What Is Radon Gas — And Why the EPA Says Every American Home Should Be Tested

In 1984, a nuclear plant engineer named Stanley Watras showed up to work and set off the radiation detectors at the plant entrance — on his way in, before the plant had even processed any nuclear material. Testing revealed the source: his own home in Boyertown, Pennsylvania was so saturated with radon that his family was receiving the radiation equivalent of 455,000 chest X-rays per year.

That discovery changed how the United States understood indoor air quality. Radon — a naturally occurring radioactive gas — wasn't just a theoretical concern. It was lurking in ordinary homes across the country, invisible and undetectable without testing.

What Exactly Is Radon?

Radon is a naturally occurring radioactive gas that forms when uranium in soil, rock, and water breaks down. Uranium is present in trace amounts in virtually all soil and rock on Earth — it's part of the planet's natural geology. As uranium decays, it produces radium, which then decays into radon gas.

Radon is chemically inert and colorless, which means you cannot see, smell, or taste it at any concentration. The only way to know whether radon is present in your home — and at what level — is to test for it.

Once radon seeps from the soil into your home, it can accumulate to dangerous concentrations, particularly in lower levels with limited ventilation. When radon decays, it produces radioactive particles called radon progeny (or radon daughters) that attach to dust, aerosols, and other particles in the air. When you breathe these in, the particles deposit on the lining of your lungs, where their radioactive decay can damage lung tissue over time.

The Health Risk: By the Numbers

The EPA estimates that radon causes approximately 21,000 lung cancer deaths in the United States each year. To put that in context:

• Radon is the number one cause of lung cancer among non-smokers
• Radon is the second-leading cause of lung cancer overall (after cigarette smoking)
• Radon kills more Americans annually than drunk driving
• Your lifetime risk of lung cancer from radon exposure at 4.0 pCi/L (the EPA action level) is about 7 in 1,000 for non-smokers — and 62 in 1,000 for smokers (the two risks are multiplicative, not additive)

The risk isn't instantaneous — it develops from long-term exposure. But precisely because radon exposure is continuous and cumulative, early detection and mitigation provides the most benefit. Reducing your radon levels now means years fewer of exposure that you won't have to count later.

Where Does Radon Come From in a Home?

Radon enters homes primarily through the soil beneath them. The dominant pathways include:

• Cracks in concrete floors and walls — even hairline cracks in poured concrete slabs allow radon to seep in
• Construction joints — where walls meet floors, or where different foundation materials meet
• Gaps around service pipes — plumbing, electrical conduits, and other penetrations through foundation walls
• Crawl spaces — an open crawl space exposes the entire floor system to soil gas
• Well water — in homes with private wells, radon can enter dissolved in water and be released when the water is used

Homes with basements tend to have higher radon levels because more of the home's surface area is in contact with radon-bearing soil. But slab-on-grade homes are not immune, and upper floors can also show elevated levels if radon infiltration is significant.

What Radon Level Is Safe?

There is no "safe" level of radon exposure — even very low concentrations carry some risk. But the EPA has established practical thresholds for action:

• Below 2 pCi/L: The national average indoor level is about 1.3 pCi/L. Below 2 pCi/L, the risk is relatively low, though not zero.
• 2–4 pCi/L: Consider mitigation. The EPA says that if you can keep your home below 2 pCi/L, that's the goal — but 2–4 pCi/L still carries meaningful long-term risk.
• 4 pCi/L and above: The EPA recommends mitigation. At 4 pCi/L, the EPA estimates a lifetime risk of 7 deaths per 1,000 non-smokers exposed. Mitigation at this level is strongly advised.
• Above 20 pCi/L: Take mitigation action within weeks, not months.

The World Health Organization sets its reference level lower — at 2.7 pCi/L — recognizing that more mitigation activity earlier provides greater health benefit.

Does Your Home Have High Radon?

The frustrating answer is: you cannot tell without testing. Radon levels vary enormously from home to home, even in the same neighborhood. Two houses on the same street, built in the same year with the same materials, can have vastly different radon levels depending on local geology, building practices, and ventilation patterns.

Some factors that tend to correlate with higher radon levels:

• Living in EPA Zone 1 states (Iowa, North Dakota, Minnesota, Pennsylvania, Colorado, and others)
• Having a basement or spending significant time in lower levels
• Living in a home with a dirt-floor crawl space or unfinished basement
• Tight construction with limited natural ventilation
• Local geology with high uranium or radium content

But none of these factors is determinative. High-risk areas have homes with low radon, and low-risk areas have homes with dangerously elevated levels. Testing is the only way to know.

Testing Is Inexpensive and Easy

This is the part that makes radon such a tractable problem. Unlike many serious health hazards, detecting radon doesn't require a professional visit, expensive equipment, or significant disruption to your home. Short-term test kits cost less than $15 and provide results from an accredited lab within a week. Long-term monitors that provide continuous readings cost $90–$200 and require no ongoing lab fees.

The EPA recommends that every home in the United States be tested for radon. If your result comes back below 2 pCi/L, you can monitor periodically to ensure that hasn't changed. If it comes back above 4 pCi/L, a radon mitigation system — typically a sub-slab depressurization system — can reduce your radon levels by 50–99%, at a cost of $800–$2,500 for most homes.

Given that radon is responsible for more deaths annually than carbon monoxide, drunk driving, and home fires combined, testing is one of the highest-impact safety investments most homeowners will ever make.