Understanding mitigation
How Does Radon Mitigation Work?
Your level is high. Here's what actually gets installed in your house, what it does physically, and how you find out afterward that it worked.
The basic idea: suction beneath the house
The most common approach is called active sub-slab depressurization. It doesn't filter radon out of your air after the fact — it reverses the natural pressure relationship between your home and the soil beneath it, so instead of your house pulling radon-laden soil gas in, the system pulls that gas out from under the slab and vents it outside before it can enter.
If you're reading this because a test came back high, it's worth knowing that installing a system usually isn't the immediate next step — start with the steps that come before a system goes in, then come back here for the mechanics.
Because an active system depends on the fan to maintain suction beneath the home, the fan is designed for continuous operation — not something you turn on occasionally.
Sources: U.S. EPA, "Consumer's Guide to Radon Reduction," on active soil depressurization. National Radon Program Services (sosradon.org): "Attached to the pipe is a quiet, continuously operating fan that discharges the radon outdoors," and the system "draws the radon-laden soil gas from beneath the foundation and exhausts it outside of the building."
Where the gas actually goes: Pennsylvania's specific rules
Pennsylvania's mitigation regulation is specific about the discharge point, precisely because a poorly placed exhaust can pull radon right back into the house or a neighboring window. The vent pipe has to terminate at least 12 inches above the roof surface where it exits through the roof, pointed vertically upward, outside the structure, discharging to open air. It also has to stay at least 10 feet from any operable window, door, or other opening into living space — or, if it's closer than that, at least 2 feet above the top of that opening. The fan itself can't be installed below grade, in a window well, or inside your conditioned living space.
Source: 25 Pa. Code § 240.308 (Radon mitigation standards) — "At least 12 inches above the surface of the roof for vent pipes that penetrate the roof"; "vertical, upward, outside the structure and discharging to the atmosphere"; "10 feet or more from an operable window unit, door or other opening into conditioned spaces unless it is 2 feet above the top of the openings."
Basements, crawl spaces, and slabs
The core idea — suction beneath the lowest level, routed outside — stays the same regardless of foundation type. What changes is where and how the suction point is created: a basement or slab-on-grade home typically gets a sub-slab suction point through the concrete floor, while a crawl space is more often sealed with a vapor barrier and depressurized as its own zone. A certified professional inspects the specific building before deciding on the approach — Pennsylvania actually requires this: a thorough visual inspection before any mitigation work begins.
Source: 25 Pa. Code § 240.308 — "The certified individual shall conduct a thorough visual inspection of the building prior to initiating any radon mitigation work."
Free York homeowner resource
Before you start calling contractors
Knowing how a system works is what lets you ask a good question. The free Radon Ready Kit turns that into something you can use on the phone: the questions to ask each provider, grouped by stage, and a worksheet for comparing the answers you get back.
Get the Free Radon Ready Kit16 pages, PDF, emailed to you. It also walks through checking a provider's DEP certification before any work starts.
How Do You Know a Radon Mitigation System Is Working?
These are two different questions, and it's worth keeping them separate:
The system appears to be operating
The fan is running, and a monitoring device on the system shows it's maintaining pressure. This tells you the equipment is functioning.
Testing confirms radon was reduced
A radon test, done afterward, actually measures the air in your home. This is the only way to confirm the number came down.
Pennsylvania requires every mitigation system to include what the regulation calls a "system failure warning device," along with written instructions for what to do if it signals a problem. In practice, this is usually a simple pressure-indicating device (often a U-tube manometer) mounted on the vent pipe. What it tells you is whether the system is pulling suction the way it's supposed to — it does not measure the radon concentration in your air. A manometer reading looks normal even on a day when, for some unrelated reason, your indoor radon level isn't actually where you'd want it. The only way to know your actual number is to test.
That's why post-mitigation testing matters, and why Pennsylvania's certified-testing protocol sets a specific window for it: not sooner than 24 hours or later than 30 days after the system is completed and turned on, with the system running the entire time you're testing. See the full testing protocol and timing requirements. Pennsylvania's mitigation labeling rule also recommends retesting at least every two years going forward, not just once after installation.
Source: 25 Pa. Code § 240.308(f) — system failure warning device and client-instructions requirement, and the information-package labeling requirement recommending retesting at least every two years. Post-mitigation testing window per 25 Pa. Code § 240.310, detailed on the Radon Testing Guide.
If the fan stops or the system needs service
If the system failure warning device signals degradation or failure, that's the cue to act — Pennsylvania requires the installer to leave you specific instructions for exactly this situation. A system that isn't maintaining suction isn't protecting your home the way it was designed to, even if it looks like it's still running.
Understand what a system would cost before you decide
Once you understand how mitigation works, cost is usually the next real question.
See what affects mitigation cost