What Is Radon? The Complete Guide to Radon Gas in Your Home

Radon is one of the most significant indoor air quality concerns in homes across the United States, yet most people have never heard of it or do not fully understand what it is. Because radon gas is invisible, odorless, and tasteless, it can build up inside a home for years without anyone noticing. The only way to know whether your home has a radon problem is to test for it.

This guide covers everything homeowners need to know about radon: what it is, where it comes from, how it enters buildings, why it matters for your health, and what you can do about it. Every claim in this guide is grounded in data from the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO).

What is radon

Radon is a radioactive gas with the chemical symbol Rn and atomic number 86. It belongs to the noble gas family on the periodic table, which means it does not bond with other elements under normal conditions. Radon is produced naturally as part of the radioactive decay chain that begins with uranium-238, an element found in varying concentrations in nearly all soil and rock on Earth.

What makes radon different from most household air pollutants is its radioactivity. When radon gas is inhaled, it continues to decay inside the lungs, releasing alpha particles that can damage lung tissue at the cellular level. This damage accumulates over months and years of exposure, which is why radon is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), the same category as asbestos and tobacco smoke.

Radon is colorless, odorless, and tasteless. You cannot see it, smell it, or taste it in your indoor air. It does not irritate the eyes or throat the way volatile organic compounds sometimes do. This invisibility is precisely why testing is so important: without a measurement device, there is no way to detect radon in your home.

Outdoors, radon disperses quickly into the atmosphere and remains at harmlessly low concentrations, typically around 0.4 pCi/L (picocuries per liter). Indoors, however, the gas can accumulate because buildings trap soil gases. The national average indoor radon level in the United States is approximately 1.3 pCi/L, but individual homes can test well above 4.0 pCi/L, which is the EPA's recommended action level.

Where does radon come from

Radon originates from the natural radioactive decay of uranium, which is present in virtually all soils and many types of rock. The full decay chain works like this: uranium-238 decays slowly over billions of years into a series of intermediate elements. One of those intermediates is radium-226. When radium-226 decays, it produces radon-222, the isotope most commonly associated with indoor air problems. Radon-222 has a half-life of about 3.8 days, which means it is constantly being produced in the ground and constantly decaying into short-lived radioactive particles called radon progeny (also known as radon daughters).

The concentration of uranium and radium in soil varies significantly by region and even within individual neighborhoods. Granite, shale, phosphate-bearing rock, and certain types of glacial deposits tend to have higher uranium content. This geological variability is why radon levels can differ dramatically from one house to the next, even on the same street.

It is worth noting that radon is not something humans have created or released. It has been seeping out of the ground for billions of years. The problem is modern: when we build enclosed structures over soil that produces radon, the gas accumulates indoors instead of dispersing into the open atmosphere.

In some regions, radon can also dissolve into groundwater. When that water is used for household supply, particularly from private wells, radon can be released into indoor air during showering, dishwashing, and other water use. Waterborne radon is generally a smaller contributor than soil gas, but it can add to overall indoor levels in homes served by well water.

How radon enters your home

Radon moves from the soil into your home through a straightforward mechanism: pressure differentials. Indoor air pressure is typically slightly lower than the pressure in the soil surrounding and beneath your foundation. This difference creates a gentle suction effect that draws soil gases, including radon, upward through any available pathway.

Common entry points include:

• Cracks in poured concrete foundations and slabs. Even hairline cracks that appear during normal concrete curing can allow radon to enter.
• Gaps around service pipes and utility penetrations. Plumbing, electrical conduits, and gas lines that pass through the foundation create small openings.
• Construction joints. Where the floor slab meets the foundation wall (the cove joint) is one of the most common entry paths.
• Sump pits and floor drains. Open or poorly sealed sump pits provide a direct channel from sub-slab soil to indoor air.
• Crawl spaces with exposed soil. Homes built over vented or unvented crawl spaces can draw radon directly from exposed ground.
• Block wall foundations. Hollow-core concrete block walls can accumulate radon in the voids and release it through mortar joints and top openings.
• Well water. In homes with private wells, dissolved radon in water is released into indoor air when water is agitated during use.

Several factors can increase the pressure differential that pulls radon indoors. Stack effect (warm air rising in the home and exiting through upper levels), exhaust fans, clothes dryers, fireplaces, and forced-air HVAC systems all contribute to lower indoor pressure relative to the soil. Wind loading against the exterior of the home can also alter pressure dynamics and temporarily increase radon entry rates.

This is why radon levels can fluctuate seasonally and even daily. Heating season often produces higher indoor radon because closed windows and active heating systems increase the stack effect. Summer readings may be lower in some homes but higher in others depending on air conditioning use and ventilation patterns.

Why radon matters for health

The health risk from radon is not theoretical. It is one of the most thoroughly studied environmental carcinogens in the world, with evidence drawn from decades of research on uranium miners and large-scale residential epidemiological studies conducted in North America, Europe, and China.

