What is radon, and how does it get into homes?

Radon is a naturally occurring radioactive gas. You cannot see it, smell it, or taste it. It forms when uranium, found in small amounts in almost all soil and rock, breaks down. Because it is a gas, radon moves up through the ground and escapes into the air.

Why radon outdoors is not the problem

Outside, radon mixes with a huge volume of air and stays at low concentrations. EPA says the average outdoor level in the United States is about 0.4 picocuries per liter (pCi/L). The trouble starts when radon collects in an enclosed space, like a house.

How radon gets inside

A house acts a bit like a chimney. Warm indoor air rises and leaks out through the upper floors and attic, which lowers the air pressure in the lowest level. That slight vacuum draws soil gas, including radon, in through any opening in contact with the ground. EPA lists common entry points:

• cracks in solid floors and walls
• construction joints, such as where the floor slab meets the foundation wall
• gaps around service pipes and wires
• cavities inside walls
• sump pits and floor drains
• crawl spaces with exposed soil
• the water supply, mainly in homes on private wells

Any home can have a radon problem: old or new, drafty or tight, with or without a basement. EPA says the national average indoor level, from a survey completed in 1991, is about 1.3 pCi/L, but individual homes range from near zero to many times the action level.

Why neighbors can have very different levels

Radon levels depend on the uranium content and permeability of the soil right under the house, how many entry points the foundation has, how the house is heated and ventilated, and the weather. Two houses on the same street can test very differently. That is why EPA’s county-level radon zones can only suggest where radon is common, not what is in your home.

Radon changes over time

Levels rise and fall with seasons and weather. They are often higher in winter, when windows are closed and the stack effect is strongest, and can change after heavy rain, frozen ground, or renovations that change airflow. A single short test is a snapshot; see short-term vs long-term tests.

What makes radon harmful

Radon itself decays into radioactive particles, called radon decay products or progeny, that can lodge in the lungs when you breathe. As they decay they release energy that can damage lung tissue, and over years that damage can lead to lung cancer. EPA estimates radon is the second leading cause of lung cancer in the United States. See radon health risks.

What to do

1. Check your county’s zone with the ZIP lookup to get a sense of local radon potential.
2. Test your home, whatever the zone. Test kits are inexpensive and widely available.
3. If your level is 4 pCi/L or higher, fix the home. Consider fixing between 2 and 4 pCi/L.

Radon by the numbers

• Average indoor level in U.S. homes: about 1.3 pCi/L (50 Bq/m³), based on EPA's national residential radon survey.
• Average outdoor level: about 0.4 pCi/L (15 Bq/m³).
• EPA action level: 4 pCi/L (150 Bq/m³). EPA recommends fixing homes at or above this level, and considering a fix between 2 and 4 pCi/L.
• Estimated U.S. lung cancer deaths each year: about 21,000, according to EPA.
• Symptoms: none immediately. EPA says lung cancer usually occurs 5 to 25 years after exposure.

Units: picocuries and becquerels

U.S. labs report radon in picocuries per liter of air (pCi/L). Most other countries use becquerels per cubic meter (Bq/m³). One pCi/L is 37 Bq/m³, so 4 pCi/L is about 150 Bq/m³. If you read international guidance, such as the World Health Organization's reference level of 100 Bq/m³, convert before comparing: 100 Bq/m³ is about 2.7 pCi/L.

How radon behaves inside a house

Radon is a noble gas. It doesn't react chemically with other substances, and air movement and pressure push it around easily. EPA notes that although radon is about 7.5 times heavier than air, in a house with forced-air heating and cooling it can be distributed throughout the home. Several forces draw it in:

• The stack effect. Warm air rises and leaks out of upper parts of the house, creating slight negative pressure near the foundation that pulls soil gas in. This is stronger in cold weather.
• Exhaust fans and combustion appliances. Bathroom and kitchen fans, clothes dryers, and fireplaces push air out, and makeup air can be drawn from the soil.
• Wind and barometric pressure changes, which can raise or lower soil gas entry from day to day.

That's why radon levels often run higher in winter and at night, and why a single short-term test is a snapshot.

Where radon gets in

Common entry points include:

• cracks in slabs and foundation walls;
• gaps around pipes, wires, and sump pits;
• floor-wall joints;
• hollow block walls;
• crawl spaces with exposed soil;
• floor drains;
• well water, in some homes, when radon dissolved in the water is released during showers and other water use.

Radon and other gases

Radon is sometimes confused with carbon monoxide or natural gas. They're very different:

• Carbon monoxide comes from burning fuel and causes immediate poisoning symptoms. CO alarms detect it.
• Natural gas is odorized with a sulfur smell so leaks can be noticed.
• Radon is odorless, comes from soil, and causes harm over years. Only a radon test detects it. CO alarms and smoke detectors don't.

The good news

Radon problems are fixable. EPA says some radon reduction systems can reduce levels by up to 99 percent, and most homes can be fixed for about the same cost as other common home repairs. The first step is a simple test. See how to test your home for radon.

Testing is quick and inexpensive, so there's no reason to guess.