Short-term vs long-term radon tests: which one should you use?

A short-term radon test stays in place for 2 to 90 days. A long-term radon test stays in place for more than 90 days. That split is how the U.S. Environmental Protection Agency defines the two durations in A Citizen's Guide to Radon (p. 4), and it is the same split on the CDC radon testing page.

EPA's Citizen's Guide says a long-term test is “more likely to tell you your home's year-round average.” Health Canada recommends a long-term test of 3 to 12 months, with at least 91 days of that test in the heating season, in its guide for radon measurements in residential dwellings.

The two countries do not give the same instruction for which of those tests should drive a mitigation decision. This page sets the U.S. EPA rules beside the Health Canada rules, then compares the device types those sources name: charcoal, alpha-track, electret ion chambers, and electronic monitors. Canadian province pages, for readers placing the Health Canada measurement guide next to provincial construction rules, are listed on the radon code index.

EPA's Citizen's Guide, Step 1, starts with a short-term test. Health Canada's homes guide starts with a long-term test, and it says short-term measurements “are not appropriate for informing mitigation decisions.” The table keeps each agency's rule in its own column and names the source for the minimum length, the decision number, the season limit, and the home-sale protocol.

On length, EPA's floor is 48 hours for any test, in the Citizen's Guide (pp. 4–5) and in the Home Buyer's and Seller's Guide. A test inside EPA's short-term window can run anywhere from 2 days up to 90 days. A test that stays out more than 90 days is EPA's long-term test, the one the Citizen's Guide calls more likely to tell you the year-round average. Health Canada's long-term window is 3 to 12 months, and the homes guide's minimum is the 91-day test it labels “Long-term radon test: at least 91 days.”

On the decision itself, Citizen's Guide Step 3 uses 4 pCi/L twice. A long-term result at or above 4 pCi/L means fix the home. An average of two short-term tests at or above 4 pCi/L means consider fixing. Health Canada states its action number in becquerels: a long-term average above 200 Bq/m³ means mitigate within 1 year. Each threshold stays in the unit that agency published.

Season is a separate constraint. EPA says a short-term test of 2 to 3 days should not run during severe storms or high winds. Health Canada wants the 91 days to fall in the heating season. When a Canadian test is shorter than 91 days and the result is below 200 Bq/m³, the homes guide says to re-test during the next heating season.

For a home sale in the United States, the Home Buyer's and Seller's Guide (EPA 402/K-13/002, March 2024) specifies the test. Use two passive short-term tests, run at the same time or back to back, each for 48 hours or more, or use one continuous monitor for 48 hours or more. Fix the home if the average of that real-estate test is at or above 4 pCi/L. The homes guide has no separate real-estate protocol.

In the Citizen's Guide and in the Consumer's Guide to Radon Reduction (p. 3), EPA lists passive devices and active devices. Passive devices on that list are charcoal canisters, alpha-track detectors, charcoal liquid scintillation devices, and electret ion chambers. Active devices are continuous radon monitors and continuous working-level monitors. The notes below follow that list by mechanism.

A charcoal canister is a short-term passive device. National Radon Program Services(NRPS) notes that “Some charcoal technologies are prone to interference by high humidity.” EPA lists the charcoal canister among the passive devices. On the short-term path in the Citizen's Guide, Step 3 considers fixing when the average of two short-term tests is at or above 4 pCi/L.

An alpha-track detector uses plastic or film that records the damage tracks alpha particles leave behind. A laboratory counts those tracks. EPA describes alpha-track detectors as commonly used for long-term tests. Health Canada recommends long-term alpha-track detectors and says the measurement has to be finished before the printed expiry date on the device (homes guide).

Because the detector is left out for the long-term window, an alpha-track test lines up with Health Canada's 3-to-12-month recommendation and with EPA's definition of a long-term test as anything kept in place more than 90 days. The expiry date is a device limit from the homes guide, separate from the 91-day minimum: the test still has to end before the date printed on that detector.

An electret ion chamber measures the voltage drop on a charged disk. The homes guidedescribes the electret as reusable until the electret is spent, and as usually read through a laboratory. NRPS says electret ion chambers are “usually only available through laboratories” (device page).

