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What Is Radon Gas? Where It Comes From and What It Does to You
The short version. What is radon? It's a radioactive gas that leaks up out of the ground into houses. You can't see it, smell it, or taste it. Over years of exposure, it's the #1 cause of lung cancer in people who never smoked (EPA, U.S. Surgeon General) — but a simple 48-hour test tells you exactly where your home stands. That's the whole game: test, get your number, act on it if you need to.
Nothing about the way a house looks, smells, or feels can tell you whether radon is inside. Two houses on the same street can read completely differently. Radon comes from uranium that's been under Alabama since the earth formed — it's geology, not anything you or the previous owner did wrong. What matters is your number, in your house, right now.
~21,000US deaths / yr
EPA estimate, radon-induced lung cancer
4.0pCi/L
EPA action level
1.3pCi/L
U.S. indoor average
The Basics
What Is Radon, in One Paragraph
Radon (chemical symbol Rn) is a gas. Chemists call it a "noble gas," which just means it doesn't react with other things — it won't burn your lungs, won't set off a chemical alarm, and won't smell like anything to your nose. What makes radon dangerous is that it is radioactive. Radon atoms are unstable and break apart on their own over a few days, releasing tiny high-energy particles (called alpha particles) as they go. When you breathe radon in, those particles and the radioactive dust they leave behind can damage the cells lining your lungs. Do that for years, and the damage can add up to lung cancer. That's the whole story — no immediate drama, but no shortcuts either.
Where It Comes From
Where Radon Actually Comes From: The Uranium Chain in Plain English
Radon isn't man-made. No factory puts it in the air, no product releases it, and nothing you or the last owner did around your house causes it. It's part of a natural chain that's been running under everyone's feet, everywhere on earth, since the planet formed.
- 01Uranium-238
A trace element in almost every rock and soil on earth. It's been there since the planet formed and takes about 4.5 billion years to break down — so it's not going anywhere.
- 02Radium-226
Uranium slowly breaks down through several steps and eventually becomes radium, a solid metal locked inside soil and rock grains. Radium is what actually produces the radon gas that ends up in the air you breathe.
- 03Radon-222
When a radium atom breaks apart (called radioactive decay), it releases a radon atom. Radon is a gas — the only step in this whole chain that is — so it can escape its rock grain, drift up through the soil, and end up inside a house. It's colorless, odorless, and tasteless.
- 04Polonium, lead, bismuth (the leftovers from radon)
Radon itself breaks down within a few days into short-lived radioactive dust particles — polonium-218, lead-214, bismuth-214. Those tiny particles stick to household dust and get pulled deep into the lungs when you breathe. They're the part that actually damages lung tissue.
The key thing to notice about that chain: radon is the only step in it that's a gas. Every other step is a solid, locked inside the rock or soil grain it formed in. Radon can slip out of that grain, drift up through tiny spaces in the ground, and eventually reach the surface — or the underside of your slab, or an open sump pit, or a hairline crack in a foundation wall.
Watch This 60 Seconds
The CDC's Short Explainer on Radon
The CDC put together a quick public-service video that explains what radon is and why testing matters. It's short, plain, and clearer than anything we could write — so we included it here. Full attribution is below the video.
How It Gets In
How Radon Gets Inside a House
Radon gets into houses because a warm home acts like a slow chimney. Heated air rises and escapes out the top of the house through the attic, ductwork, and any upper-floor gap it can find. That outflow leaves a slight vacuum down at the foundation, and that vacuum pulls air (and whatever is in it — including radon) up through the ground and into your living space. Simple physics, working around the clock.
That happens no matter how your house is built. Slab, crawlspace, basement — all three pull the same way. The old belief that "radon is just a basement problem" isn't true. Slab-on-grade homes across Jefferson and Shelby counties routinely test above the EPA action level (4.0 pCi/L — the level where the EPA says to fix your home). Here are the most common ways radon sneaks in:
- Cracks in the slab or foundation wall — The single biggest entry point in most homes. Even hairline cracks are enough — the pressure difference between the ground and the inside of the house pulls soil gas through them all day, every day.
- The seam where the slab meets the foundation wall — That perimeter joint is almost never sealed against gas. Mitigation contractors treat it as a leak by default.
- Sump pits and floor drains — An open sump pit is basically a hole straight down into the soil. Sealing the lid is one of the first things a mitigation contractor does.
- Utility penetrations — Every place a plumbing line, gas line, electrical conduit, or HVAC duct passes through the slab or foundation is a potential radon path.
- Crawlspaces — A crawlspace with a bare dirt floor lets soil gas move straight up under the living floor. Sealing it off with a heavy plastic liner (called "encapsulation") is a common fix.
Why It Matters Here
Why the Birmingham Metro Has Real Radon Numbers
Radon isn't the same everywhere. It depends on how much uranium sits in the bedrock and how easily gas can travel through the ground above it. The Birmingham metro sits in what geologists call the Valley and Ridge area — folded, cracked bedrock the Geological Survey of Alabama ranks moderate to high for radon potential (GSA, EPA Region 4 Radon Stakeholders Meeting, April 2025).
