What Is Radon and Why Should You Care?

Radon is a naturally occurring radioactive gas formed by the decay of uranium in soil, rock, and water. It is odorless, colorless, and tasteless, which means you cannot see, smell, or taste it. Because of these properties, radon can accumulate indoors without any obvious warning signs. The primary health concern is that prolonged exposure to elevated radon concentrations significantly increases the risk of developing lung cancer. The U.S. Environmental Protection Agency (EPA) estimates that radon is responsible for about 21,000 lung cancer deaths annually in the United States, making it the second leading cause of lung cancer after smoking. Understanding how radon enters homes and what levels are considered safe is essential for protecting your family.

How Radon Moves Into Residential Spaces

Radon migrates from the ground into buildings through cracks in concrete slabs, joints between floors and walls, gaps around service pipes, floor drains, and even through the water supply in some cases. The primary route is through the soil beneath the foundation. Because radon is a gas, it moves from areas of higher pressure underground to lower pressure inside the home. This pressure difference is often driven by the stack effect, where warm air rises and creates a slight vacuum at lower levels, drawing radon inside. Homes of any age, construction type, or location can have elevated radon levels, which is why testing is the only way to know the actual concentration.

Health Risks: The Science Behind Radon Exposure

When radon decays, it emits alpha particles that can damage the DNA in lung tissue when inhaled. Over many years of exposure, this damage can lead to mutations that cause cancer. The risk is dose-dependent: higher concentrations and longer exposure times increase the chance of developing lung cancer. Smokers face a particularly elevated risk because the combination of tobacco smoke and radon multiplies the danger. According to the World Health Organization (WHO), radon is estimated to cause between 3% to 14% of all lung cancers globally, depending on the average radon levels in a given country.

Key Factors That Influence Radon Risk

  • Duration of Exposure: Living in a home with high radon for years presents a greater risk than short-term stays.
  • Concentration Level: The measured radon level in picoCuries per liter (pCi/L) directly correlates with risk.
  • Smoking Status: Smokers exposed to high radon have a risk that is far higher than the sum of either risk alone.
  • Ventilation and Building Structure: Well-sealed homes with poor airflow can trap radon, leading to higher indoor concentrations.

Measuring Radon Levels: Short‑Term vs. Long‑Term Testing

Accurate measurement is the cornerstone of radon management. Two main testing approaches exist:

Short-Term Tests

Short-term tests typically last from 2 to 7 days. They use activated charcoal or electret ion chambers to capture radon and its decay products. These tests are useful for quick screening, especially during real estate transactions. However, because radon levels can fluctuate daily with weather, soil moisture, and building operation, short-term tests may not provide a reliable annual average. For the most accurate baseline, short-term tests should be conducted under closed-house conditions (windows and doors closed, normal HVAC operation) and ideally during cooler months when windows are less likely to be open.

Long-Term Tests

Long-term tests run for 90 days to one year. They provide a much better estimate of the average annual radon concentration because they account for seasonal variations. Many health authorities, including the EPA, recommend long-term testing for making informed decisions about mitigation. After installing a radon reduction system, a long-term test is also the best way to verify that the system is working effectively over time.

Safe Thresholds: What the EPA and WHO Recommend

The most widely accepted action level in the United States is 4 picoCuries per liter (pCi/L). The EPA advises homeowners to take corrective action if testing shows radon levels at or above this threshold. The WHO, however, recommends a more stringent reference level of 2.7 pCi/L (100 Bq/m³). Because there is no known completely safe level of radon – any exposure carries some risk – the goal should be to reduce radon as much as practical, even if the level is below 4 pCi/L.

Understanding pCi/L and Bq/m³

In the United States, radon levels are reported in picoCuries per liter (pCi/L). Internationally, the unit becquerel per cubic meter (Bq/m³) is common. The conversion is: 1 pCi/L = 37 Bq/m³. So an EPA action level of 4 pCi/L equals 148 Bq/m³, and the WHO recommendation of 2.7 pCi/L equals 100 Bq/m³.

Risk Ranges for Residential Spaces

  • Below 2 pCi/L (74 Bq/m³): Low relative risk. Remediation is not typically recommended, but some precautionary measures (like sealing cracks) may still be beneficial.
  • 2 pCi/L to 4 pCi/L (74–148 Bq/m³): Moderate risk. The EPA suggests considering mitigation, especially if you have a smoker in the home or if you plan to sell the property.
  • 4 pCi/L (148 Bq/m³) and above: High risk. The EPA strongly recommends professional radon mitigation to reduce levels as quickly as possible.
  • Above 10 pCi/L (370 Bq/m³): Urgent action required. Levels this high pose a severe lung cancer risk, especially with long-term occupancy.

