by age 75 at usual radon concentrations of 0, 100, and 400 bq/m3 would be about 0.4%, 0.5%, and 0.7%, respectively, for lifelong nonsmokers, and about 25 times greater (10%, 12%, and 16%) for cigarette smokers. there is great uncertainty in applying risk estimates derived from studies in miners to the effects of residential radon, and direct estimates of the risks of residential radon are needed. as with the miner data, the same confounding factor of other carcinogens such as dust applies. studies on domestic exposure average radiation doses received in germany. radon accounts for half of the background dose; and medical doses reach the same levels as background dose. the largest natural contributor to public radiation dose is radon, a naturally occurring, radioactive gas found in soil and rock, which comprises approximately 55% of the annual background dose. radon gas levels vary by locality and the composition of the underlying soil and rocks. radon (at concentrations encountered in mines) was recognized as carcinogenic in the 1980s, in view of the lung cancer statistics for miners' cohorts. although radon may present significant risks, thousands of persons annually go to radon-contaminated mines for deliberate exposure to help with the symptoms of arthritis without any serious health effects. radon as a terrestrial source of background radiation is of particular concern because, although overall very rare, where it does occur it often does so in high concentrations. some of these areas, including parts of cornwall and aberdeenshire have high enough natural radiation levels that nuclear licensed sites cannot be built there—the sites would already exceed legal limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste. people in affected localities can receive up to 10 msv per year background radiation. this led to a health policy problem: what is the health impact of exposure to radon concentrations (100 bq/m3) typically found in some buildings? detection methods when exposure to a carcinogenic substance is suspected, the cause/effect relationship on any given case can never be ascertained. lung cancer occurs spontaneously, and there is no difference between a "natural" cancer and another one caused by radon (or smoking). furthermore, it takes years for a cancer to develop, so that determining the past exposure of a case is usually very approximative. the health effect of radon can only be demonstrated through theory and statistical observation. the study design for epidemiological methods may be of three kinds: the best proofs come from observations of cohorts (predetermined populations with known exposures and exhaustive follow-up), such as those on miners, or on hiroshima and nagasaki survivors. such studies are efficient, but very costly when the population needs to be a large one. such studies can only be used when the effect is strong enough, hence, for high exposures. alternate proofs are case-control studies (the environment factors of a "case" population is individually determined, and compared to that of a "control″ population, to see what the difference might have been, and which factors may be significant), like the ones that have been used to demonstrate the link between lung cancer and smoking. such studies can identify key factors when the signal/noise ratio is strong enough, but are very sensitive to selection bias, and prone to the existence of confounding factors. lastly, ecological studies may be used (where the global environment variables and their global effect on two different populations are compared). such studies are "cheap and dirty": they can be easily conducted on very large populations (the whole usa, in dr cohen's study), but are prone to the existence of confounding factors, and exposed to the ecological fallacy problem. furthermore, theory and observation must confirm each other for a relationship to be accepted as fully proven. even when a statistical link between factor and effect appears significant, it must be backed by a theoretical explanation; and a theory is not accepted as factual unless confirmed by observations. epidemiology studies of domestic exposures a controversial epidemiological study, unexpectedly showing decreased cancer risk vs. radon domestic exposure (5 pci/l ≈ 200 bq/m3). this study lacks individual level controls for smoking and radon exposure, and therefore lacks statistical power to draw definitive conclusions. because of this, the error bars (which simply reflect the raw data variability) are probably too small. among other expert panels, the who's international agency for research on cancer concluded that these analyses "can be rejected." cohort studies are impractical for the study of domestic radon exposure. with the expected effect of small exposures being very small, the direct observation of this effect would require huge cohorts: the populations of whole countries. several ecological studies have been performed to assess possible relationships between selected cancers and estimated radon levels within particular geographic regions where environmental radon levels appear to be higher than other geographic regions. results of such ecological studies are mixed; both positive and negative associations, as well as no significant associations, have been suggested. the most direct way to assess the risks posed by radon in homes is through case-control studies. the studies have not produced a definitive answer, primarily because the risk is likely to be very small at the low exposure encountered from most homes and because it is difficult to estimate radon exposures that people have received over their lifetimes. in addition, it is clear that far more lung cancers are caused by smoking than are caused by radon. epidemiologic radon studies have found trends to increased lung cancer risk from radon with a no evidence of a threshold, and evidence against a threshold above high as 150 bq/m3 (almost exactly the epa's action level of 4 pci/l). another study similarly found that there is no evidence of a threshold but lacked the statistical power to clearly identify the threshold at this low level. notably, the latter deviance from zero at low level convinced the world health organization that, "the dose-response relation seems to be linear without evidence of a threshold, meaning that the lung cancer risk increases proportionally with increasing radon exposure." the most elaborate case-control epidemiologic radon study performed by r. william field and colleagues identified a 50% increased lung