Indoor Radon

About Radon

Radon gas is a naturally occurring radioactive gas that is invisible and odorless. It forms from the radioactive decay of small amounts of uranium and thorium naturally present in rocks and soils so some radon exists in all rocks and soils. Certain rock types, such as black shales and certain igneous rocks, can have uranium and thorium in amounts higher than is typical for the earth’s crust. Increased amounts of radon will be generated in the subsurface at these locations. Because radon is a gas, it can easily move through soil and cracks in building slabs or basement walls and concentrate in a building’s indoor air. Areas with higher amounts of radon in the underlying rocks and soil are likely to have higher percentages of buildings with indoor radon levels in excess of U.S. Environmental Protection Agency guidelines, and incidences of very high indoor radon levels are more likely in these areas.

Breathing air with elevated levels of radon gas results in an increased risk of developing lung cancer. Radon-222 is the isotope of most concern to public health because it has a much longer half-life (3.8 days) than other radon isotopes (radon-219 at 4 seconds and radon 220 at 55.3 seconds). The longer half-life allows radon-222 to migrate farther through the soil, so much more radon-222 is usually available to enter buildings than any of the other radon isotopes. Not everyone exposed to radon will develop lung cancer, but U.S. EPA and the National Cancer Institute estimate the annual number of lung cancer deaths in the United States attributable to radon is between 7,000 and 30,000. The average concentration of radon in American homes is about 1.3 picocuries per liter and the average concentration in outdoor air is about 0.4 picocuries per liter. The U.S. EPA recommends that individuals avoid long-term exposures to radon concentrations above 4 picocuries per liter. The only way to know what the radon level is in a building or home is to test the air. Fortunately, radon testing is relatively simple and inexpensive. If indoor-air testing indicates radon levels exceeding 4 picocuries per liter, the U.S. EPA recommends remediation actions be considered.


radon - periodic table element

Radon -  In its natural state radon is an odorless, colorless, tasteless, radioactive gas. It is the only naturally radioactive gas and is the heaviest of all of the elements that occur as gasses at room temperature and pressure conditions. Radon is one of a number of intermediate radioactive elements formed during the radioactive decay of uranium-238, uranium-235 or thorium-232 isotopes to form stable, non-radioactive isotopes of lead. Radon-222 is the radon isotope of most concern to public health because of its longer half-life (3.8 days). It is one of the decay products of uranium-238. It is formed by the radioactive decay of its immediate “parent isotope” radium-226 (1620-year half-life). Radon-222 decays to its “daughter isotope” polonium-218 (3.05-minute half-life). Click on image for larger view.    

radon detectors

Radon Detectors -  The photograph shows two types of inexpensive indoor radon level detectors. The short-term detector (a) uses activated charcoal to adsorb radon from the air and are typically used for tests of 2 to 7 days duration. The long-term  alpha-track detector (b) consists of a piece of special plastic inside a container. It is typically used for tests of 91 days or more. The detectors are sent to special laboratories for analysis when the collection time has expired. Click on image for larger view.     


The California Department of Public Health Radon Program

Radon Measurements by Zip Code

In California, the Department of Public Health (CDPH), Radon Program, collects radon test data for buildings throughout the state.  The data are maintained in a digital database.  The database is used to help CDPH determine areas with excessive indoor radon levels, determine areas that may need testing, and inform the public of the results.  Click here to view the CDPH Radon Measurements by Zip Code database.  This and other reports available on the CDPH  and CGS websites require Adobe Acrobat Reader to view and download.  If you do not already have Acrobat installed you can download it for free here or click on the icon at the bottom of the page.​

CGS Collaboration with CDPH

The California Geological Survey (CGS) staff have  worked with the California Department of Public Health Radon Program since 1989 to provide that program with geologic information to help identify those areas of California with increased potential for elevated indoor-radon levels. The following maps and reports about radon in California were produced as a result of that cooperative effort.

Interactive Radon Map of California

Go to the Interactive Radon Map of California

The Interactive Radon Map provides radon potential information for areas in California where CGS has completed radon potential maps.  "Radon potential" is the chance of indoor-air in a house exceeding the U.S. EPA recommended radon gas concentration action level of 4 picocuries per liter in a two to three day screening test.  The map is informational, not regulatory.  The map cannot be used to determine the indoor-air radon level of a particular home or building.  The only way to know the radon concentration within a home or building is to actually test for it.

The Interactive Map allows one to identify locations of interest by entering an address or geographic name (for example, John Doe School) in the search box and clicking on the magnifying glass symbol.  If the location is in an area where CGS has completed radon mapping, clicking on the location symbol will cause a pop-up panel to appear containing radon potential information for the location, as well as links to the related CGS radon potential map and report. Locations near boundaries between different radon potential areas will show information for both areas.  If the top of the pop-up panel shows "1 of 2" or "1 of 3," click the small triangles to toggle back and forth between the different radon potential information boxes. Geographic Information System (GIS) files are also available for download.   Please read the "About" and the "Using the Map" information boxes before using the Interactive Map.   The sections explain how to use the map and give further information about the maps and radon.  Click here to go to the Interactive Map.

Downloadable Radon Maps and Reports

(Paper copies of radon reports with maps may be purchased through the CGS Publications) sales desk.