What is Radon Gas and Why Is It Dangerous?
Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and water. It is colorless, odorless, and tasteless. You cannot detect it without a test. Radon seeps into homes through the foundation, and when it accumulates indoors, breathing it over time causes lung cancer. The EPA estimates radon kills roughly 21,000 Americans each year, making it the leading cause of lung cancer among people who have never smoked.
Every home in the United States has some level of radon. The question is whether yours has enough to put your family at risk.
Where Does Radon Come From?
Radon originates from a natural decay chain that begins with uranium-238, an element found in nearly all soil and rock on Earth. Uranium-238 has a half-life of 4.5 billion years, so it decays extremely slowly and will continue producing radon for as long as the Earth exists. Uranium decays into radium-226, which then decays into radon-222, the isotope responsible for indoor air quality problems. Each step in this chain releases radiation and produces a new element.
Radon-222 has a half-life of just 3.8 days. That short lifespan means radon does not travel far from where it is produced. The radon gas in your home is generated in the soil directly beneath and around your foundation, not miles away. This is also why two homes on the same street can have very different radon levels: the soil composition under each foundation may differ significantly.
Because uranium is present in virtually all soil, radon gas is produced everywhere. But concentrations vary widely depending on local geology. Certain rock types contain significantly more uranium and produce more radon:
- Granite contains higher-than-average uranium concentrations, which is why areas with granite bedrock often have elevated radon
- Shale, particularly black shale formations, can be uranium-rich
- Phosphate rock deposits in Florida and other states are a well-documented radon source
- Uranium-bearing formations in the Rocky Mountain states and Appalachian region produce some of the highest radon concentrations in the country
Radon is also dissolved in groundwater. When that water is pumped into your home through a private well, radon releases into the air during showering, washing dishes, and other water use. Public water systems that draw from surface water are rarely a concern; the radon dissipates before it reaches your tap.
How Radon Enters Your Home
Radon moves from the soil into your home because of a simple physics principle: your house acts like a vacuum. Warm air rises inside the home and exits through upper floors, attics, and chimneys. As that air leaves, it creates a slight negative pressure at the lowest level of the house, pulling soil gases (including radon) in through any available opening in the foundation. Mechanical systems amplify this effect: exhaust fans, dryers, furnaces, and fireplaces all push air out of the home and increase the draw of soil gas from below.
This process is called the stack effect, and it operates 24 hours a day. The pressure difference between your home and the surrounding soil is small (often just 1 to 3 pascals), but it is persistent and enough to draw radon continuously into the living space.
The specific entry points include:
- Foundation cracks: Even hairline cracks in poured concrete or block walls allow radon entry. Settling cracks that develop over time are especially common pathways.
- Cove joints: The gap where the basement floor meets the foundation wall is present in nearly every home with a poured foundation. This joint is one of the most significant radon entry points.
- Construction joints: Cold joints (where one concrete pour meets another) and the joint between the foundation wall and the footer create gaps in the concrete envelope.
- Pipe and wire penetrations: Anywhere a water line, drain pipe, electrical conduit, or gas line passes through the foundation creates an opening.
- Sump pits: An uncovered or poorly sealed sump pit is a direct connection between your basement air and the soil beneath the slab. It is one of the easiest problems to fix and one of the most commonly overlooked.
- Crawl spaces: Homes with dirt-floor crawl spaces have no barrier between the soil and indoor air. Even with a vapor barrier, radon can enter through seams, tears, and edges.
- Well water: Radon dissolved in well water releases into household air. The EPA estimates that water with 10,000 pCi/L of radon contributes roughly 1 pCi/L to indoor air levels.
Why Radon Is Dangerous
Radon is the number one cause of lung cancer in non-smokers and the second leading cause overall, behind only cigarette smoking. According to the EPA, radon is responsible for approximately 21,000 lung cancer deaths per year in the United States. The World Health Organization estimates that radon causes 3% to 14% of all lung cancers worldwide, depending on average radon levels in each country.
How Radon Damages Your Lungs
Radon itself is a gas that you inhale and exhale. The danger comes from its radioactive decay products (called “radon progeny” or “radon daughters”): polonium-218 and polonium-214. These are solid particles that attach to dust and aerosols in the air. When you breathe them in, they lodge in your lung tissue.
Once embedded, these particles emit alpha radiation, which is a heavy, high-energy form of radiation. Alpha particles cannot penetrate skin, but inside the lungs, they are devastating. Each alpha particle tears through DNA in nearby cells. Over years of exposure, this repeated damage accumulates, and some cells eventually become cancerous.
Radon and Smoking: A Multiplicative Risk
If you smoke and live in a home with elevated radon, your lung cancer risk is not simply added together; it is multiplied. A smoker exposed to 4 pCi/L of radon has roughly 5 times the lung cancer risk of a non-smoker at the same level. The EPA estimates that for smokers, living with 4 pCi/L of radon is equivalent to smoking 8 cigarettes per day. For non-smokers, 4 pCi/L is comparable to smoking half a pack per day.
This is a dose-response relationship: there is no threshold below which radon exposure is proven safe. Every reduction in radon level reduces your risk.
Putting the Risk in Perspective
The EPA compares radon risk to other causes of death to illustrate the scale of the problem:
- Radon causes more deaths per year than drunk driving (~10,000)
- Radon causes more deaths per year than home fires (~2,600)
- Radon causes more deaths per year than drowning (~3,900)
Unlike many cancer risks, radon exposure is one you can measure and control.
Radon Levels Across the United States
The EPA divides every U.S. county into three zones based on predicted average indoor radon levels:
- Zone 1 (highest potential): Predicted average indoor level above 4 pCi/L. These counties have the greatest number of homes expected to exceed the action level.
