January is Radon Action Month, and the Cook County Department of Public Health is urging residents to test their homes for radon.

The colorless, odorless, radioactive gas is the second leading cause of lung cancer in the U.S. after smoking, and is blamed for 21,000 lung cancer deaths nationwide per year, according to the U.S. Environmental Protection Agency.

Because it is colorless and odorless, radon can accumulate to dangerous levels in homes without the people who live in those homes ever knowing it. This is why testing is crucial, the county public health department said.

The Cook County Department of Public Health offers residents of suburban Cook County free radon test kts. Residents elsewhere can purchase kits through the American Lung Association, the department said.

"Radon is a silent threat, but it is preventable," Dr. Kiran Joshi, chief operating officer of the department, said in a news release. "Testing your home is simple, and if high levels are detected, there are effective ways to reduce exposure and protect your health. We encourage all suburban Cook County residents to take this important step."

Chemically, radon (Rn) is a noble gas — along with helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the synthetic element oganesson (Og). Because they have stable, full electron shells, noble gas atoms usually do not form compounds with other elements.

Radon is generated naturally through the breakdown of uranium and thorium in soil, rocks, or water. The gas can seep into homes through cracks in foundations or basement floors; gaps around pipes, cables, or utility lines; construction joints where walls and floors meet; and crawlspaces or dirt floors, the county said.

Once inside, radon can accumulate in basements and result in health risks.

"Many people don't realize that their home could have elevated radon levels, even if their neighbor's home does not," Dr. Joshi said in the release. "Testing is the only way to know."

Tests showing radon levels in the amount of 4 picocuries per liter or higher warrant steps to reduce exposure, the county said. A picocurie is a trillionth of a curie, which is a measurement of radioactivity.

Levels of 2 to 4 picocuries per leader are also enough to consider taking action, the county said.

Reducing exposure can involve actions such as sealing cracks in basement floors or foundations, and installing ventilation systems designed to lower radon levels.

The county emphasized that dangerous radon levels have been found in homes in all 50 states, and smokers and nonsmokers alike are at risk for lung cancer from the radioactive gas. There are no immediate symptoms associated with radon exposure.

Buying a home is safe as long as proper radon mitigation is in place, the county said.