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 1,265 pre-mitigation radon tests from buildings in Clark County, Washington between 2008 and 2017, according to the CDC Tracking Network. 25.1% of those tests were at or above the EPA action level of 4 pCi/L, and the median result was 1.8 pCi/L. That is roughly 1 in 4 tested buildings. The county’s share of high results is about the same as the Washington figure of 25.3% and below the U.S. figure of 36.4%. Clark County is in EPA radon Zone 1 (highest potential, predicted average above 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 Clark County
| Test result | Clark County | Washington | United States |
|---|---|---|---|
| Below 2 pCi/L | 52.5% | 57.0% | 38.8% |
| 2 to 4 pCi/L | 22.4% | 17.6% | 24.8% |
| 4 pCi/L or higher | 25.1% | 25.3% | 36.4% |
About 53% of tests in Clark County came back below 2 pCi/L, and 22.4% fell between 2 and 4 pCi/L, where the EPA suggests considering a fix. The average result (3.5 pCi/L) is well above the median (1.8 pCi/L) because a smaller number of very high readings pull the average up; the highest test reported was 75.8 pCi/L.
Recent results reported by Washington
| Year | Buildings tested | Median (pCi/L) | 4 pCi/L or higher |
|---|---|---|---|
| 2019 | 414 | 1.3 | 19.3% |
| 2020 | 355 | 1.2 | 17.6% |
Washington also sends yearly radon test data to the CDC. These newer numbers come from a different collection than the 2008–2017 laboratory figures, so small differences between them are normal.
How Clark County compares with other Washington counties
| County | Tests | 4 pCi/L or higher | Median (pCi/L) |
|---|---|---|---|
| Clark County (this page) | 1,265 | 25.1% | 1.8 |
| Spokane County | 2,249 | 41.4% | 2.9 |
| Stevens County | 131 | 36.2% | 2.3 |
| Thurston County | 174 | 10.1% | 0.7 |
| Pierce County | 153 | 4.9% | 0.7 |
| King County | 950 | 2.6% | 0.7 |
| Snohomish County | 172 | 2.3% | 0.5 |
All Washington counties, the radon zone map and state contacts: Radon in Washington.
How to test your home in Clark County
- The Washington State Department of Health, Radon can tell you about free or discounted test kits and certified professionals (phone: 360-236-3200).
- 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 Clark County?
25.1% of 1,265 pre-mitigation radon tests in Clark 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 1. Any home can have high radon, so testing is the only way to know.
What is the average radon level in Clark County?
The median test result in Clark County is 1.8 pCi/L and the mean is 3.5 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 Clark County compare with the rest of Washington?
Clark County ranks 7 of 20 Washington counties (with at least 20 tests) by the share of tests at or above 4 pCi/L. Statewide, 25.3% of 5,879 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
- CDC Tracking Network Data Explorer: Radon Tests from States (annual, 2019–2020)
- U.S. EPA: EPA Map of Radon Zones (June 2024 data file)
- Washington State Department of Health, Radon