Radon Levels: What Homeowners Need to Know
When a radon test comes back, the result is a single number, and most homeowners have no idea whether it is good, bad, or alarming. Radon levels are reported in picocuries per liter, written pCi/L, and the number tells you how much of this invisible radioactive gas is accumulating in your home’s air. This guide summarizes EPA and CDPHE guidance current as of 2026 for Front Range homeowners. It is educational only; consult a certified radon professional for testing and mitigation decisions and a physician about any health concerns.
What Are Normal and Dangerous Radon Levels?
Radon is measured in pCi/L. The EPA recommends that homeowners take action to reduce radon when the level reaches or exceeds 4.0 pCi/L. The agency also notes that no level of radon is completely safe, and it suggests considering mitigation between 2.0 and 4.0 pCi/L, since reducing exposure further continues to lower risk.
Here is a practical way to read a result:
- Below 2.0 pCi/L — relatively low, though not zero risk
- 2.0 to 3.9 pCi/L — the EPA suggests considering mitigation
- 4.0 pCi/L and above — the EPA action level; mitigation is recommended
- 10 pCi/L and above — clearly elevated; prioritize mitigation
For context, the average indoor radon level in the U.S. is around 1.3 pCi/L, while outdoor air averages about 0.4 pCi/L. Colorado tends to run higher than the national average; much of the Front Range sits in EPA Zone 1, the highest-potential category, which is why local testing is so strongly encouraged. Our pillar on radon testing in Colorado explains why the region’s geology drives elevated readings across so many homes.
Why pCi/L Matters
A picocurie per liter measures the radioactivity of radon in a liter of air. Radon decays into tiny radioactive particles, sometimes called radon progeny or daughters, that can lodge in the lungs when inhaled. Over years that lodged material emits radiation that damages lung tissue. The higher the pCi/L and the longer you breathe it, the greater the cumulative risk.
This is why a single number, while useful, is best understood as a long-term average rather than a fixed property of the house. Radon levels swing with weather, season, and how the house is operated. A snapshot taken during a stormy winter week can read higher than a summer average. A short-term test gives a quick estimate; a long-term test or a follow-up test gives a more reliable picture of what you actually breathe day to day across the year.
What Affects Your Radon Levels
Radon enters through the foundation, drawn in by the slight vacuum a heated house creates as warm air rises and escapes upstairs. Several factors influence the reading:
- Soil and rock — uranium-bearing geology produces more radon, common across the Front Range
- Foundation type — basements and slab cracks offer entry points
- Season — winter levels are often higher because homes are closed up and heated
- House operation — exhaust fans, fireplaces, and HVAC affect pressure and airflow
- Construction details — sump pits, crawl spaces, and unsealed penetrations
Because basements are closest to the soil and often the least ventilated, they typically show the highest levels, which is why testing protocols focus on the lowest livable area. A finished basement bedroom or playroom raises real-world exposure because people spend long hours there. Homeowners with elevated basement readings often start with our guide to reducing radon levels in the basement.
How to Measure Your Radon Level
You cannot see, smell, or taste radon, so testing is the only way to know your level. Two main approaches exist, and they serve different purposes.
Short-Term Tests
These run from two to seven days and give a fast estimate. They are common during real estate transactions when time is short and a closing deadline looms. Because levels fluctuate, a short-term result is a screening figure rather than a final answer, and a single low short-term test does not guarantee a low annual average.
Long-Term Tests and Continuous Monitors
Long-term tests run more than 90 days and better reflect your year-round average across seasons. Continuous electronic monitors track levels over time and show daily patterns, which helps separate a true average from a temporary spike. For ongoing awareness, many homeowners use a radon monitoring system after their initial test to keep an eye on changes.
What to Do With Your Result
If your test is at or above 4.0 pCi/L, the EPA recommends mitigation. The standard fix is an active soil depressurization system: a vent pipe and fan draw radon from beneath the foundation and exhaust it above the roofline before it enters the home. Properly installed systems routinely bring levels well below 4.0 and often under 2.0 pCi/L, and they run continuously in the background.
If your result falls between 2.0 and 3.9 pCi/L, consider mitigation, especially if children or long-term occupants spend significant time in lower levels of the home. If your result is below 2.0, retest periodically, since radon can change over time and after renovations that alter the foundation or airflow. After any mitigation, a follow-up test confirms the system is working, and a reputable contractor will expect to provide one.
Radon Levels and Home Sales on the Front Range
Radon testing is a routine part of Front Range real estate transactions. Buyers frequently request a test, and a result at or above 4.0 pCi/L often leads to negotiation over who pays for mitigation. Because so much of the region sits in Zone 1, buyers and sellers alike benefit from testing early and addressing elevated levels with a documented, professionally installed system. A clean post-mitigation test reassures everyone at the closing table and removes a common point of friction. For sellers, installing a system proactively can be a selling point rather than a liability.
