Radon-222 has been classified by International Agency for Research on Cancer as being carcinogenic to humans.[43] In September 2009, the World Health Organization released a comprehensive global initiative on radon that recommended a reference level of 100 Bq/m3 for radon, urging establishment or strengthening of radon measurement and mitigation programs as well as development building codes requiring radon prevention measures in homes under construction.[44] Elevated lung cancer rates have been reported from a number of cohort and case-control studies of underground miners exposed to radon and its decay products but the main confounding factor in all miners' studies is smoking and dust. Up to the most of regulatory bodies there is sufficient evidence for the carcinogenicity of radon and its decay products in humans for such exposures.[45] However, the discussion about the opposite results is still going on,[46][47] especially a recent retrospective case-control study of lung cancer risk showed substantial cancer rate reduction between 50 and 123 Bq per cubic meter relative to a group at zero to 25 Bq per cubic meter.[48] Additionally, the meta-analysis of many radon studies, which independently show radon risk increase, gives no confirmation of that conclusion: the joined data show log-normal distribution with the maximal value in zero risk of lung cancer below 800 Bq per cubic meter.[49]