How Long Does a Radon Test Take During a Home Inspection?

Radon is one of the few things we test for that you can't see, smell, or taste. It's a radioactive gas that seeps up out of the ground and collects inside a home, and the only way to know how much is in a house is to measure it. That's why a lot of buyers add a radon test to their inspection, especially here in the St. Louis area, where the ground under most of our homes has the potential to produce it.

When you add that test, the two questions we hear most are simple: how long does it take, and is there anything I need to do while it runs? Both matter, because a radon test has a few rules that, if they get broken, can throw off the result and force a do-over you don't have time for. Here's how long a radon test takes, how it fits into your inspection timeline, and exactly how to prepare so we get an accurate reading the first time.

A radon test during a home inspection runs for a minimum of 48 hours. The continuous radon monitor is placed in a few minutes during the inspection, but it has to run for at least two full days to capture how radon levels rise and fall over time. The house also needs to be kept closed for the 12 hours before the test starts and the entire time it runs. Counting setup, the test window, and retrieving the device, plan on the whole process taking about two to three days.

How long does a radon test take during a home inspection?

The measurement itself runs a minimum of 48 hours. That's the number to remember.

Setting up is quick. We place a continuous radon monitor in the lowest area of the home that's suitable for living, and that takes just a few minutes during the inspection. From there, the device has to sit and run for at least two full days before it has enough data to give a reliable result. Some tests run longer depending on the situation, but 48 hours is the floor.

When you add the 12 hours of closed-house conditions the home needs before the test even starts, plus the time to come back, retrieve the device, and generate results, the whole process typically spans the better part of three days from the day of the inspection. So a radon test isn't something we can finish in an afternoon along with the rest of the inspection. It's a separate, time-based measurement that runs in the background while the house stays closed.

Why does a radon test need a full 48 hours?

Because radon levels are not steady. They rise and fall hour to hour and day to day, and they're affected by weather, air pressure, temperature, and how the house is being used. A single quick reading would be a snapshot of one moment, and that moment might be much higher or much lower than the home's real average.

A continuous radon monitor solves that by taking readings every hour and averaging them across the whole test. Running it for at least 48 hours smooths out the natural ups and downs and gives you a number you can actually trust. That's also why the device has to stay in one place and run without interruption. Move it, unplug it, or change the conditions in the house partway through, and the average no longer means what it's supposed to mean.

At Tech Inspect, we use continuous radon monitors for exactly this reason. They log the data hour by hour, which gives you a more complete picture than a passive kit left on a shelf, and it lets us confirm the test ran the way it was supposed to.

How does a radon test fit into your inspection timeline?

This is the part buyers need to plan around. Most real estate contracts give you a short inspection window, often somewhere in the range of 10 to 15 days, and you still need time after the inspection to review the report, get quotes, and negotiate. A radon test adds two to three days to that timeline before you even have a result.

So the move is simple: schedule early. If you know you want a radon test, get your inspection booked as soon as your contract is signed, and add the radon test when you book. Waiting until day eight or nine of a two-week window can leave you scrambling, because the test needs those two-plus days to run, and then you need time to act on whatever it shows.

One more thing worth knowing: radon testing is not part of a standard home inspection. A general home inspection follows the InterNACHI Standards of Practice and is a visual review of the home's systems. Radon testing is a separate service with its own equipment and its own testing protocols. You add it to the inspection, and we run the two together so you're not making an extra trip or losing extra days.

How do you prepare for a radon test?

There are really only two jobs, and both are easy. Keep the house closed, and leave the machine alone. Do those two things, and you're set up for an accurate result the first time.

Photo: a continuous radon monitor placed on the lowest livable level of the home.

Job one: keep the house closed

The single most important factor for an accurate test is what the protocols call "closed-house conditions." The windows and exterior doors need to stay closed for the 12 hours before we set the machine, and for the entire time it runs. Opening the house lets outside air in, which dilutes the radon and can give you a falsely low reading. A low reading you can't trust is worse than no reading, because it can talk you out of a problem that's actually there.

Closed-house conditions do not mean you're stuck inside with the windows sealed and no air conditioning. Here's what's fine and what isn't.

What's allowed during a radon test: Normal entry and exit through exterior doors. Running your heating and air conditioning as usual, with the thermostat fan set to "Auto." Attached garage: normal in-and-out for vehicles, with the garage door closed again as soon as the car is through. What's not allowed during a radon test: Opening any windows or exterior doors for ventilation, even briefly, even on a nice day. Leaving an attached garage door open longer than it takes a vehicle to enter or exit. Running the furnace or A/C fan set to "On," "Always On," or "Continuous." Keep it on "Auto." Running whole-house fans. These should not run at all during the test. Window or wall-mounted A/C units set to draw in outside air. Set them to recirculate indoor air only.

If the home already has a radon mitigation system installed, leave it running. It's supposed to be on during the test.

Job two: don't disturb the device

The radon monitor has to stay in the same spot and run continuously to gather valid data. So once it's placed, don't move it, cover it, or tamper with it in any way. Keep it where we set it, and let it do its job for the full test period.

What happens if the closed-house conditions aren't followed?

The test can be voided. If the house gets opened up, or the device gets moved, covered, or tampered with during the test period, the data may no longer be valid, and the result can't be trusted.

