Details: It's a 3 story building. Each apartment has its own HVAC system. There is no shared ventilation system. The foundation is a slab. The radon levels on all three floors were measured simultaneously. The first floor was 7.3 pCi/L. The 2nd floor was 6.7, and the 3rd was 9.2. Each measurement was in the same horizontal place, near the center of the building. It was done when the weather was cold enough to be sure all windows were likely to be closed.

Other tests have had similar results, but were not done as rigorously. E.g., not simultaneously, not in the same part of the building, etc. The results vary a lot with the weather and probably other factors. But it doesn't seem logical that the 3rd floor should ever have higher levels than the other two floors.

4/12/2016 ETA:

Tester101 asked if there is a radon mitigation system in place. How would I find out? What does a radon mitigation system look like in a 3 story building? In other words, even if I saw one, how would I recognize it? Can I recognize it by something coming out on the roof? What would that look like? What would the difference be between that and ordinary vents such as drain vents?

Is it likely that a radon mitigation system could be malfunctioning and dumping radon onto the 3rd floor?

I read somewhere that people don't have to worry about radon on 3rd floors and higher because by the time it gets that high its levels are drastically reduced. But that contradicts another answer to this question, which implied it would be normal for the levels to be higher depending on ventilation etc.

I also read somewhere that if the levels aren't lower on higher floors, the radon could be from building materials instead of coming up from the ground. How could I tell if that's a factor here?

  • Is there a radon mitigation system in place? If so, where does it discharge?
    – Tester101
    Mar 14, 2016 at 11:25
  • 1
    Radon rises , how is the 3rd floor sealed. Are there any marble or granite tiles or countertops on the 3rd floor ? I know of 1 countertop that had to be removed because it was out gassing at dangerous levels.
    – Ed Beal
    Mar 14, 2016 at 12:06
  • In any case, even the lowest reading is high enough to justify investing in mitigation, or to service/improve whatever you have now.
    – keshlam
    Mar 14, 2016 at 13:45
  • Relatedly, does anyone know how stable the measurement is? If you measured the first floor again would it read the same? Mar 16, 2016 at 21:11
  • Before doing anything I would take radon readings right outside of the house and then again in the house after it has had its windows open for a half day. You need a baseline.
    – DMoore
    Jul 12, 2016 at 19:20

3 Answers 3


Those are some high numbers, 4-pico's is the standard "something needs to done" threshold. But, the difference in readings has to do with ventilation & thereby natural convection.

All air will rise or be drawn-up in a 3-story building, drawing is what's happening with the 2nd floor's lower readings & they're being added to the 3rd floor. Additionally, this rise or draw is magnified when exhaust fans are used or windows & exterior doors are opened.

It's best to treat or remove Radon at it's before it enters the structure so the entire structure is then cleared or reduced of Radon levels. However, this could result in more tenant turnover & higher rents...I'm just saying (I'm joking).


Don't worry about the 3rd level being higher than the others. This kind of range is normal for separate apartments in the same building. Different tennant habits will easily cause this kind of difference, all it would take is the tennant to forget to run bath/kitchen exhaust fans while showering/cooking and they'll have completely different levels than the rest of the tennants. It's not hard to have that kind of difference in levels between floors.

All three of your tests returned radon levels higher than recommended, so you should find a radon mitigation solution that lowers these numbers to below 4.0 pCi/L.

It doesn't sound like you have any existing radon mitigation operating, or the lower level would be less than 4.0 pCi/L. It's possible the fan isn't operating.


Check for radon in the water supply to the building. If that's high, it could cause the problem you see. Every time water is used in toilets, washing, showers (especially), cooking, etc some of the radon in the water will pass into the air. It's estimated that for every 1,000 pCi/L in water an additional 1 pCi/L is added to the air in the average home. So a level of 2,000 would contribute 2 pCi/L, etc.

  • 1
    Showers release the most radon from the water, so likely the 3rd floor residents are taking more showers. Radon comes from nuclear decay of radium . Radium is associated with calcium ie, limestone. No doubt some or all of the water comes from a limestone aquifer. Nov 30, 2017 at 20:24

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