Reuben's Home Inspection Blog


Could My Radon Monitor Be Broken?


One of the more awkward conversations that I had with a home seller this year dealt with a high radon test at his home in Minnesota.  We performed a radon test for the buyer, and the test came up high (over 4.0 pCi/L), so the buyer asked the seller to install a radon mitigation system.  The seller wasn’t happy about this, so he called us up to complain.

We had performed a radon test for the seller when he bought the home four years ago, and the test was below 4.0 pCi/L.

I could understand the seller’s frustration; I would have been frustrated too.  How could this be?  Was our electronic radon monitor broken?  No, it really just came down to timing.  Radon levels constantly fluctuate throughout the day, and even throughout the year.  There are many factors that affect radon test results - so many that you'll never get the exact same results twice.  A few of the larger factors include:

  • Where the test was placed in the home.
  • Barometric pressure.  Rainy weather = lower pressure = higher radon levels
  • Ventilation systems.  An HRV running at full speed can cut radon levels in half.
  • Windows open vs closed.  Houses always act like chimneys; warm air rises.  This creates negative pressure in the basement and positive pressure at the upper levels.  When windows are open at the upper levels, the house will act even more like a chimney, which increases radon levels.  That's right; opening windows on the upper levels can actually increase radon levels in the basement, which is where the test is placed.

With all of these factors, why even bother with a short term radon test? Because it helps to decide whether or not radon is a problem that needs to be dealt with.   The overall seasonal average typically won't be too far off from the short term test.

I performed a radon test at my own house for 96 hours, and as you can see from the graph below, the levels didn't vary all that much.  The overall average came out to 2.6 pCi/L.  If you were to only look at the highest levels in a 48 hour period for this test, the average would still only be 2.9 pCi/L.  I've performed many tests at my house throughout the year, and the average is consistently between 1 and 3.

Reuben's radon

For the record, we have every one of our electronic radon monitors calibrated annually.  While radon levels may be volatile, the accuracy of our tests isn't.


Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 32 commentsReuben Saltzman • October 19 2010 06:08AM


Good information...thanks Reuben for sharing this...Open windows can elevate radon....I'll bet lots of folks think that will dissipate radon...

Posted by Brin Realty Associates Team At Bean Group, Amherst NH homes and Southern NH real estate (Bean Group | Brin Realty Associates) over 8 years ago

Good Morning Reuben, thank you for posting the excellent information on testing.

Posted by Dan Edward Phillips, Realtor and Broker/Owner (Dan Edward Phillips) over 8 years ago

Hi Rene - you got it.  It's just one more reason for the seller to comply with the EPA requirements for radon testing... if being honest isn't enough.

Dan - thanks for reading.  

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

One open window can sway the result of a test as you illustrate...conditions change...and mitigation systems are less than $1000 in our area.

Posted by Sally K. & David L. Hanson, WI Realtors - Luxury - Divorce (EXP Realty 414-525-0563) over 8 years ago

Reuben, I have also been told that the system CAN break, and the fan can wear out -- that every 4-7 years we should have it retested -- I just heard this, and of course, I have been in my house for 8 years, so I need to get it this true? Thanks!

Posted by Marney Kirk, Towson, Maryland Real Estate (Cummings & Co. Realtors) over 8 years ago

Great blog Reuben.  It fluctuates!  People don't realize how inaccurate a short-term test can be.  The shortest LONG TERM test would take 3-6 months.  The lab I purchase my E-perm electrets from says 6-9 is better, and nobody has that kind of time on a sales contract.

Also, radon's half life inside the home is only slightly less than 3.5 days, so opening a window during peak periods can help a lot.  Or even turning on a ceiling fan!

Whoops!  Did I just give away a trade secret?

It's a bummer, Reubs, but I was going to suggest this for a feature and it already was posted there!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 8 years ago

I am surprised more people are not asking for radon tests yet.

Posted by Chuck Carstensen, Minnesota Real Estate Expert (RE/MAX Results) over 8 years ago

In our area almost 2/3's of house fail a Radon test so mitigation is very  important !!!

Posted by Michael J. Perry, Lancaster, PA Relo Specialist (KW Elite ) over 8 years ago



What is the cost of a radon gas mitigation system? Just curious.



