These two detectors alert you to the presence of different, separate dangers. A carbon monoxide detector will sound the alarm only in the presence of carbon monoxide, an odorless, colorless gas. A smoke detector alarms when it detects smoke particles from a fire--or as often happens in our household, because we produced a little smoke while cooking. Homes and apartments commonly have smoke detectors, however carbon monoxide detectors are less common. While dual-purpose detectors do exist, smoke detectors DO NOT detect carbon monoxide, and carbon monoxide detectors DO NOT detect smoke.