How to Test for Radon: A Complete Homeowner's Guide
Radon testing is the only way to know if your home has dangerous radon levels. This guide covers every testing method, how to interpret your results, and the critical mistakes that invalidate tests.
Why Testing Is Non-Negotiable
Radon is invisible, odorless, and tasteless. You cannot detect it without a test. The EPA estimates that 1 in every 15 US homes has radon levels above the 4.0 pCi/L action threshold. Even homes in "low-risk" EPA zones can have dangerously elevated readings due to localized geological conditions.
Testing is inexpensive ($15–$200 depending on method), takes minutes to set up, and provides a definitive answer. There is no scenario in which skipping this test makes sense.
The 3 Types of Radon Tests
Short-Term Test Kits (2–7 Days)
Charcoal canisters or alpha-track detectors that absorb radon over 2–7 days. You mail the kit to a certified lab, which returns your result in about a week. This is the most common first test.
Long-Term Test Kits (90+ Days)
Alpha-track detectors or electret ion chambers that measure radon over 90 days to one year. These provide the most accurate long-term average, accounting for seasonal fluctuations. The EPA recommends a long-term test for the definitive measurement.
Continuous Radon Monitors (CRM)
Electronic devices that continuously sample air and display real-time radon readings. Consumer-grade models like the Airthings Wave or Corentium Home provide hourly measurements. Professional monitors used during real estate transactions (like the Sun Nuclear 1027) log data for 48+ hours.
How to Place Your Test Kit: The 5 Rules
Test placement directly affects accuracy. An improperly placed test can produce a falsely low reading, giving you a dangerous false sense of security. Follow these rules exactly:
- Lowest livable level. Place the test in the lowest level of the home that is regularly occupied — usually the basement or ground floor. Garages and storage rooms don't count.
- 2–5 feet above the floor. Radon is heavier than air but mixes continuously. Testing at breathing height (a table or shelf) gives the most relevant result.
- Away from exterior walls, windows, and doors. At least 20 inches from exterior walls and 12 inches from any window or door. Drafts dilute local readings.
- Away from sump pits and foundation cracks. These are radon entry points and will produce artificially high readings that don't represent the room's average level.
- Closed-house conditions. Keep windows and doors closed for 12 hours before the test begins and throughout the test duration. Normal entry/exit is fine. Do not run whole-house fans.
⚠ Common Invalidation Mistakes
- • Leaving windows open — even cracked windows dilute radon readings by 30–50%
- • Testing during renovation — open walls and disturbed foundation material can spike readings
- • HVAC changes mid-test — switching from AC to heat (or vice versa) alters air pressure dynamics
- • Placing near humidity sources — bathrooms and laundry areas produce moisture that interferes with charcoal kits
Understanding Your Results
| Reading (pCi/L) | Risk Level | Recommended Action |
|---|---|---|
| Below 2.0 | Low | No action needed. Re-test every 2 years. |
| 2.0 – 4.0 | Moderate | Consider mitigation (WHO recommends action at 2.7). Confirm with long-term test. |
| 4.0 – 8.0 | High | Mitigate within a few months. Confirm with second short-term test. |
| Above 8.0 | Very High | Mitigate immediately. Limit time in lower levels until fixed. |
If your short-term test reads above 4.0 pCi/L, the EPA recommends confirming with a second short-term test or a long-term test before committing to mitigation. However, if your reading is above 8.0, the risk is high enough that the EPA recommends acting immediately rather than waiting for confirmation.
When to Re-Test (Even If Your First Test Was Low)
Radon levels fluctuate. A one-time test provides a snapshot, not a guarantee. The EPA recommends re-testing in these scenarios:
- Every 2 years as routine maintenance — even if previous tests were low
- After major renovation — especially if you've altered the foundation, waterproofing, or HVAC system
- After any real estate transaction — tests conducted under "closed house" conditions for sales may not reflect normal living patterns
- Seasonal change — radon levels typically peak during winter when homes are sealed and heating systems create negative pressure. A summer test may underrepresent your actual annual average by 20–50%
Professional Testing vs. DIY
Professional radon testing (by a certified NRPP or AARST professional) costs $150–$300 and uses calibrated continuous monitors that log hourly data with tamper-detection. This is the standard required for real estate transactions in most states. However, for personal screening purposes, a $15–$30 charcoal canister provides a reliable initial reading. If the result is borderline (near 4.0 pCi/L), follow up with professional testing or a long-term kit.
Already have your test results?
Check your radon level against EPA and WHO thresholds, then get an itemized mitigation cost estimate specific to your county.
📊 Sources: EPA "A Citizen's Guide to Radon" (EPA 402/K-12/002), EPA "Home Buyer's and Seller's Guide to Radon", WHO Handbook on Indoor Radon (2009). All testing protocol recommendations follow ANSI/AARST MAH 2019 standards.