Carbon monoxide is a colorless, odorless gas that can be deadly. Because carbon monoxide is found in the fumes produced when fuel is burned, it is present in and around homes. As a result, homeowners should be aware of carbon monoxide and make every effort to detect its presence.
CO forms most readily when there is insufficient oxygen to complete combustion and produce carbon dioxide. Hot water closets, furnaces in crawlspaces, heating appliances in attics, and other contained areas are common areas where CO can form.
The U.S. Centers for Disease Control and Prevention says that thousands of people visit emergency rooms and are hospitalized because of CO poisoning every year. While CO is a risk for just about anyone, infants, the elderly, those with breathing problems or chronic heart disease, and people with anemia are most likely to get sick from CO.
CO has earned the moniker “the silent killer” because it cannot be identified without the presence of a carbon monoxide detector. If a person believes he or she is smelling carbon monoxide, that person is probably mistaking the odor for other combustion byproducts that the human nose can sense. 
CO is a byproduct of vehicle exhaust, boat engines, stoves, lanterns, grills, furnaces, and fireplaces. CO is produced anytime something is burning. That is why it is essential that products designed to be used outdoors are used exclusively outside, and that indoor appliances are properly vented to the outdoors. CO can build up indoors and poison people and pets who breathe it in.
Some people may not recognize that CO is problematic in a home until multiple residents start complaining of similar symptoms. Common CO poisoning symptoms include nausea and vomiting, dizziness, chest pain, confusion, headache, and other flu-like symptoms, advises the consumer advocacy group Carbon Monoxide Kills. Those with repeated exposure to high levels of CO may eventually develop cerebral edema, which is a swelling of the brain. CO can compress brain cells and destroy them, leading to neurological issues and death. CO poisoning is actually the result of the head and heart not receiving sufficient oxygen.
CO detectors can save lives and should be installed in all homes and apartments. The National Fire Protection Association says CO detectors “shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms.” Individuals should follow the manufacturer instructions regarding where on the wall or ceiling the CO detectors should be mounted. As an added safety precaution, CO detectors should be placed on every floor of the home. 
Gas sensors in CO alarms have limited life spans, so they should be replaced generally every five to six years, because calibrating and testing for CO is more difficult than simply replacing the alarms.
Installing or replacing carbon monoxide detectors is an easy improvement that can help save lives.