Washington — 4.0M metro

Radon in Seattle,
WA

Washington State sits primarily in EPA Zone 2. King County (Seattle) shows moderate radon risk, with Washington DOH reporting 20% of tested homes exceeding action levels statewide. East King County communities near the Cascades show higher rates than the urban core.

Risk Level

Moderate Risk

1–3 pCi/L

Typical range (area avg.)

EPA action level: 4 pCi/L

Why Seattle Has Moderate Risk Radon

Western Washington's geology is dominated by volcanic and marine sedimentary rocks with moderate uranium content. The Cascade foothills east of Seattle contribute some radon, while Puget Sound lowlands sit on thick glacial deposits of moderate uranium content.

Local Housing and Radon Risk

Seattle's hilly terrain means many homes are built into slopes with partial daylight basements or full walk-out basements. These below-grade spaces can concentrate radon even in a lower-risk region. Older Craftsman homes in Queen Anne, Capitol Hill, and Ballard frequently have unfinished basement storage areas.

Testing Advice for Seattle Residents

Washington DOH provides certified radon contractor listings. Seattle's frequent rain actually suppresses radon entry in some homes by saturating surface soils, but this effect is less reliable than testing. Test during dry winter months for accurate results.

How to Test Your Seattle Home

Step 1: Choose Your Test Type

For initial screening, a charcoal short-term test kit ($12–$25) placed for 48–96 hours provides a quick result. For the most accurate reading, a long-term alpha-track test ($20–$35) left in place for 90+ days accounts for seasonal variation — especially important in Seattle's climate.

Step 2: Place the Test Correctly

Test in your lowest livable level — basement if you have one, otherwise the first floor. Keep the test 20 inches from the floor, away from windows and exterior walls. Close windows and doors 12 hours before and during the test for accurate results.

Step 3: Interpret Your Results

Below 2 pCi/L: low concern, retest every 2 years. 2–4 pCi/L: consider mitigation, retest in 2 years. 4 pCi/L or higher: the EPA recommends mitigation. Above 8 pCi/L: prioritize mitigation promptly.

Step 4: Hire a Certified Mitigator if Needed

If your test result is at or above 4 pCi/L, hire a certified radon mitigator. In Washington, always verify your contractor's state certification. Sub-slab depressurization — the most effective mitigation method — typically costs $950–$1,800 installed in the Seattle area.

Frequently Asked Questions — Radon in Seattle

Is radon a real problem in Seattle?

Yes. Washington State sits primarily in EPA Zone 2. King County (Seattle) shows moderate radon risk, with Washington DOH reporting 20% of tested homes exceeding action levels statewide. East King County communities near the Cascades show higher rates than the urban core. The EPA recommends all homes be tested regardless of location — radon levels can vary significantly even between neighboring properties.

How much does radon mitigation cost in this area?

In the Seattle area, professional radon mitigation (sub-slab depressurization) typically costs $950–$1,800 installed. This is a one-time cost that reduces radon by 50–99%. The system requires minimal maintenance and a mitigator can typically complete the work in 3–5 hours.

Should I test if I have a new home?

Absolutely. New homes built with radon-resistant features still show elevated levels in testing — construction conditions and local geology vary too much to assume any home is safe. Test within the first year of occupancy, especially if your home has a basement.

Can I test radon myself, or do I need a professional?

DIY testing is reliable and recommended as a first step. Short-term kits ($12–$25) and long-term kits ($20–$35) are accurate when placed and returned correctly. Professional testing is only required for real estate transactions in some states. If your DIY test shows elevated levels, use a certified professional for both confirmation testing and mitigation.