What Causes Radon in Basements?

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Radon is a naturally occurring radioactive gas that comes from the decay of uranium that is contained in the rock and soil beneath the basement (under the foundation) of every home. A home's basement is in constant contact with the ground, which makes it a primary entry point for radon gas.

Sources of High Levels of Radon in Basements

What is radon and why does it accumulate in homes?

Radon gas can seep into basements through multiple entryways, often going unnoticed until levels become hazardous. The most common pathways include:

• Cracks in concrete floors and walls.
• Gaps around pipes and wiring.
• Poorly sealed sump pump pits.
• Uncovered crawl spaces.
• Porous foundation materials like cinder block or concrete.
• Expansion joints and wall-floor interactions.

Contributing Factors of High Radon in Basements

Since radon is a dense gas, it accumulates in low-lying areas, making radon in basement spaces a widespread concern. Conditions inside your basement can contribute to the radon level as well.

• Air pressure differential. When the pressure inside your basement varies greatly from the pressure outside it can create what is called "stack effect". This process draws air more forcefully between the high and low-pressure areas and can pull more air from the rock and soil beneath your home impacting your radon levels.

• Structural feature in a home can also increase the risk of high radon level infiltration. Expansion joints, wall-floor intersections and porous foundation materials all create opportunities for radon to enter. Homes with aging foundations or extensive underground utility connections may also experience a radon problem due to unsealed access points.

• Appliances in the basement. Exhaust fans, water heaters, furnaces, fireplaces and dryers that are in your basement reduce the air pressure indoors. This increases the stack effect, especially in airtight homes.

Because these pathways allow radon to move freely into indoor air, monitoring radon concentration is essential. Homeowners should periodically check for foundation cracks, maintain proper sealing in vulnerable areas and use radon test kits to track fluctuations over time. Identifying and addressing radon exposure risks early can help minimize health concerns.

See Related Article: Is Radon Only in Basements?

Prevent High Levels of Radon in Basements

Radon gas is present in every home at varying levels, and it is nearly impossible to have a radon level of zero. Make sure that you measure with a radon detection kit in the lowest most lived-in areas of your home, which can often be the basement. If a radon test detects levels of 4.0 pCi/L or higher in the United States or 200 Bq/m3 or higher in Canada, immediate action is necessary to lower exposure.

Simple ways to lower your radon levels include:

• Installing, replacing or sealing a sump well cover can reduce your levels because it closes an exposed area of the earth with direct access into your basement.

• Opening windows and doors (when the weather permits), to allow radon gas to dissipate with the fresh air.

• Maintain consistent indoor temperatures and seal entry points rather than relying solely on temperature adjustments to reduce the stack effect (especially when there are extreme outdoor temperatures).

What to do After Radon Detection

If you have consistently high radon levels a radon mitigation system installed by a certified radon mitigation professional is the only way to reduce a dangerous level.

If a radon test detects levels of 4.0 pCi/L or higher in the United States or 200 Bq/m3 or higher in Canada, immediate action is necessary to lower exposure.

• Improve ventilation and seal common entry points to help reduce radon buildup.

• Sub-slab depressurization is the most widely used mitigation technique, involving a vent pipe and fan system that redirects radon gas from beneath the foundation to the outside air.

• Crawl space encapsulation and increased airflow through HVAC adjustments – can also help reduce radon concentration in homes.

Working with a qualified and certified radon mitigation specialist is recommended to assess the situation accurately and install the proper system. Professionals have the tools and expertise needed to create a long-term solution that effectively lowers radon levels and maintains indoor air quality.

Health Risks of Radon Exposure

Long-term exposure to elevated radon levels poses serious health risks. As radon decays, it releases radioactive particles that can be inhaled into the lungs, damaging tissue over time.

• Radon causes of lung cancer. The EPA identifies radon as the second leading cause of lung cancer, responsible for thousands of deaths every year.

• Leading cause of lung cancer in nonsmokers. Radon is the #1 cause of lung cancer in nonsmokers.

• Greater risk to smokers. Smokers face an even greater danger when exposed to high radon levels. The combination of smoking and radon exposure dramatically increases the likelihood of developing lung cancer compared to either risk factor alone.

• Radon has no odor, taste or visible presence. Homeowners must take a proactive approach to detection and radon mitigation. Addressing radon concerns early can significantly reduce exposure and protect long-term health.

Regional and Soil Conditions That Increase Basement Radon Risk

Radon levels vary dramatically from one home to the next, and a big part of the explanation comes down to regional geology and soil conditions. Even within a single neighborhood, one house may measure safe levels while the property next door records an elevated radon level.

• Geological formations
  Radon is created naturally as uranium breaks down in soil and rock. Some regions sit atop granite, shale, or limestone with uranium deposits, which release more gas into the soil.

• Soil composition
  While environmental factors explain broad risk patterns, local differences in rock and soil type make every property unique. Loose, sandy soil allows gas to travel freely, while dense clay slows its movement. Moisture content matters because waterlogged soil may temporarily hold gas in place, while dry conditions increase the ease of movement. These variations help explain why your indoor radon level might change across seasons or even during the same month.

• Foundation design
  Homes built with porous building materials such as cinder block provide more openings for gas entry. Even poured concrete is not airtight and can allow small amounts of gas to pass through. Over time, cracks and settling increase the number of radon entry points in foundation slabs and walls, raising the risk of exposure.

• Soil gas pressure
  Radon gas pressure beneath homes builds naturally as uranium decays underground. When the air inside your house is at lower pressure than the soil outside, this air pressure differential pulls gas upward. Basements are especially vulnerable because they are in direct contact with the soil, which is why testing often shows radon seepage into basement spaces at higher concentrations than upper floors.

• Basement concentration
  Radon gas accumulates in the lowest areas because it is dense and because basements often have more cracks, sumps, and utility penetrations. Many of the common causes of basement radon trace back to construction practices – unsealed joints, settling cracks, or penetrations for plumbing and wiring.

Slab-on-grade homes are also vulnerable. Tiny fissures allow gas to travel upward, creating a measurable problem of radon in concrete slab basements. While some homeowners assume unfinished areas are less important, testing consistently shows that radon levels in unfinished basements can still migrate to the upper floors and impact the overall air quality of the home.

EPA radon zone maps are a helpful starting point, but they are not definitive. A house in a low-risk area may still record high levels due to its soil and construction. Testing is the only way to know the actual conditions in your home. Learning how to test for radon gives you practical steps to measure exposure and decide whether mitigation is necessary.

Regional geology, soil permeability, and building design all interact to determine the level of risk. Homeowners who understand these factors are better prepared to monitor their properties and take decisive action when testing shows exposure.

Radon Testing and Its Importance 

Why test for radon after a home renovation? Any structural changes – including new additions, basement remodels or foundation repairs – can alter airflow patterns and create new radon entry points. Testing helps homeowners identify and address any newly introduced radon risks.

Two main types of radon testing are available:

• Short-term tests provide quick results within 2–7 days, making them useful for initial assessments.

• Long-term tests measure radon levels over 90 days or more, offering a more accurate representation of seasonal variations.

After testing, results should be carefully reviewed. If radon concentration levels are at or above 4.0 pCi/L, homeowners should take corrective action, starting with sealing entry points and considering a radon mitigation system if needed. Regular testing – even in homes with previously low readings – is essential for maintaining a safe indoor environment.

Exposure to radon gas is a preventable health risk, and taking proactive steps through testing and radon mitigation can significantly reduce potential harm.