Short answer
Yes, a short-term radon test can be thrown off by opening windows, running fans that pull in outside air, or moving the device, but that changes the reading, not the radon. Such interference can invalidate the result and is illegal in some states, and testers use monitors, seals and agreements to detect it.
Next step: pick the right test for your home. Which test to use
Disturbing or interfering with the test device, or with
A lowered reading does not lower the radon: once windows are closed, levels most often return to where they were within about 12 hours.
What to do next: run the test by the testing checklist, and if the level is high, fix the house rather than the number.
At a glance · 1 min read
A short-term radon test is only valid under
| What happens | Effect on the reading | Why it does not last |
|---|---|---|
| Windows or doors left open | Outdoor air dilutes indoor air while they stay open | Once they are closed, levels most often return to where they were within about 12 hours |
| Fans that bring in outside air | The same dilution, while they run | Not allowed during a short-term test; a radon system's own fan or brief small exhaust fans may run |
| Device moved, covered or handled | The sample no longer reflects the room | The device should not be disturbed at any point during the test |
| Any of the above | A result that does not describe the home | Interfering with the device or closed-house conditions can invalidate the result and is illegal in some states |
The radon is still there. A reading lowered by an open window describes a few days of unusual air, not the year the next occupant will breathe. That is why every radon test, and especially one in a home sale, should include steps to prevent interference.
At a glance · 1 min read
Interference is a known risk in real estate transactions, and testers have a set of ways to prevent or detect it.
| Method | What it catches |
|---|---|
| A device that frequently records radon or decay products | Unusual swings in the level during the test |
| A motion detector | Whether the device was moved or testing conditions changed |
| A proximity detector | People in the room, which may line up with changes in the level |
| Barometric pressure records | Weather that may have affected the test |
| Temperature records | Whether doors and windows were opened |
| Tamper-proof seals on windows | Whether closed-house conditions held |
| A signed non-interference agreement | The seller's or occupant's commitment not to interfere |
Devices differ in how hard they are to fool. Some passive devices resist interference better than others, and some active monitors are designed specifically to deter and detect it, with reports that show unusual or abnormal swings during the test.
In a picture · 1 min read
A disturbed test does not count. If the seller or tester cannot confirm that every item on the testing checklist was completed, the answer is another test, and either party should be able to show that testing conditions were not violated. A good tester tells the occupants plainly that a reliable result depends on their cooperation, because interference with the test or with closed-house conditions will invalidate it.
If you are testing in a real estate transaction and you need results quickly, any of the following three options for short-term tests are acceptable in determining whether the home should be fixed.
Option 1. Two tests at the same time
PassiveTake two short-term tests at the same time in the same location for at least 48 hours.
What to do nextFix the home if the average of the two tests is 4 pCi/L or more.
Two tests taken at the same time (simultaneous) would improve the precision of this radon test.
Option 2. One test, then a second
PassiveTake an initial short-term test for at least 48 hours. Immediately upon completing the first test, do a second test using an identical device in the same location as the first test.
What to do nextFix the home if the average of the two tests is 4 pCi/L or more.
One test followed by another test (sequential) would most likely give a better representation of the seasonal average.
Option 3. A continuous monitor
ActiveTest the home with a continuous monitor for at least 48 hours.
What to do nextFix the home if the average radon level is 4 pCi/L or more.
Both active and passive devices may have features which help to prevent test interference.
Any real estate test for radon should include steps to prevent or detect interference with the test device. Your state radon office can help you decide which option is best.
Put the testing in the contract: where the test goes, who conducts it, what type, when, how buyer and seller share the results and costs, and when
At a glance · 1 min read
The only lasting way to a lower result is a lower radon level. Radon in most homes can be brought to 2
| If you are | What to do |
|---|---|
| Selling | Test now and keep the results; if the level is high, fix it before you sell. Where radon problems have been fixed, sales have not been blocked |
| Buying | Buyers often want the test done by someone not involved in the sale, and an independent, qualified tester for the check after mitigation |
| Using one firm to test and fix | Be aware of the potential conflict of interest; some states may require the homeowner to sign a waiver |
How a fix works is covered in how to reduce radon; buying a home that already has one is in should I buy a home with a radon mitigation system.
Questions · 1 min read
By lowering the radon, which means a mitigation system. Ventilating the house during a test only changes the reading while the air is open, and interfering with closed-house conditions can invalidate the result and is illegal in some states.
It is not allowed. For a 2- to 4-day test, windows and outside doors stay closed for at least 12 hours before the test and throughout it. Once windows are closed again, levels most often return to where they were within about 12 hours.
They can try, which is why testers use monitors that record swings, motion detectors, window seals and a signed non-interference agreement.
It can show the signs. Active monitors reveal unusual or abnormal swings during the test, and a temperature record helps show whether doors and windows were opened.
It should be repeated. If every checklist item cannot be confirmed, take another test; interference invalidates the result.
Buyers often want a test by someone who is not involved in the sale, and independent, qualified testers are recommended. See should I get a radon test with my home inspection.
How a radon test works, who can run one, and how to read the number it gives you.
How a professional radon test works, what it proves and whether it is worth it.
Read the guideWhat counts as high and how to bring them down.
Read the guideEPA's action level, WHO's reference level and what counts as acceptable.
Read the guide