Why Is Radon Higher in Some Areas? A Clear Explanation

Radon levels are higher in some areas because the ground beneath your home naturally contains more uranium, which decays into radon gas. This gas then seeps through cracks in foundations and walls to build up indoors.

We found that local geology is the main driver—places with granite or shale soil often have higher radon. Many experts say that testing your home is the only way to know for sure if you’re at risk.

Radon comes from natural uranium decay in soil and rocks.

Why Radon Levels Vary So Much From Place to Place

The amount of radon in your home is a local story. It depends on what’s happening in the soil and rock right under your feet. While radon exists everywhere, some neighborhoods have much higher concentrations. Let’s break down the exact reasons why.

The Geology Under Your Home Is the Biggest Factor

Think of the ground beneath your house as a giant sponge. That sponge is filled with rocks and soil that contain tiny amounts of uranium. As this uranium naturally breaks down, or decays, it creates radon gas. The type of rock and soil you have determines how much uranium is there in the first place.

Areas with certain types of bedrock simply produce more radon. We found that the U.S. Geological Survey has mapped high-radon zones across the country, often tied to specific geological features. Your location isn’t random—it’s written in the rocks.

Common High-Radon Rock and Soil Types

Granite: This common bedrock, especially its older forms, can have higher uranium content. Many parts of New England have granite geology.
Shale and Phosphate Rock: These sedimentary rocks often contain more uranium than others. They are common in the Midwest and parts of the Appalachian Mountains.
Uranium-Rich Soils: Glacial deposits and soils derived from certain volcanic rocks can also be sources.

It’s not just about the bedrock, though. The layer of soil on top of it matters too.

How Soil Type and Permeability Affect Radon Movement

Radon is a gas. To get into your home, it has to move through the soil. Two things control this: how much radon the soil can hold and how easily the gas can travel through it.

Imagine two different sponges. One is dense and waterlogged, the other is dry and full of air pockets. Gas moves easily through the dry, airy sponge. The same is true for soil.

Soil Characteristic	Effect on Radon
Loose, sandy, or gravelly soil	High permeability. Radon gas travels easily through the gaps, making it more likely to reach your foundation.
Dense clay soil	Low permeability. It acts more like a barrier, slowing gas movement. However, if the clay is cracked or sits on permeable rock, radon can still find a path.
Soil with high moisture content	Water can fill the air pockets in soil, potentially slowing radon’s movement. But heavy rain can also push radon out of the ground.

So, a home on sandy soil over granite is a classic high-risk setup. A home on dense clay might have a lower risk, but it’s not a guarantee.

Entry Points: How Radon Gets Inside

Even with a perfect geology setup, radon needs a way in. Your home’s foundation is like a filter. If it’s perfect, very little gas gets through. But no foundation is perfect.

We found that the EPA identifies common entry points as cracks in solid floors, construction joints, gaps around service pipes, and even tiny cracks in walls. It can also seep through the water supply if your home uses a private well.

Common Cracks and Gaps to Look For

Floor-wall joints: Where the concrete slab meets the basement wall.
Cracks in the slab: Any visible crack in your basement or garage floor.
Gaps around pipes: Openings where plumbing or utility lines enter the house.
Sump pump pits: These are direct drains to the earth below.

The stack effect in your home, where warm air rises and exits, can actually create a suction that pulls radon gas in through these small openings. It’s like your house is breathing the soil air.

The Role of Water in Radon Levels

While soil is the primary source, radon can also dissolve in groundwater. If you get your water from a private well, that water can carry radon into your home. When you use that water—for showering, washing dishes, or drinking—it releases the radon into your indoor air.

Many experts say this is a bigger concern for people on private wells than for those on public water systems, which often treat for a variety of contaminants. The CDC notes that radon in water typically contributes a small fraction of the overall radon risk compared to soil gas, but it’s still worth knowing about.

Climate and Seasonal Changes

Have you heard that radon levels are higher in winter? It’s true, and the reason is simple physics. When you seal up your house against the cold, you create a pressure difference. Warm air inside your home rises and escapes through the attic.

