text: 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/m (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/m 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/m) 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/m 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.