A short-term radon test takes 2 to 7 days of actual measurement time, while a long-term test runs for a minimum of 91 days and ideally 6 to 12 months. But the total time from start to getting your results also includes preparation before the test and lab processing afterward, so the full process is longer than the device’s measurement window alone.
Short-Term Testing: 2 to 7 Days
Most people start with a short-term test. These use either charcoal canisters or continuous radon monitors placed in the lowest livable level of your home, typically a basement or ground-floor room. The device sits undisturbed for a minimum of 48 hours, though many are left in place for the full 7 days to capture a more stable reading. Continuous monitors record radon levels hour by hour and can give you results on the spot once the test period ends.
If you’re using a mail-in charcoal canister kit (the most common DIY option), you’ll seal the device and send it to a lab after the measurement period. Charcoal tests are typically processed and reported in one to three business days once the lab receives them. Factor in a few days for shipping each way, and you’re looking at roughly 10 to 14 days total from the moment you start your test to the moment you see a number.
Long-Term Testing: 3 to 12 Months
Long-term tests measure radon for a minimum of 91 days and are most useful when left in place for 6 to 12 months. They use alpha track detectors, which accumulate data over time rather than capturing a snapshot. The big advantage here is accuracy: a long-term test gives you a much better estimate of your home’s annual average radon concentration, which is what actually determines your health risk.
Once you mail the detector back to the lab, results typically arrive within three to seven business days. So the total timeline is essentially the measurement period itself plus about a week.
Why Radon Levels Change With the Seasons
Radon concentrations in most homes follow a seasonal pattern, running higher in winter and lower in summer. This happens because homes are sealed up tighter in cold months, trapping more of the gas indoors. A short-term test done in January could read significantly higher than the same test done in July, even though nothing about your home changed.
Labs can apply seasonal correction factors to adjust a short-term reading into an estimated annual average, but these corrections are imperfect. They’re based on national or regional averages, and local geology can cause your home’s seasonal pattern to differ from the norm. Roughly 10 to 20% of homes don’t follow the typical winter-high, summer-low pattern at all. This is the main reason a long-term test is considered the gold standard for understanding your actual exposure.
If a short-term test comes back elevated, the common next step is either a second short-term test to confirm or a long-term test to get a more reliable number before committing to mitigation.
Preparation Adds 12 Hours
Before the test device even starts recording, you need to maintain closed-building conditions for at least 12 hours. That means keeping all windows and exterior doors shut (normal entry and exit is fine). This lets radon levels in your home stabilize so the test reflects typical indoor conditions rather than a freshly aired-out space.
You’ll also want to place the test device at least 20 inches off the floor and away from exterior walls, drafts, and areas of high humidity like bathrooms. The lowest level of the home that you actually use or plan to use as living space is the right location. If you have a finished basement, test there. If the basement is just storage, test the ground floor.
Closed-building conditions need to continue for the entire duration of the test, not just the 12-hour lead-in. Running your HVAC system is fine, but avoid using whole-house fans or opening windows.
What Your Results Mean
The EPA recommends taking action if your home tests at 4 pCi/L (picocuries per liter) or higher. At that level, installing a radon mitigation system is strongly advised. The EPA also suggests considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe level of radon exposure. Every reduction in concentration lowers your risk of lung cancer.
A professional mitigation system, which typically involves a vent pipe and fan pulling radon from beneath your foundation, can reduce levels by up to 99%. Most installations take a single day. After mitigation, a follow-up short-term test confirms the system is working, so you’ll go through the 2-to-7-day testing cycle one more time.
Quick Timeline Summary
- Prep time: 12 hours of closed-building conditions before starting
- Short-term test: 2 to 7 days of measurement, plus 1 to 3 business days for charcoal kit lab results
- Long-term test: 91 days to 12 months of measurement, plus 3 to 7 business days for lab results
- Professional continuous monitor: 48 hours minimum, with results available immediately on-site