and transferred to blood. They then circulate within the body; most are released from the blood into the lung and exhaled, but some remain in the blood and accumulate in organs and tissues, which receive an absorbed dose from alpha, beta, and gamma radiation.
There is a direct implication between high doses of radiation and health effects in humans. For example, excess cancers have been observed in a cohort of survivors of the atomic-bomb blasts in Japan (National Research Council 1990a). A relationship between lung cancer and inhalation of radon decay products has been demonstrated in underground miners (Lubin and Boice 1997). Recent epidemiologic evidence suggests that inhalation of radon decay products in domestic environments could also be a cause of lung cancer (National Research Council 1999; Lubin and others 1995). Although the studies do not specifically identify health effects at low doses, there is compelling circumstantial evidence that they occur.
Under ambient conditions of low dose and low dose rate, any health effects associated with exposure to radon in air or water can be expected to occur from the passage of single alpha particles through individual cells. Any given cell is hit only once or not at all. An increase in exposure increases the number of cells that are hit, but it will not affect the primary damage experienced by each cell. Therefore, the initial events depend linearly on exposure or dose.
Exposed cells experience local damage in the form of DNA breaks and the products of reactive oxygen. The damage is metabolized by cellular-repair systems, and some fraction of it results in permanent genetic changes. Those changes can lead to the development of cancers; a cancer usually originates in a single transformed cell.
Risk projection models have been developed to predict the risk in situations where direct evidence is not available (National Research Council 1999; 1990a). The nature of the exposure to indoor radon, the kinds of DNA damage inflicted by alpha particles, and the extent of repair are consistent with the absence of a threshold for cancer induction. The preferred model is a straight line that reaches zero risk only when the dose or exposure is zero; it is referred to as the linear no-threshold (LNT) model.
In 1988, Congress passed the Indoor Radon Abatement Act. Its stated goal was to reduce indoor radon concentrations to outdoor levels. The Environmental Protection Agency (EPA) was authorized to implement policies described in the law. In 1987 and again in 1992, EPA published A Citizen's Guide To Radon (EPA 1992a). The document summarized the risks associated with inhalation of