There are two types of smoke alarm sensors available, ionization and photoelectric, and there is a major difference between the two. Photoelectric smoke alarms are far more effective at saving lives and should be installed in every home. Tyler already blogged about this topic last year (How do Smoke Detectors Work?), but it’s a topic worth repeating. Most smoke alarms in use today are the ionization type, but they’re being questioned more and more as a valid detection method; today they’re no longer allowed as the only type of residential smoke alarms in Iowa, Vermont, and Massachusetts.
Why all the bad press about ionization smoke alarms?
They give off too many false alarms – mostly from cooking and showers. When a smoke alarm goes off every time someone cooks bacon, people remove the battery or take down the smoke alarm. Manufacturers require them to be installed away from kitchens and bathrooms, but there is no hard and fast rule for clearance requirements across different manufacturers.
Another problem with ionization smoke alarms is that they take a long time to respond to a smoldering fire; this difference can mean tens of minutes (page 17). Estimates show that at least one third of home fatal fires involve a significant smoldering period (page 14). This is the major reason that there is such a push to replace ionization smoke alarms with photoelectric smoke alarms. The diagram and text below comes from a handout published by the CPSC, showing how a photoelectric smoke alarm will give occupants much more time to escape in the event of a smoldering fire.
In the event of a smoldering fire, a photoelectric smoke alarm clearly outperforms ionization. So why do we even have ionization alarms? Mostly cost. Photoelectric smoke alarms cost a little more than the ionization type. Ionization smoke alarms will also outperform photoelectric alarms in the event of a fast flaming fire. The faster reaction time can be measured in tens of seconds. The diagram below, again from the CPSC, helps to illustrate this.