Posted by Brian Madigan, LL.B., Broker (RE/MAX West Realty Inc., Brokerage (Toronto)) over 8 years ago

Ruben, I had one listing a while back where there was a radon issue. The owner had a venting system put in which resolved the problem. A pretty easy fix.

Posted by Michael Setunsky, Your Commercial Real Estate Link to Northern VA over 8 years ago

Good explanation of the reasons for higher test result; this will help explain it to sellers when buyer want a test.

Posted by Mary Strang over 8 years ago

Sally & David - prices for mitigation systems are typically around $1500 in my area, but I've certainly heard of some costing well under $1000.

Marney - yes, the systems can go bad.  There should be some type of indicator installed to show if it goes bad, typically a tube shaped like a "U" with red liquid in it (see below).  As long as one side is higher than the other, the system is working properly.   If you don't know, you can always get a do-it-yourself test.  Those work very well for homeowners.

I found this image at

Jay - great points.  Thanks for the 'vote of confidence' !

Chuck - it seems to be happening more and more, especially now that passive radon mitigation systems are required on new construction now here in Minnesota.

Michael - wow, and I thought we had high levels of radon here in Minnesota.  That's a crazy high number.

Brian - on average, $1500 here in Minnesota.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Michael - great point.  Correcting high radon levels is usually a very straightforward fix, and it almost always works very well.  It also makes for a healthier house (less moisture).

Mary - I fully intend to use this post as a reference the next time I need to give an explanation :)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Great to get the word out. I had at least 5 radon issues last summer. Your last point is very important. Radon machines need to be calibrated. I always ask that question of the inspector when we get a high reading.

Posted by Barb Szabo, CRS, E-pro Realtor, Cleveland Ohio Homes (RE/MAX Trinity Brecksville Ohio) over 8 years ago

Good post today, thanks for getting it out to us today.  There are many reasons why the radon level might be different that's for sure.  I've bookmarked this post for the future.

Patricia/Seacoast NH

Posted by Patricia Aulson, Realtor - Portsmouth NH Homes-Hampton NH Homes (BERKSHIRE HATHAWAY HOME SERVICES Verani Realty NH Real Estate ) over 8 years ago

Reuben... I also have run into the short term issue and have had to explain this to both buyers and sellers.  The media is a good one to stir things up and give a partial answer for people.

Posted by William Feela, Realtor, Whispering Pines Realty 651-674-5999 No. (WHISPERING PINES REALTY) over 8 years ago

Good post, Reuben. In this world of "faster is better" many people don't understand the advantages of tests longer than 48 hours. And, yes, it's very important for monitors to be calibrated.

Posted by Janet E. Campbell (Second Mile Radon Testing) over 8 years ago

We had a house tested twice during the winter time and twice in August whey my buyers purchased it. All four readings were performed by different inspectors. All four test results were so different that it left no trust in any short term tests.

What I learned so far is that if your radon reading is high, it's better to use the mitigation system. If your radon reading is lower than 4.0, it doesn't mean that it won't be higher than 4.0 if you test it at different time of year or under different weather condition.

Posted by Svetlana Stolyarova, Local-n-Global Realty, Broker 216-548-4663 (Local-n-Global Realty, Cleveland and International Real Estate Solution) over 8 years ago

The home inspection people in our area use the canister and set them in place for 48 hours.  Then they pick them up, receive the print out which then comes up with an average for those 48 hours.  I have had buyer clients still want to put in a system even though the average was below 4 however during the high periods it was over 6. 

Posted by Kimberly Thurm, Broker / Relocation Consultant ABR, CRS, GRI, SFR (Berkshire Hathaway HomeServices KoenigRubloff, Naperville, IL) over 8 years ago

Thanks for the very interesting post. In our area radon is pretty common so I deal with the test a lot.

Posted by Christa Ross, Helping you buy and sell Pittsburgh's Best Homes (RE/MAX Select Realty - REALTOR and Green Homes Specialist) over 8 years ago

Reuben.. I've run into this a few times too, where the levels were higher.  Only once has an mitigation system been put into place.

One question...What does HRV stand for?

