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Radon – What are acceptable levels?

safe radon

For many real estate professionals and the public at large, the answer to what constitutes an acceptable level of radon seems easy.  Anything at 4.0 pCi/L or above warrants mitigation.  Any number under 4.0 is considered safe.

Or is it?

EPA’s Action Level for Radon

One of the first things that we need to straighten out is that there is no such thing as a “harmless” level of radon.  A truly harmless level would be 0.0 pCi/L – not anything below 4.0 pCi/L. The EPA uses 4.0 pCi/L as its “Action Level”.  When above the Action Level, the building should have a mitigation system installed. We must realize that something “non-actionable” is not the same thing as “harmless”.  

The EPA states that the average indoor radon level is 1.3 pCi/L and the average outdoor level is roughly 0.4 pCi/L (but it depends greatly on location).  The eventual goal in a lot of public policy is to have the indoor levels reduced to outdoor levels. Although this may not always be achievable, the EPA’s Home Buyers and Sellers Guide to Radon states that it is possible that most homes can have radon reduced to 2.0 pCi/L or less with the proper mitigation system.  Although the EPA has an Action Level of anything over 4.0, they still advise reducing levels even if radon is between 2.0 and 4.0 pCi/L. 1

International Guidelines

The World Health Organization (WHO) has gone so far as to recommend that radon levels be reduced to 2.7 pCi/L. (2)  This is ⅓ less than what is recommended in the United States. The WHO Handbook on Indoor Radon reports that if this were the new guideline in the US, the number of homes needing a mitigation system would nearly double, but more than 9,000 deaths per year would be prevented. 2   

No Thresholds for Harm

It bears repeating that there is no such thing as a harmless level of this gas. Although lower levels are less risky, there is no threshold below which is risk-free.  This is commonly thought to be true for radiation and other carcinogens. Therefore, many organizations use the principle of “ALARA” for radiation: As Low As Reasonably Achievable.  

Radon gas fluctuates month by month, week by week and even hour by hour.  Testing represents a snapshot in time of the gas levels. We recommend retesting and possibly mitigating of your home if you have a reading of anything over 2.0 pCi/L, especially if you spend a significant amount of time in a basement.  


Life is a series of risk assessments.  Every day we make choices in our lives that may or may not directly affect our health.  Risk is a function of probability and severity. Radon is risky because exposure is probable and its health effects are severe (lung cancer). Compare this to a plane crash that has high severity, but low probability. Radon is responsible for about 21,000 lung cancer deaths every year in the US3, whereas there were no commercial plane crashes in 20174.  The next time you put your seatbelt on an airplane, remember that it is even more important to test your home for radon and mitigate any problems.

  1. https://www.epa.gov/sites/production/files/2015-05/documents/hmbuygud.pdf
  2.  https://ccaps.umn.edu/documents/CPE-Conferences/Radon/WHO-Handbook-On-Indoor-Radon.pdf
  3. https://www.epa.gov/radon/health-risk-radon
  4. https://www.washingtonpost.com/news/dr-gridlock/wp/2018/01/02/air-travel-was-miserable-in-2017-but-at-least-nobody-died-in-a-commercial-jet-crash/?noredirect=on&utm_term=.f0c0b30afd10

Ian Cull

Ian Cull is a nationally recognized expert in the field of indoor air quality. He is the Chief Science Officer of Indoor Science, a company he started in 2004. He speaks around the world on air quality topics and is a training provider of the Indoor Air Quality Association. Mr. Cull is a Licensed Professional Engineer (PE) and Certified Industrial Hygienist (CIH). His degree is in Environmental Engineering from the University of Illinois - Urbana Champaign. Mr. Cull has developed 50 air quality related courses for the IAQA University and is the author of the book, “Fundamentals of Mold Remediation”. In his words… “Besides being passionate about indoor air quality, I enjoy cycling, music, the Chicago Bulls, and having fun with my three kids.”

3 thoughts on “Radon – What are acceptable levels?

    It was helpful when you explained that radon levels under 4.0 pCi/L are considered safe by real estate professionals, although it’s recommended by the EPA to have it mitigated to 2.0 or less. My husband and I are planning on buying an older home near our current neighborhood that we can restore in our spare time. Thanks for sharing this info so I can be more informed if we choose to work with a radon mitigation service for the home!

    I appreciate your information on acceptable radon guidelines. I found it helpful when you said that there is no harmless level with radon gas. My husband and I should consider getting our house tested soon to make sure we are not being exposed to it.