Radon Level Chart: Understanding pCi/L Ranges and Action Levels

Understanding what your radon test result means is the first step toward making informed decisions about your home's air quality. Radon levels are measured in picocuries per liter (pCi/L) in the United States, and different concentration ranges correspond to different levels of health risk and recommended action timelines.

This guide provides a comprehensive radon level chart, explains what each range means for health and safety, covers the EPA and WHO action thresholds, and addresses how radon levels affect real estate transactions. All data is sourced from the U.S. Environmental Protection Agency and the World Health Organization.

Radon level chart (pCi/L)

The following chart shows radon concentration ranges, associated risk levels, and recommended action timelines based on EPA guidance and health risk research.

For reference, the outdoor ambient radon level is approximately 0.4 pCi/L, and the U.S. national average for indoor radon is around 1.3 pCi/L. These baseline levels help put your test result in perspective.

Understanding the ranges

0 to 2.0 pCi/L: Low Risk

Radon levels below 2.0 pCi/L are generally considered low. This does not mean zero risk—there is no known safe level of radon exposure—but the incremental lung cancer risk at these concentrations is small enough that the EPA does not recommend immediate mitigation. Homes in this range should still be retested every two to five years, as radon levels can change due to soil conditions, foundation settling, or changes in home pressurization.

2.0 to 3.9 pCi/L: Moderate / Consider Fixing

This is the gray zone. The EPA states that homeowners should "consider" fixing at these levels, particularly when confirmed by a long-term test. The World Health Organization's reference level of 2.7 pCi/L falls within this range, reflecting a more conservative risk threshold used in many countries. If you have young children, plan to live in the home long-term, or have family members who are current or former smokers, mitigation in this range is worth serious consideration.

4.0 to 7.9 pCi/L: High / Fix Recommended

This is the EPA's official action level. At 4.0 pCi/L or above, the agency recommends taking steps to reduce radon. The health risk at this concentration is measurable: long-term exposure to 4.0 pCi/L roughly doubles the lung cancer risk compared to the national average. Mitigation is strongly advised, and most systems can reduce levels by 80 to 99 percent. If you are buying or selling a home, 4.0 pCi/L is the threshold most commonly used in real estate negotiations and radon clauses.

8.0 to 20.0 pCi/L: Very High / Fix ASAP

Levels in this range represent a significant health hazard with prolonged exposure. The EPA recommends urgent action, typically meaning you should contact a certified radon mitigation contractor within days or weeks and have the system installed as soon as scheduling allows. Do not delay. While you are waiting for the mitigation system to be installed, increase ventilation by opening windows when practical, though this is only a temporary and partial measure.

20.0+ pCi/L: Extreme / Emergency

Radon levels above 20 pCi/L are rare but not unheard of. Homes in this range require immediate attention. The EPA recommends considering temporary relocation, especially for vulnerable individuals (children, pregnant women, those with compromised respiratory health), while a mitigation system is being installed. Contact a qualified contractor immediately and request expedited service. Post-mitigation testing should confirm that levels have dropped well below the action level.

EPA action level (4.0 pCi/L)

The EPA established 4.0 pCi/L as the radon action level in 1986 based on a risk-benefit analysis that balanced health risk reduction with technical feasibility and cost. At the time, this threshold represented a practical level at which mitigation technology was proven effective and widely available. The 4.0 pCi/L level remains the official U.S. standard today.

From a health perspective, 4.0 pCi/L is not a bright line between "safe" and "unsafe." Risk increases continuously with concentration. A person living at 4.0 pCi/L has roughly twice the lung cancer risk of someone living at 1.3 pCi/L (the national indoor average), and about ten times the risk of someone living at the outdoor ambient level of 0.4 pCi/L. The EPA acknowledges in its guidance documents that any reduction in radon reduces risk proportionally, which is why levels between 2.0 and 4.0 pCi/L are flagged for consideration rather than dismissed as acceptable.

