Guide

Short-Term vs Long-Term Radon Tests

What each test is actually measuring

Short-term, 2 to 7 days

Charcoal canisters, liquid scintillation vials, alpha-track and electret devices, or a continuous radon monitor. It captures the concentration during a short window under closed-house conditions, and it is deliberately a worst-case-ish snapshot: windows shut, weather whatever it happens to be. Cheap, fast, and the standard tool for a transaction.

Long-term, 90 to 365 days

Usually an alpha-track detector left in place. It integrates across seasons, weather systems and how the house is actually lived in. The result is closer to your true average exposure, which is what the health risk is a function of. Slower, and useless when a decision is due in two weeks.

Why the same house gives different numbers

Radon concentration indoors is not a property of the house alone. It moves with:

• Season. Winter readings run higher, because the stack effect is strongest when indoor and outdoor temperatures are furthest apart and windows stay shut
• Barometric pressure. Falling pressure ahead of a storm draws more soil gas into the building
• Wind, which pressurises one side of a house and depressurises the other
• Heating and ventilation. Running an exhaust fan, a dryer or a combustion appliance pulls make-up air in from below
• Whether anybody opened a window
• Where in the house the device sits, since basements read higher than upper floors

Two short-term tests in the same house in different weeks can differ substantially. That is the measurement working correctly, not failing.

PA DEP's guidance by result

DEP's stated guidance for a result in the 4 to 8 pCi/L band is to "do a second long-term (365 days) test to get a better idea of the annual average or do another short-term test (2-7 days)." Above that band, waiting a year to confirm is the wrong trade, and a second short-term test is the sensible confirmation.

DEP also recommends a retest roughly once every two years after mitigation, generally in the basement and preferably during the heating season. The decision table by result lays out the whole range.

Closed-house conditions, and how tests get invalidated

A short-term test is only meaningful if the house is kept closed. That means:

1. Windows and external doors shut, except for normal entry and exit, for 12 hours before the test starts and throughout it
2. Heating and air conditioning operating normally, but no whole-house fans, window fans or attic fans
3. The device 20 inches to 6 feet above the floor, at least 3 feet from exterior walls, windows and doors
4. Away from draughts, heat, direct sunlight and high humidity, so not on a windowsill, not next to a dryer, not in a bathroom
5. In the lowest lived-in level of the house, or the lowest level that could reasonably become living space
6. Undisturbed, and not moved partway through

In a real estate test these conditions are not always respected, and the ones that get broken are rarely accidental. A device found on a porch, near an open window, or several feet from where the report says it was, is a test that needs redoing. This is one reason a buyer's own test, run by a DEP-certified measurement professional, carries more weight than a kit left out by a seller. More on that in radon and home sales.

Which one to actually buy

If you have never tested: a short-term kit, about fifteen dollars, this week. It answers the only question that matters at this stage, which is whether there is anything to look into.

If a short-term test came back between 4 and 8: either a long-term test if nothing is on a deadline, or a second short-term if you would rather decide now.

If it came back above 8: confirm short-term and move to mitigation. If it came back above 20, treat it as urgent.

If you are buying or selling: short-term, run by a DEP-certified measurement professional, because the result has to be defensible to somebody else. General testing procedure is covered on radon testing.

Common questions

Which test should I start with?

A short-term test. It is cheap, it is fast, and it tells you whether you are in a range where anything further is needed. DEP's guidance for a 4 to 8 pCi/L result is then to follow with either a long-term 365-day test or a second short-term test.

Why did my two tests give different numbers?

That is normal and not a defect. Radon varies with season, barometric pressure, wind, heating and whether windows have been open. Short-term tests capture a slice of that variation. It is the reason the long-term average exists.

Can the test be wrong?

A test can be invalidated by closed-house conditions not being maintained, by placement in the wrong spot, by a device near a draught, a heat source or high humidity, or by tampering. Real estate tests are the ones most often disturbed, deliberately or not.