Co

Combustion

o3

Outside air, photocopying machines, electrostatic air cleaners

so2

Combustion

Particles

Combustion

Asbestos

Building materials, handheld hair dryers

Microorganisms

Air conditioners, cool-mist humidifiers

206pb STABLE

FIGURE 15.1 The major radioactive decay paths in (a) the uranium series leading to the formation of 222 Rn and (b) of 222 Rn to form 206 Pb. Half-lives are shown to the left of each process.

206pb STABLE

FIGURE 15.1 The major radioactive decay paths in (a) the uranium series leading to the formation of 222 Rn and (b) of 222 Rn to form 206 Pb. Half-lives are shown to the left of each process.

made, with the basement typically having the highest concentrations (e.g., Liu et al., 1991). Interestingly, because homes are often warmer than their surroundings, a "chimney effect" occurs that draws gases, including radon, into the house from the surroundings (e.g., Osborne, 1987; Nero, 1989; Turk et al., 1990; Hintenlang and Al-Ahmady, 1992).

Other sources include building materials such as concrete that are made from the earth's crustal materials and hence can contain significant amounts of uranium and radium (Nazaroff and Nero, 1988). Radon dissolves in water, and hence degassing from household water can also be a source. For example, Osborne (1987) reported that the radon concentration in a bathroom increased by more than two orders of magnitude during a f5-min period that a shower was running.

The health concerns associated with 222 Rn are primarily associated with its radon daughters. As a noble gas, radon is unreactive in air and is both readily inhaled and exhaled. However, a significant portion of its daughters are positively charged ions that are expected to attract water vapor and become hydrated; the formation of clusters with other ions is also likely (e.g., Castleman, 1991). Uptake on existing aerosol particles also occurs readily, and such particles can then be deposited in the respiratory tract, providing a source of radioactive emissions directly to the lung. Effects such as lung cancer may then ensue.

The health effects associated with radon, as well as sources and mitigation measures, are discussed in detail in several National Research Council reports (1988, 1991), in the book edited by Nazaroff and Nero (1988), and in the International Commission on Radiological Protection Report (1994). Initial risk assessments were based on data from underground miners who were exposed to relatively high levels of radon and its progeny. However, there has been considerable controversy over the extrapolation to lower levels in homes [e.g., see summaries by Nazaroff and Teichman (1990) and Peto and Darby (1994)].

FIGURE 15.2 Distribution of 222Rn measured in homes in the United States (adapted from Nero et al., 1986).

FIGURE 15.2 Distribution of 222Rn measured in homes in the United States (adapted from Nero et al., 1986).

A number of studies have been carried out to determine the concentrations of radon in homes (e.g., see Nero et al., 1986; Alter and Oswald, 1987; Turk et al., 1990; Liu et al., 1991; and Mose and Mushrush, 1997). Units used to express the concentration of 222 Rn are picocuries per liter (pCi L_l), with 1 pCi being the amount of substance that gives 2.2 radioactive decays per minute, or becquerels per cubic meter (Bq m~3), where 1 pCi L_1 =37 Bq m~3 (e.g., see Nazaroff and Teichman, 1990).

Figure 15.2 shows the distribution of levels measured in homes in the United States (Nero et al., 1986). As might be expected given the variables that affect 222 Rn concentrations in homes, the concentrations vary widely, from <0.1 to >8 pCi L_l. Indeed, Alter and Oswald (1987) report a few single measurements of up to 4000 pCi L-1. For comparison, the U.S. EPA recommends levels below 4 pCi L 1 (150 Bq m~3), which is exceeded by about 7% of U.S. homes (Nero et al., 1986), and the International Commission on Radiological Protection (1993) recommends 5.4-16 pCi L~' (200-600 Bq m~3). Typical outdoor concentrations in continental areas are 0.1-0.4 pCi L 1 (Nazaroff and Nero, f988).

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