The EPA estimates that radon exposure causes approximately 21,000 lung cancer deaths in the United States each year, making it the leading cause of lung cancer among people who have never smoked and the second leading cause overall after tobacco use. The WHO similarly identifies residential radon as a major contributor to lung cancer incidence worldwide.

How does a gas cause cancer? When you breathe in radon, the gas itself passes through the lungs relatively quickly. The real danger comes from radon's decay products, specifically polonium-218 and polonium-214. These short-lived radioactive particles attach to dust and aerosols in indoor air and become lodged in the bronchial epithelium when inhaled. As they decay further, they emit alpha particles that strike nearby lung cells. Alpha radiation is heavy and energetic enough to break DNA strands. Over years of repeated exposure, this cumulative DNA damage can initiate cancerous cell growth.

The risk increases with both concentration and duration of exposure. A person living in a home with 4.0 pCi/L for 20 years faces a meaningfully higher risk than someone exposed to 1.0 pCi/L for the same period. Smokers exposed to radon face a compounded risk because tobacco smoke damages the same lung tissues, making them more susceptible to radiation-induced mutations.

For a deeper look at radon's health effects, see our guide to radon health risks.

EPA radon zones explained

The EPA divides the United States into three radon zones based on predicted average indoor radon levels at the county level. These zones were developed using geological data, indoor radon measurements, and building characteristics.

• Zone 1 (highest potential): Predicted average indoor levels above 4.0 pCi/L. Counties in this zone have geological conditions most likely to produce elevated radon.
• Zone 2 (moderate potential): Predicted average indoor levels between 2.0 and 4.0 pCi/L.
• Zone 3 (lower potential): Predicted average indoor levels below 2.0 pCi/L.

These zones are useful for general awareness but come with important limitations. The EPA itself states that zone designations should not be used to determine whether a specific home needs testing. Homes in Zone 3 counties can have dangerously high radon levels, and homes in Zone 1 counties can test well below the action level. The geology beneath an individual home varies based on localized soil composition, depth to bedrock, and water table conditions that county-level maps cannot capture.

You can check your area's general radon potential using our Radon Risk Lookup tool, but always follow up with an actual test. No map replaces a measurement inside your home.

How to test for radon

Testing is straightforward, inexpensive, and the only reliable way to determine your home's radon level. There are two main categories of tests: short-term and long-term.

Short-term tests

Short-term radon tests typically measure for 2 to 7 days. Charcoal canisters and electret ion chambers are the most common types. These tests are useful as an initial screening tool because they provide results quickly. However, because radon levels fluctuate daily and seasonally, a single short-term test provides a snapshot rather than a long-term average.

Closed-house conditions are required for short-term tests. This means keeping all windows and exterior doors closed for at least 12 hours before the test begins and throughout the entire testing period. Normal entry and exit through doors is acceptable.

Long-term tests

Long-term tests measure radon over 90 days to one year. Alpha track detectors and electret ion chambers are common long-term devices. These tests provide a more representative picture of your average annual exposure, which is more meaningful for health risk assessment. The EPA recommends long-term testing when an initial short-term test returns a result between 2.0 and 3.9 pCi/L.

DIY vs. professional testing

DIY test kits are available at hardware stores and online for roughly ten to thirty dollars. They are adequate for routine screening. Professional testing uses calibrated continuous radon monitors (CRMs) that record hourly readings and include tamper indicators. Professional tests are generally recommended for real estate transactions, legal documentation, and post-mitigation verification.

Placement guidance

Place the test device in the lowest lived-in level of the home, at least 20 inches above the floor and away from exterior walls, drafts, high humidity, and direct sunlight. Do not test in kitchens, bathrooms, or laundry rooms. If you have a finished basement that you use regularly, test there. If your lowest level is an unfinished basement you rarely enter, test on the main floor.

For a step-by-step walkthrough, see our complete radon testing guide.

Radon action levels

Understanding what your radon test result means is essential for deciding next steps. Radon is measured in picocuries per liter (pCi/L) in the United States and becquerels per cubic meter (Bq/m³) in most other countries.

• Below 2.0 pCi/L: Generally considered low. No immediate action required, but retesting every 2 to 5 years is still recommended because levels can change over time.
• 2.0 to 3.9 pCi/L: The EPA says homeowners should consider fixing at these levels, especially with a long-term test confirming the reading. Risk is not negligible at these concentrations.
• 4.0 pCi/L and above: The EPA recommends taking action to reduce radon. This is the official U.S. action level and corresponds to a meaningful increase in lung cancer risk with long-term exposure.

The World Health Organization uses a lower reference level of 2.7 pCi/L (100 Bq/m³), reflecting a more conservative assessment of acceptable risk. Several countries have adopted the WHO threshold in their national building codes. In practical terms, the EPA acknowledges that there is no known safe level of radon exposure and that any reduction in concentration reduces risk proportionally.

To interpret your specific test result, use our Understand Your Results tool.

For more detail on what different radon numbers mean, see our guide to radon levels and safety.

