Radon is a gas that you can't see or smell, but is present in nearly all the air around us. Everyone breathes in radon every day, usually at low levels. People who inhale high levels of radon are at increased risk for developing lung cancer. Radon is a problem in homes in which higher concentrations collect. Find out more about radon by taking this quiz, based on information from the Environmental Protection Agency (EPA) and the National Cancer Institute (NCI).