Asbestos Exposure Risk Assesment
- Asbestos Fibers
What is Mesothelioma?
"Mesothelioma (malignant)"
Mesothelioma is a rare form of cancer. It affects the linings of the cavities around the lungs, stomach, and heart. It is caused by inhaling asbestos fibers, but the cancer usually does not appear until 10 to 40 years after a person first inhales asbestos.
Uncertainties in Risk Assessment
Errors in estimates of past asbestos exposure The current mortality from asbestos-related cancer is the consequence of exposures of 20- 50 years ago, or even longer. There is no doubt that the exposure levels in the distant past were considerably higher than those of today. As an example, Table 5 shows the concentrations measured in mines and towns of Canada in the period 1973 to 1995 (63).
Results in Parentheses Ð personal communication In the distant past, the techniques of exposure measurement did not specify asbestos fibres, but referred to either gravimetric concentration to particles in the air, expressed in grams or mg per m3 or, later, to count concentrations of particles (not fibres) expressed in million particles per cubic foot of air (mppcf). Thus, the early method measured all particles, of which fibres constituted only a minor fraction.
As exposure levels in the past must be taken into account in the quantitative risk assessment, various authors estimates assumed specific concentrations of airborne asbestos fibres converting the measured gravimetric or count concentrations of total particles to the currently defined fibres using a number of mathematical conversions.
These conversions relied on many dubious assumptions and approximations, and included errors of several orders of magnitude into the mathematical estimates of historical airborne fibre concentrations.
This is one of the main reasons why I cautioned Ð quite early Ð that the quantitative risk assessment equations and particularly low dose extrapolations used for predicting mortality or morbidity in populations exposed to considerably lower exposure levels were very uncertain (56,57).
Table 6 shows some errors in the conversion of such concentrations. The first part of the table shows the relationships between asbestos fibre diameter and length and the concentration expressed in fibres per ml for the gravimetric concentration of 10 ng/ml air [based on calculations by Pott (22)].
The table shows that the same air with weight concentration of 10 ng/ml may contain 32 f/ml if the fibre diameter is 2.0 mm and the length 40 mm, while it may contain 8,200,000 f/ml of fibres with the diameter of 0.03 mm and the length 0.63 mm.
The errors involved in the conversion of weight concentrations of total particles of unknown size distribution into the count concentrations of fibres of a defined size fraction are so great that the obtained results may be complete nonsense. Table 6 also shows an example of EPA's conversion in 1986 (36).
EPA took 30 (the geometric mean of conversion factors ranging 0.5-150 obtained in six studies) as the conversion factor to be used, introducing a possible error of more than 200 in the conversion.
Robock reported in 1984 (64) that the conversion factor for converting mppcf into f/ml obtained in a large number of samples was between 0.5 and 47.8, which introduces a hundredfold error into conversions.
Uncertainties and unspecifities of models Table 7 shows the estimation of lifetime risk due to lethality from mesothelioma (L: excess deaths per million population) induced by the asbestos concentration of 0.0004 f/ml for an age of 73 years, calculated by the well known equation of the National Research Council of the US National Academy of Sciences (NRC/NAS) L=C (conc.) (age) K (65).
Using the values of the coefficients C (0.85 Ð 7.22 x 10 8 ) and K (2.6-5.0), obtained in epidemiological investigations, the number of calculated excess deaths ranges from 0.2-60,000 per million population, yielding a ratio of up to 300,000 in estimated mortality per million population and rendering the risk assessment meaningless (56).
In 1991, I criticized (56) those EPA's uncertainties in risk assessments which led to their proposal of the asbestos ban. Table 8 shows the number of cancer cases expected by EPA to be avoided in 13 years following the proposed asbestos ban, as set forth by three consecutive EPA proposals. The very fact that the number of cancers varied from 1,000 in 1986 (36) to 315.8 in 1988 (66), ending with 148-202 in the Final Rule of 1989 (1), sheds strong doubt on EPA's risk estimates.