Most digital radon meters display a number in either pCi/L (picocuries per liter) or Bq/m³ (becquerels per cubic meter), and that number represents the concentration of radon gas in your indoor air. The key threshold to know: the EPA recommends taking action if your reading hits 4 pCi/L (150 Bq/m³) or higher. Understanding what your specific display is telling you, and why the number keeps changing, takes a bit more context.
Understanding the Units on Your Display
Your meter will show readings in one of two units, depending on where you live and which device you bought. In the United States, radon is measured in picocuries per liter (pCi/L). Most of the rest of the world uses becquerels per cubic meter (Bq/m³). Some meters let you toggle between the two. The conversion is straightforward: 1 pCi/L equals roughly 37 Bq/m³. So an EPA action level of 4 pCi/L is the same as about 150 Bq/m³.
Both units describe the same thing: how many radioactive decay events are happening in a given volume of air. A higher number means more radon. If your meter is set to Bq/m³ and you’re used to hearing advice in pCi/L, just divide by 37.
Short-Term vs. Long-Term Readings
Most digital radon monitors display more than one number. You’ll typically see a short-term reading and a long-term average, sometimes labeled “ST” and “LT” or “1-day” and “average.” These serve different purposes, and the long-term number is the one that matters most for your health.
A short-term reading captures radon levels over a brief window, usually two to four days. It gives you a quick snapshot but can swing dramatically based on weather, ventilation, and time of day. The long-term reading averages levels over at least 91 days, and ideally six to twelve months. Because radon levels shift with the seasons (typically higher in winter when homes are sealed up, lower in summer with windows open), a long-term average that spans both heating and non-heating seasons gives you the most reliable picture of your actual exposure.
If you just plugged in your monitor, the short-term number will bounce around quite a bit during the first few days. That’s normal. Give the device time to accumulate data before drawing conclusions.
Common Display Icons and Symbols
Digital radon monitors share a few standard screen elements beyond the main reading. When you first power on the device, many models enter a calibration phase. You may see “CAL” on the screen along with a countdown. This self-test typically lasts under 30 seconds, and the monitor won’t begin measuring until it’s complete.
A blinking indicator (often in the upper corner) means the instrument is actively sampling air. If that indicator stops blinking, the device may have paused or encountered an error. A battery icon appears when power is running low, and you should swap batteries promptly since a dying power supply can interrupt your measurement history. If the screen displays “Err” followed by a number, try pressing the reset button, then remove and reinstall the batteries after cleaning their contacts. Your user manual will list error codes specific to your model.
Why Your Reading Keeps Fluctuating
Radon levels are not static. If you watch your meter throughout the day, you’ll see the number rise and fall, sometimes significantly. This doesn’t mean the meter is broken. Several environmental factors drive these shifts.
Barometric pressure is one of the biggest influences. When atmospheric pressure drops (before a storm, for example), radon escapes from the soil beneath your home more easily, pushing indoor levels up. Temperature matters too: cold outdoor air creates a pressure difference that draws soil gases, including radon, into your basement or ground floor. Humidity and wind speed also play a role. Even opening a window can temporarily drop the reading by increasing ventilation, though closing it again will let levels climb back up.
This is exactly why a single reading taken over a couple of hours doesn’t tell you much. The number you should pay attention to is the long-term average, which smooths out all of these daily and seasonal swings.
Where to Place Your Meter for Accurate Readings
Placement matters more than most people realize. For reliable results, put your monitor at breathing height, between about 1.5 and 6.5 feet off the floor. The lowest livable level of your home (a finished basement, for instance) is the ideal location since radon concentrations are highest closest to the ground.
Keep the device away from windows, exterior doors, vents, and exterior walls. Drafts and fresh air near these spots will artificially lower your reading. Avoid bathrooms, kitchens, and laundry rooms where humidity spikes can interfere with measurement accuracy. A central spot in a room you actually spend time in gives you the most meaningful data about your real exposure.
What the Numbers Mean for Your Health
Once your meter has collected enough data for a reliable average, here’s how to interpret it:
- Below 2 pCi/L (75 Bq/m³): This is a low reading. No action is needed, though no level of radon is considered completely safe.
- Between 2 and 4 pCi/L (75–150 Bq/m³): The EPA recommends considering mitigation in this range. The risk is modest but real, especially over years of exposure.
- 4 pCi/L (150 Bq/m³) or higher: The EPA’s official action level. At this concentration, you should have a mitigation system installed. Most systems use a fan and piping to vent radon from beneath your foundation to the outdoors, and they typically bring levels down well below 2 pCi/L.
These thresholds are based on long-term averages, not momentary spikes. If your short-term reading jumps to 8 pCi/L during a rainstorm but your long-term average sits at 2.5, the long-term number is the one that reflects your actual risk. That said, if even your short-term readings are consistently above 4, you have enough information to act without waiting months for a long-term average to confirm it.
Getting the Most From Your Monitor
If you’re running your first test, keep your home in normal living conditions. Closing every window and sealing the house will give you an artificially high reading. Running fans constantly will give you an artificially low one. Just live normally and let the device collect data over time.
Check your readings periodically, but resist the urge to react to any single number. The daily swings are real but expected. What you’re looking for is the trend line: the long-term average settling into a range you can act on. If your meter offers a way to export or log historical data, reviewing that log after a few months gives you a much clearer picture than glancing at the screen on any given afternoon.