Core idea: Place an approved device in the lowest lived-in level, maintain closed-house conditions for short-term tests, and compare the average to EPA’s 4.0 pCi/L action level. Professional continuous monitors add hour-by-hour detail useful for real estate and troubleshooting.1
Common test device types
| Type | Typical use | Notes |
|---|---|---|
| Charcoal canister / liquid scintillation | Short-term (often 2–7 days) | Integrates exposure; mailed to a lab |
| Alpha track | Long-term (91+ days) | Best picture of annual average |
| Electret ion chamber | Short or longer deployments | Lab or specialist readout |
| Continuous radon monitor (CRM) | Short-term professional / real estate | Hourly data; tamper-aware options |
For a deeper comparison of timelines, see short-term vs long-term tests and DIY kits vs professional testing.
Placement rules that protect accuracy
EPA and measurement protocols emphasize:
- Lowest level of the home that is used regularly (finished basement family room, etc.)
- Breathing-zone height—typically 20–72 inches off the floor
- Away from drafts, exterior walls when possible, sump pits, kitchens, baths, and laundry rooms
- At least 3 feet from exterior doors and windows that open
- Not in closets, cupboards, or crawl spaces you never occupy (unless diagnosing entry)
Closed-house conditions
For short-term tests, windows stay closed. Exterior doors are used normally for entry/exit but not propped open. HVAC runs as usual for the season; whole-house fans and window A/C units that exchange outdoor air are typically avoided. Many protocols ask for closed-house conditions starting 12 hours before a short-term test begins.
Violating closed-house rules can artificially lower results—especially risky in a purchase inspection.
Reading your result
- ≥ 4.0 pCi/L: Fix the home. Schedule mitigation planning.
- 2–4 pCi/L: Consider mitigation for extra margin; retest to confirm.
- < 2 pCi/L: Good news—retest periodically and after major renovations.
Short-term spikes and dips happen. A single mid-winter two-day test can differ from a summer test. When a number will drive a five-figure decision or a purchase, professional CRM testing and/or a follow-up measurement is wise.
Common mistakes we see in the Black Hills
- Testing a spare bathroom or unfinished storage area nobody uses
- Leaving a window cracked “just a little” during a short-term test
- Placing a kit on the floor next to a sump
- Assuming a 10-year-old result still applies after finishing a basement
- Skipping post-mitigation verification
Sources
- U.S. EPA. A Citizen’s Guide to Radon and home testing protocols. epa.gov/radon
- American Association of Radon Scientists and Technologists (AARST) measurement standards overview.
Need a local next step? Call (605) 600-6282 to talk through testing, real estate timing, or mitigation for your Rapid City–area home.