How do I definitively spot radon test tampering?

I have difficulty deciding if radon and environmental readings show tampering. This is a basement test location, and the suspected time is around 14:00 on July 29 through about 10:00 July 30. House was vacant and not occupied during the test. The seller does have access and lives near by. Heat was off. There is no AC. There is a HRV, but room vents and unit are on the main floor.

One thing that seems to be lacking in radon testing is how to spot tampering, in this case opening up the windows. I have been unable to find definitive information about determining what patterns suggest tampering/opening windows.

This does seem like opening a window, but how can I be sure? I see radon goes down, and environmental readings, especially temperature, suddenly increase. Humidity and temperature go up/down together, not opposite to each other. Is that definitive, or speculative? Again, this one is a basement.

The weather has been rather warm at night for my area, northern MN, and not scorching hot during the day. Lows in the 50’s, highs mid 70’s.

Here’s a more normal test, IMO there is no tampering. The temperature/humidity goes up/down with day/night. Temperature goes up, humidity goes down, a pattern I understand and like to see. This is a main floor test location over a crawl space:

I have contacted my State supervisor, being in a licensed State. I’ll not get his response until after 8th August, as he’s on vacation.

That’s the problem with trying to interpret readings of a sampling system. Yes, I know it’s called a Continuous Radon Monitor, but each one of those changes in the line are a sample point. What you need to do is have a CYA in your Radon contract that relieves you of responsibility for any tampering. Just let the average fall where it may. You did your job. As an aside i provide an informational item to the seller that suggest that opening windows will increase the readings by stack effect. This usually thwarts any idea they may have had about tampering.

This is a basement window opening concern, so opening a basement window will reduce radon readings. I did see a pattern a while back that was confirmed to be the result of leaving an outside basement access door slightly open for a few hours.

I do have the CYA issues covered. We have a check list on our State report to the client, two items which address tampering. Of course, we also have a signed cooperation agreement with the owner addressing tampering.

So who closed the window? And remember, there first 4 hours are discarded from EPA average.

I didn’t. It easily could have been the seller/owner. I set my device with a 4 hour delay start, and it automatically ends at the test duration time I set, here 48 hours. I sometimes have to set a 12 hour delay start when closed house conditions are not present at the start of a test. Nice to be able to do that!

I’m a little ambiguous on my pick up notifications left at the property in order to keep the “tamperer” out of balance per timing of their mischief.

I use the Corentium Pro as well. One thing I don’t like is the scale they use for the graphs. The scale is so condensed it makes changes look super-dramatic sometimes.

For instance, your chart looks like a large immediate jump in temp, but it was actually from about 62.75 degrees to about 65 degrees. So just over 2 degrees over the course of 4 hours or so. Maybe not so crazy considering the home had non-operating heat/air and an operating (assumption based on your post) HRV.


I do radon testing, but don’t promote it because I don’t really agree with the testing timeline and procedures. A 48-72 hour test will never give accurate results and is geared more to a money ploy by the those in the mitigation industry.

True levels of radon should be tested through all climate seasons (where applicable) and this isn’t going to happen in 48 hours, tampered with or not. Anyone that study and took Radon classes should know that this is a LONG TERM exposure health hazard. For that reason, if my 48 hour test is above the average, I recommend at least 6-12 months with canisters or a CMR. I don’t recommend mitigation based on a 2 or 3 day test cycle.

1 Like

A piece of tape in an innocuous location can indicate if the window was opened. Ultimately it’s not your responsibility to prove tampering.


Here’s my take…If the home tests at over 4 pCi/L, I can confidently say that at least some point during the year, the radon level is reaching suggested mitigation levels. And we may have caught it during a low period so it may be much higher other times of the year.

Installation of a mitigation system is relatively cheap and easy and they work very well. If we are reaching mitigation levels during our test, be on the safe side and install the system.

Where I have trouble, is when the test comes back at 2 or 2.5 and everyone thinks everything is good. I always tell my client a longer term test should be done in that case.


That is a good idea. However, the first 4 hours after the start of the machine are discarded if your machine produces an “EPA Average”.

When you produce your report, does it say “overall average” as well as “EPA average”? Mine does and they are often different averages. The delay is why.

1 Like

Right, the EPA says 4.0 - mitigate
between 2-4 mitigation should be considered or a long term test.

1 Like

He uses the Corentium Pro. They are always on. So the 4 hour delay is basically telling the software to not start considering data for the first 4 hours.

1 Like

Right, which is a built in guideline. Not to be confused with a “start testing delay”.

But it should be based on an long average, and not something that may have spiked at a particular time. I have had readings in my own house that have spiked above 5, but stayed under 1 for a many months. Go figure… :thinking:

1 Like

Yeah, it’s always the toughest if it’s hovering right around 4. But I stand by my position to just go ahead and mitigate because it is easy and relatively cheap. There is no downside to lowering radon. As a matter of fact the EPA explicitly states there is no known safe level of radon. If the test in in the teens (as most are around me), it makes it way easier to make the mitigation decision.

Right, a spike above the 4.0 during a test is not to be alarming or even commented on in my opinion. The EPA Average is the only actionable level in my opinion.


I follow EPA recommendations; “A Citizens Guide to Radon”,

EPA Recommends the Following Testing Steps:
Step 1. Take a short-term test. If your result is 4 pCi/L or higher, take a follow-up test (Step 2) to be sure.
Step 2. Follow up with either a long-term test or a second short-term test:
• For a better understanding of your year-round average
radon level, take a long-term test.
• If you need results quickly, take a second short-term test.
The higher your initial short-term test result, the more certain you can
be that you should take a short-term rather than a long-term follow up
test. If your first short-term test result is more than twice EPA’s 4 pCi/L action level, you should take a second short-term test immediately.
Step 3. • If you followed up with a long-term test: Fix your home if your long- term test result is 4 pCi/L or more.
• If you followed up with a second short-term test: The higher your short-term results, the more certain you can be that you should fix your home. Consider fixing your home if the average of your first and second test is 4 pCi/L or higher (see also page 7 under Home Sales).

Putting it off to a 6 month test, which does not require closed house conditions, means that the test probably will not be done and the home not mitigated. Radon does kill; I know of two cases with high home radon.

I’m still not confident on the definitive determination relative to tampering on this test, my original question. Determining if tampering happened is not critical, because the average was around 6.5 pCi/L. High enough to recommend mitigation, relative to the 4.0 threshold, and the 2.0 consider mitigation threshold.

I’ve tested my home short term and 6+ months, and results were very close - within 0.5 pCi/L of each other.

It would be a 91+ day test (3 month minimum).

I know they are already sensitive to the results or they would not have paid for the test in the 1st place. So yes, they probably would do the long term test. Provide them with a link to the EPA Buyer’s and seller’s guide to radon. Then give them the results, the EPA’s recommendation, and move along.

I don’t see evidence of tampering myself, but do see what looks like a possible weather event. High pressure ridge moving through the area or some sustained winds generating positive pressure within the home, thereby keeping the Rn at bay for a bit. IF windows were opened anywhere near the unit, you would think the drop off would be considerably more (well below 4 pCi/L)…

A 2nd test would need to be performed to have a comparison, and/or have a side by side using the same monitor as the one used here in that side by side comparison. Everything set up nearly identical to the original test. But that’s up to you…


There was an event, which is typical. No need to retest though…its above the 4.0…they should mitigate.