Radon Level: What It Means and How It’s Measured
Your home’s radon level is the concentration of radon gas in its indoor air, and it is the single number that determines whether you need to act. Radon is an invisible, odorless radioactive gas that seeps up from the ground, and because you cannot sense it, testing is the only way to know your level. This guide is the comprehensive anchor for understanding radon levels: what the number means, how it is measured, the thresholds that matter, and what to do at each. It summarizes EPA and CDC guidance current as of 2026 and is informational only; consult a radon professional for testing and mitigation decisions and your physician for any health concerns.
What Is a Radon Level?
A radon level is a measurement of how much radon gas is present in a given volume of air, expressed in the United States as picocuries per liter (pCi/L). The picocurie is a unit of radioactivity, so the number reflects how much radioactive decay is happening in the air you breathe. A higher pCi/L value means more radon and a greater dose of radiation to your lungs over time.
Radon enters homes from the soil and rock beneath them, drawn in through foundation cracks, sump openings, and gaps around pipes by the slight pressure difference between a home and the ground. Because it comes from geology, a radon level depends heavily on where a home sits and how it is built, not on how clean or new it is. Two neighboring homes can have very different levels.
How Radon Levels Are Measured
Short-Term Tests
Short-term tests run from about two to seven days and give a quick snapshot of a home’s radon level. They use devices like charcoal canisters or short-duration electronic monitors. Short-term testing is common for initial screening and for real estate transactions, where a fast result is needed. Because radon fluctuates with weather and conditions, a short-term test reflects only the period it ran, so a result near the action level is often confirmed with a second test.
Long-Term Tests
Long-term tests run more than 90 days and capture how radon changes across seasons. They give a more accurate picture of a home’s typical year-round level because they average out short-term swings. Long-term testing is ideal when you want the most reliable estimate of your actual exposure, though it requires patience. Many experts consider a long-term result the best representation of true risk.
Continuous Radon Monitors
Continuous radon monitors measure and log levels over time, displaying how readings rise and fall with weather, season, and household activity. They provide ongoing visibility rather than a single averaged number, which helps homeowners understand their home’s behavior and verify that a mitigation system keeps levels low. Professional-grade continuous monitors are also used in real estate testing for tamper resistance and detailed data.
Understanding the pCi/L Unit
The unit behind every radon level deserves a moment of explanation, because it is unfamiliar to most homeowners. A picocurie is a measure of radioactive decay, and “per liter” ties that decay to a volume of air, so pCi/L expresses how much radioactive activity is present in the air you breathe. You do not need to understand the physics to use the number; what matters is that higher pCi/L means more radon and more radiation dose. Some countries use a different unit, becquerels per cubic meter, but in the United States pCi/L is the standard, and the EPA’s thresholds are all expressed in it.
The Thresholds That Matter
The EPA’s action level of 4 pCi/L is the key threshold. At or above 4 pCi/L, the EPA recommends installing a mitigation system to reduce radon. Between 2 and 4 pCi/L, the EPA suggests considering action, because measurable risk remains. Below 2 pCi/L, risk is lower, and homeowners often simply retest periodically.
For perspective, the EPA notes the average indoor radon level is around 1.3 pCi/L and the average outdoor level is roughly 0.4 pCi/L. No level is completely safe, since radon risk has no true floor, but these averages show the realistic low end homes can reach. The thresholds are guidance for action, not lines where radon becomes harmless. Our overview of radon testing for Colorado homeowners puts these numbers in local context.
What a Radon Level Tells You About Risk
Radon’s danger comes from its radioactive decay. As radon breaks down, it releases tiny radioactive particles that can lodge in lung tissue and damage cells over years of exposure. The EPA and CDC identify radon as a leading cause of lung cancer, behind smoking. The risk accumulates with both the concentration and the number of years a person is exposed, so reducing a high level lowers cumulative dose.
Because risk scales with concentration, a home at 8 pCi/L delivers roughly twice the dose of one at 4 pCi/L over the same period. This is why higher levels are treated more urgently and why the goal is to keep levels as low as reasonably achievable. For a closer look at where risk becomes pressing, see the companion guide on what level of radon is unsafe, and for typical baselines, the guide on what a normal radon level is.
Factors That Affect Your Level
Several factors shape a home’s radon level. Geology is the biggest: the amount of uranium in the underlying soil and rock determines how much radon is generated. Foundation type and condition matter, since cracks and openings provide entry paths. Home pressure, ventilation, and even weather influence how much radon is drawn in and how it accumulates.
