Radon Glossary & Plain-Language Definitions

A

Aeration System

A system used to remove radon from well water before it enters the home's plumbing. Water is sprayed through a chamber where air strips out the dissolved radon, which is then vented outside. The most effective treatment for waterborne radon — far more effective than activated carbon for high readings. Learn more about water treatment →

Action Level

The radon concentration at which the EPA recommends taking action to reduce indoor levels. Currently 4 pCi/L in the United States. The World Health Organization recommends action at the lower 2.7 pCi/L threshold. Levels below action level still carry some risk; mitigation is reasonable any time levels exceed 2 pCi/L.

Active Soil Depressurization (ASD)

The general category of radon mitigation systems that use a fan to depressurize soil beneath, around, or near a home's foundation. Sub-slab depressurization (SSD) is the most common type of ASD.

B

Becquerel (Bq/m³)

The international unit for radon concentration, used outside the United States. 1 pCi/L equals approximately 37 Bq/m³. The EPA action level of 4 pCi/L equals about 148 Bq/m³.

Block-Wall Depressurization

A mitigation technique used when a home has hollow concrete-block (CMU) foundation walls. A fan depressurizes the wall cavity itself, drawing radon from the void inside the blocks before it can leak into the basement.

C

Charcoal Canister Test

A short-term radon test (typically 2–7 days) using activated charcoal to absorb radon from the air. Inexpensive ($15–$30) and widely used. Less reliable than continuous monitors but suitable for initial screening or post-mitigation verification.

Closed-House Conditions

The required testing condition where windows and exterior doors stay closed (except for normal entry/exit) for 12 hours before and during the test. Without closed-house conditions, results are not valid for EPA action-level decisions.

Continuous Radon Monitor (CRM)

An electronic device that records radon concentration hour-by-hour. More accurate than charcoal canisters and shows level fluctuation over time. Used by professionals for diagnostic testing and by some homeowners as a permanent indoor air quality monitor ($150–$300).

Crawlspace Encapsulation

The installation of a heavy-duty polyethylene vapor barrier across a crawlspace floor and walls, sealed at all seams and to the foundation. Combined with active depressurization, encapsulation is the standard mitigation approach for crawlspace foundations. Side benefits include reduced moisture, fewer pests, and improved indoor air quality.

D

Diagnostic Testing

Pressure and airflow measurements taken at or near the planned suction point to determine sub-slab communication, fan sizing, and the number of suction points needed. A required step in professional system design.

Drain-Tile Depressurization

A mitigation technique that connects the radon fan to an existing perimeter drain-tile system rather than a slab penetration. Often more effective than standard sub-slab depressurization because the drain tile communicates around the entire foundation perimeter.

E

EPA (Environmental Protection Agency)

The U.S. federal agency that sets radon action levels and provides public guidance. The EPA publishes the Radon Zone Map and the standard "A Citizen's Guide to Radon."

Exhaust Pipe

The PVC pipe (typically 3-inch or 4-inch Schedule 40) that carries radon gas from the suction point to a discharge point above the roofline. Routed either inside the home or along an exterior wall.

F

Radon Fan

The continuously-running fan that creates and maintains negative pressure in the mitigation system. Sized based on the home's sub-slab resistance — typical models draw 25–90 watts of electricity. Lifespan is generally 10–15 years of continuous operation.

G

Grab Sample

A short-duration radon test (minutes to hours) used by professionals for diagnostic purposes. Not used for long-term assessment of homeowner risk.

H

Half-Life

The time it takes for half of a sample of radioactive material to decay. Radon-222 (the form found in homes) has a half-life of 3.8 days, which is why test timing and closed-house conditions matter.

I

Indoor Air Quality (IAQ)

The quality of air inside a building. Radon is one of several IAQ contaminants and the leading cause of lung cancer in non-smokers in the U.S.

M

Manometer (U-Tube)

The simple liquid-filled gauge installed on the mitigation pipe near the fan. The level difference between the two sides of the U-tube indicates the system is pulling negative pressure. Easily checked at a glance — equal levels mean the fan has stopped working.

