Carbon monoxide is a colorless, odorless, and tasteless gas that is toxic to humans and animals in higher concentrations. Luckily, CO detectors can help prevent tragedy, but only if used—and used properly. In general, detectors should be placed on the ceiling or high on the wall on each level of your home (including the basement). Also place them within ten feet of each bedroom door. The alarm's "test button" does not check the CO gas sensor, so you must annually test it. These detectors typically last for two to five years.