Short-Term vs. Long-Term Radon Tests: Which Number Should You Trust?
Radon isn't steady. The level in your basement swings hour to hour and season to season — it climbs in winter when the house is sealed up and the furnace is pulling air, and it drops in summer when the windows are open. That single fact is why two honest radon tests of the same house can hand you two different numbers, and why the first question isn't “what's my number” but “which kind of test gave it to me.”
The short-term test: a fast snapshot
A short-term test sits in the lowest lived-in level of the house for anywhere from 2 to 90 days — most commonly 2 to 7. It's cheap, it's what you get in a hardware-store kit, and it's the one used in real-estate deals because nobody's going to wait three months to close on a house.
The catch is that it's a snapshot. Run a 2-day test during a calm, mild week and you might read low. Run the same test during a cold snap with the house buttoned up and you might read high. Neither is lying — they're both true for those two days. That's the weakness: a short window can land on an unusually low or unusually high stretch and misrepresent the year.
The long-term test: the honest average
A long-term test runs more than 90 days — ideally spanning a couple of seasons, including winter. Because it's collecting continuously, it averages out the daily and weather-driven swings and gives you a number much closer to your real year-round exposure. Since the health risk from radon is about long-term average exposure, not any single bad day, the long-term result is the more meaningful one for deciding how you actually live in the house.
| Short-term | Long-term | |
|---|---|---|
| Duration | 2–90 days (often 2–7) | 90+ days |
| Best for | Real-estate deals, a first quick check | Knowing your true average exposure |
| Weakness | Can catch a freak high or low week | Slow — you wait months for the answer |
| Cost | Low | Low-moderate |
So which do you trust?
Use them in order. The EPA's own guidance is the practical playbook here:
- Start with a short-term test. If it comes back at 4 pCi/L or higher — the EPA action level — don't wait around. Do a follow-up.
- If the first result is high and you need an answer fast, run a second short-term test and act on the average of the two.
- If you can afford the time, follow a high short-term result with a long-term test. That's the number that tells you the truth about the year.
- Fix at 4 and think hard between 2 and 4. There's no safe level; 4 pCi/L is where the EPA says to act, and they suggest considering a fix anywhere above 2.
One more thing the fast test gets wrong constantly: people run it with the windows open to “be fair.” That invalidates it. Short-term tests need closed-house conditions — windows and outside doors shut except for normal in-and-out — for at least 12 hours before and during the test. An open-window test reads artificially low and gives you a false all-clear.
Joe — the septic-ignoring, well-neglecting legend himself — ran a 2-day radon kit in July with the basement windows cracked “for accuracy” and got a 1.8. He framed the result. Come January, sealed up tight with the furnace roaring, his brother-in-law's long-term test in the same corner read a 6.1. Same basement, same house, honest tests — Joe just measured the one week of the year that made him feel good.
The bottom line
Short-term tests are for speed; long-term tests are for truth. Test short first, test with the house closed up, and if the number's high, either average two short tests or let a long-term test settle the argument before you decide the basement is fine. The EPA's radon guide walks through the whole sequence in plain English: EPA — Radon.