How to Read Your Radon Test Results

The Unit: pCi/L

Radon is measured in picocuries per liter (pCi/L) — a measure of radioactivity per unit of air. One picocurie is one trillionth of a curie, and one curie is the radioactive decay rate of one gram of radium.

You don't need to understand the physics. What matters is the number and where it falls relative to established benchmarks.

The Benchmarks

Outdoor air: ~0.4 pCi/L (the unavoidable background level)

EPA action level: 4 pCi/L — At or above this level, the EPA recommends fixing your home.

EPA consider fixing: 2–4 pCi/L — Risk is present; mitigation is worth serious consideration.

WHO recommended level: 2.7 pCi/L — The World Health Organization uses a lower threshold than the US EPA.

Post-mitigation target: below 2 pCi/L — This is what certified mitigators aim for after installation.

Reading Your Lab Report

If you used a charcoal canister or alpha track test, your results come from a certified lab. Here's what the report typically shows:

Average result: The main number. This is your home's radon level in pCi/L.

Measurement uncertainty: Often shown as ±X pCi/L or as a percentage. A result of 4.2 pCi/L ±0.5 means the true value is most likely between 3.7 and 4.7 pCi/L. For borderline results, this matters.

Test duration: Short-term tests run 48–96 hours. Long-term tests run 90–365 days. Long-term tests give a more accurate annual average.

Test location: Results should be from the lowest livable level of your home (basement, first floor if no basement). Testing on upper floors gives artificially low results.

Short-Term vs. Long-Term Results

Short-term test (48–96 hours): Suitable for making a mitigation decision. Radon fluctuates day-to-day based on weather and pressure changes, so a short-term test is a snapshot, not a long-term average. Still accurate enough to act on.

Long-term alpha track test (90–365 days): The most accurate way to measure annual average exposure. If you're borderline (3–5 pCi/L on a short-term test) or want post-mitigation confirmation, this is the gold standard.

If You Used a Digital Continuous Monitor

Devices like the Airthings Corentium or Safety Siren display rolling averages. Which number matters?

Long-term average (30+ days): The most meaningful number. Short spikes from storms or pressure changes don't change your long-term risk.

Short-term average (24–48 hours): Useful for troubleshooting — checking if levels spike during specific conditions. Not the number to use for a mitigation decision.

Common Questions About Results

"My test result was 3.8 pCi/L — is that fine?"

Technically below the EPA action level, but only barely. Given measurement uncertainty, the true level could be above 4 pCi/L. The WHO recommends action at 2.7 pCi/L. If you're planning to be in the home long-term, mitigation is worth considering.

"My result was 0.5 pCi/L in the basement — should I test other floors?"

A low basement result is reassuring. Radon levels decrease on upper floors since radon enters from the ground. If the basement is low, upper floors are lower. No action needed.

"My digital monitor shows different numbers every day."

Normal. Radon fluctuates with barometric pressure, wind, and temperature. A spike to 8 pCi/L during a storm doesn't mean your average is 8 pCi/L. Look at the long-term average displayed on the device.

"My neighbor tested 10 pCi/L but my result was 1.8 pCi/L — which is right?"

Both. Radon levels vary dramatically from house to house on the same street due to foundation construction, soil permeability, and building ventilation. Your result is specific to your home.

What to Do With Your Result

• Below 2 pCi/L: Nothing required. Retest in 2 years.
• 2–4 pCi/L: Consider mitigation, especially for long-term occupancy.
• 4 pCi/L and above: Contact a certified mitigator. Find one in your state →
• 10+ pCi/L: High priority. Schedule mitigation promptly.

Find a certified mitigator near you →