Radon in California: County Zones and How to Test

EPA assigns radon zones to counties, not to whole states. On the EPA Map of Radon Zones, drawn in 1993 and not revised since, California's 58 counties split 2 in Zone 1 (highest potential), 29 in Zone 2 (moderate) and 27 in Zone 3 (low). The zones describe predicted county averages, and EPA's own instruction is: "The map should not be used to determine if a home in a given zone should be tested for radon. Homes with elevated levels of radon have been found in all three zones. All homes should be tested for radon."

California has 2 Zone 1 counties on the 1993 EPA map: Santa Barbara County, Ventura County. Zone 1 means EPA predicted a county average indoor screening level above 4 pCi/L; it is not a reading of any home.

No, and that answer is EPA's own. The map predicts county averages from 1993 data; it does not measure homes. EPA's words: "The map should not be used to determine if a home in a given zone should be tested for radon. Homes with elevated levels of radon have been found in all three zones. All homes should be tested for radon." A short-term test costs little and is the only way to know your own number.

EPA's numbers, quoted directly: "The EPA recommends homes be fixed if the radon level is 4 pCi/L (picocuries per liter) (150 becquerels per meter cubed (Bq/m3)) or more. Because there is no known safe level of exposure to radon, the EPA also recommends that Americans consider fixing their home for radon levels between 2 pCi/L and 4 pCi/L (75 - 150 Bq/m3)." This site adds no threshold of its own.

January is National Radon Action Month on EPA's calendar, and winter is the practical testing season everywhere, because short-term tests want closed-house conditions. EPA's test instructions: "Follow the instructions that come with your test kit. If you are doing a short-term test, close your windows and outside doors and keep them closed as much as possible during the test." Cold months are when windows and doors stay shut anyway, in California as anywhere.

Here is EPA's answer, verbatim: "Radon is the number one cause of lung cancer among non-smokers, according to EPA estimates. Overall, radon is the second leading cause of lung cancer. Radon is responsible for about 21,000 lung cancer deaths every year. About 2,900 of these deaths occur among people who have never smoked." The EPA and the U.S. Surgeon General recommend testing every home. Read more at https://www.epa.gov/radon/health-risk-radon, and talk to your doctor about personal risk, particularly if you smoke.

The 1993 EPA zones on this page are the newest county-level assignment published as data for California: its radon program had no county-level test result summary published as data as of 2026-09-01. The state program (California Department of Public Health, Indoor Radon Program) is the right place to ask for anything newer.