Radon Mitigation Strategies That Work

If testing reveals high radon levels, don’t panic. Radon reduction systems are effective and usually affordable. The most common technique is sub-slab depressurization (SSD). This involves installing a vent pipe through the foundation slab and connecting it to a fan that continuously draws radon from beneath the house and vents it safely above the roofline. Other approaches include:

Sealing Entry Points

Cracks in the foundation, gaps around utility lines, and joints between walls and floors should be sealed with polyurethane caulk or similar materials. While sealing alone is rarely sufficient to bring high radon levels down to safe thresholds, it is an important complementary measure that improves the efficiency of active mitigation systems.

Improving Ventilation

Increasing the air exchange rate in a home can dilute radon concentrations. This can be achieved through heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs), which bring in fresh outdoor air while exhausting stale indoor air. However, this method is usually less effective than sub-slab depressurization for homes with elevated radon coming from the soil.

Active Soil Depressurization (ASD)

This is a variation of SSD that can be used for homes with crawlspaces or slab-on-grade foundations. ASD systems create a vacuum under the building envelope, preventing radon from entering. Professional installers typically run the vent pipe up through an interior closet or along an exterior wall to minimize aesthetic impact.

Radon Reduction in Water

In homes using private well water, radon can also enter via the water supply. Aeration systems or granular activated carbon (GAC) filters can remove radon from water before it is used. This is only necessary if water testing shows radon levels above 10,000 pCi/L (370,000 Bq/m³), which is exceptionally rare.

Post-Mitigation Testing and System Maintenance

After a radon mitigation system is installed, a follow-up test (preferably long-term) should be performed within 30 days to confirm that radon levels have dropped below 4 pCi/L, and ideally below 2 pCi/L. The system’s fan should run continuously; a manometer (U-tube gauge) is often installed to show that the fan is operating correctly. Homeowners should inspect the manometer monthly and schedule professional check-ups every few years to ensure the system remains effective. If the system fails or the fan stops working, radon levels can return to previous highs within days.

Radon Testing During Real Estate Transactions

Because radon is a health hazard and testing is inexpensive, many home buyers and sellers include radon testing as part of the inspection process. The EPA recommends that any home be tested before a sale, and that the results be shared with the buyer. If high levels are discovered, the buyer may request that the seller install a mitigation system. Most states do not require radon disclosure, but many do, and the practice is becoming standard in regions with high radon potential. Testing during a real estate transaction should be performed by a certified, independent professional to ensure neutrality.

Regional Variations in Radon Potential

Radon levels vary greatly by geographic location. The EPA's Radon Zone Map divides the United States into three zones: Zone 1 (highest potential) includes areas like the upper Midwest, Northeast, and some Rocky Mountain states. Zone 2 has moderate potential, and Zone 3 has low potential. Even within a high-risk zone, individual homes can have low levels, and homes in low-risk zones can have high levels. Local geology, such as granite bedrock or uranium-rich soil, strongly influences radon production. Therefore, every home should be tested regardless of its location.

Myths About Radon That Need to Be Cleared Up

Several misconceptions persist about radon. One common myth is that only older homes have radon problems. In reality, newer homes can be even more airtight, trapping radon effectively. Another myth is that radon is only a problem if you have a basement. Radon can enter homes with slab-on-grade foundations or even crawlspaces. Yet another false belief is that neighboring homes with safe levels guarantee your home is also safe. Radon levels can vary dramatically between adjacent houses due to differences in construction and soil conditions. Finally, some people think opening windows is a permanent solution. While ventilation helps temporarily, it is not a reliable long-term fix, especially during winter months when windows remain closed.

Practical Steps for Every Homeowner

  1. Test your home. Purchase a do-it-yourself radon test kit from a hardware store or online, or hire a certified professional. Start with a short-term test; if the result is 4 pCi/L or higher, follow up with a long-term test.
  2. Consider mitigation if levels are elevated. Even levels between 2 and 4 pCi/L may warrant mitigation, especially if you have children, elderly residents, or smokers in the home.
  3. Hire a certified radon mitigation contractor. Look for professionals with certification from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB).
  4. Retest after mitigation to confirm that levels have dropped.
  5. Retest your home every two years, or after any major renovation that affects the foundation or ventilation.
  6. Test as part of any real estate transaction to protect both buyers and sellers from unexpected liability.

Summing Up: Maintaining Safe Radon Levels

Radon is a serious but manageable health hazard. Understanding radon levels and safe thresholds is the first step toward ensuring your indoor environment is as healthy as possible. The EPA action level of 4 pCi/L serves as a clear benchmark, but achieving levels below 2 pCi/L is a worthwhile goal. Regular testing, prompt mitigation, and periodic system maintenance are the keys to keeping radon out of your home. Because radon is invisible and odorless, you cannot rely on your senses to detect it. Only testing provides the data needed to make informed decisions. With the right knowledge and actions, you can significantly reduce the lung cancer risk posed by this naturally occurring gas.

For the most current recommendations and to find a certified professional, visit the EPA’s Radon page or the WHO’s radon fact sheet.