What Causes High Radon in a Home?

Where Radon Comes From

Radon (Rn-222) is a radioactive noble gas produced by the radioactive decay chain of uranium-238, which is present in essentially all soil and rock on earth — just in varying concentrations. Uranium decays to radium, which decays to radon gas. Radon then seeps upward through soil pores and rock fissures toward the surface.

At the surface, radon disperses harmlessly into outdoor air (average: 0.4 pCi/L). When it encounters a building, it either disperses around it or — if the building creates a pressure differential — gets drawn inside.

Why Your Home Has the Level It Has

Your home's radon level is determined by four interacting factors:

1. Geology — the source

The uranium content of the rock and soil beneath your home determines how much radon is available. Granite, schist, gneiss, and uranium-bearing sedimentary rocks produce far more radon than limestone, basalt, or alluvial river deposits.

This is why Colorado (granite Front Range) averages 6.6 pCi/L while Florida (limestone coastal plain) averages 1.2 pCi/L. It's also why neighbors on the same street can have very different readings — one might sit over a granite intrusion, the other over a limestone pocket.

High-uranium geology doesn't guarantee high indoor radon — it just means there's more to work with. What actually gets into your home depends on the other three factors.

2. Foundation type and condition

Radon needs a pathway from the soil into your home. The foundation type determines what pathways exist:

• Slab-on-grade: Radon enters through cracks in the slab, control joints, the slab-wall joint, utility penetrations (plumbing, conduit), and porous concrete itself. More entry points = more radon.
• Basement: Similar to slab, plus the exposed wall-floor joint, floor drain, sump pit (a major entry point — essentially a direct opening to the soil), and block walls if hollow-core construction.
• Crawl space: Open soil exposure — radon rises directly from the dirt or gravel floor into the crawl space and then through the subfloor into the living space.

A well-maintained slab with properly sealed penetrations lets in less radon than a cracked, heavily penetrated one. This is why foundation age and condition matter — older foundations accumulate more cracks and gaps over time.

3. Sub-slab material

The material between the bottom of your slab and the native soil determines how easily radon moves:

• Gravel or coarse aggregate: Excellent communication. Radon moves freely under the entire slab — which sounds bad, but actually makes mitigation very effective. One suction point can depressurize a large area.
• Packed clay or fine sand: Poor communication. Radon doesn't move freely, but mitigation is harder too — suction from one point doesn't travel far, potentially requiring multiple suction points.
• No sub-slab material (direct slab on native soil): Variable. Depends on native soil type.

4. Building pressure and ventilation

Your home operates as a pressure system. When indoor air pressure is lower than outdoor pressure — which happens naturally as warm air rises and is exhausted through upper-level openings — the house acts like a vacuum, drawing air (and radon) upward from the soil.

Factors that increase this "stack effect" and pull more radon in:

• Tighter construction (modern energy-efficient homes, or older homes that have been retrofitted for efficiency)
• Exhaust fans (kitchen, bathroom, whole-house) that depressurize the lower level
• Forced-air heating with returns concentrated on the upper floors, exhausting lower-level air
• Cold outdoor temperatures — the temperature differential between indoors and outdoors drives stronger stack effect in winter

This is why radon is higher in winter in most US climates — tighter building operation and stronger stack effect together pull more radon in.

Why Neighbors Have Different Levels

Two houses on the same street can test 2 pCi/L and 18 pCi/L. The reasons:

• Different foundation crack patterns (one had settling, one didn't)
• Different sub-slab material (one has gravel aggregate, one was poured direct on clay)
• Different HVAC systems creating different pressure dynamics
• One has an open sump pit (major entry point), one doesn't
• Different construction vintages — an older home may have more cracks than a newer one

You cannot predict your home's radon level from your neighbor's result. Only testing tells you where you stand.

What You Can Do

You can't change the geology under your home. What you can change:

1. Test — know your actual level before assuming high or low
2. Seal entry points — cracks, penetrations, sump pits
3. Install a mitigation system — sub-slab depressurization prevents soil gas from entering regardless of geology or building pressure

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