Drinking Water and Health, Volume 5 (1983) / Chapter Skim
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Asbesto
Asbesto
Pages 123-148
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From page 123... ...
An excess of gastrointestinal tract cancers has also been.found in these occupationally exposed groups. The general population may be exposed to asbestos fibers in "air, be`,erages, drinking water, food and pha~..aceutical and dental preparations and by consumer use of asbestos containing products" (International
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From page 124... ...
caution should be exercised when drawing conclusions from the table.) The asbestos in these supplies was derived from a variety of sources: minTABLE III-1 Asbestos Concentrations in Drinking Water from 365 Cities in 43 States, Puerto Rico, and the District of Columbia, as Measured by Transmission Electron Microscopya Asbestos Concentration, Number of Percentage of 106 fibers/liter Cities Samples Below detectable limits 110 30.1 Not statistically significant 90 24.7 Less than 1 90 24.7 From 1 to 10 34 9.3 Greater than 10 41 11.2 TOTAL 365 100.0 .
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From page 125... ...
The size distribution of asbestos fibers in water varies by source (Millette et al., 1980~. The smallest fibers are found in water contaminated by the natural erosion of serpentine rock.
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From page 126... ...
The committee concurs with these reviews. Assuming that the exposed and unexposed workers have the same general risk factors for GI cancers and that their observed GI cancer rates are re and ru, respectively, then the GI cancer burden from the exposure can be expressed either as a ratio of rates, or relative risk, i.e., RR = rears, or a difference of rates, i.e., DR = re—rU.
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From page 127... ...
Table III-3 shows the estimates derived either from the published studies or from personal communication with the authors. CONVERTING RISE TO ASBESTOS WORKERS TO RISK FROM SWALLOWED ASBE STO S In converting the observed risk of GI cancers in asbestos workers to risk of Gl cancer from ingested asbestos fibers, a number of steps should be clearly distinguished.
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From page 128... ...
1979 21.5 1.49 1.49 Seidman et al., 1979; U.S. EPA, 1979 Newhouse and Berry, 1979 Henderson and Enterline, 1979 34.0 1.18 1.32 39.9 1.38 1.55 150 = esophagus 151 = stomach 152 = small intestine 153 = large intestine 154 = rectum 155 = liver 156 = gallbladder 157 = pancreas 158 = retropentoneum and peritoneum IS9 = gastrointestinal ban, not otherwise specified Standardized mortality ratio = 100 X RR (relative risk)
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From page 129... ...
, and expressed as numbers of LM fibers/ml air times Y It is not selfendent that equal doses measured in this way must have equal effects (e.g., cigarette smoking measured as pack-years does not have a fixed effect on lung cancer incidence but is greater at a low intensity for a long time)
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From page 130... ...
This overestimation will generally be small, and as long as we adopt the same convention when calculating the risk from asbestos in the ~ ater supply, the error will have a very small effect on the computation of risk. Step 2: Conversion of Dose of Asbestos Inhaled to Dose of Asbestos Swallowed Since the excess GI cancers in the workers are assumed to be caused by the asbestos fibers that these workers swallowed rather than simply inhaled, the dose calculated in Step 1 must be converted to fibers swallowed.
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From page 131... ...
, which applies to measurements made by the LM, may thus be expressed as follows for doses swallowed, as measured by the TEM: RR = 1 + [a/~588 X 106 X 50~] X dose (in TEM fibers swallowed)
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From page 132... ...
makes direct estimates of relative risks observable in epidemiological studies correlating exposure to asbestos in drinking water and GI cancer mortality rates. It is also reasonable to assume that the relative risks derived from this equation will be approximately correct if applied to studies of cancer incidence.
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From page 133... ...
CONVERTING RR S TO LIFETIME GI CANCER RISKS As discussed above, expressing the risk from any agent in terms of relative risks implies that the effect of the agent is to multiply whatever "normal" or background cancer rate exists. Table III-5 shows the results of calculations based on equation (6)
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From page 134... ...
and (7) e Duluth Electron microscope studies revealed that during 1973 the Duluth, Minnesota, water supply contained from 1 X 106 to 30 X 106 TEM fibers/liter, depending on weather conditions on Lake Superior (Cook et al., 1974~.
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From page 135... ...
since it assumes that the level of asbestos contamination reached the 1973 level immediately after dumping began, which is not likely. Predicted relative risks for the findings of Masson et al.
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From page 136... ...
is the maximum duration of exposure and 0.7 X 106 TEM fibers/liter is the maximum observed asbestos contamination of the water supplies. As for Duluth, such relative risks are far too small to be sensibly detected by any epidemiological study.
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From page 137... ...
The authors attributed this to "the carcinogenic potential of ingested asbestos fibers which migrate to the lungs...." It is, however, at least as likely that this observed lung cancer effect merely demonstrates that the adjustments for other risk-modifying factors are not adequate. This possibility is strengthened by their observation that cancer of the endometrium has a negative association with the asbestos content of the water supply.
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From page 138... ...
predicts a relative risk of 1.41, which is in close agreement with the observed proportional incidence ratios. TABLE III-7 Odds Ratios for Gastrointestinal Cancer incidence (1974-1977)
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From page 139... ...
A man breathing 1 LM fibers/ml air at work is considered by the EPA to swallow an average of 200 X 8 X 106 X s/7 = 1,142.9 X 106 TEM fibers/day, where 1 LM fiber = 200 TEM fibers, 8 = m3 of air breathed per day at work, 106 = ml in m3, and s/7 = proportion of working days in a week. (The committee used a conversion factor of SO for LM to TEM, a factor of 49/s2 to represent working weeks in a year, and 0.3 as the proportion of inhaled asbestos fibers that are subsequently swallowed.
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From page 140... ...
The EPA then equates X with an additional lifetime cancer risk of X The required TEM fibers/liter for an additional lifetime cancer risk of 1 in 100,000 is then obtained by simple proportion, i.e.: X/(calculated TEM fibers/liter over 70 years)
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From page 141... ...
suggests that the relative risk of GI cancers from asbestos exposure of humans may be written as: RR = 1 + 0.05 X h, where h X 10~2 is the number of TEM fibers swallowed. If we assume that the daily dose to an animal over a lifetime had an effect equivalent to that in humans exposed for 70 years, then the "Maximum Daily Dose" column of Table III-8 may be multiplied by 1.28 X 10-9 = 0.05 X 70 X 36S.25 X 10-~2 to give the expected excess relative risk (RR—1)
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From page 142... ...
142 ~n ~_ ~: ._ s" x oo ._ o C~ ~D £ o~ o _, ~: C)
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From page 143... ...
Peritoneal mesotheliomas are associated with occupational exposure to asbestos, and indeed asbestos may be virtually the sole cause of this tumor. If peritoneal mesotheliomas are caused by asbestos fibers migrating from the gastrointestinal tract, which must be considered a definite possibility, then risk of such tumors needs to be included in evaluating the total cancer risk from swallowed asbestos.
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From page 144... ...
Adequate animal feeding studies conducted to date have failed to confirm a cancer risk from ingested asbestos. Further work aimed at understanding the inconsistent results of GI cancer excess in occupationally exposed groups is clearly warranted.
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From page 145... ...
1981. Community Health Associated with Arsenic in Drinking Water in Millard County, Utah.
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From page 146... ...
1978. An investigation of the use of asbestos cement pipe for public water supply and the incidence of gastrointestinal cancer in Conneetieut.
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From page 147... ...
1982. Cancer incidence in relation to asbestos in drinking water in the Puget Sound region.
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