What is radon?

Radon is a radioactive gas that forms naturally when uranium, thorium or radium break down in rock, soil and water. You can’t see or smell it, and the only way to know whether your home has a radon problem is to test for it (EPA, Basic Radon Facts).

Everyone breathes some radon every day, usually at very low levels (National Cancer Institute). It becomes a concern when it builds up indoors, because breathing high levels over time raises the risk of lung cancer.

Is radon a gas? Is it radioactive?

Yes to both. Radon is a noble gas, which means it doesn’t react chemically with other substances. It comes from the natural decay of uranium and thorium, and it keeps decaying through a chain of other elements until it reaches a stable form of lead. The common form of radon, radon-222, has a half-life of 3.8 days: half of any given amount decays in that time (ATSDR).

That decay is why radon matters. As radon breaks down in the air, it produces radioactive particles. When you breathe, those particles settle on the cells lining your airways, where they can damage DNA and potentially cause lung cancer (WHO).

What does radon smell or look like?

It doesn’t smell or look like anything. Radon has no smell, color or taste (WHO), so you won’t notice it in your home. If you can smell something, radon isn’t what you’re smelling. Radon also causes no immediate symptoms, which is why a test is the only way to find out.

Where radon comes from and how it gets in

Radon from the soil is the main cause of radon problems in homes. It moves up through the ground and gets in through cracks and other holes in the foundation, and the house then traps it inside. Less often it enters through well water, and in a small number of homes building materials give off radon too (EPA, Citizen’s Guide).

Common entry points include floor cracks, floor-wall joints, gaps around pipes and cables, hollow-block walls, sumps and drains (WHO).

Any home can have a problem: new or old, well sealed or drafty, with or without a basement. High levels have been found in every state, and levels can differ greatly even between houses next door to each other (EPA, Basic Radon Facts). To see how often tests come back high near you, look up your county.

Is radon heavier than air?

Radon is a heavy gas (ATSDR), but that doesn’t mean it stays at floor level or that upper floors are safe. Basements and first floors typically have the highest levels because they are closest to the ground, where radon comes from (National Cancer Institute). Considerable levels can also be found above the ground floor (WHO). See radon in basements for more.

How much radon is normal?

In the US, radon is measured in picocuries per liter of air (pCi/L), a measure of radioactivity.

All from EPA, Basic Radon Facts. These aren’t lines between safe and unsafe: EPA says any radon exposure carries some risk (EPA, Citizen’s Guide). See radon levels explained for the full chart.

Why radon matters

Radon is the number one cause of lung cancer among people who have never smoked and the second leading cause overall, according to EPA. EPA estimates it causes about 21,000 lung cancer deaths a year in the US, about 2,900 of them among people who have never smoked (EPA). Worldwide, WHO estimates radon causes 3% to 14% of lung cancers in a country, depending on its average radon level and how many people smoke (WHO).

Smoking makes the risk much greater: WHO estimates smokers are 25 times more at risk from radon than non-smokers. Lung cancer is the only cancer proven to be linked to breathing radon (National Cancer Institute).

What to do about it

1. Test your home. EPA recommends testing all homes, and most tests take only a few days. See how to test for radon.
2. Fix it if it’s high. Most homes can be fixed for about the same cost as other common home repairs (EPA, Basic Radon Facts). See how radon mitigation systems work.
3. Building a home? Simple features added during construction can reduce radon entry. See radon-resistant new construction.