Chemicals lifetime cancer risks

Travis and Hester (1990) use the estimated lifetime exposures of the U.S. population to these "background" chemicals (as estimated by EPA exposure studies) combined with the estimated potency of the exposures (derived from the animal experiments of the NTP) to conclude that lifetime cancer risk from these chemical exposures is between 0.14 and 0.50 percent. If the risk from exposure to background chemicals is added to the risk from exposure to naturally occurring radioisotopes—the background incidence of cancer is between 1.0 and 1.5 percent in the U.S. population. ... 

Table 8.4—Estimated lifetime cancer risks association with occupational exposure limits for various chemical and physical agentsf ...

Table IV lists the results of risk calculations provided in the preliminary proposal for the substances that were proposed as potential carcinogens in the regulatory context at that time (44). 1,1-Dichloroethylene was later converted to a listing of equivocal evidence of carcinogenicity. The table includes calculations made by the USEPA CAG and the NAS Safe Drinking Water Committee. These calculations attempt to project concentrations of each chemical in drinking water that, if consumed for a lifetime (70 years) at the rate of 2 L of water per day would contribute an excess lifetime cancer risk of up to 1 in 100,000 and up to 1 in 1,000,000. The quality of evidence of carcinogenicity ranging from sufficient in humans to limited in animals is also included for each chemical. Provisional ADI values calculated from chronic toxicity data only are included for the sake of comparison.

Excess lifetime cancer risks considered negligible have values in the range of about 1CU4 to 10 6 or below, depending on the exposure situation intakes of noncarcinogenic hazardous chemicals less than RfDs are considered negligible. 

Okrent and Xing (1993) estimated the lifetime cancer risk to a future resident at a hazardous waste disposal site after loss of institutional control. The assumed exposure pathways involve consumption of contaminated fruits and vegetables, ingestion of contaminated soil, and dermal absorption. The slope factors for each chemical that induces stochastic effects were obtained from the IRIS (1988) database and, thus, represent upper bounds (UCLs). The exposure duration was assumed to be 70 y. Based on these assumptions, the estimated lifetime cancer risk was 0.3, due almost entirely to arsenic. If the risk were reduced by a factor of 10, based on the assumption that UCLs of slope factors for chemicals that induce stochastic effects should be reduced by this amount in evaluating waste for classification as low-hazard (see Section 7.1.7.1), the estimated risk would be reduced to 0.03. Either of these results is greater than the assumed limit on acceptable risk of 10 3 (see Table 7.1). Thus, based on this analysis, the waste would be classified as high-hazard in the absence of perpetual institutional control to preclude permanent occupancy of a disposal site. 

Chemical USEPA Cancer Classification21 Dietary Inhalation Total Excess Lifetime Cancer Risk Acceptable Range of Excess Lifetime Cancer Risk (USEPA)... 

Chemical USEPA Cancer Classification15 LADD (total, mg kg-1 bw d 1)d Slope factor6 Excess lifetime cancer risk Acceptable range of excess lifetime cancer risk (USEPA) ... 

Risk characterization includes a comparison between toxicity values and/or exposure criteria and exposure (dose or media concentration) to determine whether the exposure is acceptable. US EPA developed a formalized system that is commonly used to determine whether chemicals are likely to present an unacceptable risk based on current and likely future use of the property. The estimated dose is used to calculate an additional lifetime cancer risk for each chemical regulated as a carcinogen. Typically, a total site risk (sum of the risk associated with all carcinogens identified at the site) is presented. Acceptable risk is defined by the agency, in the appropriate laws, or by regulations that govern the site. Acceptable risk is a function of policy or law but is supposed to be rooted in science. 

Many chemical risks such as those of chloroform in drinking water, are calculated, not measured - that is, they are based not only on scientific data, but also on various sets of assumptions and extrapolation models that, while scientifically plausible (they fall within the bounds of acceptable biological theory), have not been subjected to empirical study and verification. Indeed, the results of most risk assessments - whether expressed as an estimate of extra cancer risk or an ADI - are scientific hypotheses that are not generally testable with any practicable epidemiological method. There is, for example, no practical means to test whether chloroform residues in chlorinated drinking water increase lifetime cancer risk in humans by 8 in 1000000, as hypothesized above. The tools of epidemiology are enormously strained, indeed, when called upon to detect the relatively low risks associated with most environmental chemicals. Without such a test, these risks remain unverified. 

Every chemical exposure resulting in an estimated lifetime cancer risk of 4 in 1000 (one in 250) was subjected to some type of regulatory action. 

The concentration of a chemical in drinking water that is not expected to cause any adverse noncarcinogenic effects for a lifetime of exposure. The concentration of a chemical in drinking water corresponding to an estimated lifetime cancer risk of 1 in 10,000. 

TABLE 4.1. Estimated Lifetime Cancer Risks of Chemicals in Drinking Water at the Federal MCL... 

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