Natural gas is a combustible product that can produce carbon monoxide (CO) if not burned correctly. Between 1999 and 2010, unintentional exposure to CO caused an average of 430 deaths annually across the United States and that number has held steady through today according to the Centers for Disease Control and Prevention. Carbon monoxide poisoning is potentially fatal. CO can escape from poorly-maintained or badly-fitted appliances, blocked ducts and chimneys, natural gas fires, water heaters, barbecues, furnaces, and generators, to name a few. Carbon monoxide is colorless and odorless—unlike methane mixed with mercaptan—and can escape detection for months. Low-level exposure to CO may provoke the physical symptoms listed above. Remember that the longer your exposure to CO, the sicker you become, affecting your judgments and emotional state. High-level exposure to carbon monoxide is much more dangerous. Mental states change rapidly, and damage to the nervous system and brain can impair physical prowess. Loss of consciousness and death may follow quickly.