Putting Your Number in Perspective
A high radon reading is a reason to act, not a reason to panic. Unlike a gas leak or carbon monoxide, radon causes harm through long-term, cumulative exposure, so a few weeks spent arranging mitigation does not meaningfully change your lifetime risk. The goal is to reduce the years of exposure ahead of you. At the same time, indefinitely ignoring a reading of 4.0 or higher does add up over time, which is why the EPA frames mitigation as recommended rather than optional. Reading your number through that lens, urgent enough to address but not an emergency, keeps the decision in proportion.
Short-Term Versus Long-Term Readings
One of the most common sources of confusion is why two tests of the same home produce different numbers. The answer usually comes down to timing and test type. A short-term test captures a few days, and those days might be unusually high or low depending on weather and how the house was operated. A long-term test averages months and smooths out those swings, so it better represents your true year-round exposure.
A practical strategy looks like this. Start with a short-term test for a quick read. If it comes back well below 2.0, your risk is likely low, though periodic retesting is still wise. If it lands near or above 4.0, either follow up with a second short-term test or move to a long-term test to confirm before deciding on mitigation. During a real estate transaction, where time is short, a short-term test under controlled closed-house conditions is the norm, and a high result typically prompts mitigation rather than a months-long wait. Matching the test type to your situation prevents both false alarms and false reassurance.
How Often to Retest Your Radon Level
A radon level is not a permanent property of a home. It can change as the house settles, as new foundation cracks form, after renovations that alter airflow, or after a basement is finished and used differently. For that reason, a single low test years ago does not guarantee a low level today. General guidance points toward retesting every couple of years, and sooner after specific events:
- After any major renovation, especially one touching the foundation or basement
- After installing or significantly changing an HVAC system
- After finishing a basement or converting lower-level space to a bedroom
- If you have a mitigation system, to confirm it is still performing
- When buying a home, regardless of any prior test on file
Retesting is inexpensive relative to the protection it provides, and a fresh number gives you current information rather than relying on a measurement that may no longer reflect how you live in the home. For households that want continuous reassurance, an electronic monitor turns periodic testing into an ongoing readout.
When to Call a Professional
Call a certified radon professional if your test is at or above 4.0 pCi/L, if you are buying or selling a home, or if you want a reliable long-term measurement rather than a DIY snapshot. A qualified measurement provider can place tests correctly and interpret the result, and a certified mitigation contractor can design a system suited to your foundation type. Periodic retesting after mitigation or major renovations keeps you confident the number stays low over the years.
Placing the Test for an Accurate Reading
An accurate radon level depends as much on where and how the test is placed as on the device itself. The standard is to test the lowest livable area, a basement if it is used, or the ground floor in a slab-on-grade home. Place the detector at breathing height, away from exterior doors, windows, drafts, and high humidity, and keep it clear of fans or vents that would disturb the air around it. For short-term tests, closed-house conditions are typical: windows and exterior doors stay shut except for normal entry and exit, which prevents outdoor air from artificially diluting the reading. A test placed in the wrong spot or under the wrong conditions can read misleadingly low and give false reassurance, so following the kit instructions or hiring a certified provider to position the test correctly is worth the small effort.
How Radon Levels Compare to Other Home Risks
Homeowners sometimes struggle to know how seriously to take a radon number compared with the other hazards a house can hold. Radon is unusual because it produces no immediate symptoms and no detectable odor, yet the EPA ranks it as a leading cause of lung cancer, behind only smoking. That combination, serious long-term risk with zero short-term warning, is exactly why testing matters so much.
Unlike a gas leak or a carbon monoxide problem, which can harm you in hours, radon works over years. A useful way to hold both facts at once is to treat a high reading as important but not urgent in the emergency sense. You should act, but you have time to test, choose a contractor, and install a proper system rather than scrambling. The cumulative nature of the risk also means that every year you spend at a lower level after mitigation genuinely reduces your lifetime exposure. Framed against other home risks, radon deserves the same routine seriousness as maintaining smoke detectors: a known hazard with a reliable, affordable fix, worth addressing precisely because it is so easy to overlook. For Front Range homeowners in a high-radon region, that routine seriousness is well placed.
References
- What Is Radon Gas and Is It Dangerous — U.S. Environmental Protection Agency
- Radon in Colorado Homes — Colorado Department of Public Health and Environment
- Health Risk of Radon — U.S. Environmental Protection Agency
- Radon and Lung Health — American Lung Association
If you are a Denver-metro or Front Range homeowner who wants an accurate radon measurement or help interpreting a result, get in touch through our contact page for a vetted local professional.