chapter, the committee discusses some specific areas where action appears warranted, but it first summarizes the significance of the uncertainty around preventive action.

RECOGNIZING UNCERTAINTY OF BENEFITS AND RISKS

Potential for Introducing New Hazards or Risks

A key concept to remember when evaluating a particular risk associated with a particular factor is that an action that is aimed at eliminating the specific risk of concern may result in a substitution of one risk for another, or perhaps shifting risk from one group to another. Any risk of the alternative action thus needs to be considered and weighed against the risk that the change is intended to reduce or eliminate.

The complexity of trade-offs from substitutions can be illustrated with the case of contamination of potable ground water sources with pesticides or industrial chemicals shown to be carcinogenic in experimental animals or humans. Reducing exposures to potentially carcinogenic substances in drinking water from groundwater sources seems to be a logical, health-protective action, even if the actual or perceived risk from the contaminants is small. A typical action to reduce the potential cancer risk from using the contaminated ground water is to switch the consumer to an alternative source of potable water, such as a public water supply system. However, such systems require disinfection, usually by chlorination, and chlorination of surface water introduces trace levels of disinfection by-products (DBPs). Several DBPs have been found to be carcinogenic in animal bioassays (e.g., NTP, 2007a,b), and some epidemiologic studies have suggested that long-term exposure to DBPs is associated with an increase in bladder cancer (reviewed in Richardson et al., 2007), especially in a subset of the population with specific genetic polymorphisms (Cantor et al., 2010).

In this scenario, one would have to consider many factors, including (1) the relative carcinogenic potency of the groundwater contaminant(s) versus that of the DBPs, (2) the concentrations of the groundwater contaminants or the DBPs in the drinking water and indoor air following use, and (3) the duration and frequency of likely exposure to a given drinking water source over a lifetime. Depending on these values, it is possible that a comparative risk assessment would show that switching from the contaminated groundwater supply to the uncontaminated but disinfected surface water supply actually increased, rather than decreased, potential cancer risks to the exposed population.

This example, however, also illustrates the challenges in assessing trade-offs in population- and individual-level risks and benefits. There are certainly no clear benefits, at least to the individual consumer, of drinking



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