MW-RN-2020 (1)

TECHNICAL BACKGROUND 1.0 INTRODUCTION Radon ( 222 Rn) is a naturally-occurring radioactive decay product of 226 Ra, and a radioisotope in the 238 U decay series (Figure 1). Natural radiation accounts for the majority of human exposure to radiation, with inhaled radon decay products being the largest contributor in spite of the increase in use of manmade radiation in industry and medicine. The earth's crust contains trace amounts of 238 U and 232 Th, which produce the decay products 222 Rn and 220 Rn (thoron) gas, respectively. 222 Rn and two of its progeny, 218 Po and 214 Po, are high- energy alpha-particle emitters, while 214 Pb and 214 Bi are less- damaging beta-particle emitters. Most of the decay products are also emitters of gamma radiation. Inhalation of the alpha- emitting progeny of radon and thoron pose a radiation health hazard to the lungs. However, thoron is often ignored in studies because of its short half-life (t 1/2 = 55 s), difficulty to measure, and that it often occurs in lower concentrations than 222 Rn (t 1/2 = 3.8 d) in aquatic environments. The 222 Rn concentration in water is due to the decay of 226 Ra that is associated with proximal rock and soil. The radon gas percolates through the soil/rock and dissolves in the water. Therefore, the concentration of radon in water is higher than one would expect if the activity were due only to dissolved 226 Ra in the water (i.e., radium solubility in groundwater is typically low). To date, no maximum permissible concentration levels exist nationally for radon in drinking water, although radon was proposed to join radium ( 226 Ra and 228 Ra) on the U.S. EPA list of regulated radionuclides in the 1996 Amendments to the Safe Water Drinking Act (42 USC §300f to 300j-26). The maximum contaminant level (MCL) for radon was to be relatively low (300 pCi/L), although application of a multimedia mitigation program would allow 222 Rn concentrations in a public water supply to be up to 4000 pCi/L (USEPA 2011). The proposed MCL for waterborne radon was withdrawn in 1997 but re-issued in 1998 to include a proposed Alternate Maximum Contaminant Level (AMCL) of 4,000 pCi/L to be permitted only if the affected community implemented a multimedia program. The multimedia program entailed reduction on airborne radon concentrations in homes to below the action level (e.g., 4 pCi/L) to achieve radon risk reductions that meet or exceed risk reduction achieved if implementing the MCL value of 300 pCi/L for radon in water. The proposed MCL with the AMCL of 4,000 pCi/L is still in pending status. The radon in your water supply poses an inhalation risk and a small ingestion risk. Most of your risk from radon in water comes from radon released into the air when water is used for such household activities as showering, dishwashing, and laundry. Breathing the radon that is released from the water into the air is much larger than your risk of stomach cancer from swallowing water with radon This released radon, plus that from soil gas, accumulates in indoor air and poses an increased health risk for lung cancer. If one is concerned about radon from groundwater, consider testing for radon in both air and water. By testing for radon in both air and water, the results could enable one to more completely assess the radon mitigation option(s) best suited to the situation. Some of the devices and procedures for testing a home's water supply are different from those used for measuring radon in air. Radon released from tap water is typically a small contributor to radon levels in indoor air. However, breathing radon released to air from household water uses increases the risk of lung cancer over the course of your lifetime. Ingestion of drinking water containing radon also presents a risk of internal organ cancers, primarily stomach cancer. This risk is smaller than the risk of developing lung cancer from radon released to air from tap water. A report on radon in drinking water (NRC 1999), estimated that radon in drinking water causes about 168 cancer deaths per year, 89 1,600 yr 3.85 d 3.05 m 26.8 m 19.7 m 0.16 ms 22 yr 5.0 d 138 d Stable Figure 1 – The 226 Ra decay portion of the 238 U decay chain contains radon and its progeny