The Danbury Department of Health & Human Services is offering free radon test kits to Danbury residents. Officials say it's important to know the radon levels in a home because it's 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.  There is a $10 deposit fee, which will be reimbursed when the radon test kit is returned.