Arsenic in Drinking Water 2001 Update (2001) / Chapter Skim
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5 Quantitative Assessment of Risks Using Modeling Approaches
5 Quantitative Assessment of Risks Using Modeling Approaches
Pages 169-213
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of zero for arsenic in drinking water and a maximum contaminant level (MCL) for arsenic of 10 fig in drinking water (EPA 2001~.
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and, therefore, addresses the "Iong-term, chronic effects of exposure to Tow concentrations of inorganic arsenic in drinking water." With respect to Tong-term effects, EPA concludes that "arsenic is a multisite human carcinogen by the drinking water route," and on the basis of epidemiological studies of Asian, Mexican, and South American populations, those "with exposures to arsenic in drinking water generally at or above several hundred micrograms per liter are reported to have increased risks of skin, bladder, and lung cancer." EPA also notes that increased risk of liver and kidney cancer have been associated with arsenic exposure and that skin cancer has been associated with inorganic arsenic contamination in Argentina (reviewed by Neubauer 1947, as cited in EPA 2000a) , in Poland (EPA 2000a)
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of arsenic in drinking water in which there was a statistically significant increase in prostate-cancer mortality, but no increase in bladder or lung cancer mortality. EPA also discussed a study by Kurttio et al.
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still considered the southwestern Taiwan data to be the critical data set for conducting a quantitative risk assessment for exposure to arsenic in drinking water. Dose-Response Modeling Model Choice and Selection of a Comparison Group In its proposed arsenic rule, EPA concluded, on the basis of the NEC (1999)
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is in part a policy decision. For the proposed rule, EPA used the bladder cancer risk estimates presented in the NRC (1999)
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Because EPA did not present theoretical lifetime excess bladder or lung cancer risk estimates, the subcommittee used linear extrapolation from the EDo~s presented in Morales et al.
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(2000) a Arsenic Bladder Cancer Lung Cancer Concentration (,ug/L)
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To account for the intake of arsenic from food, EPA multiplied the Tower-bound risk estimates by the fraction of arsenic consumed per kilogram contributed by drinking water (calculated by dividing the arsenic ingested from drinking water (pa/kg/day) by the total arsenic consumed from drinking water, cooking water, and food)
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Therefore, when calculating the bladder cancer cases avoided at a given MCL, EPA adjusted the upper bound by a factor of 1.25 to reflect the mortality for bladder cancer. With respect to lung cancer, EPA concluded that "because lung cancer "mortality]
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. Because exposures to other forms of arsenic can produce health effects, the SAB recommended that future risk assessments provide quantitative information on how the intake of inorganic arsenic is related to the concentration of arsenic metabolites in the urine and to bladder cancer.
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The latter is particularly relevant if the latency period for cancer development from low arsenic exposure is long or if the appropriate dose metric involves a less-than-lifetime exposure as discussed in Chapter 4. Although the SAB recognized that children differ from adults in many ways that could make them more susceptible to toxic chemicals, "the majority of the tSAB]
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Issues to be addressed include choice of end point (bladder and lung cancer) , and the use of the southwestern Taiwanese study as the basis for EPA's risk assessment.
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1998) , the southwestern Taiwanese studies documenting bladder and lung cancer (Chen et al.
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Quantitative Risk Estimates The usefulness of the Chiou et al. study to quantitative risk analysis are limited by its follow-up period compared with that of the studies from southwestern Taiwan (Chen et al.
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This property is particularly advantageous for computing EDs in settings where raw data are unavailable but where estimates of relative risks and their associated confidence intervals (CIs) have been published (e.g., lung cancer odds ratios reported by Ferreccio et al.
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184 cd o · _ U' U ¢ a' To ,~ ._ n On .O ._ a Ct Cal Ct v CC o a an TO OF o a of Ct a ^ ~ V ~ onto ~' cq Ct an a .= a' ._ a an ~ ~ ~ ~ ~ + + ~ + ~ ~ ~^ ,^ T~^ ~ 0^ ~ ~ ~ ~ ~ ~ ~ ~ ='—~ 0——= _~—= =—==—— _ ~ ~ oo _ ~ _ — ~ t— —~ ~} tS, ~ oo c~]
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(~3) The lifetime cancer mortality risk for an individual exposed to an arsenic concentration of a, ,ug/L (Rd)
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for lung cancer were 0.076 and 0.046 for mates and females, respectively. The corresponding risks for bladder cancer were 0.007 and 0.003.
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For simplicity, it is assumed here that the relative risks are the same for cancer mortality and incidence. That assumption is reasonably accurate in the case of lung cancer, which has a very high case mortality.
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Results are also included for the southwestern Taiwanese data previously reported by Chen et al.
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Use of an unexposed external comparison population also minimizes the impact of exposure misclassification in the Tow-dose range within the study population. A potential disadvantage, however, of using an external comparison group is that the analysis can be biased if the study population differs from the comparison population in important ways.
