There are no physical signs of carbon monoxide, as it is a colorless and odorless gas that cannot be detected by a person's senses, according to the Minnesota Department of Health. The only way to detect the gas indoors is by using a carbon monoxide sensor.
Due to the fact that it cannot be detected, the Center for Disease Control and Prevention estimates that more than 20,000 people in the United States are treated for carbon monoxide poisoning each year. Furthermore, more than 4,000 of these people are hospitalized due to the poisoning, while around 400 people die from carbon monoxide poisoning each year.
Carbon monoxide is produced as a result of the incomplete burning of organic matter, which includes fuels. In homes and other buildings, carbon monoxide normally builds up when fuel or wood burning stoves are not properly vented or operated. Improper maintenance may also lead to the gas building up indoors. There are a huge number of potential sources of carbon monoxide indoors, including gas cooking stoves, furnaces, water heaters, clothes dryers and fireplaces. Barbecue grills, vehicles, lawn mowers and generators are also sources of carbon monoxide, which is why they should never be used in enclosed spaces.