Radon – What are acceptable levels?

Oct 9, 2018

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. https://www.epa.gov/sites/production/files/2015-05/documents/hmbuygud.pdf

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.  https://ccaps.umn.edu/documents/CPE-Conferences/Radon/WHO-Handbook-On-Indoor-Radon.pdf    

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 US https://www.epa.gov/radon/health-risk-radon , whereas there were no commercial plane crashes in 2017 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 .  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.