Data sources: EPA, WHO, CDC Last updated: March 2026

What Is Radon?

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and water. It is colorless, odorless, and tasteless — impossible to detect without testing.

Radon seeps into buildings through cracks in foundations, gaps around pipes, sump pits, and other openings where the building contacts the ground. It can also enter through well water. Once inside, radon accumulates to concentrations that significantly exceed outdoor levels.

The EPA estimates that radon causes approximately 21,000 lung cancer deaths per year in the United States — making it the second leading cause of lung cancer after smoking, and the #1 cause among non-smokers (EPA Radon Overview).

Health Effects

Lung Cancer

Radon decays into radioactive particles (radon progeny) that lodge in lung tissue when inhaled. These particles emit alpha radiation that damages DNA in lung cells, which can lead to cancer over time.

Risk by exposure level (EPA estimates for lifetime exposure):

Indoor Radon Level	Lung Cancer Deaths per 1,000	Comparable Risk
20 pCi/L	36 (non-smokers) / 260 (smokers)	35x average outdoor level
10 pCi/L	18 / 150	Comparable to smoking half a pack per day
4 pCi/L	7 / 62	5x drowning risk
2 pCi/L	4 / 32	2x average indoor level
1.3 pCi/L (avg outdoor)	—	Background level

Synergy with smoking: A smoker exposed to radon has a dramatically higher risk than either exposure alone. Radon + smoking is multiplicative, not additive. If you smoke and have elevated radon, mitigation and quitting smoking together provide the largest risk reduction.

Radon in Water

Radon dissolved in water poses two risks:

1. Inhalation — radon releases from water into indoor air during showering, laundry, and dishwashing (the larger risk)
2. Ingestion — drinking water containing radon (a smaller risk)

The EPA estimates a ratio of approximately 10,000 pCi/L in water = 1 pCi/L added to indoor air. If your well water contains 40,000 pCi/L of radon (not uncommon in granite regions), it adds about 4 pCi/L to your indoor air — matching the EPA action level from water alone.

EPA Guidelines

Radon is not regulated like other contaminants — there is no enforceable MCL for radon in indoor air. Instead, the EPA publishes guidelines:

Guideline	Level	Recommendation
EPA Action Level	4 pCi/L	Mitigate — install a radon reduction system
EPA Consider Mitigation	2–4 pCi/L	Consider fixing, especially in frequently occupied areas
WHO Action Level	2.7 pCi/L (100 Bq/m³)	Mitigate (stricter than EPA)
Average U.S. indoor	1.3 pCi/L	For comparison
Average U.S. outdoor	0.4 pCi/L	Background level

For radon in water, the EPA proposed (but never finalized) an MCL of 300 pCi/L for systems without indoor air programs, and 4,000 pCi/L for systems with air mitigation programs. Some states have set their own standards.

Who Is at Risk?

Radon is found in every state, but concentrations vary dramatically by geology:

EPA Radon Zones

The EPA classifies U.S. counties into three zones based on predicted average indoor radon levels:

• Zone 1 (High potential): Predicted average ≥4 pCi/L. Includes much of the northern tier (Minnesota, Wisconsin, Iowa, North Dakota, Montana), Appalachian region, and parts of the Rockies
• Zone 2 (Moderate potential): Predicted average 2–4 pCi/L
• Zone 3 (Low potential): Predicted average <2 pCi/L

Critical caveat: Zone classification is a starting point, not a guarantee. Homes with 20+ pCi/L have been found in Zone 3 areas, and homes in Zone 1 may test below 2 pCi/L. Geology varies at the neighborhood level — the only way to know is to test.

Check your ZIP code report for the radon zone classification of your area.

Factors That Increase Radon Entry

• Granite and shale bedrock — higher uranium content
• Well-drained soil — allows radon gas to move easily through the ground
• Basement or slab-on-grade construction — more contact with soil
• Negative pressure in the home — heating, exhaust fans, and stack effect pull soil gas indoors
• Cracks and gaps in the foundation — the primary entry pathway
• Well water — dissolved radon enters through the plumbing. Relevant for private well owners

How to Test

Short-Term Test (2–7 Days)

Best for: Initial screening, real estate transactions

• Cost: $15–$30 (DIY kit); $150–$300 (certified professional with continuous monitor)
• Method: Charcoal canister or continuous radon monitor placed in the lowest livable level
• When to use: Quick answer, especially during a real estate transaction
• Limitations: Results vary with weather, ventilation, and season. A short-term test is a snapshot, not a long-term average

Long-Term Test (90+ Days)

Best for: Determining actual annual average exposure

• Cost: $25–$40 (alpha track or electret detector)
• Method: Device placed in the lowest livable level for 90 days to 1 year
• When to use: After moving in, to get the most accurate average
• Advantage: Accounts for seasonal and weather variations. The long-term average is the basis for health risk estimates

Professional Testing

For real estate transactions, hire a certified radon measurement professional (NRPP or NRSB certified):

