The City of Danbury Department of Health & Human Services is offering free radon test kits to residents. Radon is an odorless and invisible poisonous gas. Radon gas is the leading cause of lung cancer in non-smokers. Radon is formed from the decay of naturally occurring uranium and found in rock, soil and water. Danbury residents will need to provide a refundable $10 deposit for the free radon test kit.