health effects of radon health effects of radon radon, a radioactive, colorless, odorless, tasteless noble gas, has been studied by a number of scientific and medical bodies for its effects on health. a naturally-occurring gas formed as a decay product of radium, radon is one of the densest substances that remains a gas under normal conditions, and is considered to be a health hazard due to its radioactivity. its most stable isotope, radon-222 (222rn), has a half-life of 3.8 days. due to its high radioactivity, it has been less well studied by chemists, but a few compounds are known. radon is formed as part of the normal radioactive decay chain of uranium into 206pb. uranium has been present since the earth was formed, and its most common isotope has a very long half-life (4.5 billion years), which is the time required for one-half of uranium to break down. thus, uranium and radon will continue to occur for millions of years at about the same concentrations as they do now. radon is responsible for the majority of public exposure to ionizing radiation. it is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location. radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics and basements. it can also be found in some spring waters and hot springs. according to a 2003 report epa's assessment of risks from radon in homes from the united states environmental protection agency, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon-induced u.s. lung cancer deaths per year—second only to cigarette smoking. thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant. occurrence concentration units 210pb is formed from the decay of 222rn. here is a typical deposition rate of 210pb as observed in japan as a function of time, due to variations in radon concentration. radon concentration in the atmosphere is usually measured in becquerels per cubic meter (bq/m3), which is an si derived unit. as a frame of reference, typical domestic exposures are about 100 bq/m3 indoors and 10–20 bq/m3 outdoors. in the us, radon concentrations are often measured in picocuries per liter (pci/l), with 1 pci/l = 37 bq/m3. the mining industry traditionally measures exposure using the working level (wl) index, and the cumulative exposure in working level months (wlm): 1 wl equals any combination of short-lived 222rn progeny (218po, 214pb, 214bi, and 214po) in 1 liter of air that releases 1.3 × 105 mev of potential alpha energy; one wl is equivalent to 2.08 × 10−5 joules per cubic meter of air (j/m3). the si unit of cumulative exposure is expressed in joule-hours per cubic meter (j·h/m3). one wlm is equivalent to 3.6 × 10−3 j·h/m3. an exposure to 1 wl for 1 working month (170 hours) equals 1 wlm cumulative exposure. a cumulative exposure of 1 wlm is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 bq/m3. the radon (222rn) released into the air decays to 210pb and other radioisotopes. the levels of 210pb can be measured. the rate of deposition of this radioisotope is dependent on the weather. natural radon concentration next to a uranium mine radon concentrations found in natural environments are much too low to be detected by chemical means: for example, a 1000 bq/m3 (relatively high) concentration corresponds to 0.17 picogram per cubic meter. the average concentration of radon in the atmosphere is about 6×10−20 atoms of radon for each molecule in the air, or about 150 atoms in each ml of air. the entire radon activity of the earth's atmosphere at any one time is due to some tens of grams of radon, constantly being replaced by decay of larger amounts of radium and uranium. its concentration can vary greatly from place to place. in the open air, it ranges from 1 to 100 bq/m3, even less (0.1 bq/m3) above the ocean. in caves, aerated mines, or poorly ventilated dwellings, its concentration can climb to 20–2,000 bq/m3. in mining contexts, radon concentrations can be much higher. ventilation regulations try to maintain concentrations in uranium mines under the "working level", and under 3 wl (546 pci 222rn per liter of air; 20.2 kbq/m3 measured from 1976 to 1985) 95 percent of the time. the concentration in the air at the (unventilated) gastein healing gallery averages 43 kbq/m3 (about 1.2 nci/l) with maximal value of 160 kbq/m3 (about 4.3 nci/l). radon emanates naturally from the ground and from some building materials all over the world, wherever there are traces of uranium or thorium, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. in every 1 square mile of surface soil, the first 6 inches (150 mm) (of depth) contains about 0.035 oz of radium (0.4 g per km2) which releases radon in small amounts to the atmosphere. sand used in making concrete is the major source of radon in buildings. on a global scale, it is estimated that 2,400 million curies (91 tbq) of radon are released from soil annually. not all granitic regions are prone to high emissions of radon. being an unreactive noble gas, it usually migrates freely through faults and fragmented soils, and may accumulate in caves or water. due to its very small half-life (four days for 222rn), its concentration decreases very quickly when the distance from the production area increases. its atmospheric concentration varies greatly depending on the season and conditions. for instance, it has been shown to accumulate in the air if there is a meteorological inversion and little wind. because atmospheric radon concentrations are very low, radon-rich water exposed to air continually loses radon by volatilization. hence, ground water generally has higher concentrations of 222rn than surface water, because the radon is continuously replenished by radioactive decay of 226ra present in rocks. likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone because of diffusional losses to the atmosphere. as a below-ground source of water, some springs—including hot springs—contain significant amounts of radon. the towns of boulder, montana; misasa; bad kreuznach, and the country of japan have radium-rich springs which emit radon. to be classified as a radon mineral water, radon