Radon Testing: Short-Term vs Long-Term Tests Explained

Radon is an invisible, odorless radioactive gas that seeps into homes through cracks in the foundation. It’s the second leading cause of lung cancer in the United States, and testing is the only way to know if your home has elevated levels. Here’s how the two main testing approaches compare.

Short-Term Tests (2 - 7 Days)

Short-term tests give you a quick snapshot of radon levels. The most common type uses activated charcoal canisters placed in the lowest livable level of the home for 48 to 96 hours. After exposure, the canisters are sealed and mailed to a lab for analysis.

Continuous radon monitors (CRMs) are another short-term option, typically used by certified professionals. CRMs record radon levels hour by hour and deliver results immediately. They’re the standard for real estate transactions because they’re tamper-resistant and provide more detailed data than passive canisters.

Short-term tests are useful for initial screening and real estate deals, but they only capture a narrow window. Radon levels fluctuate with weather, soil moisture, and ventilation patterns.

Long-Term Tests (90+ Days)

Long-term tests — usually alpha track detectors — measure radon over 90 days to a full year. Because they average out seasonal fluctuations, they give a more accurate picture of your actual annual exposure. If you’re not in a rush (no real estate deadline), a long-term test is the better way to decide whether you need mitigation.

The EPA Action Level: 4 pCi/L

The EPA recommends taking action if your home tests at or above 4 picocuries per liter (pCi/L). They also suggest considering mitigation for levels between 2 and 4 pCi/L, since there’s no truly safe level of radon exposure. The World Health Organization sets its reference level even lower at 2.7 pCi/L.

When to Test

Every home should be tested at least once, regardless of age, construction type, or geographic location. You should retest if you’ve made structural changes to the foundation, installed a new HVAC system, or haven’t tested in more than two years. Homes with previous mitigation systems should be retested every two years to ensure the system is still working.

For cost information, see our radon mitigation cost guide.

Real Estate Testing

During a home purchase, a short-term CRM test is standard. The monitor is placed for 48 hours under closed-building conditions. If results come back at or above 4 pCi/L, buyers typically negotiate mitigation as part of the sale.

Find certified radon testing professionals in your area to get your home tested.

Codes and Standards Worth Knowing

The EPA’s radon program sets the national action level (4 pCi/L) that drives nearly every mitigation decision, and the American Association of Radon Scientists and Technologists publishes the consensus standards that mitigation systems are designed and tested against, including ANSI/AARST RMS-LB for residential mitigation and MAH for measurement that mitigation systems are designed and tested against. A mitigator who is AARST-certified and references the current ANSI/AARST standard for your installation type is working at trade-association level, not freelancing.

Frequently Asked Questions

How much does a radon test cost? DIY charcoal canister tests cost $15–$30 from a hardware store or through your state radon office. Professional short-term CRM tests run $150–$300 and are standard for real estate transactions. Long-term alpha track tests cost $25–$50 and provide the most accurate annual average. For home sales, a professional test is worth the extra cost because the results carry more weight in negotiation.

Where in my home should radon be tested? Always test in the lowest livable level of the home — typically a basement, or the first floor if there’s no basement. The EPA defines “livable” as any area used regularly for sleeping, recreation, or daily activities. Testing in a crawl space or utility room gives misleading results.

What do I do if my radon test comes back high? If your result is 4 pCi/L or above, get a second confirmatory test (short-term or long-term), then contact a certified radon mitigation contractor for an estimate. Mitigation is highly effective and most systems reduce levels by 80–99%. If your result is between 2 and 4 pCi/L, the EPA recommends considering mitigation, especially in homes with young children or anyone with elevated lung cancer risk. For a full week-by-week response plan, see our 30-day action plan for high radon results.

Can radon levels change over time? Yes. Radon fluctuates with seasons, soil moisture, atmospheric pressure, and ventilation patterns. Levels tend to be highest in winter when homes are sealed and lowest in summer with open windows. See radon levels in winter for why the cold season drives readings up and why it is the best time to test. A long-term test (90+ days) gives the best average. Homes with mitigation systems should be retested every two years to confirm ongoing performance.