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Radon is a naturally occurring radioactive gas found in soils, rock, and water throughout the U.S.It is produced from the natural radioactive decay of uranium, which is found in rocks and soil. Radon can also be found in water.

Radon escapes easily from the ground into the air, where it disintegrates through short-lived decay products called radon progeny. As radon progeny decay, they emit radioactive alpha particles and attach to aerosols, dust and other particles in the air. As we breathe, radon progeny are deposited on the cells lining the airways where the alpha particles can damage DNA and potentially cause lung cancer.

Radon can seep into homes through cracks in the foundation, walls, and joints, gaps in the floors, small pores in hollow-block walls, sumps and drains. Radon levels are usually higher in basements, cellars or other structural areas in contact with soil.

Radon concentrations can vary between adjacent homes, and can vary within a home from day-to-day and from hour-to-hour. Because of these fluctuations, estimating the annual mean concentration of radon in indoor air requires measurements of mean radon concentrations for at least three months.

EPA recommends that home owners take mitigation action when radon levels exceed above 4 picocuries per liter (pCi/L). To assist homeowners, EPA has divided the country into three radon zones based on expected concentrations of radon gas. Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (picocuries per liter). Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L while Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L.

It is important to note that the EPA recommended action level of 4 pCi/L does not mean that radon concentrations below the action level are safe. In fact, EPA says there is no safe level of exposure to radon. EPA estimates  indoor radon gas is responsible for an estimated 21,100 lung cancer deaths in the United States which makes radon the second leading cause of cancer behind smoking. The cancer estimate was based on an average exposure of 1.3 pCi/l which is obviously below the recommended action level. For example, 2003 study estimated that exposure to 4 pCi/L of radon gas would result in 73 deaths from lung cancer out of 10,000 people who are non-smokers. For exposures at 0.2pCi/L, the risk of death from lung cancer for non-smokers was calculated to be 37 out of 10,000. Even at 1.25 pCi/L, the study estimated that the cancer risk dropped to 23 out of 10,000.

The EPA work was confirmed by World Health Organization  indoor radon study completed in 2009. The study reported that radon in homes is the second leading cause of lung cancer worldwide. The WHO estimated that 3% to 14% of all lung cancers worldwide are attributable to indoor radon.

As a result, EPA recommends that home owners consider “fixing” their homes when the radon level is between 2 and 4 pCi/L. Both EPA  and the U.S. Surgeon General recommend testing all homes below the third floor for radon regardless of geographic location or zone designation.