Three things about the local geology drive the numbers up:
- Rock types that hold more uranium. The Geological Survey of Alabama specifically flags certain carbon-rich shales and granite-type rocks under Birmingham as our higher-radon source rocks — they carry more uranium than average sandstones do.
- Faulted, cracked bedrock. The Valley and Ridge is famous for folded, faulted rock. The GSA notes those cracks and faults act like elevator shafts, letting radon travel up fast from source rock toward the surface.
- Sinkhole country (karst limestone). Parts of Jefferson and Shelby sit on limestone that's been slowly eaten away by water over millions of years. All those little cracks, dissolution channels, and small cavities give radon fast paths upward.
The Alabama Department of Public Health lists Jefferson and Shelby among Alabama's 15 highest-potential radon counties. In the CDC's 2008–2017 data, about 1 in 9 reported tests in Jefferson and Shelby came back at or above the EPA's "fix it" line of 4.0 pCi/L. Jefferson's highest recorded reading in that period was 24.6 pCi/L — more than six times that line. That's what turns radon here from a national statistic into a Birmingham problem.
For the county- and ZIP-level breakdown, seeAlabama radon levels by ZIP code.
What The Numbers Mean
How Radon Is Measured: pCi/L in One Paragraph
Radon is measured in picocuries per liter — a measure of how much radon is in the air, written pCi/L. Don't worry about the math. Just know the reference points. Outside, the U.S. average is about 0.4 pCi/L — that's normal background you can't do anything about. Inside, the U.S. average is about 1.3 pCi/L. The EPA action level — the line where the EPA and U.S. Surgeon General say to fix your home — is 4.0 pCi/L. The World Health Organization draws its line lower, at 2.7 pCi/L (they use metric units — 100 Bq/m³). None of those numbers are zero, and there's no indoor level considered fully "safe" — but risk goes up predictably with the number. So the practical question is always: what does your specific house read?
For the full breakdown of what every level means, seewhat is a safe radon level. For the middle band most homes fall into, seeradon level between 2 and 4 pCi/L.
What To Do Next
What a Homeowner Actually Does About Radon
There's one first step, and it's the same for every home no matter how it's built:test. Radon is invisible, and two houses on the same street can read completely differently — the Alabama Department of Public Health puts that exact point on the front page of its radon site. Nothing about the age of your house, its foundation, its size, or your neighborhood's average rules out an elevated reading today.
The standard test is a 48-hour continuous monitor — a small sealed device that sits on your lowest lived-in floor for two days and counts the gas hour by hour, then gives you a single pCi/L number. If that number lands at or above 4.0 pCi/L, EPA and the Surgeon General say fix the home. A properly designed mitigation system (a small fan and pipe that vents radon out from under the slab — the trade calls it "sub-slab depressurization") routinely gets homes well below 2 pCi/L. If the number is between 2.0 and 4.0 pCi/L, EPA and ADPH tell homeowners tothink about fixing. Those are their actual words, not our marketing.
For the mechanics of what a mitigation system actually does, seefixing radon. For the mechanics of what a real test looks like end to end, see how radon testing works.
Test your home — $295 flat, sealed monitor, independent lab.
FAQ
Frequently Asked Questions
What is radon in simple terms?
Radon is a radioactive gas that forms naturally when uranium in soil and rock slowly breaks down. It has no color, no smell, and no taste. It seeps up out of the ground and can build up inside buildings. Radon is the second-leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked (EPA, U.S. Surgeon General).
Is radon really that dangerous?
The EPA estimates radon causes about 21,000 lung-cancer deaths in the United States every year. The World Health Organization classifies it as a Group 1 human carcinogen — the highest cancer-risk category. But the risk is long-term, not immediate: a single afternoon in a home with elevated radon isn't the problem. Decades of nightly sleep in a bedroom with elevated radon is.
Where does radon come from?
Radon comes from the slow natural breakdown of uranium in soil and rock. Uranium is present in tiny amounts almost everywhere on earth, but some rock types have more of it than others. In the Birmingham metro, the Valley and Ridge geology is ranked moderate to high for radon potential by the U.S. Geological Survey and the Geological Survey of Alabama — faulted rock, limestone that's been eaten away over time (karst), and certain carbon-rich or granite-type bedrock all contribute.
How does radon get inside a house?
Warm air inside your house rises and slips out through the top of the building. That creates a slight vacuum down at the foundation, and the vacuum pulls soil gas — including any radon in it — up through cracks in the slab, the slab-to-wall seam, sump pits, floor drains, pipe penetrations, and any exposed dirt in a crawlspace. Slab-on-grade homes are not exempt: radon works on slab, crawlspace, and basement foundations alike.
How do I know if my house has radon?
There's no way to tell without a test. Radon is invisible and has no smell, so nothing you can see, smell, or feel about your home — musty basement, damp corners, cracks in the slab — will tell you. The only way to know your radon level is to measure it. The standard short-term test is a 48-hour continuous monitor that produces a single number in picocuries per liter (pCi/L) — a measure of how much radon is in the air.