and 1 L per day for "infants" (individuals of 10 kg body mass or less) as default drinking water intakes (EPA 1994b). The combined mean value of 1.2 L d-1 consists of both direct use, such as tapwater ingestion as well as indirect use, such as juices and other beverages that contain tapwater, such as coffee. A National Research Council committee (1977) has suggested that daily consumption of water can vary with extent of physical activity and fluctuations in temperature and humidity and that people who live in warmer climates might have higher intakes of water.

Numerous studies have developed data on drinking-water intake. All the studies that are available were based on short-term survey data. One of the more commonly cited studies on water intake is the Ershow and Cantor (1989) study. They estimated water intake on the basis of data collected by the US Department of Agriculture 1977–1978 Nationwide Food Consumption Survey and calculated daily intake and total water intake by various age groups of males, females, and both sexes combined. They defined tapwater as "all water from the household tap consumed directly as a beverage or used to prepare foods and beverages" and defined total water intake as tapwater plus "water intrinsic to foods and beverages." Table 4.1 summarizes data from the Ershow and Cantor study.

The combined mean value of 1.2 L d-1 (table 4.1) is for all uses of tapwater, which consists of both direct use (i.e., direct ingestion) and indirect use, i.e. making coffee, tea, etc. As noted by the EPA (1994b), the concern about radon dissolved in water is largely for the water that is ingested directly. The EPA has estimated that slightly more than half of the tapwater use is directly ingested. The committee has adopted a value of 0.6 L d-1 for direct use. This value is similar to that used by the EPA in their Multimedia Risk Assessment (EPA 1994b). However, the committee has conservatively assumed for direct use, that all of the radon in the tapwater remains dissolved in the process of transferring the water from the tap to the stomach.

Physicochemical Properties Of Radon

Radon, a noble gas, is essentially chemically inert. Unlike the other noble gases, radon has no known stable isotope. Rather it has 36 radioactive isotopes and isomers, which range in mass number from 198 to 228. The radon isotope of interest here is 222Rn (physical half-life, 3.825 d), a member of the decay series beginning with the primordial radionuclide 238U. 222Rn emits alpha particles as it spontaneously decays to a series of short-lived radioactive decay products, which are followed by a longer-lived series headed by 210 Pb (half-life, 22.3 y), as shown earlier in figure 1.1. The cumulative energies of the radiation emitted by the members of the decay series (alpha particles, electrons, and photons) are shown in table 4.2. The tabulated values represent the average or expected energy of the indicated radiation emitted per atom of 222Rn initially present. The entry for a particular member includes the contribution of the member and its precursors

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