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7 Risk Assessment
Pages 200-236

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From page 200...
... Nevertheless, when the data base is good, the risk est imate s can be suf f ic lent ly informal ive to aid policy judgments. Some of the factors that enhance the usefulness of the data include dose-response information based on several accurately known exposure levels; knowledge of physiologic and metabolic factors that affect exposure of body tissues; an understanding of the mechanism by which the substance results in toxicity; knowledge of the extent to which experimental systems mimic the human response; and an understanding of the properties of a complex and variable substance that account for its toxicity.
From page 201...
... "Population risk estimates" describe the expected frequency or incidence of a harmful effect in a specific group of humans under defined conditions of exposure . The amount and complexity of informal ion needed increase as we progress from hazard identification to dose-response assessment to population risk estimation, although each step builds on the preceding one.
From page 202...
... A variety of mathematical models have been developed for using data at high doses, usually only available from studies in animals, to est imate risks for humans at low doses (Armitage, 1982; Cornfield et al., 1978; Crump et al., 1976; Fishbein, 1980; Food Safety Council, 1980; Krewski and Van Ryzin, 1981; Van Ryzin, 1980~. Because there are extensive data on the effects of asbestos and some other fibers in humans, the quantitative risk assessments in this chapter are based exclusively on data from epidemiological studies in humans, whereas the comparative risk assesement~ also take into consideration data from laboratory studies.
From page 203...
... - Substances vary "Arkedly both in the quantity required to produce a toxic effect and in the rapidity with which the incidence of toxic effects decreases with decreasing dose, i.e., the shape of the dose-response curve. In an experiment covering a sufficiently wide range of exposure levels, it is possible to find some levels that are toxic and some lower leve Is at which no toxic ity is observed.
From page 204...
... Accurate documentation of exposure is important for determining the dose-response curves for toxicity in animals or humans and also for estimating population risks. Errors in the estimation of exposure will lead to errors in He f ining the dose-re spouse curve and in making quantitative risk estimates for individuals or specific populations.
From page 205...
... That dose-response relationship is then applied to a hypothetical, but reasonable, exposure level to show potential population risk levels in populations of arbitrary size. In the final section, the committee assesses ricks for other types of fibers and, in same cases, for other tiaeases by qualitative comparisons with the base case of a generalized asbestos exposure.
From page 206...
... Next, there is a review of several assessments for asbestos that were based on such models. Finally, these assessments and the committee.e own analyses are applied to the information presented in earlier chapters to produce quantitative risk estimates for nonoccupational exposures to asbestos in ambient air.
From page 207...
... , the cumulative incidence function must be fitted to daea -- preferably to data based on observations in human populations. The multistage model described above has been fitted successfully to many sets of cancer data, including data on asbestos, and appears at present to be a generally adequate model for assessing cancer risk.
From page 208...
... Lung Cancer Rick from Nonoccupat tonal Environmental Exposures The following summary of risk a~sesamento for lung cancer from asbestos exposures is based on data on exposure of worker populations. These data suggest that the function I(t,&)
From page 209...
... for an added risk of 0.033: due to asbestos exposure. Mesothelioma Risk from Nonoccupational Environmental Exposures The committee reviewed two estimations of mesothelioma risk, one by Peto and his colleagues (Peso, 1982; Peto et al., 1982)
From page 210...
... Corresponding Lifetime Risk (~) b byAgeat First Exposure (yes )
From page 211...
... and the supporting data in the papers cited above suggest that mesothelioma mortality does not depend on sex or smoking history, but does depend ~ trongly on age at fire t exposure. Life time Risk Estimates for Lung Cancer and Mesothelioma Table 7-2 summarizes lifetime risk estimates for lung cancer and mesothelioma for nonoccupational environmental exposures to 0.0004 fibers/cm3 (a median level)
From page 212...
... A mix of indoor ant outdoor measured exposure levels was used to select the median value of 0.0004 fiber6/cm3 and the high value of 0.002 fibers/cm3 as the reference levels. · Use of the linear model.
From page 213...
... · Individual lifetime risk estimates for lung cancer from nonoccupational environmental exposures to 0.0004 fibers/cm3 are much lower than the risks observed for smoking. me basis for the calculations in Table 7-2 is discussed in detail in the following two subsections.
From page 214...
... (10) The increased lifetime risk attributable to asbestos exposure at ~ fiber/cm3 for ~ year is 0.0022, i.e., 0.1122 - 0.1100.
From page 215...
... 215 TABLE 7-3. Estimated Increase in Lung Cancer Risk per Unit of Exposure to Asbe s tosa Study Occupation of No.
From page 216...
... The value of P ~ 2 used in the calculations in Table 7-2 falls near the bottom of this range, but is within a factor of 5 of the top of the range. If we use P = 5, which in the middle of the range, trebling cancer risk estimates in Table 7-2 would be multiplied by a factor of 2.
From page 217...
... The lower limit is taken as 0, which is always a possible lower limit, especially if the low-dose ilnear assumption in equation (5) overestimates the individual lifetime risk.
From page 218...
... TABLE 7-5. Sensitivity of Estimates for Lifetime Riskea of Mesothe 1 ions to Values of k and c Lifetiae Risk Estimates x 106, Using k Values from Various Studies Peto et reto et This Study Schneider This Study al., 1982 This Study al., 1982 Nicholson, (low)
From page 219...
... if the actual sizes of fibers in ambient air were considered. If we assume that all fibers are equally potent, then the risk estimates would be correspondingly higher.
From page 220...
