Maximum Contaminant Level Goals

Maximum contaminant level goals (MCLGs) must be set at a level at which "no known or anticipated adverse effects on the health of persons occur and which (sic) allows an adequate margin of safety" (Cotruvo 1987, 1988). First, the highest NOAEL is based upon an assessment of human or animal data (usually from animal experiments). To determine the RfD for regulatory purposes, the NOAEL is divided by an uncertainty factor (UF). This process corrects for the extrapolation of animal data to human data, the existence of weak or insufficient data, and individual differences in human sensitivity to toxic agents, among other factors.

For MCLG purposes, the NOAEL must be measurable in terms of concentration in drinking water (e.g., milligrams per liter). An adjustment of the RfD, which is reported in milligrams per kilograms (body weight) per day to milligrams per liter, is necessary. This adjustment is made by factoring in a reference amount of drinking water consumed per day and a reference weight for the consumer. The NOAEL, in milligrams per liter, is called the drinking water equivalent level (DWEL).

DWELs are calculated as follows:

where:

NOAEL = no observed adverse effect level 70 kg = assumed weight of an adult 2 L/day = assumed amount of water consumed by an adult per day

UF = uncertainty factor (usually 10, 100, or 1000)

(Note: An uncertainty factor of 10 is used when good acute or chronic human exposure data are available and supported by acute and chronic data in other species; an uncertainty factor of 1000 is used when acute or chronic data in all species are limited or incomplete [National Academy of Science, 1977]).

To determine the MCLG, regulators account for the contribution from other sources of exposure, including air and food. Comprehensive data are usually not available on exposures from air and food. In this case, the MCLG is determined by

MCLG = DWEL

X (percentage of the drinking water contribution) 4.6(2)

When specific data are not available, regulators often use a 20 percent drinking water contribution.

Maximum contaminant levels (MCLs) are set "as close to" the MCLGs "as is feasible." The term feasible means feasible with the use of the BAT, which is a technology standard rather than an ACL, taking costs into consideration (Cotruvo 1987, 1988). The general approach to setting MCLs is to determine the feasibility of controlling contaminants. This approach requires (1) evaluating the availability and cost of analytical methods, (2) evaluating the availability and performance of technologies and other factors related to feasibility and identifying those that are best, and (3) assessing the costs of applying the technologies to achieve various concentrations.

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