January is National Radon Action Month. Radon is a naturally occurring, potentially dangerous gas that may build up to unhealthy levels inside the home. During home sales, this is a common inspection done in tandem with others, and costs $150-200. It is typically a 48 hour test done with machine or cannisters. You can also have this test done at home anytime.

The EPA has set a level at which if the test results are over, they recommend "remediation" which consists of a mitigation system to be installed in the lowest level of the home that lowers the buildup to acceptable levels. Installation should be done by a certified contractor and can cost $700-1200 and should include a re-test after install and warranty.

EPA: https://www.epa.gov/radon

CDC: https://www.cdc.gov/.../fea.../protect-home-radon/index.html