Testing for Radon: How to Find Out What's in Your Home's Air
Radon can’t be seen or smelled — but it’s remarkably easy and cheap to measure. And because levels differ several-fold even between directly neighboring houses, only your own test gives you a real answer: neither the EPA zone map nor your neighbor’s reading tells you what’s in your bedroom. The good news: a meaningful test costs less than a pizza night and asks almost nothing of you — set out a detector, wait, mail it in.
The test methods at a glance
1. Passive test kits — the classic route. These are small detectors without power, electronics, or maintenance. Two types matter:
- Charcoal canister tests (2–7 days): activated carbon absorbs radon; the lab measures what accumulated. Cost: about $15–30 including lab analysis. Fast, cheap — but sensitive to the day-to-day swings radon is famous for, so treat the result as a screening value, not a verdict.
- Alpha-track detectors (3–12 months): a small plastic film records the tracks of alpha particles over months. Cost: about $25–40 including analysis. This is the test that captures the long-term average that actually determines your risk.
Many state radon programs hand out free or discounted test kits, especially during January’s National Radon Action Month — a quick search for your state’s radon program is worth it before buying anything. For results that count in a real estate transaction, hire a certified measurement professional (listed with the NRPP or NRSB); they use calibrated continuous monitors, follow closed-house protocols, and their report carries weight in negotiations.
2. Digital monitors — the ongoing view. Consumer devices from about $150–250 display the level continuously, usually with an app and history curves. Their strength isn’t official proof but observation: How do readings respond to airing out? Do they climb overnight in the bedroom? Did sealing that basement crack actually help? If you’re mitigating or changing ventilation habits, you see the effect within days instead of months. They don’t replace a certified measurement, though — their accuracy is lower and the reading isn’t certified.
When and where to test
The value of your test depends almost entirely on when and where the detector sits:
- Test in the heating season: from fall to spring, closed windows and running heat maximize the stack effect that pulls soil gas indoors — that worst-case season is exactly what you want to know. A summer-only test systematically understates your exposure.
- Short-term first, long-term for certainty: a charcoal test gives a first answer within a week. If it comes back near or above 4 pCi/L, follow up — with a second short-term test if a transaction is pressing, or better, an alpha-track test over 3–12 months for the annual average.
- Test the lowest lived-in level: the bedroom or family room in the basement or on the first floor — where you actually spend hours. An unused crawl space is not the benchmark: readings there are nearly always the highest, but you don’t sleep there.
- Place it right: at breathing height on a shelf or table, away from drafts, windows, exterior walls, heat sources, and high humidity (no kitchens or bathrooms). Follow the closed-house conditions on the kit’s instructions for short-term tests: windows closed, normal door use, starting 12 hours before the test.
- Live normally for long-term tests: heat and ventilate as usual. If you air out extra diligently just for the test, you measure a scenario that has nothing to do with your life — and buy yourself false reassurance.
Reading the result
What matters is the long-term average compared against the EPA benchmarks:
| Result (long-term average) | What it means |
|---|---|
| below 2 pCi/L (~75 Bq/m³) | Unremarkable — the WHO target of 100 Bq/m³ sits in this range |
| 2–4 pCi/L | EPA says: consider fixing. Mitigation at this level is often worthwhile, since no level is truly risk-free |
| 4–10 pCi/L | Action level exceeded — fix your home, see Reducing radon |
| above 10 pCi/L | Well into the range where mitigation is urgent — get quotes from certified mitigators promptly |
Two reading aids:
- A single high day on a monitor is not an alarm. After an unventilated weekend or in certain weather (temperature inversions, pressure swings), readings spike and fall again. Health risk comes from the average over years, not from peaks.
- Just under 4 pCi/L is not a free pass. The action level is a decision threshold, not a line between “dangerous” and “safe” — risk rises continuously with concentration. The EPA explicitly recommends considering mitigation from 2 pCi/L up. If a modest fix takes you from 3.5 to 0.8 pCi/L, it’s worth doing.
And if the number is high?
No reason to panic — but a clear to-do list. In order:
- Confirm the measurement: if the source was a single short-term kit or a consumer monitor, follow up with a second test — ideally long-term or professional. Mitigation decisions deserve solid numbers.
- Simple measures first: more consistent ventilation of the lowest floors, sealing visible cracks and penetrations, keeping the basement door closed and weather-stripped. This alone often shaves off a meaningful share.
- Mitigate professionally if needed: for persistently elevated levels, a certified mitigation contractor (NRPP/NRSB listed; many states license them) installs an active sub-slab depressurization system — a pipe and fan that vent soil gas above the roofline. Typical cost: $800–2,500 one-time, cutting levels by 80–99 percent. Details: Reducing radon.
- Verify success: after any fix, retest — a monitor is fine for watching trends, a certified test for the record. Then retest every two years, as the EPA recommends, since fans fail and houses settle.
Frequently asked questions
Is one detector enough?
Two are better: one in the bedroom, one in the most-used living area. Radon distributes unevenly through a house, and kits are cheap enough that a second data point costs less than the uncertainty it removes. In homes with a finished basement, add a third there.
I rent — can I just test?
Yes. Setting out a detector isn’t a building modification and needs no permission. If the result exceeds the action level, your landlord is the right addressee for mitigation; a certified test report is your negotiating basis. Several states (and some cities) have radon disclosure or testing rules for rentals — your state radon program’s website will say.
Can’t I test in summer?
You can — say, when a home purchase won’t wait for winter. Just read the result as a lower bound: heating-season values typically run well above it. For the final verdict, retest in the next heating season.
Are cheap digital monitors from online marketplaces any good?
For trends, yes; for absolute numbers, with caution. Established consumer brands generally stay within their stated specs; no-name devices without verifiable specifications can be far off. When in doubt, run the monitor next to an alpha-track detector for a few months — you’ll see how well it’s calibrated.
How often should I retest?
After any relevant change to the house: energy retrofit (tighter envelope!), new windows, basement remodel, new HVAC or ventilation equipment — and otherwise every two years, per EPA guidance. Tightening a building can raise radon levels because the gas escapes more slowly, and mitigation fans wear out without announcing it.