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Get Informed About Radon

Learn more about radon and other home maintenance items. Purchase the California Building Performance Guidelines at

General Subject Information: 
Radon is a naturally occurring radioactive gas that is a result of decaying thorium and uranium.  Radon typically comes from rocks containing uranium and thorium, like certain granites or shales.  The colorless and odorless gas can be found in the air, or it can be absorbed into ground water and then subsequently released in the air.  Radon is considered to be chemically inert, meaning it does not readily combine with other chemicals.  However, certain levels of radon exposure can be hazardous to human health.

Why is it important to know about radon? 
Radon is classified as a human carcinogen by the Environmental Protection Agency (EPA). It is the leading cause of lung cancer among non-smokers and the second leading cause among smokers.  However, any cancer resulting from inhaling radon is not likely to become apparent for at least 20-30 years after initial exposure.  The level of radon exposure, duration of exposure, and use of tobacco (smoking) are factors in determining the risk of developing lung cancer.  Exposure to radon does not result in acute respiratory symptoms such as colds, asthma, or allergies.

A standard unit of measurement for radon is picocuries per liter of air (pCi/L). In the United States the average level of radon found indoors is 1.3 pCi/L, but it can range from 0.25 to over 3,000 pCi/L. There is insufficient data to define a “safe” or harmless level of radon, though it is accepted that the greater the level of exposure and the longer duration of exposure, the greater the health risk.  The EPA guideline states that radon levels should not exceed 4 pCi/L indoors.  If the radon level of your home measures above 4pCi/L, the homeowner should consider a radon mitigation system.

Radon gas enters the home through the soil from cracks and openings in concrete slabs, crawl spaces, floor drains, sumps, and concrete blocks.  Generally, living areas that are closest to the soil will have the highest levels of radon, as compared to living areas or rooms on second stories.  Radon can also be present in tap water, as it can be absorbed into the ground water from soil containing radon.  Radon present in water can be released when showering, washing dishes, or washing clothes.  Radon can also be present in water when the water source is a well that is exposed to uranium and radium rock strata.  Radon is of more of a concern when it comes from this type of source.  A granite countertop is not considered harmful because of its low level of emission.

Builder Responsibility:
Within certain counties of the following states, the builder is required to install an approved radon mitigation system:   
• Florida
• Maine
• Maryland
• Michigan
• Minnesota
• New Jersey
• Oregon
• Virginia
• Washington

Each state has slightly different requirements.  As the harmful effects of radon become more widely known more states are expected to institute radon regulations.  The builder is not responsible for the presence of radon.

Homeowner Responsibility: 
Whether the house contains a radon mitigation system or not, it should be tested.  It is not possible to test for radon on a lot before a house is built.  Results can vary from house-to-house, and from street-to-street. 

  • Homeowners may also consult a government agency to help them determine the amounts of radon present in the home and any recommended subsequent actions.   To get more information on radon testing, call 1-800-SOS-RADON.
  • A water test should be considered, especially if the indoor air levels of radon are at or above the EPA guideline of 4pCi/L.  The water company that supplies the house should have information about the source of the water and any radon tests performed.  If the house has water supplied by a well, homeowners should contact a laboratory certified for radon testing to perform a water test. 
  • For more information on radon, visit
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