Carbon monoxide is a colorless, odorless, and tasteless gas. It is toxic to humans, and in lower doses it can cause headaches and flu-like symptoms. There is a slight danger of carbon monoxide poisoning in residential homes, especially during the winter months because the home is sealed, and the furnace is on. A faulty furnace can release carbon monoxide. As a result, many states suggest having carbon monoxide detectors inside your home.
Carbon monoxide is also a danger when working in coal mines. Before the days of manufactured carbon monoxide detectors, coal miners knew that certain animals could act as living carbon monoxide detectors. Canaries and mice were often used, although canaries were the primary choice. Their small size and fast rate of respiration makes them more susceptible to the gas than humans, and they can act as an early warning system. Canaries will exhibit stressed behavior at low levels of carbon monoxide. At high levels, they will fall off of their perch. The first person into the mine would carry the canary cage. If the canary started acting oddly, miners knew to evacuate immediately or to put on a ventilator mask. Canaries were still officially used in British mines until the end of 1986 when official detection devices replaced them. Although canaries are no longer used in coal mines, the phrase, "canary in a coal mine" is still used to describe something that is a predictor of danger to come.