An electronic radon monitor shows continuous readings on the device or in an app. Health Canada recommends only devices listed by the Canadian National Radon Proficiency Program (C-NRPP) when an electronic monitor is used for a long-term test. The homes guide suggests replacing an electronic radon monitor five years after purchase unless it is recalibrated, and it notes that many consumer electronic monitors cannot be recalibrated.

EPA and NRPS describe active devices, the category that includes continuous radon monitors, as devices that “usually require operation by trained testers” and that “often cost more than passive testing.” For a home sale under the EPA guide, one continuous monitor run for 48 hours or more is an accepted option next to two passive short-term tests. For a Health Canada long-term result, the monitor has to be C-NRPP-listed, and the figure used is the long-term average after at least 91 days.

C-NRPP publishes its approved electronic-monitor list through c-nrpp.ca, and EPA's find a radon test kit or measurement and mitigation professional page is the U.S. directory for a test kit or a tester.

EPA, CDC, and Health Canada publish different placement figures. The table lists each source on its own row, with that source's height in that row.

For an EPA test, the Citizen's Guide (p. 5) procedure is: put the device on the lowest lived-in level, in a regularly used room, outside the kitchen and the bathroom; keep it at least 20 inches above the floor; keep it away from drafts, high heat, high humidity, and exterior walls. For any EPA short-term test, close windows and outside doors. For an EPA test of 2 to 3 days, those closed-house conditions start at least 12 hours before the test begins. The storm-and-wind limit on a 2-to-3-day test, from the same guide, sits on top of that closed-house step.

CDC's testing page gives its own sentence: the device should be “raised three feet off the ground, in the middle of the room.” Three feet is the CDC height. Twenty inches is the EPA height.

Health Canada's homes guide, Appendix II, places the device in the breathing zone: at least 30 cm above the floor, at least 30 cm from walls and ceilings, and at least 1 m from vents, windows, and doors. Appendix II also keeps the device out of kitchens, laundry rooms, and bathrooms. The homeowner brochure, radon long-term test instructions, says 0.5 to 2 m from the floor. Appendix II states the 30 cm minimum. The homeowner brochure states the 0.5 to 2 m range. Both figures are Health Canada's, from those two documents.

EPA's Citizen's Guide (p. 6) works a two-test case: when the short-term pair averages 4.1 pCi/L, the chance that the year-round average sits somewhat under 4 pCi/L is about 50 percent. That page 6 case, and the guide's risk tables, are in the Citizen's Guide. Reading a picocurie result against the action level is the subject of what radon levels mean.

The NJ DEP radon basicsdocument reports New Jersey data: a short-term result of 4 pCi/L or more was confirmed by later testing “in 80% of cases.”

On repeat measurements, Health Canada's homes guide records the allowance in the CSA and ANSI standards: repeat results may differ by up to plus or minus 25 percent. That allowance is set next to the action numbers on what radon levels mean.

If the first short-term result is already high, Citizen's Guide Steps 2 and 3 are the follow-up. Step 3 is the rule already in the comparison table: fix when the long-term result is at or above 4 pCi/L, and consider fixing when two short-term tests average at or above 4 pCi/L. When the short-term result is above 8 pCi/L, which is twice 4, take the second short-term test immediately.

The CDC testing page says state radon offices may offer free or discounted kits. National Radon Program Services sells short-term kits that include laboratory analysis, and it sells long-term alpha-track kits. The U.S. lookup for a kit or a professional is EPA's find a radon test kit or professional page, and the Canadian list for a long-term electronic monitor is the C-NRPP list reached from c-nrpp.ca.

Health Canada's homes guide, under “Re-testing,” lists these occasions:

• After renovations that affect the structure or the ventilation.
• After energy retrofits.
• After a mitigation system is installed.
• Every 5 years when an active system is in place.
• For a new home, 2 years after the year of construction.

EPA's Citizen's Guide (p. 6) and the CDC testing page add two occasions: test again if you start living on a lower level, and test again after fixing.

On a new building, the post-construction test is also what decides whether passive radon control is enough. That decision, passive stack versus an added fan, is covered in passive versus active radon control in new construction. The duration and device rules on this page are the measurement side of the same test.

After the test has a result, the next question is what that number means under the action levels the same agencies publish. That discussion is what your number means.