Many questions have been raised in recent years regarding smoke detectors because they sometimes don’t appear to provide early warning to the building occupants. Many people don’t realize that not all smoke detectors are created equal. Which is the best type of smoke detector for your practice and can carbon monoxide detectors play a role in smoke detection?  As a fire fighter for over twenty years and a fire department officer for approximately ten years, I have been asked these questions many times. To understand the concerns, let’s first talk about the three different types of smoke detectors.  Most people have ionization smoke detectors in their homes and offices.  For many years, the ionization smoke detector was the detector of choice – it appeared to do the job and alerted occupants to the presence of smoke or fire in their homes.   IONIZATION SMOKE DETECTOR Inside the ionization smoke detector chamber there are two metal plates or electrodes a small distance apart from each other.  One of the metal plates carries a negative charge and the other metal plate carries a positive charge. A very small amount of electrical current continuously runs between the two electrodes inside the detector.  If smoke enters the detector, the smoke particles attach themselves to the positive atoms. The positive atoms become neutrally charged, and the electrical current flowing between the electrodes is decreased. The reduction in electrical current causes the smoke detector to go into alarm (see diagram).   PHOTOELECTRIC SMOKE DETECTOR The next type of smoke detector is known as a photoelectric smoke detector.  There is a T-shaped chamber inside the photoelectric smoke detector.   At one end of the chamber there is a light emitting diode (LED), and the bottom leg of the “T” has a photo cell installed. Under normal conditions, the LED provides a light source that travels continuously through the upper portion of the “T” and doesn’t affect the photo cell at the bottom of it.  If smoke or smoke particles enter the smoke detector chamber, the small beam of light from the LED is scattered downward into the bottom leg.  Any light entering this area is directed towards the photo cell, initiating an electrical current that in turn activates the smoke detector alarm (see diagram).     One of the most current concerns associated with using ionization smoke detectors is that it doesn’t alarm during the smoldering phase of most fires. Smoldering fires occur much more frequently and are typically the most dangerous and deadly.  A smoldering fire can produce large quantities of smoke and toxic gases which can quickly affect an individual’s respiratory system.  Another concern is that the ionization smoke detector can’t determine the difference between smoke and steam. That may not sound like a problem, but if an ionization detector is placed too close to a shower or kitchen, it can produce false alarms. This can lead to individuals developing complacency regarding smoke detector activations.  In some cases, if the smoke detector is wired into a fire alarm system, false alarms can result in potential fines from the local fire marshal or fire department.  Ionization smoke detectors can however provide a warning to the building occupants if the fire is flaming or free-burning. The photoelectrical smoke detector can provide an early warning of a smoldering fire and give building occupants the chance to exit the building prior to the development of a serious fire.   COMBINATION OR DUAL PURPOSE SMOKE DETECTOR The third type of smoke detector is called a combination or dual sensor detector.  It incorporates both ionization and photoelectric technologies in a single sensing device.   This provides detection of both free-burning fires and smoldering fires.   CARBON MONOXIDE DETECTOR Carbon monoxide detectors, unlike smoke detectors, function to detect the presence of carbon monoxide (CO) since it is colorless, tasteless and odorless. They protect from being overcome by exhaust gases when a fossil-fuel-burning appliance (furnace, hot water heater, boiler, fireplace, etc.) malfunctions, or a chimney/flue pipe is blocked or leaks inside our homes. In one building fire several years ago, the fire had started in a dental practice on the ground floor.  Smoke and hot gases rose to the floor above and actually activated the CO detector on the second floor corridor of the building which housed several apartments.  This activated the CO detector because of the higher level of CO being generated by the fire occurring in the dental practice below, thereby alerting the apartment occupants to the fire so they could safely evacuate the building. The ionization smoke detection system never activated prior to the arrival of the fire department. So, there is a possibility that CO detectors can provide some warning of a fire, but they should not be used in lieu of smoke detectors. In short, the combination or dual purpose smoke detector affords the best warning against either a smoldering or free burning fire. If you already have ionization smoke detectors, you could install additional photoelectric smoke detectors in the same areas. The Massachusetts Board of Fire Prevention Regulation has passed a new law effective April 5, 2010 requiring owners of certain residential buildings to install and maintain both types of smoke detectors or dual senor devices. Although this is a Massachusetts law right now, it seems likely that it could be   adopted in other states throughout the country in the coming years.