What Does My Radon Test Result Mean?

What is a pCi/L?

pCi/L stands for picocuries per liter. It's the standard unit used to measure radon concentration in the air. One picocurie equals one trillionth of a curie — a very small amount of radioactivity, but radon's health impact comes from continuous, long-term exposure rather than a single high reading.

For context:

• 0.4 pCi/L — Average outdoor radon level in the US
• 1.3 pCi/L — Average indoor radon level in US homes
• 2.7 pCi/L — Average indoor level worldwide
• 4.0 pCi/L — EPA action level for residential homes

What is the EPA action level for radon?

The EPA recommends taking action to reduce radon if your home tests at or above 4.0 pCi/L. This is the threshold at which they recommend installing a mitigation system. The EPA also suggests considering mitigation for results between 2.0 and 4.0 pCi/L — the risk is lower but still present.

There is no fully "safe" level of radon — any amount of radioactive gas exposure carries some risk. However, 4.0 pCi/L is the level the EPA considers achievable to remediate through standard mitigation in virtually any home.

Should I retest before mitigating?

It depends on what kind of test you ran:

• Short-term test (2–7 days) — These are screening tests. If your short-term test came back at or above 4 pCi/L, the EPA recommends following up with either a long-term test (90+ days) or a second short-term test before mitigating, unless the level is very high (above 10 pCi/L), in which case act immediately.
• Long-term test (90+ days) — These reflect a year-round average and are more accurate. If a long-term test shows above 4 pCi/L, mitigate.
• Continuous radon monitor — These give a real-time average. If your monitor shows above 4 pCi/L sustained for several weeks, mitigate.

How much does radon mitigation cost?

The most common system — sub-slab depressurization — typically costs $800 to $2,500 installed. Costs vary by:

• Foundation type (basement, crawl space, slab)
• Home size and complexity
• Local labor rates
• Whether you need cosmetic finishing (interior pipe routing, etc.)
• Geographic location

Operating costs are minimal — a typical radon fan uses about $20-30 of electricity per year. Some homeowners qualify for tax credits or low-interest loans for radon mitigation. See our full cost guide for state-by-state estimates.

How effective is radon mitigation?

A properly installed mitigation system typically reduces indoor radon by 90% or more. Most homes that start above 4 pCi/L can be reduced to below 2 pCi/L — often below 1 pCi/L. The system runs continuously and quietly, drawing radon from beneath the foundation before it can enter the home.

Always retest after mitigation to verify the system is working. The EPA recommends retesting:

• Within 30 days of installation
• Every 2 years thereafter
• After any major foundation work or HVAC changes

What if my result is very high (10+ pCi/L)?

Levels above 10 pCi/L are elevated but not uncommon, particularly in homes located in EPA Zone 1 areas. Some homes test in the 20–40 pCi/L range or higher. The good news: standard mitigation systems handle these levels effectively, often reducing them by 95% or more.

In the meantime, you can reduce exposure by:

• Sealing visible cracks in foundation walls and floors
• Increasing ventilation, especially in the lowest level
• Avoiding extended time in basements
• Running HVAC systems with outdoor air intake

These are temporary measures — only a professional mitigation system will reliably reduce radon long-term.