This creates a slight vacuum at the lower levels of your house. That vacuum pulls more radon gas from the soil through any available crack. It’s why winter is often the recommended season for testing—you’re likely measuring the highest risk period.

Heavy rain can also cause short-term spikes as water saturates the soil and pushes radon gas upward toward the surface.

Your Home’s Construction and Ventilation

A tightly built, modern home might seem safer, but it can trap radon more effectively. Older homes with more natural drafts might have more air exchange, but that’s not a reliable way to lower radon.

Proper ventilation and mitigation systems are the key. A mitigation system doesn’t prevent radon from entering; it intercepts it before it fills your living spaces. It’s a direct answer to the problem geology creates.

How to Find Out Your Specific Risk

You can’t see, smell, or taste radon. The only way to know what’s in your home is to test. This is the single most important step you can take. The EPA recommends testing for at least 48 hours, with longer-term tests of 90+ days giving a better annual average.

Short-term kits are available at hardware stores or online. They’re easy to use: you place the detector in the lowest lived-in area, leave it undisturbed, and mail it to a lab. Many people find the process straightforward.

Basic Radon Testing Checklist

Choose the right location: Test in the lowest level you regularly use, like a basement family room or bedroom.
Close all windows and doors: Except for going in and out, keep them shut during the test.
Avoid disrupting the detector: Don’t place it near drafts, heat, or humidity.
Test for the right duration: 48 hours for a quick snapshot, 90+ days for a more accurate yearly average.
Follow the kit instructions exactly: Each kit has specific directions for placement and mailing.
Retest if you get a high reading

What Do the Numbers Mean?

Your test results will be in picocuries per liter (pCi/L). The EPA has set an action level of 4 pCi/L. If your test is at or above this level, they strongly recommend installing a mitigation system.

Even levels below 4 pCi/L aren’t zero risk. The EPA suggests considering mitigation between 2 and 4 pCi/L. The goal is to get your exposure as low as reasonably achievable.

So, Why Is Your Neighbor’s Radon Different?

Two homes side-by-side can have different radon levels. This happens because of small changes in geology, soil disturbance during construction, or differences in the foundation. The path of least resistance for radon might be right under your neighbor’s slab and not yours.

This is why blanket statements about entire towns can be misleading. The only truth is your home’s own measurement.

Key Takeaways for Homeowners

Radon is a natural, geologic phenomenon. You can’t change the rock under your house, but you can take control of the risk. The combination of high-uranium soil, permeable ground, and foundation cracks creates the perfect conditions for indoor radon.

Testing is simple, affordable, and the only path to peace of mind. If you find a problem, proven mitigation systems can reduce radon levels by over 90%. You have the power to address this invisible issue effectively.

Conclusion

Radon levels vary because the soil and rock under your home hold different amounts of uranium. The geology creates the source, while soil permeability and foundation cracks allow the gas to enter your living space. Remember, you can’t see or smell this risk. The only way to know your home’s specific level is to conduct a proper test. We recommend you start with an affordable short-term test kit. If levels are high, a professional mitigation system can effectively reduce your exposure by over 90%. Your next step is simple: test your home and take action based on the results.

Frequently Asked Questions

How often should I test my home for radon?

The EPA recommends testing your home every two years or after any major renovations. You should also test if you change your ventilation system or the structure of your home. A new test provides a current snapshot of your risk.

Can two homes right next to each other have different radon levels?

Yes, this is very common. Small changes in the soil’s geology, cracks in one foundation but not another, or even different construction styles can lead to different radon levels. That is why testing every home individually is so important.

Is radon a problem in public water systems?

Public water systems are typically treated for various contaminants, which often reduces radon levels. The risk is generally much lower than from soil gas. Private well owners should consider testing their water for radon separately.

Does a mitigation system really work?

Yes, professional mitigation systems are very effective. They work by drawing radon from the soil before it enters your home and venting it safely outside. We found that these systems can reduce indoor radon levels by over 90 percent.

Why are radon levels higher in winter?

During winter, you seal your home against the cold, which creates a pressure difference. As warm air rises and escapes, it pulls more radon gas from the soil into your house. This is why winter is a recommended season for testing.