cancer risk with prolonged radon exposure at the epa's action level of 4 pci/l. iowa has the highest average radon concentrations in the united states and a very stable population which added to the strength of the study. for that study, the odds ratio was found to be increased slightly above the confidence interval (95% ci) for cumulative radon exposures above 17 wlm (6.2 pc/l=230 bq/m3 and above). the results of a methodical ten-year-long, case-controlled study of residential radon exposure in worcester county, massachusetts, found an apparent 60% reduction in lung cancer risk amongst people exposed to low levels (0–150 bq/m3) of radon gas; levels typically encountered in 90% of american homes—an apparent support for the idea of radiation hormesis. in that study, a significant result (95% ci) was obtained for the 75–150 bq/m3 category. the study paid close attention to the cohort's levels of smoking, occupational exposure to carcinogens and education attainment. however, unlike the majority of the residential radon studies, the study was not population-based. errors in retrospective exposure assessment could not be ruled out in the finding at low levels. other studies into the effects of domestic radon exposure have not reported a hormetic effect; including for example the respected "iowa radon lung cancer study" of field et al. (2000), which also used sophisticated radon exposure dosimetry. intentional exposure "radon therapy" is an intentional exposure to radon via inhalation or ingestion. nevertheless, epidemiological evidence shows a clear link between breathing high concentrations of radon and incidence of lung cancer. arthritis in the late 20th century and early 21st century, some "health mines" were established in basin, montana, which attracted people seeking relief from health problems such as arthritis through limited exposure to radioactive mine water and radon. the practice is controversial because of the well-documented ill effects of high-dose radiation on the body. radon has been alleged to induce beneficial long-term effects, although proper clinical trials have not been performed. bathing radioactive water baths have been applied since 1906 in jáchymov, czech republic, but even before radon discovery they were used in bad gastein, austria. radium-rich springs are also used in traditional japanese onsen in misasa, tottori prefecture. drinking therapy is applied in bad brambach, germany. inhalation therapy is carried out in gasteiner-heilstollen, austria, in kowary, poland and in boulder, montana, united states. in the united states and europe there are several "radon spas", where people sit for minutes or hours in a high-radon atmosphere in the belief that low doses of radiation will invigorate or energize them. radiotherapy radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made in particle accelerators and nuclear reactors. radon has been used in implantable seeds, made of gold or glass, primarily used to treat cancers. the gold seeds were produced by filling a long tube with radon pumped from a radium source, the tube being then divided into short sections by crimping and cutting. the gold layer keeps the radon within, and filters out the alpha and beta radiation, while allowing the gamma rays to escape (which kill the diseased tissue). the activities might range from 0.05 to 5 millicuries per seed (2 to 200 mbq). the gamma rays are produced by radon and the first short-lived elements of its decay chain (218po, 214pb, 214bi, 214po). radon and its first decay products being very short-lived, the seed is left in place. after 11 half-lives (42 days), radon radioactivity is at 1/2000 of its original level. at this stage, the predominant residual activity is due to the radon decay product 210pb, whose half-life (22.3 years) is 2000 times that of radon, and its descendants 210bi and 210po, totaling 0.03% of the initial seed activity. health policies federal radon action plan the federal radon action plan, also known as frap, was created in 2010 and launched in 2011. it was piloted by the u.s. environmental protection agency in conjunction with the u.s. departments of health and human services, agriculture, defense, energy, housing and urban development, the interior, veterans affairs, and the general services administration. the goal set forth by frap was to eliminate radon induced cancer that can be prevented by expanding radon testing, mitigating high levels of radon exposure, and developing radon resistant construction, and to meet healthy people 2020 radon objectives. they identified the barriers to change as limited public knowledge of the dangers of radon exposure, the perceived high costs of mitigation, and the availability of radon testing. as a result, they also identified major ways to create change: demonstrate the importance of testing and the ease of mitigation, provide incentives for testing and mitigation, and build the radon services industry. to complete these goals, representatives from each organization and department established specific commitments and timelines to complete tasks and continued to meet periodically. however, frap was concluded in 2016 as the national radon action plan took over. in the final report on commitments, it was found that frap completed 88% of their commitments. they reported achieving the highest rates of radon mitigation and new construction mitigation in the united states as of 2014. frap concluded that because of their efforts, at least 1.6 million homes, schools, and childcare facilities received direct and immediate positive effects. national radon action plan the national radon action plan, also known as nrap, was created in 2014 and launched in 2015. it is led by the american lung association with collaborative efforts from the american association of radon scientists and technologists, american society of home inspectors, cancer survivors against radon, children's environmental health network, citizens for radioactive radon reduction, conference of radiation control program directors, environmental law institute, national center for healthy housing, u.s. environmental protection agency, u.s. department of health and human services, and u.s. department of housing and urban development. the goals of nrap are to continue efforts set forth by frap to eliminate radon induced cancer that can be prevented by expanding radon testing, mitigating high levels of radon exposure, and developing radon resistant construction. nrap also aims to reduce radon risk in 5 million homes, and save 3,200