Updated November 2026 · Data: CDC National Environmental Public Health Tracking Network (radon test results from laboratories, 2008–2017), EPA Map of Radon Zones · Sources and method
Radon testing laboratories reported 280 pre-mitigation radon tests from buildings in San Mateo County, California between 2008 and 2017, according to the CDC Tracking Network. 9.2% of those tests were at or above the EPA action level of 4 pCi/L, and the median result was 0.9 pCi/L. That is roughly 1 in 11 tested buildings. The county’s share of high results is about the same as the California figure of 10.7% and below the U.S. figure of 36.4%. San Mateo County is in EPA radon Zone 2 (moderate potential, 2 to 4 pCi/L).
These are area results, not your home’s level. Homes next to each other can have very different radon levels. The only way to know yours is to test.
Radon test results in San Mateo County
| Test result | San Mateo County | California | United States |
|---|---|---|---|
| Below 2 pCi/L | 74.1% | 76.3% | 38.8% |
| 2 to 4 pCi/L | 16.7% | 13.0% | 24.8% |
| 4 pCi/L or higher | 9.2% | 10.7% | 36.4% |
About 74% of tests in San Mateo County came back below 2 pCi/L, and 16.7% fell between 2 and 4 pCi/L, where the EPA suggests considering a fix. The average result (2.0 pCi/L) is well above the median (0.9 pCi/L) because a smaller number of very high readings pull the average up; the highest test reported was 58.2 pCi/L.
How San Mateo County compares with other California counties
| County | Tests | 4 pCi/L or higher | Median (pCi/L) |
|---|---|---|---|
| San Mateo County (this page) | 280 | 9.2% | 0.9 |
| El Dorado County | 224 | 30.3% | 1.6 |
| Nevada County | 203 | 25.0% | 2.0 |
| Santa Barbara County | 207 | 17.6% | 1.2 |
| Ventura County | 203 | 11.9% | 1.0 |
| Los Angeles County | 1,367 | 9.5% | 0.7 |
| Santa Clara County | 447 | 9.2% | 0.9 |
| Contra Costa County | 177 | 8.8% | 0.9 |
| Sacramento County | 131 | 7.7% | 0.9 |
All California counties, the radon zone map and state contacts: Radon in California.
How to test your home in San Mateo County
- The California Indoor Radon Program (CDPH) can tell you about free or discounted test kits and certified professionals (phone: 916-716-3697).
- Short-term test kits take 2 to 4 days. Put the kit in the lowest lived-in level of the home, with windows and doors closed. See how to test for radon.
- If the result is 4 pCi/L or higher, the EPA recommends fixing the home; between 2 and 4 pCi/L, consider a fix. Read what radon level is safe.
- A radon mitigation system usually lowers levels by 80% to 99%. Typical prices are in our radon mitigation cost guide.
Frequently asked questions
Is radon a problem in San Mateo County?
9.2% of 280 pre-mitigation radon tests in San Mateo County from 2008 to 2017 were at or above 4 pCi/L, according to CDC Tracking Network laboratory data. The county is in EPA radon Zone 2. Any home can have high radon, so testing is the only way to know.
What is the average radon level in San Mateo County?
The median test result in San Mateo County is 0.9 pCi/L and the mean is 2.0 pCi/L (CDC Tracking Network, 2008–2017). For comparison, the average indoor radon level in U.S. homes is about 1.3 pCi/L, according to the EPA.
How does San Mateo County compare with the rest of California?
San Mateo County ranks 19 of 32 California counties (with at least 20 tests) by the share of tests at or above 4 pCi/L. Statewide, 10.7% of 5,663 laboratory tests reached that level.
Should I test if my neighbor’s result was low?
Yes. Radon depends on the soil and rock under each building and on how the building is built, so two houses on the same street can have very different results.
Sources and method
The CDC National Environmental Public Health Tracking Network collects radon test results from radon testing laboratories and summarizes them by county over 10-year periods; the latest period available is 2008–2017. Numbers describe buildings that were tested (mostly homes tested by their owners), not a random sample of every home, and counties with few tests are less reliable. We only publish county pages where at least 100 buildings were tested. State and U.S. figures on this page are weighted by the number of tests in each county. Data retrieved in November 2026.
- CDC Tracking Network Data Explorer: Radon Tests from Labs (median, mean, maximum, number of buildings tested and percent by level, 2008–2017)
- CDC: Radon Testing data
- U.S. EPA: EPA Map of Radon Zones (June 2024 data file)
- California Indoor Radon Program (CDPH)