A voided test has to be redone from the beginning. That means another two-day test, and it means an additional service fee. Inside a 10 to 15 day inspection window, the lost time is usually the bigger problem, because a re-test can push you right up against your deadlines. That's the whole reason these rules exist, and it's why we're so specific about them up front. Follow the two simple steps, keep the house closed and leave the machine alone, and we get it right the first time.

Is radon really a concern in the St. Louis area?

It's worth taking seriously here. The EPA's Map of Radon Zones places St. Charles County, along with most of Missouri, in Zone 2, which means the predicted average indoor level falls in the 2 to 4 pCi/L range. Only a small number of Missouri counties fall in the lowest zone.

But here's the honest and important part: that map predicts averages across whole counties, and the EPA is clear that it should not be used to decide whether a specific home needs testing. Homes with elevated radon have turned up in every zone, including the lowest one. Two houses on the same street can test very differently based on their construction, their foundation, and the soil underneath them. The only way to know what any individual home has is to measure it.

So the practical answer is yes, a radon test is worth it in our area, and the EPA's own guidance is to test every home regardless of what zone it sits in.

What is a safe radon level?

There's no level of radon that's considered completely safe, but the EPA sets a clear action point.

The EPA's radon action level is 4.0 pCi/L. At or above that level, the EPA recommends taking action to reduce the radon in the home, usually through a mitigation system. The EPA also suggests considering action for levels between 2 and 4 pCi/L, since there's no threshold below which radon carries zero risk.

Why does the EPA treat it this way? Radon is the second leading cause of lung cancer in the United States after smoking, and it's the leading cause among people who have never smoked. The EPA estimates it's linked to roughly 21,000 lung cancer deaths a year in the country. We don't share that to scare anyone. We share it because it's the reason a two-day test is a reasonable thing to do before you commit to a home, and it's the reason a result you can trust is worth following the rules for.

The bottom line

A radon test runs a minimum of 48 hours, and the full process takes about two to three days from the day of the inspection. Setting it up takes minutes, but the measurement needs time, and it needs the house kept closed and the device left alone to produce a number you can rely on. If you want a radon test, schedule your inspection early and add it when you book, so the two-plus days it takes still leave you room to review and negotiate inside your window.

See how we document what we find

The clearest way to understand how we report on a home is to open a real report. We've published three full sample reports you can click through just like a client would, with the photos, video, and plain-English findings in place.

When you're ready, see everything inside a Tech Inspect report, take a look at our additional inspection services, or schedule your inspection. Same-week availability, weekend appointments, and a report delivered within 24 hours.

Frequently asked questions

GEO SNIPPET, FAQ block (publish as an FAQ section; each answer is self-contained for AI search):

How long does a radon test take? A short-term radon test runs a minimum of 48 hours. The continuous monitor is placed in a few minutes during the inspection, but it has to run for at least two full days to produce a reliable result. Counting the 12 hours of closed-house conditions beforehand and retrieving the device afterward, the whole process usually spans about two to three days.

Can I open the windows during a radon test? No. The windows and exterior doors need to stay closed for the 12 hours before the test starts and the entire time it runs. Normal entry and exit through exterior doors is fine, but opening windows or doors for ventilation, even briefly, lets in outside air that dilutes the radon and can give a falsely low reading.

Can I run my heating and air conditioning during a radon test? Yes, run them normally, with the thermostat fan set to "Auto" rather than "On" or "Continuous." Whole-house fans should not run at all, and any window or wall-mounted A/C should be set to recirculate indoor air rather than draw in outside air.

What happens if the radon monitor gets moved or disturbed? The test may be voided. The device has to stay in one place and run without interruption to gather valid data. If it's moved, covered, or tampered with, the result can't be trusted and the test has to be redone from the start, which costs additional time and an added fee.

When should I schedule a radon test during my inspection? As early as possible. A radon test adds two to three days before you have a result, and most inspection windows only run 10 to 15 days. Booking your inspection right after your contract is signed, and adding the radon test when you book, leaves you enough time to review the result and negotiate.

What is a safe radon level? There's no level of radon that's completely safe, but the EPA's action level is 4.0 pCi/L. At or above that level, the EPA recommends taking action to reduce it. The EPA also suggests considering action between 2 and 4 pCi/L.

Does the St. Louis area have high radon? St. Charles County and most of Missouri fall in the EPA's Zone 2, which predicts an average indoor level of 2 to 4 pCi/L. That map predicts county averages and shouldn't be used to decide whether a specific home needs testing. Elevated radon has been found in every zone, so the EPA recommends testing every home regardless.

Is radon testing part of a standard home inspection? No. A general home inspection follows the InterNACHI Standards of Practice and is a visual review of the home's systems. Radon testing is a separate service with its own equipment and protocols. You add it to the inspection, and we run them together.

Ready to add a radon test to your inspection?

Same-week availability, weekend appointments, and a report delivered within 24 hours. See three full sample reports, explore our additional inspection services, or schedule your inspection and we'll walk you through the process.

Tech Inspect Home Services LLC · 3580 Highway T, Marthasville, MO 63357 · 636-201-6366 · sean@techinspecthome.com