Radon levels also vary by season and over time. Winter often shows higher readings because homes are sealed and heating systems create pressure differences that pull more radon inside. A home that tested low years ago can test higher after settling, renovation, or changes in soil conditions. This variability is why periodic retesting is wise even after a reassuring result.
How Radon Enters and Accumulates
Understanding why a level is what it is starts with how radon gets indoors. The gas is produced continuously in soil and rock as uranium decays, and it migrates upward through the ground. A home sits over that source, and because heated indoor air rises and escapes through upper levels, it creates a slight negative pressure at the foundation, a gentle vacuum that pulls soil gas, radon included, in through cracks, joints, sump pits, crawl spaces, and gaps around pipes and wiring.
Once inside, radon accumulates because the home encloses it. The lowest levels typically read highest, since they are closest to the soil and the entry points. This is also why levels often climb in winter: sealed windows and active heating intensify the pressure difference that draws radon in while reducing the fresh-air exchange that would dilute it. The same home can read meaningfully higher in January than in July, which is one reason a long-term measurement gives a truer annual picture than a single summer test.
How to Test Correctly
Accurate results depend on following EPA protocols. Place the test device on the lowest livable level of the home, the level people occupy, not necessarily the basement if it is unfinished and unused. During a short-term test, keep windows and exterior doors closed as much as possible and avoid running whole-house fans that draw in outside air. Do not disturb or move the device during the test period.
Choose your test type to match your goal. Use a short-term test for a quick screen or a transaction, a long-term test for the most accurate year-round picture, and a continuous monitor for ongoing visibility. If a short-term test lands near the action level, confirm with a second test before deciding. A qualified radon professional can perform testing to recognized standards when a documented, tamper-resistant result is needed.
Short-Term and Long-Term Results Compared
Because radon fluctuates, the type of test shapes how you should read the number. A short-term result is a snapshot of a few days under closed-house conditions; it is fast and useful for screening or a transaction, but it can land higher or lower than the home’s true annual average depending on weather and the season it was run. A long-term result, averaged over more than 90 days, smooths out those swings and better represents the exposure occupants actually receive year-round.
A sensible strategy uses both. A short-term test screens quickly; if it returns a borderline or elevated result, a follow-up confirms it, and a long-term test can refine the picture when time allows. When a short-term reading is well above the action level, however, waiting months for a long-term average is not warranted; the prudent move is to confirm and mitigate. Matching the test type and your response to the situation is part of reading a radon level wisely rather than reacting to a single number out of context.
What to Do With Your Radon Level
If your level is at or above 4 pCi/L, the EPA recommends mitigation. If it falls between 2 and 4 pCi/L, consider mitigation based on how long you will live there and who lives with you. Below 2 pCi/L, your risk is lower, but periodic retesting remains a good habit since levels change.
Mitigation is a proven, effective process. The most common method is active sub-slab depressurization, which uses a fan and vent pipe to draw radon from beneath the foundation and release it safely above the roofline before it enters the home. A qualified radon mitigation professional designs and installs the system, and a follow-up test confirms the level dropped. Testing tells you your radon level; mitigation, where needed, brings a high one down to a low-risk range.
Radon Levels During a Home Sale
Radon levels come up frequently in real estate, where a test is a common part of a buyer’s inspection. A short-term test is typical here because transactions move quickly, and professional-grade continuous monitors are often used for their tamper resistance and detailed data. A result at or above the action level commonly leads to a negotiated mitigation, with the seller installing a system or crediting the buyer to do so before or after closing.
For sellers, a tested and, if necessary, mitigated home removes a frequent point of friction. Documentation of a mitigation system and a confirming post-mitigation test reassures buyers that the level is under control. For buyers, understanding what a radon level means turns a test result from a source of anxiety into a clear, negotiable item. Either way, the action level of 4 pCi/L is the shared reference that frames the conversation, so knowing how levels are measured and what they signify is valuable on both sides of a transaction.
References
- What radon is and why levels matter — U.S. Environmental Protection Agency
- Radon exposure and health effects — Centers for Disease Control and Prevention
- Colorado radon testing and mitigation resources — Colorado Department of Public Health and Environment
If you want a vetted Front Range professional to test or help interpret your radon level, you can get in touch through our contact page.