• Uses continuous radon monitors (CRMs) that log hourly readings
• Provides tamper-evident setup
• Results in 48 hours
• Report accepted by lenders, sellers, and real estate attorneys

Testing Protocol

1. Close windows and exterior doors 12 hours before and during the test (except normal entry/exit)
2. Place the test in the lowest livable level (finished basement, or ground floor if no basement)
3. Do not place in kitchens, bathrooms, laundry rooms, or near drafts (these dilute readings)
4. Do not operate whole-house fans during the test
5. Normal HVAC operation is fine — keep the heating/cooling system running as usual

Radon in Water

Radon in water primarily affects private well users. Public water systems that draw from groundwater may also have elevated radon, but aeration during treatment typically reduces it.

Testing

• When to test: If you have well water in an EPA Zone 1 or 2 area, or if your indoor air radon test is elevated
• Method: Collect a sample in a special radon-in-water kit (the sample must not be exposed to air, which allows radon to escape)
• Cost: $40–$80
• Labs: State-certified labs that offer radon-in-water analysis

Treatment

Method	Removal	Cost	Notes
Granular Activated Carbon (GAC)	95%+	$1,000–$3,000	Most common for residential. The carbon bed holds radioactive radon progeny — requires periodic replacement and proper disposal
Aeration	95–99%	$3,000–$7,000	Bubbles air through water to strip radon. No radioactive waste. Higher cost but lower maintenance

For more on radon as a water contaminant: Radon contaminant page. For product-level comparisons of short-term kits, long-term alpha-track detectors, and continuous monitors, see Best Radon Test Kits.

Mitigation Systems

Sub-Slab Depressurization (SSD)

The most common and effective system for homes with basements or slab foundations:

How it works:

1. A hole is drilled through the basement slab into the gravel layer beneath
2. A PVC pipe runs from below the slab, up through the house, and exits through the roof
3. An inline fan creates negative pressure under the slab, drawing radon out before it enters the home
4. Radon is vented above the roofline where it disperses harmlessly

Performance: Reduces radon levels by 80–99%. Most homes achieve post-mitigation levels below 2 pCi/L.

Cost: $800–$2,500 installed (most homes: $1,000–$1,500)

Operating cost: ~$50–$100/year electricity for the fan. The fan runs continuously.

Lifespan: Fan replacement every 5–10 years ($150–$300). The piping lasts indefinitely.

Sub-Membrane Depressurization

For homes with crawl spaces and exposed dirt floors:

• A heavy polyethylene sheet is sealed over the dirt floor
• A pipe and fan system depressurizes the area below the membrane
• Same principle as SSD but applied to exposed soil

Cost: $1,500–$3,500 (larger area = higher cost)

Heat Recovery Ventilator (HRV)

For homes where sub-slab systems are not feasible:

• Exchanges indoor air with outdoor air while recovering heat
• Dilutes radon rather than actively removing it
• Less effective than SSD (typically achieves 25–50% reduction)
• Better suited as a supplemental measure

Sealing Alone Is NOT Sufficient

Sealing cracks and gaps in the foundation is helpful as a complement to depressurization, but sealing alone does not reduce radon to safe levels. The pressure differential across the slab is the primary driver — sealing without depressurization just shifts the entry point.

Choosing a Contractor

• Hire a NRPP or NRSB certified radon mitigation professional
• Get 2–3 quotes — pricing varies significantly
• Ask for post-mitigation testing (reputable contractors include this)
• Verify the contractor carries liability insurance
• State licensing requirements vary — check your state radon program

Radon in Real Estate

Disclosure Requirements

As of 2026, radon disclosure requirements vary by state:

• Some states require disclosure of known radon test results (e.g., Illinois, Indiana, Maine, Minnesota)
• Other states include radon in general property disclosure forms but don't require testing
• Federal law does not require radon testing or disclosure for residential sales

Regardless of disclosure requirements, the EPA recommends testing before buying any home.

Real Estate Testing Protocol

During a home purchase:

1. Request radon testing as part of your inspection contingency — 48-hour CRM test is standard
2. Negotiate based on results — if radon exceeds 4 pCi/L, request seller-funded mitigation ($800–$2,500)
3. Post-mitigation test — verify the system works (usually included in mitigation contract)
4. Re-test after moving in — your living patterns (HVAC use, window opening) affect radon levels

Negotiating Mitigation

If radon exceeds 4 pCi/L:

• Seller installs mitigation before closing (most common resolution)
• Price credit for buyer to install after closing ($1,500–$2,500 credit)
• Escrow holdback — funds held until mitigation is complete and verified
• Walk away — rare for radon alone, since mitigation is effective and relatively inexpensive

Radon above 4 pCi/L should NOT be a deal-breaker. Mitigation systems work, they're affordable, and they reduce levels to well below 2 pCi/L. It's a solvable problem — unlike some structural issues.

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