- Zone 2 (moderate potential): Predicted average between 2 and 4 pCi/L.
- Zone 3 (low potential): Predicted average below 2 pCi/L.
States with the largest percentage of Zone 1 counties include Iowa, Pennsylvania, Colorado, Minnesota, Ohio, Indiana, Nebraska, and North Dakota. Iowa consistently records the highest average indoor radon levels of any state, with some counties averaging above 8 pCi/L.
However, EPA zone maps are based on averages and geology, not individual homes. A Zone 3 county can have homes testing at 20 pCi/L, and a Zone 1 county can have homes at 1 pCi/L. The only way to know your home's level is to test it.
Radon levels also vary by season. In most of the country, indoor radon levels are highest during winter, when homes are closed up and the temperature difference between indoors and outdoors (which drives the stack effect) is greatest. Summer levels are often 30% to 50% lower. This seasonal variation is one reason the EPA recommends long-term testing (90 days or more) for the most accurate picture of your annual average exposure.
For reference:
- Average outdoor radon level in the U.S.: 0.4 pCi/L
- Average indoor radon level in the U.S.: 1.3 pCi/L
- About 1 in 15 U.S. homes (roughly 6 million) have radon levels at or above 4 pCi/L
How Much Radon Is Too Much?
There is no known safe level of radon. Any exposure carries some risk. However, two organizations have set action thresholds to guide homeowners:
| Organization | Action Level | Equivalent |
|---|---|---|
| U.S. EPA | 4.0 pCi/L | 148 Bq/m³ |
| World Health Organization | 2.7 pCi/L | 100 Bq/m³ |
The EPA recommends taking action to reduce radon when levels reach 4 pCi/L or higher, and to consider mitigation between 2 and 4 pCi/L. The WHO sets a lower threshold of 2.7 pCi/L (100 Bq/m³), reflecting the evidence that cancer risk is significant even below the EPA's action level.
At different radon levels, the EPA estimates these approximate lung cancer risks over a lifetime of exposure:
| Radon Level | Non-Smoker Risk | Smoker Risk |
|---|---|---|
| 20 pCi/L | 36 out of 1,000 | 260 out of 1,000 |
| 10 pCi/L | 18 out of 1,000 | 150 out of 1,000 |
| 8 pCi/L | 15 out of 1,000 | 120 out of 1,000 |
| 4 pCi/L | 7 out of 1,000 | 62 out of 1,000 |
| 2 pCi/L | 4 out of 1,000 | 32 out of 1,000 |
| 1.3 pCi/L | 2 out of 1,000 | 20 out of 1,000 |
These numbers make two things clear: radon is dangerous at any level above background, and the combination of radon and smoking is far worse than either alone.
What To Do About Radon
Radon is a solvable problem. The steps are straightforward: test, fix if needed, and re-test periodically.
Step 1: Test Your Home
Every home should be tested for radon, regardless of its age, location, or construction type. New homes can have high radon, old homes can have low radon, and two identical houses next door to each other can have completely different levels. The geology under each foundation is what matters, not the age or condition of the building.
Short-term test kits (2 to 7 days) are available at hardware stores for $15 to $40, or you can hire a certified radon measurement professional for more accurate results ($125 to $350). Long-term tests (90+ days) give you a better picture of your year-round average. Place the test device on the lowest lived-in level of your home, with windows and doors closed as much as possible during the test period. For a detailed walkthrough, see our guide to radon testing.
Step 2: Mitigate If Above 4 pCi/L
If your test results come back at or above the EPA's action level of 4 pCi/L, hire a certified radon mitigation professional to install a radon reduction system. The most common type, active sub-slab depressurization, uses a fan and PVC piping to pull radon from beneath your foundation and vent it above the roofline. These systems reduce radon levels by 80% to 99% and cost $800 to $2,500 for most homes. For details on how these systems work, see our radon mitigation guide.
If your results fall between 2 and 4 pCi/L, the EPA still recommends considering mitigation. Given that the WHO's threshold is 2.7 pCi/L and there is no proven safe level, reducing radon below 2 pCi/L is a reasonable goal.
Step 3: Re-Test Every Two Years
Radon levels fluctuate with weather, soil conditions, and changes to your home. Even homes with mitigation systems should be re-tested every two years. Check your mitigation system's manometer monthly to confirm it is still running.
Find a Professional
Whether you need testing or mitigation, work with a certified professional. Look for NRPP or NRSB certification, which ensures the technician has met national competency standards. Find a certified radon professional near you through our national directory.
Frequently Asked Questions
What is radon gas?
Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and water. It is colorless, odorless, and tasteless. Radon seeps into homes through cracks and openings in the foundation, and long-term exposure causes lung cancer.
Is radon dangerous?
Yes. Radon is the leading cause of lung cancer among non-smokers and the second leading cause overall. The EPA estimates that radon causes approximately 21,000 lung cancer deaths per year in the United States. The risk increases with higher concentrations and longer exposure.
Where does radon come from?
Radon comes from the natural radioactive decay of uranium, which is present in nearly all soil and rock. Uranium decays into radium, which then decays into radon gas. Certain rock types, including granite, shale, and phosphate deposits, produce more radon than others.
What level of radon is safe?
There is no known safe level of radon. The EPA recommends taking action when indoor radon reaches 4 pCi/L, and the World Health Organization sets its threshold at 2.7 pCi/L. Reducing radon to the lowest achievable level is always beneficial.
How do I know if my home has radon?
The only way to know is to test. Radon has no color, odor, or taste. You can use a DIY test kit ($15 to $40 at hardware stores) or hire a certified radon measurement professional. Every home should be tested, regardless of age, location, or construction type.