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Short-term vs long-term radon test: differences that matter

By Radon411 Editorial TeamUpdated 6 min readHow we research

Short answer

A short-term radon test stays in your home for 2 to 90 days and gives a fast first answer, while a long-term test stays for more than 90 days and is more likely to reflect your year-round average. For most homes the order is a short-term test first, then a long-term or second short-term test to confirm a high result.

Key takeaways

  • A short-term test stays in your home for 2 to 90 days, depending on the device, and never less than 48 hours. Most charcoal kits run 2 to 7 days.
  • A long-term test stays for more than 90 days. Its result is more likely to tell you your home's year-round average.
  • The usual order for most homes: a short-term test first, then a follow-up (long-term, or a second short-term test if you need speed) if the first is at or above 4 pCi/L.
  • Short-term tests need closed-house conditions. During a long-term test, you live normally.
  • Neither is the better test in general; they answer different questions at different speeds.

The short version

Short-term and long-term radon tests at a glance
ComparedShort-term testLong-term test
How long it runs2 to 90 days, depending on the device; at least 48 hoursMore than 90 days; usually 3 months to 1 year
Usual devicesCharcoal canister, charcoal liquid scintillation, electret, alpha track, continuous monitorAlpha track, electret
What the result reflectsThe level during those daysA result more likely to reflect your year-round average
Rules while it runsClosed-house conditions; for tests under four days, from 12 hours beforeLive normally
Main useA first answer, a quick follow-up, a home saleConfirming a result, deciding on a fix with time to spare
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What each test is

Short-term radon test
A test that stays in your home for two to 90 days, depending on the device. Every radon test should run for at least 48 hours, and some devices need longer.
Long-term radon test
A test that stays in your home for more than 90 days. In Canada, a long-term test means at least 91 days.

In practice the two groups overlap less than the 90-day boundary suggests. Short-term kits typically run 2 to 7 days and long-term kits 3 months to 1 year, with 100 to 120 days a common choice for an alpha track detector. Most charcoal kits are short-term; most alpha track kits are long-term; electret devices can be either, depending on the electret.

Short-term and long-term kits we cover10

All test kits

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What the result can and cannot tell you

The core difference is seasons. Radon levels tend to vary from day to day and season to season, so a is less likely than a to tell you your year-round average. A long-term test spans more of that variation, so its reading is more likely to match the average you actually live with.

The uncertainty near the line is real: if the average of two short-term tests is 4.1 , there is about a 50 percent chance that the year-round average is somewhat below, or above, 4 pCi/L. Canada goes further: its guidance treats short-term readings as possibly much lower or much higher than the real average, and Health Canada recommends basing decisions on tests at least 91 days long.

The usual order: which test to use first

  1. Step 1 of 4:

    Take a short-term test

    The first step for most homes. If the result is at or above 4 pCi/L, EPA recommends a follow-up test to be sure.

  2. Step 2 of 4:

    Follow up

    Either a long-term test, for a better understanding of your year-round average, or a second short-term test if you need results quickly.

  3. Step 3 of 4:

    Choose the follow-up by how high the first result was

    The higher your initial short-term result, the more certain you can be that a short-term follow-up, rather than a long-term one, is the right choice.

  4. Step 4 of 4:

    Decide on the result

    EPA recommends fixing a home when the radon level is 4 pCi/L or higher. EPA recommends considering a fix when the level is between 2 and 4 pCi/L.

Flowchart of EPA's radon testing steps: take a short-term test; at or above 4 pCi/L follow up with a long-term or a second short-term test; then fix the home or consider fixing based on the result or the average of the two tests.
The testing steps: a short-term test, a follow-up, then a decision.
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  1. 1Step 1: Take a short-term test.

    • Below 4 pCi/LEven if your test result is below 4 pCi/L, you may want to test again sometime in the future.
    • At or above 4 pCi/LIf your result is at or above 4 pCi/L, take a followup test to be sure.

  2. 2Step 2: Follow up with either a long-term test or a second short-term test.

    • Long-term testFor a better understanding of your year-round average radon level, take a long-term test.
    • Second short-term testIf you need results quickly, take a second short-term 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.

  3. 3Step 3: Decide whether to fix your home.

