Radon is a colorless, odorless gas that occurs naturally in the earth. As elements like uranium and radium break down in the rock and soil beneath your house, they release radon gas. Uranium and radium are radioactive. The same is true for radon gas, which can cause lung cancer. Radon exposure is the second leading cause of lung cancer in the United States.

Radon isn’t a problem outdoors, because it quickly dissipates into the atmosphere, leaving only tiny amounts in the air you breathe. But if your house is tightly sealed (as many modern homes are thanks to high-quality insulation and efficient air sealing), then you could be at risk. That’s because radon can become trapped inside and build up to unsafe levels.

Because radon is odorless and invisible, the only way to know if you have a problem is to conduct a radon test. Both the Centers for Disease Control (CDC) and the Environmental Protection Agency (EPA) recommend checking the radon level in your home for safety, even if you don’t live in an area of elevated risk.