Putnam County health officials are urging residents to test their homes for radon, a colorless, odorless gas that is the leading cause of lung cancer among non-smokers. The Putnam County Department of Health says the county falls in the highest-risk zone on the Environmental Protection Agency radon map, with about one-third of homes believed to have elevated levels. Officials say testing is the only way to know, and low-cost kits are available through the New York State Department of Health and the American Lung Association.