You're standing in the hardware-store aisle looking at two radon kits. One says "2-day test" on the box. The other says "90-day test." They cost about the same. Which one do you buy?
The right answer depends on what you're trying to learn, what your timeline is, and how seasonal Colorado's radon swings actually are. This page walks through when to use which — and why a quick short-term test sometimes misses what a 90-day test catches.
The short version
| Short-term | Long-term | |
|---|---|---|
| Duration | 2 to 7 days | 90+ days (often 3 to 12 months) |
| Cost | $15–$40 | $30–$60 |
| How it works | Activated charcoal or short alpha-track detector | Long alpha-track detector |
| Result reflects | Levels during the test window | Average across seasons |
| Best for | Quick first read, real-estate timing, post-mit verification | Accurate annual exposure picture, borderline results, year-over-year tracking |
| Closed-house required? | 12 hours before + during test | Normal living, no closed-house requirement |
Short-term tests: speed over precision
A short-term test captures whatever the radon level is during a 2 to 7-day window. That window can be windy and warm (lower readings) or still and cold (higher readings). Same house, same year — two short-term tests run a month apart can land 30% apart from each other.
That doesn't mean short-term tests are unreliable. It means they answer a specific question well: "Right now, is my home above the action level?" If the answer is clearly yes (say, 8 pCi/L), no further short-term tests will change that. If the answer is clearly no (1.2 pCi/L), the home is probably fine.
Where short-term tests get tricky is the middle — a 3.6 pCi/L reading in March doesn't tell you whether the annual average is 2.4 or 4.8. That's where long-term tests earn their keep.
Long-term tests: averaging across seasons
Radon levels in Colorado homes change with the seasons. The biggest driver is the stack effect: in winter, the temperature difference between warm indoor air and cold outdoor air creates suction at lower levels of the home, which pulls more soil gas (and radon) inside. In summer, the effect weakens.
For a typical Colorado home:
- Winter readings can be 30–50% higher than summer readings on the same home.
- A short-term test in winter will read closer to your seasonal peak.
- A short-term test in summer will read closer to your seasonal trough.
- A long-term test averages both, giving you a more accurate picture of your annual exposure.
EPA notes that the lung cancer risk from radon is cumulative — it depends on average exposure over years, not on the level during any single 2-day window.[1] A long-term test is closer to that average.
When each test type is the right choice
Use a short-term test when:
- You've never tested before and want a quick first read.
- You're in a real estate transaction (though a professional continuous monitor is preferred).
- You need to confirm a previous test result quickly.
- You're running a post-mitigation test (per EPA, 2–7 days under closed-house conditions).[2]
- Your previous reading was clearly high (10+ pCi/L) and you want to verify before mitigating.
Use a long-term test when:
- Your short-term result was borderline (3.0–4.5 pCi/L) and you're not sure whether to mitigate.
- You want a defensible annual exposure picture (not under real-estate deadline pressure).
- You're tracking levels year-over-year in a home with previous high results.
- You want to capture a Colorado-specific seasonal average.
Stacking the two for a complete picture
Many Colorado homeowners use both:
- Short-term first. Get the quick read. If it's clearly low (under 2.0) or clearly high (over 6.0), you have your answer.
- Long-term follow-up when the short-term lands in the borderline zone. A 90-day or year-long test gives you the average that the short-term snapshot couldn't.
- Short-term post-mitigation. After installing a system, a 2–7 day closed-house short-term test verifies the system worked. EPA explicitly recommends short-term for this purpose.[2]
A homeowner in Stetson Hills tests in early February with a short-term kit. The result: 5.4 pCi/L — above the action level. Concerned but not panicked, they place a long-term alpha-track detector in the basement office where they spend most of their work day. Ninety days later (early May), the long-term test comes back at 3.1 pCi/L. The winter short-term captured the seasonal peak; the long-term captured the seasonal average. Both are real readings. The homeowner decides to mitigate, given the basement is daily living space and the borderline-zone-plus-winter-peak combination. The post-mitigation short-term test (closed-house, 48 hours) comes back at 0.9 pCi/L.