Short-Term vs Long-Term Radon Tests: Which Number Should You Trust?
You put a radon test in the basement, wait a few days, and get a number. But your neighbor tested the same week and got something totally different — and a friend who did a 90-day test got a third answer. So which number is real? The short version: they are all real, they just measure different things. Knowing the difference decides whether you are about to spend a couple thousand dollars on a mitigation system you may or may not need.
Radon is not steady. It seeps up out of the soil and it swings with the weather, the season, the barometric pressure, and whether your windows are open. A single snapshot can land high or low depending on the day you happened to catch. The EPA's action level is 4 picocuries per liter (pCi/L) — at or above that, they recommend fixing the home — so which side of that line you land on genuinely matters, and a swingy reading can put you on the wrong side.
The short-term test: fast, cheap, jumpy
A short-term test runs anywhere from 2 to 7 days (some go up to 90). These are the charcoal canisters and the little electronic monitors you can buy at a hardware store or get from your state radon program. They are cheap, and you get an answer this week.
The catch is that a two-day test is measuring two days of a moving target. Test during a cold, closed-up stretch and the number reads high; test with the windows cracked in mild weather and it reads low. To keep it honest, you run a short-term test under "closed-house" conditions — windows and outside doors shut except for normal in-and-out — starting at least 12 hours before the test and staying that way the whole time.
The long-term test: slow, patient, honest
A long-term test runs more than 90 days — often a full 3 to 12 months. These use alpha-track detectors that quietly log exposure the whole time. Because they average across seasons, they smooth out the daily and weekly swings and give you a number much closer to your true year-round average exposure. And year-round average is exactly what your lung-cancer risk actually depends on.
The downside is obvious: you wait months for an answer. That does not work when you are in the middle of a home sale with a closing date.
Which one should you use?
| Situation | Best test | Why |
|---|---|---|
| Buying/selling a home, tight timeline | Short-term (often two placed together) | You need an answer in days, not months |
| You own the home and want the real picture | Long-term | Averages out seasonal swings — closest to your true risk |
| Short-term came back near 4 pCi/L | Follow up before you decide | A borderline snapshot deserves a second, longer look |
The EPA's own guidance is practical about this. If you need a quick answer, do a short-term test. If that first short-term result is 4 pCi/L or higher, they suggest confirming it — either with a second short-term test or, better, a long-term test — before committing to mitigation. And if the first result comes back really high (roughly double the action level or more), skip the wait and confirm with another short-term test right away.
Joe — the radon-ignoring legend, same fellow who let his septic and his well go over on the other two blogs — ran a two-day test in July with the windows open and the whole-house fan roaring. Got a 1.8 and declared victory. Come January, closed up tight against the cold, his basement was pulling well over 6. He had been reassured by a number that measured his ventilation habits, not his radon. A cheap long-term detector would have told him the truth the first time.
Bottom line
Neither test is wrong — they answer different questions. Use a short-term test when the clock is against you, but if you actually live in the house, a long-term test is the number that reflects the air you breathe across the seasons. Whatever you use, test in the lowest lived-in level of the home and follow up on anything borderline. The EPA lays out the whole testing protocol here: Health Risk of Radon (EPA).