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Here in Ottawa, it seems like lately it has been raining more often than not. It is even raining right now as I write this article.

Don’t worry though, I would not waste your time by talking about how depressing rainy days can be. No, this article will address a unique concern when it comes to radon present in homes – higher levels of radon when it’s raining. The reason why it is a concern is because being exposed to radon, any amount of radon, is bad; moreover, the more radon you are exposed to, the higher the risk of developing lung cancer.

Radon is present in every single home or building everywhere, but the question is… how much? That leads to the main issue or body of this article…

Recently I was asked by a customer, after she noticed the incline of radon measurement on her radon monitor, “Is it normal for my radon concentration levels to be higher when it rains?” In short, the answer is yes, it is expected that radon concentration levels will be slightly higher, within your home, on rainy days. After searching the internet and speaking with other radon mitigation professionals, here are 3 theories and 3 concepts derived from what I learned.

3 Theories As To Why Radon Concentration Levels Are Higher When It Rains

  • Radon can be released into ground water below your home. Rain can move and change the levels of the ground water. Radon in water that comes in contact with air is released, perhaps in greater amounts when it’s raining.
  • Radon in the air could potentially be “caught” by water droplets while falling down, thereby carrying more radon towards the surface of the earth, and when seeping down to the ground water beneath a home.
  • When it rains, people have a tendency to close their windows and increase their heating. Both of these can lead to more radon entering the house from below and keeping more radon trapped inside the house.

3 Concepts to Explain How Radon Concentration Levels Are Higher When It Rains

  1. The water displaces soil gas and, as a result, raises soil gas pressure so it enters the home more easily.
  2. Another concept is that a rising water table can similarly push the soil gas ahead and into the house.
  3. Water saturated soil at ground level could block the convection of radon from coming out of the ground; therefore, it is more inclined to move laterally over to the house.

When rain poors, radon grows!

So if you just mitigated your house of radon and you use a continuous radon monitor and you noticed that the radon reading has increased only on rainy days, don’t worry, that’s normal. If that slight increase puts you above the recommended guideline set out by the E.P.A., Health Canada, or W.H.O. (depending on which one you follow), then consider calling a radon professional to replace your current radon fan with a more powerful fan. This should help you greatly and give you piece of mind on those dreary, rainy days.


  1. Joel

    We have a radon mitigation system. It does well until the past few weeks. We live in New Hampshire and we just experience a really severe drought. Over the past few weeks it has finally started to rain. My levels went from 1.25 to 7.7 in the basement. After opening windows in the basement we for this level down to 3.8 in a 24 hour period.

    My upstairs levels have been under .5 however. After a rain storm last night they jump to 2.8. We typically keep the windows in then house shut year round and even have air sealing done. This is the first time we have seen these levels this high since buying the home.

    I have all the windows open and am trying to keep the radon levels in check. But the radon system we have is doing nothing it seems.

    What can be done?

    1. Simon

      There are many things that can be done to improve your situation. Send us an email at [email protected] to talk about it.

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