In Today's homes there are more synthetic materials like nylon and polyester that tend to smolder for a long time and then burn faster than natural materials like cotton and wood.  The most common smoke alarms use an ionization method which is best for detecting fast buring flaming fires but may be slow to detect smoldering.  Smoldering fires like mentioned above are picked up better by photoelectric alarms.  It may be advisable to have one of each type on each floor or a combination unit which is also available.

Almost half the home fire deaths still occur in the small percentage of homes without any smoke alarms.  Athur Cote, chief engineer for the NFPA (www.nfpa.org) said "Almost all the time you find a "failure", the detector has been disabled.  They recommend replacing alarms after ten years.