    • After a long-term testFix your home if your long-term test result is 4 pCi/L or more.
    • After a second short-term testConsider fixing your home if the average of your first and second test is at or above 4 pCi/L.
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The rules while each test runs

What each test asks of the household
ComparedShort-term testLong-term test
Windows and outside doorsClosed except for normal entry and exit; for tests under four days, from at least 12 hours beforeNormal use
FansRecirculating heating and cooling fans may run; fans that bring in outside air should notNormal use
WeatherAvoid tests under four days during severe storms or high windsSeasons are part of the measurement; in Canada, at least 91 days in the heating season
WhereLowest level that could be used regularly; at least 20 inches above the floorA normal occupancy area of the lowest lived-in level; off the floor, away from an exterior wall
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During a long-term test you should not change how you live in the home. The point of a long-term test is to measure the home as you live in it.

Short-term and long-term tests in a home sale

A sale rarely leaves 90 days, so there are three accepted short-term options for when results are needed quickly: two short-term tests at the same time in the same place for at least 48 hours, two tests one after the other with identical devices, or a continuous monitor for at least 48 hours. Tests taken at the same time improve precision, while tests one after the other better represent the seasonal average, and that any real estate test should include steps to prevent or detect interference.

EPA's three short-term radon testing options for a real estate transaction: two tests at the same time, one test then a second, or a continuous monitor, each for at least 48 hours, with what to do after each.
The three short-term options for a home sale, with what to do after each.
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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.

  1. Option 1. Two tests at the same time

    Passive

    Take 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.

  2. Option 2. One test, then a second

    Passive

    Take 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.

  3. Option 3. A continuous monitor

    Active

    Test 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.

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If time permits, a long-term test can confirm an initial short-term result. A buyer who moves in can run one through the first heating season. See buying a house with radon.

Which one fits your situation

Neither test comes out ahead; each fits different situations. Each list has five points.

When each test fits, five situations each
ComparedA short-term test fits ifA long-term test fits if
Where you startYou have never tested and want a first answerYou want the closest estimate of your year-round average
Your timelineYou are buying or selling and the deadline is days awayA short-term result came back near or above 4 pCi/L and you have time
After a high resultYou need a quick follow-up to a high first resultYou want to decide on a fix with seasons accounted for
Life during the testYou can keep the house closed for a few daysYou would rather live normally while you test
After a fix or a low resultYou want to check a system soon after it is installedYou want to confirm a low result through a heating season
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Table comparing a short-term radon test, a long-term radon test and a continuous radon monitor: how long each runs and what each is for.
Short-term tests, long-term tests and continuous monitors compared.
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TestHow long it runsWhat it is for
Short-term testStays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term testStays in the home for more than 90 days.It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitorLogs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.Logs a graph over time, so you can see trends and spot anomalies rather than a single number.
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Every short-term and long-term kit we cover is on radon test kits, with more on each type at short-term kits and long-term kits. A digital monitor can serve as either, depending on how long you run it; see charcoal kit vs digital monitor.

Questions about short-term and long-term tests

Is a short-term or long-term radon test better?

Neither in general. A short-term test gives a fast first answer; a long-term test is more likely to reflect your year-round average. The usual order uses both: a short-term test first, then a follow-up if it is high.

How long is a short-term radon test?

Two to 90 days depending on the device, and at least 48 hours. Most charcoal kits run 2 to 7 days.

How long is a long-term radon test?

More than 90 days. Most long-term kits run 3 months to 1 year.

Is a short-term radon test accurate?

It can measure the level during its days accurately, but radon varies from day to day and season to season, so it is less likely to reflect your year-round average. That is why a high result calls for a follow-up.

Should I do a long-term test if my short-term test was high?

Follow up with either a long-term test or a second short-term test. Choose the long-term test for a better picture of your year-round average, or a second short-term test if you need results quickly or the first result was very high.

Do I keep windows closed during a long-term radon test?

No. Closed-house conditions are for short-term tests. During a long-term test, live as you normally do.

Which season should a long-term radon test cover?

The heating season, when indoor levels are typically highest. Canada's guidance asks for at least 91 days of the test to fall in it.

Can a radon monitor be a long-term test?

Yes, if it runs for more than 90 days and you use its long-term average. Run a monitor for at least 3 months for that.

Which test do I use when buying a house?

Usually a short-term option, because of the deadline: two passive tests together or one after the other, or a continuous monitor for at least 48 hours.

Why did my short-term and long-term results differ?

Because they measured different periods. Radon changes with weather and seasons, and a long-term test averages more of them.