Most U.S. state radon programs, building codes that address radon-resistant new construction, and real estate disclosure requirements align with the EPA's 4.0 pCi/L threshold. It is the most commonly cited benchmark in home inspections, buyer-seller negotiations, and contractor quotes.

For more on the science and policy behind action levels, see our guide to radon levels and safety.

WHO recommendation (2.7 pCi/L)

The World Health Organization uses a reference level of 100 becquerels per cubic meter (Bq/m³), which converts to approximately 2.7 pCi/L. This lower threshold reflects a more conservative assessment of acceptable risk and is based on epidemiological evidence showing measurable increases in lung cancer incidence at concentrations below the EPA's 4.0 pCi/L action level.

Several countries, including members of the European Union, have adopted the WHO's 2.7 pCi/L guideline (or its equivalent in Bq/m³) into their national building codes and public health recommendations. In practice, this means homeowners in those regions are encouraged to mitigate at lower levels than their U.S. counterparts.

The difference between the EPA and WHO thresholds is not a scientific disagreement about risk. Both agencies agree that radon is a serious carcinogen with no safe exposure level. The difference is in policy: where to draw the line for recommended action, balancing health protection with economic and practical considerations. If your test result falls between 2.7 and 4.0 pCi/L, you are in a zone where international guidance suggests mitigation, even if U.S. policy uses softer language like "consider fixing."

Homeowners who want to minimize risk, particularly those with young children or long-term occupancy plans, may choose to use the WHO threshold as their personal action level. Mitigation systems work just as effectively at 3.0 pCi/L as they do at 5.0 pCi/L.

Radon levels in real estate

Radon testing has become a standard part of home inspections in many parts of the United States, particularly in states with high radon potential. Understanding how radon levels affect real estate transactions is important whether you are buying or selling.

Disclosure requirements

State laws vary widely on radon disclosure. Some states require sellers to disclose known radon test results regardless of level. Others have specific thresholds (often 4.0 pCi/L) above which disclosure is mandatory. A few states have no disclosure requirement at all. In practice, many real estate contracts include contingency clauses that allow buyers to request radon testing and negotiate repairs or credits if results exceed 4.0 pCi/L.

Negotiation thresholds

The 4.0 pCi/L EPA action level is the most common benchmark in real estate negotiations. Buyers typically request mitigation or a seller credit to cover installation costs when test results meet or exceed this level. Results between 2.0 and 3.9 pCi/L are less clear-cut: some buyers will negotiate, particularly in higher-priced homes or if they have specific health concerns, while others may accept the level as-is or request a retest.

Sellers who have already installed a mitigation system and have post-mitigation test results below 4.0 pCi/L are generally in a strong position. A functioning radon system can be a selling point, especially in high-radon regions where buyers expect to encounter the issue.

Testing during transactions

Real estate radon testing is almost always conducted as a short-term test using a continuous radon monitor (CRM) placed by a certified professional. These tests run for 48 to 96 hours under closed-house conditions and provide results quickly enough to fit within typical inspection contingency timelines. Buyers should ensure the test is conducted by a certified tester and follows EPA protocols for device placement and closed-house conditions.

For a detailed walkthrough of radon in home buying and selling, see our guide to radon and real estate.

Seasonal variation

Radon levels in homes are not constant. They fluctuate daily and seasonally in response to several environmental and building operation factors.

Why levels change

The primary driver of seasonal radon variation is the stack effect. During heating season, warm indoor air rises and exits through upper-level leaks (attic vents, gaps around windows, exhaust fans), creating negative pressure in the lower levels of the home. This negative pressure draws more soil gas, including radon, up through the foundation. In summer, the temperature differential between indoor and outdoor air is smaller, reducing the stack effect and often lowering radon levels.

