis of possible asbestosis were certified as suffering from asbestosis by the pneumoconiosis Medical Panel within 3-5 yr.

The most reliable data relate to men first employed after 1950; 6•6% of men in this group had possible asbestosis after an average length of follow-up of 16 yr and an average exposure to 5 fibre/cm3 where the dust levels were determined by static area samplers. The forced expiratory volume and forced vital capacity declined significantly with exposure, after allowing for age and height, but there was no decline in the total lung capacity. The transfer factor also declined with exposure, but not to a statistically significant extent. The non-smokers and light smokers as a group had less crepitations, asbestosis and small opacities on the chest radiograph than heavier smokers with similar exposure. Combining dust concentrations to form the cumulative dose may not be completely satisfactory, and a family of measures was investigated which allows for elimination of dust from the lungs and includes the cumulative dose as a special case. Because the rate of elimination of dust from the lungs is unknown and cannot be estimated from the data, this approach leads to a wide range of possible interpretations of the data; for example the concentration such that possible asbestosis occurs in no more than 1% of men after 40 years' exposure could be as high as 1•1 fibres/cm3 or may have to be as low as 0•3 fibres/cm3. This range is wide because the data relate to higher dust levels, and a shorter period of follow-up. Until data are available on groups exposed to lower levels it will not be possible to assess the effects of the current standard with any certainty. However, the results of this study show that it is important to continue to reduce dust levels to values as low as possible."

If you found any of these excerpts, please read them in their entirety.  We all owe a debt of gratitude to these researchers.