Any home with a natural gas or wood burning appliance needs to be fitted with a carbon monoxide detector. Many home owners will replace, as they should, their obsolete 10 year old smoke alarms with a combination smoke alarm and carbon monoxide (CO) detector. So convenient but oh so wrong.
Smoke alarms mounted high on the wall or ceiling is correct, as smoke rises and fills the ceiling cavity first.
However carbon monoxide is heavier than air and settles low on the floor first. A receptacle plug-in CO detector, within 18 inches of the floor, is the most appropriate mounting. In contrast, by the time a ceiling mounted CO detector triggers an alarm, it may be too late for home occupants sleeping in a bed.
Retailers and suppliers should do better with their packaging and instructions. Life safety is serious business and we can certainly do better informing both contractors and DIY'ers on proper application of this technology.