Mitigation

The general term for any system, technique, or modification designed to reduce indoor radon levels. The most common mitigation is sub-slab depressurization (SSD).

N

NRPP (National Radon Proficiency Program)

The leading certification body for radon professionals in the United States. NRPP-certified mitigators have completed required training, passed examinations, and maintain continuing education. Maine homeowners should always work with NRPP-certified contractors for mitigation.

P

pCi/L (Picocuries per Liter)

The U.S. unit for radon concentration in air or water. The EPA action level is 4 pCi/L of indoor air. For waterborne radon, the proposed EPA limit is 4,000 pCi/L (a different threshold because water radon is largely a problem when it volatilizes into indoor air).

Post-Mitigation Test

A radon test performed 24 hours to 30 days after mitigation system installation to confirm radon levels have dropped below the action level. Required for documentation in real estate transactions.

Pressure Field Extension (PFE)

The diagnostic measurement that confirms a single suction point can pull negative pressure across the entire sub-slab area. Poor PFE indicates a need for additional suction points.

R

Radon

A naturally-occurring radioactive gas formed by the decay of uranium and radium in soil, rock, and water. Colorless, odorless, and tasteless. The leading cause of lung cancer in non-smokers in the United States. Particularly common in Maine due to the state's granite-rich geology.

Radon Zone (EPA)

The EPA's three-tier classification of geographic radon risk. Zone 1 (highest risk) covers most of inland Maine. Zone 2 (moderate-high risk) includes coastal Maine including Knox and Hancock counties. Zone 3 (lowest risk) — Maine has no Zone 3 areas.

S

Sub-Slab Depressurization (SSD)

The most common radon mitigation technique. A suction point is drilled through the basement slab, and a continuously-running fan creates negative pressure in the soil beneath the slab — drawing radon away from the home before it can enter living space. Effective in 95%+ of installations. Detailed explanation →

Suction Point

The hole drilled through the basement slab where the mitigation pipe connects to the soil beneath. Most homes need one suction point; larger or more complex foundations may need two or more.

Sealing (Slab Cracks)

Filling cracks, gaps, and openings in the basement slab and around penetrations (sump pits, drains, pipe boots). Sealing alone doesn't reliably reduce radon, but it improves the performance of an active mitigation system by preventing pressure short-circuits.

Sealed Sump Cover

A gasketed lid that seals an existing sump pit, preventing it from acting as a leak path for soil gas. Often part of a mitigation installation when a sump is present.

T

Radon Testing

The measurement of indoor radon concentration using a charcoal canister, alpha-track detector, or continuous monitor. The EPA recommends testing every Maine home at least once, retesting every 2–5 years, and after any major renovation or HVAC change. Note: Central Maine Radon & Water Mitigation does not provide testing services — we focus exclusively on mitigation. Testing is performed by certified radon measurement professionals.

U

Uranium

A naturally-occurring radioactive element. As uranium in soil and rock decays, it produces radium, which decays into radon gas. Maine's bedrock is uranium-rich, which is the underlying reason for the state's high radon levels.

V

Vapor Barrier

A heavy-gauge polyethylene sheet (typically 10–20 mil) sealed across a crawlspace or sub-slab area to block soil gas (including radon) from entering the home. Required for crawlspace mitigation; sometimes used in sub-slab applications where the slab is unfinished.

W

Waterborne Radon

Radon dissolved in well water, which is released into indoor air when the water is agitated (showers, dishwashers, washing machines). A separate problem from soilborne airborne radon and requires a separate aeration treatment. Typical contribution: roughly 1 pCi/L of indoor air for every 10,000 pCi/L of waterborne radon. Compare water vs air →

Working Level (WL)

An older unit of radon exposure used primarily by occupational health regulators. 1 Working Level approximately equals 200 pCi/L for short-term exposure calculations. Rarely used in residential contexts.