What Is pCi/L?
Radon is measured in picocuries per liter (pCi/L) — a unit of radioactive decay rate per unit of air. One picocurie equals 0.037 radioactive disintegrations per second. In practical terms: the higher the pCi/L, the more radon is decaying in the air you breathe, and the greater the risk of inhaling radon decay products that lodge in lung tissue.
The average outdoor air level in the U.S. is approximately 0.4 pCi/L. The average U.S. indoor level is about 1.3 pCi/L. The EPA action level is 4 pCi/L — ten times the outdoor average and about three times the national indoor average.
For context: a result of 10 pCi/L means radon is decaying in your home's air at roughly 25 times the outdoor rate. This is why even modest mitigation — bringing a home from 8 to 1.5 pCi/L — produces meaningful risk reduction.
Low
Consider Mitigating
Act Now
Act Urgently
Short-Term vs. Long-Term Tests: Which Do You Have?
The type of test you used matters — it changes how you interpret the result and whether you need a confirmatory test before acting.
Short-Term Tests (2–7 days)
Charcoal canister tests (the most common DIY kit) and electret ion chamber tests are short-term devices. They are inexpensive ($15–$30 for mail-in charcoal kits at hardware stores), fast, and widely used as initial screening tools.
The limitation: a 2–7 day test is a snapshot. Radon levels vary by day, by weather, and by season. A single short-term test can read 30–50% higher or lower than your actual annual average, depending on when and how the test was conducted. EPA requires short-term tests to be conducted under closed-house conditions — windows shut, doors used only for entry and exit — for at least 12 hours before and during the test.
Long-Term Tests (90+ days)
Alpha track detectors (small plastic badges with a track-etch film inside) are left in place for 90 days to a full year. They measure cumulative radon exposure and produce a result that closely approximates your true annual average. Long-term tests are the gold standard for understanding actual exposure.
Long-term tests cost $25–$50 for a mail-in kit, take longer, but remove seasonal and weather variability from the equation. If time permits, confirming your short-term result with a long-term test before committing to mitigation is the most conservative approach — unless your short-term result is 8 pCi/L or above, in which case do not wait.
Continuous Monitors
Electronic radon monitors (devices like Airthings Wave or Safety Siren) provide ongoing real-time or rolling-average readings. Useful for understanding patterns and tracking mitigation performance, but consumer-grade devices are not calibrated to laboratory standards and should not be used as the sole basis for a mitigation decision. Use a lab-analyzed test for official results.
Reading Your Lab Report
Most mail-in radon test labs return a simple one-page report. Here is what the key fields mean:
| Field on Report | What It Means |
|---|---|
| Result / Average (pCi/L) | Your radon concentration — compare to the action table below |
| Test Start / End Date | Confirms test duration; short-term tests should be 48+ hours for validity |
| Test Type | Charcoal canister, alpha track, electret, etc. |
| Lab Accreditation | Should show NRPP or NELAC accreditation — confirms the lab is quality-verified |
| Uncertainty / Error Range | Some reports include ± uncertainty (e.g., 4.2 ± 0.6 pCi/L). The stated result is the center of this range. |
| Placement Location | Should be lowest livable level of the home. If placed in attic or second floor, the result is not valid for mitigation decisions. |
Action Table: What to Do Based on Your Result
| Result (pCi/L) | Risk Level | Recommended Action | Timeline |
|---|---|---|---|
| Below 2.0 | Low | No action required. Re-test every 2 years as precaution. | Next test in 2 years |
| 2.0–3.9 | Elevated | Run long-term test for confirmation. Strongly consider mitigation, especially if sleeping areas are at or below grade. | Confirm within 90 days |
| 4.0–7.9 | Act | Confirm with second short-term test if using initial short-term result. Mitigate within 1–3 months. | Mitigate within months |
| 8.0–19.9 | Act Soon | Schedule mitigation within weeks. Second test not required at this level. | Mitigate within weeks |
| 20.0+ | Act Immediately | Contact certified contractor this week. Comparable to occupational radiation exposure. Do not delay. | Mitigate immediately |
Where Was Your Test Placed? It Changes Everything.
The validity of your result depends entirely on where the test device was placed. EPA guidelines require that radon tests be conducted at the lowest livable level of the home — meaning the lowest floor where someone spends four or more hours per day.
- Finished basement used regularly: Test in the basement.
- Unfinished basement: Test on the first floor, or in the basement if it is regularly occupied.
- Slab-on-grade home (no basement): Test on the first floor.
- Crawl space foundation: Test on the first floor above the crawl space.
A test placed in an attic, second floor, or rarely-used room will produce a lower result than the area where occupants spend time. If your test was not placed at the lowest livable level, the result may underestimate your actual exposure. Repeat the test correctly before making a mitigation decision.
Why the Season Your Test Was Conducted Matters
Radon levels in most U.S. homes are highest in winter and lowest in summer. The reasons:
- Stack effect: In heating season, warm indoor air rises and exits through upper floors/attic, drawing cold outdoor air and soil gases in from below through foundation cracks and gaps.
- Closed windows: In winter, homes are sealed. In summer, ventilation from open windows dilutes indoor radon.
- Frozen ground: In very cold climates, frozen soil can temporarily concentrate radon in the pathways that remain unfrozen beneath foundations.
The practical implication: a summer test conducted with windows open may read 1.5–2.5 pCi/L lower than the same home's winter average. If you tested in June–August and your result was 2.5 pCi/L, your January–February average may well be above 4 pCi/L. Consider running a long-term test over fall/winter before concluding your home is safe.
Your Next Steps Based on Your Result
- Identify your test type. Short-term (2–7 days) or long-term (90+ days)? Was the test placed at the lowest livable level under closed-house conditions?
- Find your range in the action table. Below 4: confirm with long-term test. 4–7.9: confirm with second short-term and schedule mitigation. 8+: schedule mitigation without further delay.
- Use the Action Level Advisor tool. Enter your result and state to get state-specific guidance including permit and contractor certification requirements.
- Contact 2–3 certified contractors. Get written quotes. A standard quote should include: system type, fan model, post-mitigation test, and whether permits will be pulled.
- Check your state's permit requirements. Some municipalities require an electrical permit for the fan circuit. Your contractor should handle this, but confirm before work begins.
Download: Homeowner Action Checklist
Print-ready one-page checklist covering every step from test results to post-mitigation sign-off.
Download Free PDF