Invisible and odorless carbon monoxide gas kills over 5,000 people each year. Clogged chimneys, malfunctioning stoves, water heaters, space heaters as well as auto, boat and airplane exhaust releases carbon monoxide into the air that you breathe. Pro-Lab’s Professional Carbon Monoxide Detector contains two (2) yellow chemical buttons that turn grey when they come in contact with carbon monoxide. They will detect as little as 50 ppm (before it becomes dangerous to you and your family). This instant detector is reusable and each button will last up to three months, once opened.