Radon glossary
Plain-language definitions of the terms you'll see in radon test reports, building codes and this site's county guides.
Measurement and health terms
| Term | What it means |
|---|---|
| Radon | A radioactive gas formed when uranium in rock and soil breaks down. It has no color, smell or taste. It's the leading cause of lung cancer in people who never smoked (EPA). |
| pCi/L (picocuries per liter) | The unit U.S. radon results are reported in. |
| Bq/m³ (becquerels per cubic meter) | The unit most other countries use. 1 pCi/L = 37 Bq/m³. |
| Action level | EPA's recommended level for fixing a home: 4 pCi/L. EPA also recommends considering a fix between 2 and 4 pCi/L. |
| WHO reference level | The World Health Organization's recommended national reference level: 100 Bq/m³, about 2.7 pCi/L (WHO). |
| Radon progeny (decay products) | The radioactive particles radon breaks down into. They lodge in the lungs when inhaled and cause the health risk. |
Testing terms
| Term | What it means |
|---|---|
| Short-term test | A test that runs 2 to 90 days. The free Tennessee kit runs 3 to 5 days (CDC). |
| Long-term test | A test that runs more than 90 days. It better reflects a home's year-round average. |
| Closed-house conditions | Keeping windows and outside doors closed for at least 12 hours before and during a short-term test, except for normal coming and going. |
| Lowest lived-in level | The lowest floor of the home used regularly as living space, where a standard test is placed. |
| Continuous radon monitor (CRM) | An electronic device that records radon hourly. Professionals often use these for real estate tests. |
Mitigation and construction terms
| Term | What it means |
|---|---|
| Mitigation | Reducing the radon in a building, usually with a vent pipe and fan system. See how radon mitigation works. |
| Sub-slab depressurization | The most common system for basements and slab homes: a pipe through the floor and a fan pull soil gas from beneath the slab and release it above the roof. |
| Sub-membrane depressurization | The crawlspace version: plastic sheeting covers the soil, and a pipe and fan draw gas from beneath it. |
| Radon-resistant new construction (RRNC) | Building features, such as a gravel layer, soil-gas barrier, sealed openings and a vent pipe, that make a new home easier to keep low in radon. |
| Passive system | A vent pipe without a fan. It relies on natural airflow and can be upgraded with a fan if a test shows high radon. |
| ANSI/AARST standards | The national consensus standards for radon testing and mitigation, such as SGM-SF-2023 for existing homes (AARST). |
Codes, maps and credentials
| Term | What it means |
|---|---|
| EPA radon zone | EPA's county-level map of predicted radon potential. Zone 1 is highest (predicted average above 4 pCi/L), Zone 2 moderate (2 to 4), Zone 3 low (below 2). EPA says the map shouldn't be used to decide whether to test a particular home (EPA). |
| IRC radon appendix | The optional radon-control chapter of the International Residential Code. It's lettered Appendix F through the 2018 edition, AF in 2021 and BE in 2024. It applies only where a local ordinance specifically adopts it (Environmental Law Institute). |
| Opt-out county | A Tennessee county that voted to exempt one- and two-family homes in its unincorporated areas from the statewide residential building standards (CTAS). |
| Exempt jurisdiction | A Tennessee city or county that enforces its own residential building code instead of the state program (State Fire Marshal). |
| NRPP and NRSB | The two national organizations that certify radon professionals. Tennessee has no state license, so these are the main credentials to look for. See how to find a certified professional. |
Last reviewed October 17, 2026 by the Tennessee Radon Authority editorial team. How we research.