Based on an analysis of the available data on radon concentrations outdoors and on the transfer from water to air, the Research Council committee arrived at these additional conclusions:
- The average outdoor air concentration over the entire United States is about 15 becquerel per cubic meter (405 pCi per cubic meter or 0.4 pCi per liter).
- The contribution to radon concentration in indoor air from household usage of water is very low—only about one ten-thousandth the water concentration. The reason the resulting airborne concentration is so low is because only about half of the radon in the household water supply escapes into the air and then it is diluted into the large volume of air inside the home.
- Combining this information, the committee has determined that the level of radon in drinking water that would cause an increase of radon in indoor air that is no greater than the level of radon naturally present in outdoor air is about 150 becquerel per liter (4,050 pCi per liter). This conclusion will affect the public and water utilities in the following ways:
- People who own their own wells are not legally obliged to do anything because the Safe Drinking Water Act does not regulate private wells. However, people who are served by private wells and who wish to minimize their risk should test their water and consider taking action to reduce the radon if the concentration in the water is above the AMCL. In addition, those people should also measure the indoor air concentration in their home and consider taking actions to reduce it if it is above EPA's recommended action levels. Lastly, as the earlier NRC report concluded, stopping smoking is the most effective way to reduce the risk of lung cancer and reduce the risks associated with radon.
- Water supplies serving 25 or more people or with 15 or more connections are considered to be public water supplies. Those supplies, along with some special cases such as schools, will be subject to radon regulation if they rely on groundwater. In this case, there are three possibilities: (a) The radon in the water is already below the MCL. This will apply to the majority of people in the United States—only about 1 of every 14 individuals routinely consumes water with concentrations greater than the 1991 proposed MCL (11 becquerel per liter or 300 pCi/L). For water below the MCL, nothing needs be done. (b) The radon in the water is greater than the AMCL. In this case, radon reduction (mitigation) would be required by law after the regulation is final. Data available to the committee indicate that there are several types of water mitigation technology that could effectively reduce the radon concentration to the MCL. (c) The radon in the water is between the MCL and the AMCL. In this case, the concentration