text: Radon-222 has been classified by International Agency for Research on Cancer as being carcinogenic to humans. In September 2009, the World Health Organization released a comprehensive global initiative on radon that recommended a reference level of 100 Bq/m 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.
  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. However, the discussion about the opposite results is still going on, 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. 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.
  The primary route of exposure to radon and its progeny is inhalation. Radiation exposure from radon is indirect. The health hazard from radon does not come primarily from radon itself, but rather from the radioactive products formed in the decay of radon. The general effects of radon to the human body are caused by its radioactivity and consequent risk of radiation-induced cancer. Lung cancer is the only observed consequence of high concentration radon exposures; both human and animal studies indicate that the lung and respiratory system are the primary targets of radon daughter-induced toxicity.
  Radon has a short half-life (3.8 days) and decays into other solid particulate radium-series radioactive nuclides.
  Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.
  If the gas is inhaled, the radon atoms decay in the airways or the lungs, resulting in radioactive polonium and ultimately lead atoms attaching to the nearest tissue. If dust or aerosol is inhaled that already carries radon decay products, the deposition pattern of the decay products in the respiratory tract depends on the behaviour of the particles in the lungs. Smaller diameter particles diffuse further into the respiratory system, whereas the larger—tens to hundreds of micron-sized—particles often deposit higher in the airways and are cleared by the body's mucociliary staircase. Deposited radioactive atoms or dust or aerosol particles continue to decay, causing continued exposure by emitting energetic alpha radiation with some associated gamma radiation too, that can damage vital molecules in lung cells,
  by either creating free radicals or causing DNA breaks or damage,
  perhaps causing mutations that sometimes turn cancerous. In addition, through ingestion and blood transport, following crossing of the lung membrane by radon, radioactive progeny may also be transported to other parts of the body.
  The risk of lung cancer caused by smoking is much higher than the risk of lung cancer caused by indoor radon. Radiation from radon has been attributed to increase of lung cancer among smokers too. It is generally believed that exposure to radon and cigarette smoking are synergistic; that is, that the combined effect exceeds the sum of their independent effects. This is because the daughters of radon often become attached to smoke and dust particles, and are then able to lodge in the lungs.
  It is unknown whether radon causes other types of cancer, but recent studies suggest a need for further studies to assess the relationship between radon and leukemia.
  The effects of radon, if found in food or drinking water, are unknown. Following ingestion of radon dissolved in water, the biological half-life for removal of radon from the body ranges from 30 to 70 minutes. More than 90% of the absorbed radon is eliminated by exhalation within 100 minutes, By 600 minutes, only 1% of the absorbed amount remains in the body.