Radon terms, defined

Radon Glossary: Key Terms & Definitions

Radon comes with a lot of jargon — pCi/L, sub-slab depressurization, progeny, manometers, NRPP. This glossary defines every term in plain English so Colorado homeowners can read a test result, understand a mitigation quote, and ask a contractor the right questions. Every definition is grounded in EPA guidance and AARST-ANSI industry standards.

Measurement & Units

Picocurie per Liter (pCi/L): The standard U.S. unit for indoor radon concentration. One picocurie equals about 0.037 radioactive decays per second in one liter of air. The EPA action level is 4 pCi/L.
Becquerel per Cubic Meter (Bq/m³): The international (SI) unit for radon concentration. 1 pCi/L equals 37 Bq/m³, so the EPA action level of 4 pCi/L is about 148 Bq/m³ and the WHO reference level of 100 Bq/m³ is about 2.7 pCi/L.
EPA Action Level: The radon concentration — 4 pCi/L — at or above which the U.S. EPA recommends installing a mitigation system. The World Health Organization recommends a lower reference level of 2.7 pCi/L (100 Bq/m³).
Working Level (WL): A historical unit measuring the concentration of radon decay products (progeny) in air, used mainly in occupational and mining settings. Under typical equilibrium assumptions, 1 WL corresponds to roughly 200 pCi/L of radon.
Short-Term Test: A radon test lasting 2–90 days — commonly 2–7 days with a charcoal canister or continuous monitor — used for quick screening and most real-estate transactions.
Long-Term Test: A radon test lasting more than 90 days, usually with an alpha-track detector. It gives a more accurate year-round average because radon levels rise and fall with the seasons.

Measurement Devices

Charcoal Canister: A passive short-term test device that adsorbs radon onto activated charcoal over 2–7 days, then is sealed and sent to a lab for analysis.
Continuous Radon Monitor (CRM): An electronic device that records radon hour-by-hour. Professionals use it for fast, tamper-resistant measurement and for post-mitigation verification testing.
Alpha-Track Detector: A passive long-term device containing a plastic film that records etch marks from alpha particles. It is counted in a lab to give a multi-month average radon level.
Closed-House Conditions: A testing protocol requiring windows and exterior doors to remain closed (except for normal entry and exit) for 12 hours before and throughout a short-term test, so the reading reflects worst-case accumulation.

Systems & Components

Radon Mitigation System: An engineered system that reduces indoor radon, most often by drawing soil gas from beneath the foundation and venting it above the roofline before it can enter the home.
Active Sub-Slab Depressurization (ASD / SSD): The most common mitigation method. A continuous-duty fan creates negative pressure beneath the concrete slab so radon-laden soil gas is pulled out and vented outdoors rather than seeping inside. It accounts for roughly 80% of installs.
Sub-Membrane Depressurization (SMD): The crawl-space version of ASD. A sealed polyethylene membrane is laid over the dirt floor and a fan draws soil gas from beneath it.
Block-Wall Depressurization: A method that draws radon from the hollow interior cavities of a concrete-block (cinder-block) foundation wall.
Drain-Tile Depressurization: A method that connects the radon system to an existing perimeter drain-tile (footing drain) loop to collect and vent soil gas.
Passive Radon System: A mitigation system that relies on natural stack-effect airflow with no fan. Common in radon-resistant new construction and can be activated later by adding a fan if testing shows it is needed.
Radon Fan: The continuous-duty inline fan (for example the RadonAway GP301 at about 79 watts) that creates the suction in an active system. It runs 24/7 with a typical service life of 5–10 years.
Suction Point (Suction Pit): The location where the system pipe penetrates the slab. The contractor removes some material beneath the slab to form a cavity that helps the fan reach the whole foundation.
Manometer: A simple U-tube pressure gauge mounted on the system pipe. Unequal liquid columns confirm at a glance that the fan is actively pulling negative pressure.
Pressure Field Extension (PFE): How far the fan's negative-pressure zone reaches under the slab. A diagnostic measurement that confirms the suction covers the entire foundation footprint.
Exhaust / Discharge Point: Where the system vents radon outdoors. EPA placement rules require it to terminate above the roofline, at least 10 feet above grade and at least 10 feet from any window, door, or air intake.

Health & Science

Radon (Rn-222): A naturally occurring, colorless, odorless, radioactive gas produced by the decay of uranium and radium in soil and rock. It is the leading cause of lung cancer among non-smokers, per the EPA and U.S. Surgeon General.
Radon Progeny (Radon Daughters / Decay Products): The short-lived radioactive particles radon decays into, such as polonium-218 and polonium-214. When inhaled they lodge in the lungs and emit alpha radiation — the actual mechanism behind radon-related lung cancer.
Half-Life: The time it takes for half of a radioactive substance to decay. Radon-222 has a half-life of 3.8 days.
Alpha Particle: The type of radiation emitted by radon and its progeny. It is harmless to skin from outside the body but damages lung tissue when emitted internally from inhaled progeny.
Soil Gas: The air held in the pore spaces of soil and rock beneath a building. It is the medium that carries radon from the ground toward and into the foundation.
Stack Effect: The natural upward airflow inside a heated building — warm air rises, lowering pressure on lower floors and pulling soil gas (and radon) indoors through cracks and openings. It is strongest in winter.

Standards, Credentials & Real Estate

NRPP (National Radon Proficiency Program): A leading U.S. certification program, administered by AARST, for radon measurement and mitigation professionals.
NRSB (National Radon Safety Board): The other major U.S. certification body for radon measurement and mitigation professionals.
AARST: The American Association of Radon Scientists and Technologists — the professional body that develops national radon standards and administers the NRPP certification.
AARST-ANSI Standards: The consensus national standards (such as SGM-SF, RMS-LB, and MAMF) that define proper radon measurement and mitigation practice in the United States.
Radon-Resistant New Construction (RRNC): Building techniques — a gas-permeable layer, vapor barrier, vent pipe, and sealed openings — that make a new home passively radon-resistant or far cheaper to mitigate later.
Post-Mitigation Verification Test: A radon test performed roughly 24 hours to 30 days after a system is activated to confirm levels are below the EPA action level. AARST-ANSI standards require it before an install is considered complete.
Radon Contingency (Real Estate): A clause in a real-estate contract that lets the buyer require radon testing and, if levels are elevated, negotiate mitigation before closing.

Radon in Colorado

These definitions apply nationwide, but radon risk is local. For Colorado-specific numbers — average indoor levels, EPA radon-zone classifications, state contractor-credential rules, and disclosure requirements — see how Colorado Radon Experts sources and verifies its data on our methodology page, or start with the how radon mitigation works guide.

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