at the same time for prolonged exposure periods, and they may have a synergistic relationship.
Due to the absence of a federal risk assessment paradigm for evaluating health risks from trace contaminants in reclaimed water, private associations as well as states (particularly California) have embarked upon their own programs to use existing screening paradigms to assess health risks of contaminants in reclaimed water (e.g., Rodriquez et al., 2007a; Bruce et al., 2010; Drewes et al., 2010; Snyder et al., 2010b; Bull et al., 2011). Techniques to conduct such water quality evaluations and subsequently perform exposure and risk assessments are summarized in Khan (2010).
Rodriquez et al. (2007a,b, 2008) and Snyder et al. (2010b) used these screening health risk assessment approaches to evaluate potential health risks from chemicals in reclaimed water in Australia and the United States, respectively. In both evaluations, potential health impacts of chemical contaminants were evaluated using a combination of approaches based on extrapolating health risks using actual health effects data on a specific contaminant, as well as chemical class-based evaluation approaches in the absence of contaminant-specific data. For regulated chemicals, EPA MCLs, Australian drinking water guidelines, or WHO drinking water guideline values were used as benchmark risk values (or risk based action levels, RBALs), from which risk quotients can be evaluated (see also example in Appendix A). RBALs for unregulated chemicals with existing risk values can be based upon EPA reference doses (RfDs), WHO acceptable daily intakes (ADIs), lowest therapeutic doses for pharmaceuticals, or EPA cancer slope factors (CSFs), among other risk values. If existing risk values have not been derived, it is possible to derive risk values for noncarcinogens or carcinogens using human or laboratory animal datasets on the chemical under consideration using methods described in Boxes 6-5 and 6-6. The selection of one risk value over another (e.g., RfD vs. ADI) or selection of a specific epidemiological or toxicological dataset used to derive a RBAL generally should be based upon the critical health effect(s) identified for the specific chemical in the most sensitive species.
Potential health risks from the presence of a chemical in reclaimed water can be assessed by dividing a chemical’s RBAL by the concentration of that chemical in reclaimed water. This risk quotient is known as a Margin of Safety (MOS), with values >1 indicating that the presence of a chemical in reclaimed water is unlikely to pose a significant risk of adverse health effects. This is exampled in Chapter 7 for 24 organic contaminants in reclaimed water.
Benchmarks for unregulated chemicals without complete epidemiological or toxicological datasets or risk values were evaluated by Rodriquez et al. (2007a,b) and Snyder et al. (2010b) using class-based risk assessment approaches, including the Threshold of Toxicological Concern (TTC), FDA’s Threshold of Regulation (TOR; see Box 6-7), or EPA’s Toxicity Equivalency Factor (TEF) approach. Rodriquez et al. (2007a,b) used the TTC approach for both unregulated noncarcinogens and carcinogens without available toxicity information, while Snyder et al. (2010b) used TTC for noncarcinogens and nongenotoxic carcinogens. The Toxicity Equivalency Factor (TEF)/Toxicity Equivalents (TEQ) approach was used by Rodriquez et al. (2008) to assess potential health risks from dioxin and dioxin-like compounds in Australian reclaimed water used to augment drinking water supplies, based
Threshold of Regulation (TOR)
One class-based approach is the Threshold of Regulation, which was developed as a method to evaluate the potential toxicity of carcinogens extracted from food contact substances. The TOR is a concentration of chemicals unlikely to pose a significant risk of adverse health effects, including cancer risk (10–6) over a lifetime (FDA, 1995; Rulis, 1987, 1989). The FDA derived a threshold value of 0.5 ppb for carcinogens in the diet based on carcinogenic potencies of 500 substances from 3500 experiments of Gold et al.’s (1984, 1986, 1987) Carcinogenic Potency Database. The distribution of chronic dose rates that would induce tumors in 50 percent of test animals (TD50s) was plotted. This distribution was extrapolated to a Virtually Safe Dose (10–6 lifetime risk of cancer) in humans and is equal to 0.5 μg chemicals/kg of food, or 1.5 μg/person/day (based on 3 kg food/drink consumed/day). This value can be extrapolated to a concentration in water intended for ingestion, as follows:
TOR: 0.5 μg/kg food/day x (3 kg food/day)/ (2 L water/day) = 0.75 μg/L.