Radon glossary: 40 terms from action level to working level, explained plainly

Radon reports, contractor proposals, and government guides use a lot of specialized vocabulary. This glossary explains the terms you're most likely to see, in plain language, with pointers to our more detailed guides.

Measurement terms

Action level. The radon level at which EPA recommends fixing a home: 4 pCi/L (about 150 Bq/m³). EPA also suggests considering a fix between 2 and 4 pCi/L.

Average indoor level. EPA reports the average indoor radon level in U.S. homes is about 1.3 pCi/L, based on a national survey completed in 1991.

Average outdoor level. About 0.4 pCi/L, according to EPA.

Becquerel per cubic meter (Bq/m³). The radon unit used in most countries outside the U.S. 1 pCi/L = 37 Bq/m³.

Picocurie per liter (pCi/L). The U.S. unit for radon in air: a measure of radioactivity in one liter of air.

Working level (WL). A unit for radon decay products, used mainly in occupational and professional measurements.

Decay products (progeny). The short-lived radioactive elements formed when radon decays. They can attach to dust and lodge in the lungs, and they deliver most of the radiation dose.

Half-life. The time it takes for half of a radioactive substance to decay. Radon-222's half-life is about 3.8 days, which is why water and air samples must reach the lab quickly.

Test types and conditions

Short-term test. A test lasting 2 to 90 days. It's used for screening and real estate.

Long-term test. A test lasting more than 90 days. It gives a better estimate of year-round average exposure.

Charcoal canister. A common short-term passive device that adsorbs radon onto activated charcoal, then is analyzed by a lab.

Alpha track detector. A long-term passive device that records tracks from alpha particles on a plastic film.

Electret ion chamber. A passive device that measures radon by the drop in voltage on a charged disk. It can be short- or long-term.

Continuous radon monitor (CRM). An electronic device that records radon hourly. Professionals use calibrated CRMs, especially for real estate.

Closed-house conditions. For short-term tests, keeping windows and exterior doors closed, except for normal entry and exit, for 12 hours before and during the test.

Lowest lived-in level. The lowest floor of the home that's used, or could be used, as living space. EPA recommends testing there.

Follow-up test. A second test to confirm a first result at or above 4 pCi/L. EPA recommends fixing if a long-term follow-up, or the average of two short-term tests, is 4 pCi/L or higher.

Post-mitigation test. A test after a radon system is installed. EPA advises testing within 30 days, but no sooner than 24 hours after the system starts.

Mitigation terms

Mitigation. Reducing radon levels in a building, usually with an active soil suction system.

Active soil depressurization (ASD). The umbrella term for fan-powered soil suction systems that create lower pressure beneath the house than inside it.

Sub-slab depressurization (SSD) / sub-slab suction. A pipe through the slab into the soil or gravel beneath, with a fan that draws radon out and vents it above the roof. This is the most common method. EPA lists soil suction methods at 50 to 99 percent reduction.

Suction point. The hole in the slab, or the spot under a membrane, where the pipe draws soil gas.

Pressure field extension (PFE) test. A diagnostic test in which small holes are drilled in the slab to measure how far suction spreads, used to plan suction points.

Drain tile suction. Applying suction to existing perforated drain pipes around or under the foundation.

Sump-hole suction. Capping a sump pit and using it as a suction point.

Block-wall suction. Depressurizing the hollow cores of concrete block foundation walls.

Submembrane suction (submembrane depressurization). Covering a crawl space floor with plastic sheeting and drawing radon from beneath it. EPA calls it the most effective method for crawl space homes when properly applied.

Passive system. A vent pipe without a fan that relies on natural draft. Often part of radon-resistant new construction.

Radon fan. An inline fan designed for continuous operation in radon systems. It should be outside the living space. EPA notes fans may last five years or more, and warranties tend not to exceed five years.

Discharge point. Where the system vents radon outdoors. EPA explains it must be above the roof, because concentrations at the outlet are high but fall to near background within 3 to 4 feet.

Manometer (U-tube). A simple gauge, often a U-shaped tube of colored liquid, that shows suction in the system pipe. A change in the reading can mean a problem.

Warning device. Any device, such as a manometer, alarm, or light, that alerts you if an active system stops working. EPA says one must be installed.

Sealing. Closing cracks and openings in the foundation. EPA calls it a basic part of most approaches, but doesn't recommend sealing alone.

House or basement pressurization. Blowing air into the lowest level to keep soil gas out. EPA suggests considering it only after more common methods.

Heat recovery ventilator (HRV). A balanced ventilation unit that brings in outdoor air while recovering heat from exhaust air. EPA describes its effectiveness for radon as limited and best suited to small areas.

Building and program terms

Radon-resistant new construction (RRNC). Building techniques that include a gas-permeable layer, plastic sheeting, sealing, a vent pipe, and an attic junction box for a future fan.

EPA Map of Radon Zones. A county-level map classifying radon potential as Zone 1 (highest), 2, or 3. EPA recommends testing all homes regardless of zone.

State radon program. The state office that provides information, test kits, and lists of licensed or certified professionals.

Stack effect. Warm air rising and escaping from upper parts of a house, which draws soil gas in at the bottom, especially in cold weather.

Where to go next

If you're just starting, read what is radon and how to test your home for radon. If your test came back high, see radon mitigation systems. Bookmark this page for the next time a radon report or proposal leaves you puzzled.