Smoke rises — so alarms should be placed on ceilings or high walls in every sleeping area, outside sleeping zones, and on each level of the home.  They must avoid placement near windows, doors, air ducts, or kitchens where drafts, steam, or cooking smoke can cause false alarms.  Detectors have a useful life: most alarms should be replaced every 10 years, even if they still “work,” because sensing components degrade over time.