Long Term Radon Test: 90-Day Kits vs Continuous Monitors

Radon levels in the same basement can double between a humid July week and a closed-up January week. That swing is why a short test can mislead you. A long term radon test measures for 90 days or more, averaging out weather, heating season and daily ventilation habits to show what your family actually breathes over time.

The short answer: use a 2 to 4 day kit when you need a fast screening, such as during a home sale. Use a long-term test when a short result lands near the action level of 4 picocuries per liter (pCi/L), when you want a true annual picture, or when you are confirming that a mitigation system keeps working across seasons. A continuous electronic monitor adds real-time readings and trend data, useful when you want to watch changes rather than wait months for a lab result.

What follows explains how each option works, when to choose it, how to place it and what to do with the results.

Why a Long Term Radon Test Gives a Truer Picture

Radon is a radioactive gas from uranium in soil and rock. It enters through foundation cracks, sump pits, floor drains and gaps around pipes. How much gets in depends on soil moisture, barometric pressure, wind, and the “stack effect” created when warm indoor air rises and pulls soil gas in at the bottom of the house.

Because those factors change daily and seasonally, a 48 hour snapshot can catch a high or low moment. A long term radon test runs 90 days to a full year, so the result approximates your annual average exposure. Health guidance on radon risk is based on long exposure, which makes the long-term number the better guide for deciding whether to mitigate.

Short-Term vs Long-Term vs Continuous: How They Compare

Alpha track detectors

These passive kits contain a small piece of plastic film. As radon decays, alpha particles leave microscopic tracks on the film. After the test period, you mail the detector to a lab, which counts the tracks and calculates the average concentration. They need no power, make no noise and cost little, but you see nothing until the lab report arrives.

Electret ion chambers

A less common passive option used by some professionals. A charged disc loses voltage as radon decays nearby, and the drop is read with a special meter. They can be set up for short or long periods.

Continuous electronic monitors

A long term radon detector of the electronic kind plugs in or runs on batteries and displays short-term and long-term averages. Consumer units show daily, weekly and yearly averages; professional radon testing machines log hourly data for certified reports. They cost more up front but can be reused indefinitely.

When to Choose a 90-Day Kit

• Borderline short-term result: a reading between about 2 and 4 pCi/L is worth confirming with a long-term test before spending on mitigation.
• Annual baseline: you want a single, accurate average for your records.
• Budget priority: passive kits cost much less than monitors.
• Vacation homes and rentals: a kit can sit untouched for months without power.

Many radon programs suggest testing across the heating season, since closed windows and stack effect tend to push levels up. A 90-day test from roughly November through February often captures the high end of your range.

When a Continuous Monitor Makes More Sense

• After mitigation: watch levels drop within days, then keep an eye on them through seasonal changes.
• Checking for fan failure: a rising trend can signal that the mitigation fan has stopped.
• Remodeling a basement: track levels before and after finishing a lower level.
• Curiosity and control: seeing how opening windows or running an HVAC fan changes readings.

Consumer monitors are convenient but are not a substitute for a certified test when documentation is required, such as during a sale. Many need several days to stabilize before the readings are reliable.

How to Set Up a Long Term Radon Test

1. Pick the lowest lived-in level. Test the basement if it is used as living space or could be, otherwise the ground floor.
2. Choose the right room. Bedrooms, family rooms and offices are ideal. Avoid kitchens, bathrooms, laundry rooms and closets, where humidity and fans skew results.
3. Place it correctly. Set the detector 20 to 36 inches above the floor, at least 12 inches from exterior walls, 3 feet from windows and doors, and away from drafts, heat sources and direct sunlight.
4. Record the start date and time on the kit and in your notes.
5. Live normally. Unlike short-term tests, long-term tests do not require closed-house conditions for the whole period, though normal seasonal use of windows is fine.
6. Leave it undisturbed for at least 90 days. Do not move or handle it.
7. Seal and mail promptly. Follow the kit’s instructions, note the end date and send it to the lab within the stated window.

For a monitor, follow the same placement rules, plug it in, and wait for the long-term average to build before drawing conclusions.

A few practical habits improve accuracy. Write the kit serial number in your notes in case the label smudges. Keep pets and children from moving the detector, and do not place it on top of electronics that run warm. If you test in more than one room, use separate kits and label each location clearly so the lab results can be matched to the right space. Homes with a finished basement and a ground floor bedroom often benefit from testing both levels, since the upper floor usually reads lower and tells you how much radon reaches where people sleep. Avoid starting a test right before major foundation, window or HVAC work, since those changes can shift results partway through the period.

Reading Your Results

Results are reported in pCi/L in the United States. The federal action level is 4 pCi/L, and levels between 2 and 4 are worth considering reducing as well, since no level is completely risk-free. If your long term radon test comes back at or above 4 pCi/L, plan mitigation. Between 2 and 4, weigh the cost of mitigation against the added peace of mind, and retest periodically.

After mitigation, retest with a short-term kit within about 30 days, then follow up with a long-term test or a monitor to confirm performance through a full season. Retest every two years or after major renovations, foundation work or HVAC changes.

Troubleshooting Unexpected Readings

• Short and long tests disagree: normal, because short tests catch a moment. Trust the long-term average for decisions.
• Monitor readings jump daily: expected with weather and pressure changes. Watch the weekly and yearly averages.
• Levels rise after mitigation: check the system’s manometer and fan; a stalled fan or new foundation cracks can raise readings.
• Lab reports an invalid result: the kit may have been mailed late or exposed to heat. Retest.

Cost Ranges

Passive long-term kits commonly cost from about fifteen to fifty dollars including lab analysis. Consumer continuous monitors typically range from roughly a hundred to a few hundred dollars. Professional testing with calibrated equipment often runs one hundred fifty to a few hundred dollars. Mitigation systems, when needed, usually cost from about eight hundred to a few thousand dollars depending on foundation type.

When to Call a Licensed Professional

Hire a certified radon measurement professional for real estate transactions or whenever you need documented results. If testing shows levels at or above the action level, use a certified radon mitigation contractor. Mitigation involves drilling through slabs, routing vent pipe and wiring a fan, which may require permits and a licensed electrician for any new circuit. Many states license radon professionals, so check certification before hiring.