Carbon monoxide (CO) is a colorless, odorless gas formed when fuels such as natural gas, wood, gasoline, or propane burn incompletely. It can build up indoors when combustion appliances, engines, or generators are used without adequate ventilation. Once inhaled, CO enters the bloodstream and binds to hemoglobin, reducing the blood's ability to carry oxygen; exposure can cause headache, dizziness, nausea, confusion, and, at high levels, death. Because people cannot detect it by sight or smell, carbon monoxide alarms are an important way to provide warning. CO is distinct from carbon dioxide (CO₂), though both contain carbon and oxygen atoms.