concentration must be above a minimum of 2 nci/l (7 bq/l). the activity of radon mineral water reaches 2,000 bq/l in merano and 4,000 bq/l in the village of lurisia (ligurian alps, italy). radon is also found in some petroleum. because radon has a similar pressure and temperature curve to propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become partially radioactive due to radon decay particles. residues from the oil and gas industry often contain radium and its daughters. the sulfate scale from an oil well can be radium rich, while the water, oil, and gas from a well often contains radon. the radon decays to form solid radioisotopes which form coatings on the inside of pipework. in an oil processing plant, the area of the plant where propane is processed is often one of the more contaminated areas, because radon has a similar boiling point to propane. accumulation in dwellings typical lognormal radon distribution in dwellings typical domestic exposures are of around 100 bq/m3 indoors, but specifics of construction and ventilation strongly affect levels of accumulation; a further complication for risk assessment is that concentrations in a single location may differ by a factor of two over an hour, and concentrations can vary greatly even between two adjoining rooms in the same structure. the distribution of radon concentrations is highly skewed: the larger concentrations have a disproportionately greater weight. indoor radon concentration is usually assumed to follow a lognormal distribution on a given territory. thus, the geometric mean is generally used to estimate the "average" radon concentration in an area. the mean concentration ranges from less than 10 bq/m3 to over 100 bq/m3 in some european countries. typical geometric standard deviations found in studies range between 2 and 3, meaning (given the 68–95–99.7 rule) that the radon concentration is expected to be more than a hundred times the mean concentration for 2 to 3% of the cases. the so-called "watras incident" in 1984 is named for american construction engineer stanley watras, an employee at a u.s. nuclear power plant, who triggered radiation monitors while leaving work over several days—even though the plant had not yet been fueled, and despite watras being decontaminated and sent home "clean" each evening. this pointed to a source of contamination outside the power plant, which turned out to be radon levels of 100,000 bq/m3 (2.7 nci/l) in the basement of his home. he was told that living in the home was the equivalent of smoking 135 packs of cigarettes a day, and he and his family had increased their risk of developing lung cancer by 13 or 14 percent. the incident dramatized the fact that radon levels in particular dwellings can occasionally be orders of magnitude higher than typical. radon soon became a standard homeowner concern, though typical domestic exposures are two to three orders of magnitude lower (100 bq/m3, or 2.5 pci/l), making individual testing essential to assessment of radon risk in any particular dwelling. radon exists in every u.s. state, and about 6% of american houses have elevated levels. the highest average radon concentrations in the united states are found in iowa and in the appalachian mountain areas in southeastern pennsylvania. some of the highest readings have been recorded in mallow, county cork, ireland. iowa has the highest average radon concentrations in the united states due to significant glaciation that ground the granitic rocks from the canadian shield and deposited it as soils making up the rich iowa farmland. many cities within the state, such as iowa city, have passed requirements for radon-resistant construction in new homes. in a few locations, uranium tailings have been used for landfills and were subsequently built on, resulting in possible increased exposure to radon. jewelry contamination in the early 20th century, 210pb-contaminated gold, from gold seeds that were used in radiotherapy which had held 222rn, were melted down and made into a small number of jewelry pieces, such as rings, in the u.s. wearing such a contaminated ring could lead to a skin exposure of 10 to 100 millirad/day (0.004 to 0.04 msv/h). health effects cancer in miners relative risk of lung cancer mortality by cumulative exposure to radon decay products (in wlm) from the combined data from 11 cohorts of underground hard rock miners. though high exposures (>50 wlm) cause statistically significant excess cancers, the evidence on small exposures (10 wlm) is inconclusive and appears slightly beneficial in this study (see radiation hormesis). the health effects of high exposure to radon in mines, where exposures reaching 1,000,000 bq/m3 can be found, can be recognized in paracelsus' 1530 description of a wasting disease of miners, the mala metallorum. though at the time radon itself was not understood to be the cause—indeed, neither it nor radiation had even been discovered—mineralogist georg agricola recommended ventilation of mines to avoid this mountain sickness (bergsucht). in 1879, the "wasting" was identified as lung cancer by herting and hesse in their investigation of miners from schneeberg, saxony, germany. given that the type locality of the important uranium ore pitchblende is in the ore mountains and that region was the most important german speaking mining area at the time, it is likely the radon induced lung cancers were associated with uranium. beyond mining in general, radon is a particular problem in the mining of uranium; significant excess lung cancer deaths have been identified in epidemiological studies of uranium miners and other hard-rock miners employed in the 1940s and 1950s. residues from processing of uranium ore can also be a source of radon. radon resulting from the high radium content in uncovered dumps and tailing ponds can be easily released into the atmosphere. modern mining techniques, including better ventilation for underground mines, routine radiation monitoring as well as technologies like in-situ leaching have helped decrease the incidence of radon exposure among miners in