... 220 lb. of Sample Set · Samples 1.
From page 221...
... Risk Assessments for Special Subpopulations :} .., j Table 7-2 shows lifetime risk estimates for people who are exposed throughout their liver to levels of either 0.0004 or 0.002 fibere/cm3 in ambient air. The predominant risk is from mesothelioma, but lung cancers also contribute to the rink, especially for male smokers.
From page 222...
... If k were less than 3.2, the corresponding lifetime risk for mesothelioma would be less than 67 x lo~6. These calculations show that childhood exposures to asbestiform fibers might contribute noticeable lifetime mesothelioma risks to those so exposed.
From page 223...
... Unlike tile quantitative risk estimates for particular assumed exposure levels, the population risk estimates can easily change along with changing patterns of production and use. Even at a known population risk level, some individuals will receive higher than average exposures and stand at correspondingly greater individual risk, whereas the majority of the population will usually have lower risks.
From page 224...
... The following combinations of fiber type, effect, and route of exposure were considered for comparative risk assessments: chry~sot i le. /gas t rointe ~ t ina 1 c anc e r/ inge s t ion chrysot i le/mesothe lioma/inge st ion crocitolite/lung cancer/inhalation crocidoliee/mesothelioma/inhalation other asbestos/all cancers/both routes fibrous glass/lung cancer/inhalation fibrous glans/mesothelioma/inhalation attapulgite/lung cancer/inhalation attapulgite/mesothelioma/inhalation mineral wool/lung cancer/inhalation mineral wool/mesothelioma/~nhalation ceramic fiber/lung cancer/inhalation ceramic fiber/mesothelioma/inhalation carbon fiber/lung cancer/inhalation carbon fiber/me sothe 1 ioma/ inha let ion The committees results are expressed in comparison with the chrysotile/lung cancer/inhalation cell, hereafter called the prime cell.
From page 225...
... j 1 COMPARATIVE RISK ASSESSMENT SCORESHEET Cell Scored / / ~; ~_ c t Route Scores Comparative with Ce 11 Fibe r Expo sure S c ore R i ad i ~ no s i t ion Score . Produc t ion Use Pattern Geography Populat ion Trends Fiber Size Morpho logy Chemistry Penetrat ion Stability / Overall risk compared with cell above Overal 1 risk compared with prime ce 11 Qual ity of comparat ive ri sk as se s sment Remarks: Effect Route Ef fee t s Score ~.
From page 226...
... The sharp downtrend in asbestos exposures tends to ameliorate the population risks that might otherwise be asseased, whereas new fiber types may present enormously higher exposures in the future than they do at present. Fiber Size.
From page 227...
... Whatever the response to fiber size, it seems likely that long, thin fibers that have strength, durability, flexibility, and a high aspect ratio are more likely to cause adverse health effects than are fibers without these characteristics. The curIlness of chrysotile fiber bundles may increase their effective aerodynamic diameter, thus decreasing their respirability below that expected on the basis of fiber diameter alone.
From page 228...
... Discussion of Comparative Risks Table 7-7 anm-=rizes from a different perapectlve the information in Appendis H No cell of the fiber/effect/route matrix approaches the population risk levels associated with the prime cell (chrysotile/lung cancer/i~halation)
From page 229...
... Sw~ry of Coaperative ai~ Aseeseacot Coopared trith Chrycotile/Lung Cancer/~helation, Dats on the Factor Suesest that Populat ion Rick Should bc Much Lover Production Fibrous glese Minere1 wool Crocidolite Attepulgite Other ashestos Carbon f iber Ceramic f iber Use pattern Fibrous glass Other asbestos Crocidolite Cesemic fiber "tepulgite Carbon fiber Minere1 vool Cnrysotile/ingest ion Geogrephy Fibrous glass Other asbestos Crocidolite Minere1 wool At tepulgite Carbon fiber Ceremic fiber Cnrysot ile/ ingest ion Population Fibrous glass Crocidolite Carbon fiber Attapulgitc Other sabestos Ceramic fiber Minere1 vool Trend e Fibrous glase Other ashestos Crocidolite At tepu lgi te Mine re 1 woo 1 Carbon f ibe r Ceramic fiber Fiber sisc Crocidolite Minere1 vool Fibrous glese Other asbestos Attapulgite Carbon fiber Ceremic f iber Morphology Croc ido 1 ite t`1 1 others Chemistry No clear effect of cheeistry evident Penetration Crocidolite Cerbon fiber Minera1 vool Fibrous glass Other asbesto. Ceremic fiber Chrlrsotile/ingestion Atespulgiee Stabilit~r Crocidolite All otbere Fibrous g1~e Other asbesto.
From page 230...
... SUMMARY AND RECOMMENDATIONS The committee has made quantitative risk assessments for nonoccupational exposures to asbestos and qualitative (or comparative) risk assessments for a variety of asbestiform fibers.
From page 231...
... Based on this and various other assumptions, the individual lifetime risk for lung cancer was estimated to be between 3 in a million for female nonsmokers and 64 in a million for male smokers, and for me~othelioma it was approximately nine in a million, regardless of smoking habits or sex. However, other assumptions could decrease the risks essentially to zero, or could increase them.
From page 232...
... 1979. Asbestos exposure: Factors associated with excess cancer and respiratory mortalilty.
From page 233...
... 1981. Dose response models for quantal response toxicity data.
From page 234...
... 1982. Dose and time relationships for lung cancer and mesothelioma in relation to smoking and asbestos exposure.
From page 235...
... 1980. Quantitative risk assessment.


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