|Fig. 1: Radon decay series. (Source: Wikimedia Commons)|
Although there has historically been a large focus on outdoor air pollution, people now spend almost 90% of their time indoors, so there are many public health concerns related to the quality of indoor air and indoor air pollution.  One of the primary causes of indoor air pollution is radon and its decay products.  Radon (chemical symbol Rn) is an odorless, colorless, and tasteless radioactive gas that results from the natural decay of uranium and radium, both of which can be found in building materials, domestic water sources, and most rocks and soils.  It can enter from the ground through openings in the walls and floors of buildings, and pose short and long term effects on health.  Radon undergoes radioactive decay into radioactive radon decay products, emitting ionizing radiation that can pose both short and long term effects on health. 
The uranium isotope U-238 undergoes an α decay, two β- decays, and then another two α decay reactions to Ra-226.  Ra-226 then undergoes alpha decay to produce Rn-222 (see Fig. 1).  Because Rn-222 is a noble gas, it is relatively stable, allowing it to travel through the air.  However, the short-lived decay products of radon especially polonium, lead, and bismuth are chemically active and thus can be collected in the lungs.  The Pb-210 that results from the α decay of polonium is the most important factor in determining radiation dose, and poses the main risk. 
Radon can be found all over the United States, and it is a known human lung carcinogen, responsible for the majority of the general publics radiation exposure.  Even though most inhaled radon is quickly exhaled, the decay products are able to deposit within the lung and irradiate the cells there, increasing the risk of lung cancer.  Even small amounts of radon can lead to lung cancer, and radon is the second leading cause of lung cancer and is estimated to result in approximately 21,000 lung cancer deaths per year.  Researchers Darby et al. used a case control study conducted in nine different European countries to estimate a 16% excess risk in lung cancer per 100 Bq/m3, causing the estimated percentage of lung cancer deaths due to radon to be 9%.  There is currently no evidence showing a level of exposure below which radon is harmless, so even small exposures to radon can be very detrimental to health.  For these reasons, it is important that homeowners test and fix their homes for radon. 
There are many ways to control the indoor concentrations of Rn-222 and its progeny. The easiest method is to increase the air-exchange rate, which is the rate at which the indoor air is exchanged for outdoor air.  People can do so by opening windows to increase ventilation and installing fans within buildings.  People can also reduce indoor air pollution by decreasing radon sources and preventing the transport of radon to the interior by sealing materials that have high radiation rates and plugging cracks and holes through which air with high levels of Rn-222 can enter from the surrounding soil.  Finally, people can remove Rn-222 and its progeny from indoor air through special filtration methods.  Such methods include filtration with fiber, electrostatic, or charcoal filters, mixing the indoor air to allow Rn-222 and its progeny to deposit within the structure or ventilation system, or space-charging to remove progeny ions. 
© Catherine Raquel. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. D. Spengler and K Sexton, "Indoor Air Pollution: a Public Health Persepective," Science, 221, 9 (1983).
 J. Zhang and K. R. Smith, "Indoor Air Pollution: a Global Health Concern," Brit. Med. Bull. 68, 209 (2003).
 "EPA Assessment of Risks from Radon in Homes," U.S. Environmental Protection Agency, EPA 403-R-03-003, June 2-003.
 "Sources and Characterization of Indoor Pollution," in Indoor Pollutants (National Academies Press, 1981) p. 57.
 S. Darby et al., "Radon in Homes and Risk of Lung Cancer: Collaborative Analysis of Individual Data from 13 European Case-Control Studies," Brit. Med. J. 330, 223 (2005).