Posted by Valerie Osterhoudt, ABR, Cromwell, CT Real Estate ~ 860.883.8889 (Johnson Real Estate, Inc.) over 8 years ago

Hi Reuben~  Most people do not test for Radon here, but they should.  I had no idea about the effects of opening windows in the upper level could do to the radon levels!

Posted by Vickie McCartney, Broker, Real Estate Agent Owensboro KY (Maverick Realty) over 8 years ago

I guess the test could have been repeated. But in the end this problrm needs to be dealt with and the seller has to bite the bullet

Posted by Charlie Ragonesi, Homes - Big Canoe, Jasper, North Georgia Pros ( over 8 years ago

Barb - it's a valid question, and it's not cheap getting those machines calibrated; we pay about $140 per machine.

Patricia - I'm keeping this as a reference too :)

William - it's gotta be very frustrating as a seller to be on the receiving end of a high test.

Janet - yes, long term tests are much better, but can you only imagine asking for one when you're buying a house?

Svetlana - I completely agree with your point about having a mitigation system installed after one high test.  It's not that expensive, and it's good for the house.

Kimberly - I remember one client of ours that insisted the level be less than 2.0 because she was a smoker, and didn't want to increase her risk of lung cancer.  How's that for crazy?

Christa - it's those test results that are very close to 4 that are toughest to deal with.  When it's very low, great.  When it's very high, no brainer.  But the grey area...

Valerie - I missed a great opportunity to include a link in my post.  HRV stands for Heat Recovery Ventilator.  An outdated term for these devices is an air-to-air exchanger.  I wrote a three-part series on them - click here for part one

Vickie - I agree, radon should be tested everywhere, but it's certainly more important in some parts of the country than others.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Charlie - you're right, the bottom line is that the seller needs to bite the bullet.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Reuben, You mention windows open vs closed, but you didn't mention that they must be closed to perform a short term real estate test. Whether the windows can be open is a continued problem for me with agents. Many still believe (I wonder where they get the idea from) that only the basement windows must be closed.

Jay mentioned above about long term testing being more accurate. Actually 90% of short term test will agree with a long term test, so that is just another myth.

One other thing, in my experience it is precipitation that most influences the level of radon. You can practically track the event by looking at the tape from the monitor.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 8 years ago

James - true, I didn't mention anything about the windows being closed.  I suppose I could have slipped that in somewhere.  

You're certainly right about the rain - can you guess when it started raining based on my graph?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Thanks Reuben, Radon can be a deal killer sometimes so I try to stay educated!

Posted by Richard Campbell over 8 years ago

Short-term radon testing in general is very accurate (depending on the device used and the skills of the person performing the measurement). 'Accurate' means the results of the tests are close to the actual radon levels present during the radon test. So how could radon tests performed in the same location of the same house produce widely differing results? (In one reply above four different radon tests produced four widely different results.)

What is often misunderstood is that radon levels in a house are constantly fluctuating - hour to hour, day to day, week to week, season to season. Changes in weather (temperature, barometric pressure, wind, precipitation, etc.) and how the house is used can cause radon levels in a home to change dramatically.

Short-term radon tests can provide a clue as to whether a house has a radon problem or not. In other words, a short-term test is an indicator based on a snapshot in time (the 48-hour or 72-hour period of time during the radon test). But one must understand that another short-term radon test may yeild very different results. It is simply the nature of short-term radon testing. I always recommend long-term radon testing (91 days to one year) because the normal fluctuations are averaged out and the result is the average radon concentration the occupants are exposed to during the course of a year. (And, because door and windows do not have to be closed during a long-term test the results reflect the actual lived-in radon levels rather than the potential levels we measure during a short-term test with the house tightly closed up.)

So why do a short-term radon test if the results can vary so much? The simple answer is that in most real estate transactions the inspection period is only a week or two which is not enough time to conduct a long-term test (91 days to one year).

Posted by Bruce Breedlove (Avalon Inspection Services) over 8 years ago

Bruce - I want to say 'great summary', but your comment was just as long as my blog.  You should re-post this as blog.  Good stuff.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 8 years ago

Yeah,a simple answer can be misunderstood, especially for a topic like radon.

Posted by Bruce Breedlove (Avalon Inspection Services) over 8 years ago

I always like the way Bruce explains things.

Posted by Janet E. Campbell (Second Mile Radon Testing) over 8 years ago