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In general, estimated EDo~s tended to be lower for models that included an external comparison population, primarily because the lung and bladder cancer rates in the comparison populations were much Tower than the rates seen even in the study villages with Tow exposures. Consequently, the fitted dose-response models tended to be steeper (Morales et al.
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Furthermore, the fact remains that there is some empirical evidence suggesting that a supralinear model might indeed hold. For example, statistical goodness-of-f~t criteria applied to both the southwestern Taiwanese data (Chen et al.
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equivalent concentration (pg/L) Figure 5-1 Estimated lifetime death risk over background rates in Taiwan for male bladder cancer (A)
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Only the results for mate lung cancer are shown in Table 54. Similar patterns emerged for female lung cancer, and male and female bladder cancer.
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t) between age and log dose a Values are based on male lung cancer data from southwestern Taiwan (Chen et al.
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EPA adjusted its Tower-bound risk estimates to account for the extra arsenic, adding ~ ~ of water to drinking-water consumption to account for water used in cooking and multiplying the risk estimates by the fraction of arsenic contributed by drinking water. The subcommittee believes that the method used by EPA to account for the arsenic present in cooking water is valid and easily accomplished.
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than a typical individual in Taiwan. It should be noted, however, that the assumption of major differences in water consumption between the Taiwanese study population and the U.S.
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~ 996~. Table 5-6 shows the results of this analysis based on a multiplicative model with a linear dose effect, using the southwestern region of Taiwan as an external comparison population.
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population more generally, however, it is risk of disease incidence, not mortality, that is the endpoint of interest. As discussed earlier, EPA converted risks calculated from Taiwanese mortality data to cancer incidence by assuming an 80% mortality for bladder cancer and a 100% mortaTity for lung cancer (see Table 5-2~.
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The lifetime cancer incidence assumed for the calculations reported here are 7.85% and 5.75°/O for male and female lung cancer, respectively, and 3.42% and 1.13% for male and female bladder cancer, respectively. It is useful to note again for comparison purposes that the corresponding lifetime death rates are 7.62% and 4.85% for lifetime-lung cancer mortality (male and female, respectively)
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Risks were also estimated using the same assumptions but based on the average arsenic concentrations from 1930 to 1994. The risk estimates (per 10,000)
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Then, when comparing cancer risk estimates, it is important to be aware of how those assumptions affect the estimates. For example, the higher the ratio of water ingestion in Taiwan relative to the United States in terms of liters per body weight per day, the smaller the U.S.
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To illustrate the importance of the background rate, Table S-9 shows the same risk projections using the Taiwanese data set and the background TABLE 5-9 Theoretical Maximum-Likelihood Estimates of Excess Lifetime Risk (Incidence per 10,000 people) of Lung and Bladder Cancer for Populations Exposed at Various Concentrations of Arsenic in Droning Water, Using the Background Cancer Incidence Rate for Taiwana Arsenic Bladder Cancer Lung Cancer Concentration (,ug/L)
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Using the southwestern Taiwanese data, the risk estimates for arsenic at 3,ug/L of drinking water range from 1.7 to 4.0 per 10,000 for mares and from 1.8 to 5.4 per 10,000 for females, depending on which assumptions are used (Tables 5-8 and 5-9~. Different studies have estimated the risks of lung cancer following exposure to arsenic, and it is possible and useful to compare the risk estimates generated in the different analyses at a given arsenic concentration.
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lung cancer estimates at 10 ,ug/L are 38 per 10,000 and 21 per 10,000 in mates and females, respectively. Overall the peak period exposure data in northern Chile and the data from southwestern Taiwan yield coherent lifetime excess risk estimates ranging from 1.4 to 6.7 per 1,000 for lung cancer in the United States at a drinkingwater arsenic concentration of 10 Vigil.
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SUMMARY AND CONCLUSIONS · Since EPA issued a pending standard of 10 ,ug/L, based on lung and bladder cancer data from the southwestern Taiwanese study in 2000, two additional studies have appeared in the literature that are of sufficient size and quality and with adequate quantification of dose to be considered in computing EDs for arsenic in drinking water. One is a study that examined urinary tract cancer, and TCC in particular, in northeastern Taiwan (Chiou et al.
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~ Analysis of the data from the period of peak arsenic exposure in northern Chile and the data from southwestern Taiwan results in similar estimates of lifetime lung cancer incidence in the United States. The consistency of the results adds to the confidence in the validity of the risk estimates.
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(2000~. · Dose-response analysis of the southwestern Taiwanese data should incorporate an unexposed comparison group; the southwestern Taiwanese region is the recommended comparison group.
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1998. Cost of Lung Cancer Chapter II.V in Cost of Illness Handbook.
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2000. Lung cancer and arsenic concentrations in drinking water in Chile.
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1999. Arsenic concentrations in well water and risk of bladder and kidney cancer in Finland.
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1998. Marked increase in bladder and lung cancer mortality in a region of northern Chile due to arsenic in dying water.
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