How radon mitigation works

If your home tests at or above the EPA action level, a radon mitigation system can bring levels down dramatically. The most widely used and effective method is called active sub-slab depressurization (ASD).

ASD works by creating a negative pressure zone beneath the foundation slab. A contractor drills a small hole through the slab, inserts a suction point (typically a section of PVC pipe), and connects it to a continuously running inline fan. The fan draws radon-laden soil gas from beneath the slab and exhausts it through a vent pipe that terminates above the roofline, where the gas disperses harmlessly into the outdoor atmosphere.

This approach is effective because it addresses the source of the problem. Rather than trying to filter radon out of indoor air after it has already entered, ASD prevents the gas from entering in the first place. A well-designed system can reduce indoor radon levels by 80 to 99 percent.

Other mitigation techniques exist for specific situations. Sub-membrane depressurization is used in crawl spaces where a plastic vapor barrier is sealed over exposed soil and connected to a fan and vent pipe. Block wall suction addresses hollow-core concrete block foundations. Heat recovery ventilators (HRVs) can help in some cases by increasing ventilation while recovering energy, though they are less commonly used as a primary mitigation method.

Most residential mitigation installations are completed in a single day. After installation, a post-mitigation radon test is conducted to verify that the system has reduced levels below the action level. Ongoing maintenance is minimal: the fan should be checked periodically, and retesting is recommended every two years.

For a full walkthrough of the mitigation process, contractor selection, and cost expectations, see our homeowner guide to radon mitigation. You can also estimate costs for your area using our radon mitigation cost tool.

Common myths about radon

Misinformation about radon is widespread. Here are some of the most persistent myths and the facts that correct them.

Myth: My neighbor tested low, so my home is fine

Radon levels can vary significantly from house to house, even between adjacent properties. Differences in foundation type, soil composition directly beneath each slab, and building pressurization mean that one home's result cannot predict another's. The EPA recommends that every home be tested individually.

Myth: New homes are immune to radon

New construction does not guarantee low radon. While some new homes are built with radon-resistant features (passive radon systems with a vent pipe and gravel layer beneath the slab), these features reduce radon entry but do not eliminate it. Many new homes still test above 4.0 pCi/L. Every new home should be tested after occupancy.

Myth: Radon only affects basements

While radon concentrations are typically highest on the lowest level of a home, upper floors are not immune. Radon can migrate through interior stairwells, HVAC ductwork, and elevator shafts. Slab-on-grade homes without basements can have elevated radon on their main living level. The key factor is the connection between the building envelope and the underlying soil, not whether a basement exists.

Myth: Radon is only a problem in certain states

Elevated radon has been found in homes in all 50 states. While certain geological regions have higher average levels (parts of the Upper Midwest, Appalachian region, and Northern Plains, for example), high-radon homes exist in every state, including those that are predominantly Zone 3 on the EPA map. Geographic generalizations are not a substitute for testing.

Myth: Opening windows will solve a radon problem

Opening windows can temporarily lower radon levels by diluting indoor air with outdoor air, but it is not a reliable or sustainable solution. As soon as windows are closed, radon begins to accumulate again. In cold or hot climates, keeping windows open year-round is impractical and energy-inefficient. A properly installed mitigation system provides continuous, passive reduction regardless of weather or season.

Frequently asked questions

1. What is radon gas exactly?

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil, rock, and groundwater. It is colorless, odorless, and tasteless, which means you cannot detect it without testing. Radon can accumulate indoors and pose a health risk with prolonged exposure.

2. Is radon dangerous in my home?

Radon can be dangerous at elevated concentrations over long periods. The EPA estimates radon causes approximately 21,000 lung cancer deaths per year in the United States, making it the leading cause of lung cancer among non-smokers. The risk depends on concentration level and duration of exposure.

3. What radon level is considered unsafe?

The EPA recommends taking action to reduce radon at or above 4.0 pCi/L and suggests homeowners consider action between 2.0 and 3.9 pCi/L. The World Health Organization uses a lower reference level of 2.7 pCi/L (100 Bq/m³). There is no known safe level of radon exposure.

4. How does radon get into a house?

Radon enters homes primarily through the soil beneath and around the foundation. Common entry points include cracks in concrete slabs, gaps around service pipes, construction joints, sump pits, crawl spaces with exposed soil, and in some cases through well water. Pressure differences between indoor and outdoor air draw soil gases upward into the structure.

5. Can radon be fixed if levels are high?

Yes. Radon mitigation systems, most commonly active sub-slab depressurization, can reduce indoor radon levels by up to 99 percent. A qualified contractor installs a sealed suction point beneath the foundation and a fan that vents radon gas safely above the roofline. Most installations are completed in one day.

Sources & disclaimer: Data in this guide is drawn from the EPA's A Citizen's Guide to Radon, EPA Consumer's Guide to Radon Reduction, and the WHO Handbook on Indoor Radon. This content is educational only and does not constitute legal, medical, or professional advice. Consult qualified professionals for decisions about testing, mitigation, and health concerns related to radon exposure.