How Radon Testing Works
Short-Term vs Long-Term Testing
There are two main categories of radon tests based on duration: short-term and long-term. Each has distinct use cases and accuracy tradeoffs.
Short-Term Tests
Conducted over 2–7 days. Results are faster but capture a snapshot rather than year-round averages. Useful for real estate transactions or initial screening. Windows and doors should remain closed as much as possible during testing.
Long-Term Tests
Conducted over 90 days to a year using alpha track detectors. Provides a more accurate picture of average radon exposure over time since levels fluctuate with weather, seasons, and home conditions.
The EPA recommends following up a short-term test with a second short-term test or a long-term test before deciding on mitigation. If initial results are at or above 4 pCi/L, take action.
Passive vs Active Testing Devices
Radon testing devices fall into two categories based on how they collect samples:
- Passive devices — do not require power. They simply absorb radon over the test period and are then sent to a laboratory for analysis. Charcoal canisters (most common for short-term) and alpha track detectors (for long-term) are both passive.
- Active devices — require electricity and use a fan or pump to draw air through a detector. They provide continuous monitoring and can record hourly readings. Professional testers often use continuous radon monitors (CRMs) that provide tamper-evident data logs.
For real estate transactions where results are needed quickly and tamper-evidence matters, certified professionals typically use active continuous monitors. For routine home testing, passive devices are perfectly acceptable.
DIY Test Kits vs Professional Testing
Both options are valid, but they serve different situations:
DIY Test Kits
- Cost: $15–$40 plus lab fee
- Purchase at hardware stores or online
- Mail to an accredited lab for analysis
- Good for routine home monitoring
- Not accepted for real estate disclosures in some states
Professional Testing
- Cost: $100–$300
- NRPP or AARST certified tester
- Tamper-evident continuous monitor
- Required for most real estate transactions
- Provides official written report
Where and How to Place the Test Device
Proper placement is critical for accurate results. Follow these guidelines:
- Test in the lowest livable level of the home — the floor where people spend the most time, even if it's occasionally used (a finished basement counts)
- Place the device at least 20 inches above the floor and away from exterior walls
- Keep the device away from drafts, high humidity, and direct sunlight
- Do not place near doors, windows, fans, or air vents
- During a short-term test, keep windows and exterior doors closed as much as possible (at least 12 hours before and throughout the test)
What Happens After Testing
After the test period, the device is sent to a laboratory (for passive devices) or the professional reads results from the monitor directly. You'll receive a result in picocuries per liter (pCi/L):
- Below 2 pCi/L — low risk; retest in a few years
- 2–4 pCi/L — consider mitigation, especially for smokers
- 4 pCi/L or above — EPA recommends fixing your home
- Above 8 pCi/L — take action promptly
If your level is at or above 4 pCi/L, contact a certified radon mitigation contractor. Modern mitigation systems are highly effective — reducing radon levels by 50–99% in most homes.