A monoxide is a chemical compound containing one oxygen atom bonded to one atom of another element.

The most famous monoxide is carbon monoxide, a poisonous gas created when fuels like wood, gas, or coal don't burn completely. You can't see, smell, or taste carbon monoxide, which makes it dangerous. That's why homes have carbon monoxide detectors: small devices that beep loudly if they sense this gas, warning people to get outside immediately and call for help.

Carbon monoxide is dangerous because it tricks your body. When you breathe, your blood normally carries oxygen from your lungs to your brain and muscles. But carbon monoxide molecules can attach to your blood cells instead, blocking oxygen from getting where it needs to go. It's like someone cutting in line and taking the seat meant for oxygen.

Other monoxides exist too, like nitrogen monoxide (also called nitric oxide), which your body actually makes naturally to help blood vessels work properly. But when people say “monoxide” without specifying which kind, they almost always mean carbon monoxide. That's why furnaces, water heaters, and fireplaces need proper ventilation: to let any carbon monoxide escape safely outside rather than building up indoors where people could breathe it in.