The Tennessee Department of Environment and Conservation had planned to offer free radon test kits through July 30. However, due to such a large demand, their free supply was gone within hours of offering them on Monday. Radon is a naturally occurring gas that can seep into homes through cracks and openings in the foundation. In concentrated levels, it can threaten human health. Federal statistics show it as the No. 1 cause of lung cancer among non-smokers in the United States. The best time to use the kits is in cold weather, usually from October to March.