Source: Kolluru et al. (1996); Doull et al. (1980).

Source: Kolluru et al. (1996); Doull et al. (1980).

resulting intake is called the subchronic daily intake (SDI). In some cases, instead of estimating intake of a contaminant, the body burden will be measured directly, such as by blood or tissue sampling.

Risk Characterization Toxic effects in humans are broadly classified for risk assessment purposes as either carcinogenic or noncarcinogenic. Carcinogenic effects are the formation of tumors. Noncarcinogenic effects include all toxic responses other than tumor formation, which may include, for example, respiratory, neurological, or reproductive effects. The U.S. EPA makes the following important distinction between these two categories: Carcinogenic effects are always assumed to have no threshold; any finite dosage is capable of producing cancer. Noncarcinogenic effects, on the other hand, are always assumed to have a threshold below which no effect will occur. The characterization of risk is different for the two types. Some chemicals may exhibit both types of effect, so both need to be checked when doing a risk assessment for any chemical.

Noncarcinogenic Risk Characterization Toxicological effects other than carcinogen-esis are assumed to produce their effect only above a threshold. The threshold is determined experimentally from bioassay and epidemiological data. The highest level at which no toxic effect is found is termed the no observed adverse effect level (NOAEL). The NOAEL is often divided by a safety factor of 10 to take into account differences between the typical members of a population and its more sensitive members.

The resulting safe threshold is called the reference dose (RfD) [formerly called the acceptable daily intake (ADI)]. Appendix C gives RfDs for several important environmental toxins. Unless information to the contrary is available, noncarcinogenic risks are assumed to be additive.

The RfD is used to compute a hazard index (HI), which represents a summary of all exposures over all pathways:

where E,- is the exposure or intake in mg/kg • day. If the hazard index exceeds 1.0, a more detailed study on the risk and control measures would be required. If several chemicals are known to produce their noncarcinogenic effect on different organs, the summation may be carried out separately for each organ.

Carcinogenic Risk Characterization The different models for extrapolating carcinogenic risk, including the linearized multistage model, result in a linear relationship between risk and dose at low dose. The slope of that relationship is the carcinogenic potency factor (CPF) or slope factor (SF) and relates the probability of contracting cancer, p, to the exposure or intake:

where CDI is the chronic daily intake in mg/kg • day (equivalent to exposure E). CPFs for some pollutants are given in Appendix C. The potency factor includes a safety factor to take into account the uncertainty in "mouse-to-human" extrapolation. The value of the safety factor varies from 10 to 1000, and depends on the quality and quantity of the evidence of toxicity. If toxicity has been tested in only a single species, a larger safety factor is called for. If multiple species have been tested, a smaller safety factor, together with using the most sensitive species, would give reasonable protection. Other considerations affecting the safety factor include whether there is specific knowledge of toxic mechanisms such as specific biotransformations that occur, or of pharmacokinetic behavior. There may be different potency factors for different routes of absorption.

Both the RfD and the potency factor include safety factors and conservative assumptions. To summarize, the risk assessment is conservative because of the emphasis on data from the most sensitive species, strain, and sex; extrapolation by body surface area instead of body weight, the use of safety factors for extrapolation from animals to humans; use of the upper confidence limit of the dose-response model; and the assumption of no threshold for carcinogens.

In performing human health risk assessments for exposures involving the public, an excess lifetime risk less than 10~6 is generally considered acceptable for regulatory purposes. In some cases, such as occupational exposures or other populations that have a stake in the costs, higher risks may be acceptable. It must be noted that the public perception of risk is quite different from the probabilistic definition given here. Although engineers and scientists recognize that "zero risk" is an impossibility, the public rightfully demands that the goal should be zero risk. That is, although the goal may not be achievable, all reasonable exertions should be made to attempt it.

Example Risk Characterization CPF and RfD values for some pollutants are given in Appendix C. More complete and up-to-date information can obtained online from U.S. EPA's Integrated Risk Information Systems (IRIS) database at

Example 22.1 Estimate and characterize the risk to an adult from a lifetime of drinking water containing 1 mg/L of perchloroethylene (PCE) and 10 mg/L of carbon tetrachloride (CT).

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