Workplace Exposure to Asbestos:
Review and Recommendations
DOSE-RESPONSE RELATIONSHIPS
Memorandum on Asbestos Update and Recommended
Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to
Asbestos in Animals
IV. Biologic Effects of Exposure to
Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals
other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical
Sources of Asbestos
VIII. Dose-Response Relationships
References
Evidence available to date indicates that a large dose of asbestos will
produce a bigger health hazard than a small dose. Seidman et al. (1979),
using the length of time worked in an amosite asbestos factory as a measure
of the dosage of asbestos, reported an increased risk of dying from lung
cancer with increasing duration of employment. Henderson and Enterline
(1979), using cumulative dust exposure as an estimate of dose, reported
that the dose-response relationship for lung cancer is more likely linear.
They predicted the relationship to be SMR=100+0.658 times cumulative dust
exposure (mppcf years). Liddell et al. (1977) also reported a similar
relationship, i.e., a tendency for the mortality for lung cancer to increase
with the dose.
Berry et al. (1979) reported that the occurrence of crepitaions, possible
asbestosis, and certified asbestosis was related to the cumulative dose.
Newhouse and Berry (1973) suggested that the risk of dying from mesothelioma
increases with increasing dose. Jones et al. (1979) reported a linear
relationship between the mesothelioma rate and length of exposure. In
a study of the women workers in a wartime gas mask factory, they found
that women having a long employment period had a higher proportion of
death due to mesothelioma than those who had a short period of employment.
Although there appears to be little dispute that a larger dose of asbestos
will pose a greater health risk, the exact nature of the dose-response
relationships may be subject to considerable debate. This is so primarily
because of problems of exposure estimation. Methods of measuring dust
levels have changed over time with respect to sampling instrument (thermal
precipitation vs. midget impinger vs. membrane filter), location of sampling
(personal vs. area), and dust counting (particles vs. actual fibers) and/or
evaluation techniques (whole fields vs. eyepiece graticule). As a result,
conversion of dust levels obtained by one method to levels comparable
to another method is far from simple, and is subject to considerable error.
Another factor which may lead to differences of opinion on the exact shape
of the dose-response curve is the measure of the dose. The commonly used
measures of exposure are the cumulative dose and the duration of employment.
Since using cumulative dose as a measure of exposure gives equal weight
to the concentrations of dust experienced in each year of exposure, exposure
of many years ago is considered as important as recent exposure. This
practice is unrealistic for the chronic diseases having a long latency
period. Duration of employment has also been used as a measure of exposure
under the assumption that increasing the work time approximates increasing
the dose. This procedure has the same problem as using the cumulative
dose. Furthermore, in the absence of reliable past exposure data, the
duration of employment may not equal the total dose of asbestos.
With regard to the linear hypothesis, the British Advisory Committee
on Asbestos stated the following in 1979:
Our reasons for preferring a linear hypothesis are:
(1) It fits the data for occupational exposures
(2) it is the simplest hypothesis and the one most readily used
for extrapolation to the probable effects of low doses
(3) it is likely to lead to an overestimate rather than underestimate
of risks at very low doses.
(Final Report, Vol.2, p.14).
Data available to date provide no evidence for the existence of a threshold
level. Virtually all levels of asbestos exposure studied to date demonstrated
an excess of asbestos-related disease.
Memorandum on Asbestos Update and Recommended
Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to
Asbestos in Animals
IV. Biologic Effects of Exposure to
Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals
other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical
Sources of Asbestos
VIII. Dose-Response Relationships
References
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