Radon Glossary: pCi/L, EPA Zones, Sub-Slab Depressurization & More

Radon testing and mitigation come with their own vocabulary, and most of it shows up for the first time on an inspection report or a contractor's quote — not somewhere you'd naturally learn it first. Here's what the terms on this site actually mean, in plain language.

Action level

The radon concentration at which EPA recommends fixing a home: 4 pCi/L. EPA also says levels between 2–4 pCi/L are worth considering fixing, and that no level of radon exposure is completely risk-free — 4 pCi/L is a recommended threshold for action, not a safety cutoff below which risk disappears.

Arithmetic mean vs. geometric mean

Two different ways of averaging a set of radon readings. The arithmetic mean is the simple average — add up all readings and divide by the count. The geometric mean multiplies the readings together and takes the root, which reduces the influence of a small number of very high outlier readings. York County's state-run survey reported both: an arithmetic mean of 15.5 pCi/L and a geometric mean of 7.5 pCi/L across the same 68 homes — the gap between the two numbers itself tells you a handful of very high readings pulled the simple average up. See the full York County radon levels data.

EPA Zone 1

EPA's Map of Radon Zones sorts U.S. counties into three tiers by predicted average indoor radon level. Zone 1 is the highest category — a predicted average indoor screening level of 4 pCi/L or greater. York County sits in Zone 1.

Manometer

A small U-tube gauge mounted on a mitigation system's vent pipe. It gives a quick visual check — a liquid level difference between the two sides of the tube — confirming the system's fan is still running and the system is still under vacuum. Checking it periodically is the main maintenance step for an installed system.

pCi/L (picocuries per liter)

The standard unit for measuring radon concentration in air. It's a measure of radioactivity per volume of air — specifically, one trillionth of a curie of radioactive decay per liter. You don't need to understand the physics to use the number: EPA's action level is 4 pCi/L, and any test result you get back will be expressed in this unit.

PA DEP certification / Radon Certification Act

Pennsylvania's Radon Certification Act requires radon testing, mitigation, and lab analysis work in the state to be performed under active PA DEP certification or authorization — by a certified individual, or by a DEP-approved employee of a certified firm. It's a real, checkable credential — see how to verify a PA DEP-certified radon contractor for what it requires and how to confirm someone's status directly with DEP.

Radon-resistant new construction

Passive radon-control features — typically a layer of gas-permeable material under the slab, a sealed vapor barrier, and a vent pipe routed to the roofline — installed during a new home's construction, before occupancy. These features reduce radon entry but don't guarantee a low result on their own, and PA DEP's installer exemption for these features covers installation before occupancy only, not testing or mitigation needed after move-in.

Short-term vs. long-term test

A short-term test runs for 2–90 days (most commonly 2–7 days) and gives a faster read, useful for real estate timelines. A long-term test runs for more than 90 days and better reflects a home's year-round average, since radon levels shift with weather and season. See short-term vs. long-term radon tests for which one fits your situation.

Sub-slab depressurization (SSD)

The standard, most effective radon mitigation method. A suction pipe is inserted through the foundation slab into the soil or gravel layer beneath the house; a continuously running fan draws soil gas — including radon — out from under the home and vents it above the roofline before it can enter living space. Properly installed, SSD systems reduce indoor radon levels by 50–99%. Full explanation on sub-slab depressurization.

Sub-membrane depressurization

The crawl-space equivalent of sub-slab depressurization. Instead of penetrating a concrete slab, a sealed vapor barrier (membrane) is installed across the crawl-space floor, and a suction point draws soil gas from beneath that membrane rather than beneath a slab. See crawl space radon mitigation vs. sub-slab depressurization for how the two methods compare.

Get a Free Radon Estimate if any of this raised a question specific to your home.