Why a Continuous Radon Monitor Gives You the Full Picture on Indoor Air Quality

The Difference a Continuous Radon Monitor Makes

When I first started testing homes for radon gas back in the mid-2000s, the standard tool was a small charcoal canister. You would set it in the lowest lived-in level of the house, leave it for two to seven days, and then mail it to a lab. The lab returned a single number: the average radon level over that period. That number mattered, but it left a lot of questions unanswered.

Radon levels in a house are not steady. They bounce up and down with weather, with how the furnace runs, with whether someone opened a basement window. A short-term radon test can catch a spike or it can miss one entirely. That is why I now rely almost entirely on a continuous radon monitor for every professional radon test I perform in St. Louis homes. The difference is not subtle. It is the difference between a single snapshot and a full movie of your indoor air quality.

A continuous radon monitor, often called a CRM by people in the trade, sits in the home and takes radon readings at regular intervals - usually every hour or even more frequently. It logs each measurement and stores it. When the test is over, I can download a graph that shows exactly when radon went up, when it came down, and what the peak values were. That kind of detail helps me give homeowners clear, practical advice about what to do next.

How a CRM Differs from Other Radon Detectors

There are three common ways to measure radon in a home: the short-term radon test with a passive device like a charcoal canister or an alpha-track detector, the long-term radon test that runs for three months to a year, and the continuous radon monitor that takes active readings over a shorter period, typically 48 hours to seven days for a professional test.

Each method has its place. A long-term radon test is excellent for figuring out the true year-round average. The EPA recommends that if your initial short-term test shows 4 pCi/L or higher, you follow up with a long-term test or a second short-term test. But if you are buying or selling a home, or if you simply want a confident answer within a week, a continuous radon monitor is the right tool.

The key advantage of a CRM is that it gives you not just the average but the pattern. I have seen houses where the average over 48 hours was 3.9 pCi/L, just under the EPA action guideline, but the peak reading at 3:00 in the morning was 18 pCi/L. The average looked fine, but the occupants were breathing high radon levels for several hours every night. A charcoal canister would have missed that entirely.

Another advantage is that a CRM is tamper-resistant. The device records if it was moved or if the power was interrupted. That matters in a real estate transaction, where the seller might be tempted to open windows or run fans to lower the readings. I use Airthings and RadonAway monitors in my work, and both brands log motion and power events. That gives me confidence that the radon measurement I am reporting is honest and accurate.

Why St. Louis Homes Need a Professional Radon Test

St. Louis sits on a mix of limestone, glacial till, and shale. That geology, combined with the region's hot summers and cold winters, creates the perfect conditions for radon gas to enter homes through foundation cracks, sump pumps, and floor drains. Missouri as a whole has some of the highest radon potential in the country. The EPA has mapped most of St. Louis County and St. Louis City as Zone 1, meaning predicted average indoor radon levels above 4 pCi/L.

I have tested homes in Kirkwood, in the Central West End, in Florissant, and in Wildwood. The readings vary wildly from one block to the next. Two identical houses built by the same builder in the same subdivision can have very different radon levels because of small differences in how the concrete was poured, how the soil settled, or where the sump pump drains. That is why I never assume. Every home gets its own professional radon test with a continuous radon monitor.

When I arrive at a home, I place the CRM in the lowest livable area - usually the basement if it is finished or used regularly. I set it at least 20 inches above the floor, away from windows, doors, and HVAC vents. I leave it running for a minimum of 48 hours, though I prefer 72 hours or more when the schedule allows. The device takes a reading every hour. At the end of the test, I download the data and produce a report that includes the average, the peak, and a graph of the hourly readings.

What the Results Mean and What to Do Next

If the average radon level is below 2 pCi/L, I tell homeowners they are in good shape, but I also remind them that levels can change over time as the house settles or as they make changes to the foundation or the HVAC system. If the average is between 2 and 4 pCi/L, I recommend a long-term radon test to get a better picture of the year-round average. If the average is 4 pCi/L or higher, which is the EPA action level, the next step is radon mitigation.

Radon mitigation is not as disruptive as many people fear. The most common approach is a radon mitigation system, which typically involves a vent pipe running from below the basement slab up through the roof, with a small fan that creates negative pressure under the slab and vents the radon gas safely outside. A system like that, installed by a certified professional, can reduce indoor radon levels by 90 percent or more. Companies like RadonAway and HomeRadon manufacture the fans and piping that most mitigators use.

I do not install mitigation systems myself. My job is radon testing and measurement, not mitigation. But I work closely with several trusted contractors in the St. Louis area, and I give every homeowner a list of certified mitigators along with my report. That way, the homeowner has everything they need to take action if the results call for it.

The Value of Seeing the Full Pattern

Let me give you a real example from a home I tested last winter in South City. The homeowner had done a short-term radon test with a kit from the hardware store. It came back at 3.1 pCi/L. They were relieved. But they had a baby on the way and wanted a second opinion. I placed a continuous radon monitor in the same spot where they had put the kit. Over the next 72 hours, the monitor recorded a pattern: radon levels were low all day, around 1.5 pCi/L, but every night between midnight and 6:00 AM, they climbed to between 7 and 9 pCi/L. The average over the whole test was 4.2 pCi/L.

Without the CRM, the family would have thought their radon level was fine. With the CRM, they saw the truth: the radon was coming in during the night when the house was closed up and the furnace was running, and the levels were high enough to warrant action. They had a mitigation system installed the following month. A follow-up test with the same continuous radon monitor showed the average had dropped to 1.1 pCi/L, and the nighttime spikes were gone.

That is the power of a continuous radon monitor. It does not just give you a number. It gives you the story behind the number. And in a place like St. Louis, where radon levels can vary from hour to hour and from house to house, that story is worth having.

Choosing a Radon Detector for Your Home

If you are thinking about testing your own home, you have a few options. You can buy a short-term test kit at a hardware store for about 15 to 30 dollars. That will give you a rough idea. You can buy a consumer-grade continuous radon monitor from a brand like Airthings or HomeRadon for 200 to 300 dollars and run it yourself. That gives you hourly readings and long-term tracking, which is better than a single snapshot. Or you can hire a certified professional to perform a radon measurement with a high-end CRM that has been calibrated within the past year.

The professional route costs more upfront, typically 150 to 250 dollars for a test in the St. Louis area. But you get a calibrated instrument, a certified technician who knows where to place it and how to interpret the data, and a report that is defensible in a real estate transaction. If you are buying or selling a home, the professional radon test is the only option that most lenders and buyers will accept. If you just want to know for your own peace of mind, a consumer-grade monitor can work fine, as long as you follow the instructions carefully and keep the device away from drafts and direct sunlight.

Air Sense Environmental, my company, uses only calibrated continuous radon monitors from Airthings and RadonAway. I calibrate each device annually and I keep a log of every calibration. That attention to detail matters because the accuracy of the radon measurement is the foundation of every decision that follows. If the measurement is off, the mitigation recommendation might be off too. You might spend thousands on a system you do not need, or worse, skip a system that you do need.

Final Thoughts on Continuous Radon Monitoring

Radon gas is a serious health risk. It is the second leading cause of lung cancer after smoking, and it is completely invisible. You cannot smell it, taste it, or see it. The only way to know if it is in your home is to test. And the best way to test, in my experience, is with a continuous radon monitor that captures the full pattern of your indoor air quality over several days.

Whether you hire a professional or do it yourself, make sure the test is thorough. A single charcoal canister might tell you enough, or it might not. A CRM will tell you more. And when it comes to the air your family breathes every night while they sleep, more information is always better.