Most residential smoke alarms use one of two basic technologies. They are either ionization or photoelectric. There are very significant differences in the way these devices respond in real-world fires. Ionization alarms
are notorious for nuisance tripping from cooking and even the steam from a shower, but they are very slow to respond - if they respond at all - in a room filled with smoke from a smoldering fire.  

Ionization type smoke detectors account for well over 90% of the smoke alarms installed in residential construction in the US. This technology uses a small amount of radioactive material to detect changes in the airflow across the sensor. On average, ionization units respond about 30 seconds faster to an open-flame fire than photoelectric type alarms. However, in a smoldering fire, ionization units respond on average 30 to 60
minutes slower than a photoelectric unit. In some cases, they may not respond at all. Most residential fire fatalities occur at night and are result of smoke inhalation. The flash-over point in a fire is basically the point
where the fire goes critical. Twenty to thirty years ago, the flash-over point in a fire occurred in as little as 12-14 minutes. Due primarily to the increased use of synthetic and engineered materials, flash-over now often
occurs in as little as 2-4 minutes. This leaves the occupants significantly less time to safely exit their home in a fire.

The issue with ionization alarms is more than just the inferior response times. A recent Alaskan study shows that ionization units are up to 8 times more likely to be non-functional in the first year after installation. Because ionization units are very prone to nuisance tripping from cooking, etc., people become frustrated and intentionally disable the units - i.e. they remove the battery, etc. This leaves the home unprotected. Homes with non-functional smoke alarms account for around 2/3 of all fire deaths. Statistically, ionization alarms are the most likely to be disabled. Most of the other 1/3 of fire deaths occur in homes where a functional alarm is installed. However, in far too many cases, the alarm sounds too late to alert the occupants.

How do you know which you have? It's not always possible to know. In general, if the unit has a Hush feature, it is an ionization unit. If the label says anything about radioactive materials, Americium-241 or the model number ends in an “I” - then it is an ionization unit. When there is any doubt, there is better than a 90% chance it is an ionization unit. To be safe, simply replace all unknown units with photoelectric units.

There are combination photoelectric/ionization units on the market. These suffer the same issues as ionization only detectors. They nuisance trip due to the ionization detector. In some cases, the manufactures reduce the sensitivity/response of the units to smoke to cut nuisance tripping. The design of certain combination units is such that the response is identical to ionization only units. The International Association of Fire Fighters specifically recommends against installing combination alarms.  

There are also combination photoelectric/carbon monoxide (CO) alarms. These combination units are more expensive. From a safety standpoint, smoke alarms should be replaced every 10 years. Most carbon monoxide detectors should be replaced every 5 years. So your either replacing the smoke alarm portion more often than needed or relying on a CO detector that is past its replacement date. Separate units simply make more sense.

Don't just replace your smoke alarm batteries – replace your alarms with photoelectric units and recommend that all of your family, friends and clients do the same!

This article was provided by Ron Bishop, owner of North State Home Inspections, and was
prepared by the “California Real Estate Inspection Association”.
www.northstatehomeinspections.com
530-524-5030