Other factors that influence radon levels include:

• Soil moisture: Wet soil can block some radon pathways while increasing radon generation from radium decay. Dry soil may crack and open new pathways.
• Barometric pressure: Falling barometric pressure can increase soil gas flow into the home; rising pressure can reduce it.
• Wind: Wind blowing against the house can create localized pressure changes that temporarily increase or decrease radon entry.
• HVAC operation: Furnaces, air conditioners, and ventilation systems affect indoor pressure and air exchange rates.

Interpreting winter vs. summer results

Because radon levels are often higher in winter, a test conducted during heating season may return a higher result than one conducted in summer. This does not mean your summer level is the "true" level—both are real, and your annual average exposure includes both seasonal peaks and valleys. The EPA recommends that if you test during the summer and get a result just below 4.0 pCi/L, consider retesting in winter to see whether levels rise above the action threshold.

Long-term tests (90 days to one year) smooth out these seasonal fluctuations and provide a better estimate of your true annual average exposure, which is the most relevant metric for health risk assessment.

For more on testing protocols and how to account for seasonal changes, see our complete radon testing guide.

When to retest

Radon levels can change over time, so periodic retesting is an important part of home maintenance, even if your initial test was low.

Routine retesting

The EPA recommends retesting every two years if you have a mitigation system in place, and every two to five years if your initial test was below the action level and you do not have a system. This interval accounts for the possibility of foundation settling, soil condition changes, and alterations to the home that might affect radon entry or building pressurization.

When to test sooner

Certain events should trigger an immediate retest:

• Home renovations: Finishing a basement, adding HVAC equipment, or altering the foundation can change radon levels.
• Foundation work: Any repair or modification to the slab, crawl space, or basement walls may create new entry points or close old ones.
• Change in occupancy patterns: If you start using a previously unoccupied basement level, test that space.
• Mitigation system issues: If your system's fan stops working or you notice the pressure gauge indicating a problem, retest to confirm the system is still effective.
• Summer test near the threshold: If a summer test returned 3.0 to 3.9 pCi/L, retest in winter to see whether seasonal increases push you above 4.0 pCi/L.

Frequently asked questions

1. What radon level is considered safe?

There is no known safe level of radon exposure. However, the EPA recommends taking action at 4.0 pCi/L or above, and considering action between 2.0 and 4.0 pCi/L. The World Health Organization uses a lower reference level of 2.7 pCi/L (100 Bq/m³). Levels below 2.0 pCi/L are generally considered low risk, though not zero risk.

2. What does a radon level of 10 pCi/L mean?

A radon level of 10 pCi/L is considered very high and requires urgent mitigation, typically within a few months. At this concentration, the health risk is significantly elevated. The EPA classifies this in the 8-20 pCi/L range where immediate action is strongly recommended.

3. How quickly should I fix radon at different levels?

Timeline recommendations vary by level:

• Below 2.0 pCi/L: no immediate action needed
• 2.0–3.9 pCi/L: consider fixing within a year
• 4.0–7.9 pCi/L: fix within several months
• 8.0–20 pCi/L: fix as soon as possible (weeks to months)
• Above 20 pCi/L: extreme priority, consider temporary relocation during mitigation

4. Do radon levels change over time?

Yes. Radon levels fluctuate daily and seasonally due to changes in soil moisture, barometric pressure, temperature differentials, and home pressurization. Heating season typically produces higher readings due to stack effect and closed windows. This is why long-term testing (90+ days) provides a more representative annual average than short-term tests.

5. What radon level requires disclosure in real estate?

Disclosure requirements vary by state. Some states require sellers to disclose known radon test results regardless of level, while others have specific thresholds (often 4.0 pCi/L). Many real estate contracts include radon testing contingencies with negotiation or repair clauses at 4.0 pCi/L or above. Check your state's specific disclosure laws.

Sources & disclaimer: Data in this guide is drawn from the EPA's A Citizen's Guide to Radon, the WHO Handbook on Indoor Radon, and EPA state radon program guidance. This content is educational only and does not constitute legal, medical, or professional advice. Consult qualified professionals for decisions about testing, mitigation, and health concerns.