The three categories
| Method | Duration | Result speed | Best use |
|---|---|---|---|
| Activated charcoal | 2–7 days | 3–7 days (lab) | Inexpensive first screen |
| Alpha track | 90 days – 1 year | 1–2 weeks after mailing | True annual average |
| Continuous radon monitor | 48 hours | Immediate | Real estate, diagnostics, verification |
| Electret ion chamber | 2 days – 1 year | Immediate on site | Professional short or long term |
Activated charcoal
A canister adsorbs radon decay products over a few days and is mailed to a lab. Cheap and widely available. Weaknesses: sensitive to humidity, weighted toward the final day of exposure, and useless if the closed-house rules are broken. Good for finding out whether you have a problem at all.
Alpha track detectors
A plastic film records etch marks from alpha particles over months. Because it integrates across seasons, a long-term alpha track test is the best representation of the exposure you actually receive year over year. Weakness: you wait. It is not a transaction tool.
Continuous radon monitors
An electronic instrument samples continuously and logs hourly readings, usually with temperature, humidity, and motion or tilt sensing. You see not just the average but the pattern — overnight peaks, a drop when someone opened a window, a spike during a storm. This is what should be used for a real estate transaction and for post-mitigation verification.
Conditions that invalidate a test
- Windows or exterior doors left open during the test or the 12 hours before it.
- An evaporative cooler, whole-house fan, or window fan running.
- The device placed in a kitchen, bathroom, laundry, or closet.
- Placement within 3 feet of an exterior wall or window, or below 20 inches from the floor.
- Severe storms or high winds during a short-term test, which can skew readings in both directions.
Which method should you use?
- 1Never tested before: a short-term test to find out where you stand.
- 2Short-term result between 2.0 and 4.0: follow with a long-term test to see your annual average.
- 3Short-term result at or above 4.0: a second short-term test or proceed directly to mitigation.
- 4Buying or selling: a professional 48-hour continuous monitor test.
- 5System just installed: a 48-hour verification test after 24 hours of runtime.
pCi/L reference table
Levels reflect EPA guidance for residential indoor air.
| Level | What it means | Recommended action |
|---|---|---|
| 0.0 – 1.9 pCi/L | At or below typical US indoor average (1.3 pCi/L) | No action needed. Retest in about two years. |
| 2.0 – 3.9 pCi/L | Elevated but below the action level | EPA suggests considering mitigation. A long-term test helps confirm. |
| 4.0 – 7.9 pCi/L | At or above the EPA action level | Mitigation recommended. |
| 8.0 – 19.9 pCi/L | Two to five times the action level | Mitigate promptly. |
| 20+ pCi/L | Substantially elevated | Treat as urgent; mitigate as soon as possible. |