The American Society of Home Inspectors contacted me with its concerns regarding ionization smoke detection.

According to the National Fire Protection Association, ionization alarms have a small amount of radioactive material between two electrically charged plates that ionizes the air and causes current to flow between the plates.

When smoke enters a chamber, it disrupts the flow of ions, reducing the flow of current, activating the alarm. Though such alarms respond best to “flaming fires,” the group said, photoelectric smoke detection is more responsive to fires that begin with smoldering. Photoelectric-type alarms aim a light source into a sensing chamber. Smoke enters the chamber, reflecting light onto the sensor, triggering the alarm.

A fire-protection association study found that ionization alarms produced far more nuisance alarms than photoelectric alarms. As the association and Underwriters Laboratories study alarm performance to determine whether the standards should be changed, the home inspectors’ group is advocating replacement of ionization alarms with photoelectric ones.

Skip Walker, the home-inspection group’s point person, said that in one UL study, “ionization alarms failed to trigger at all in 91 percent of tests for smoldering fires in synthetic materials such as mattress foam and nylon carpet.”

Although the fire-protection association recommends both varieties and supports use of the dual alarms now available, a Consumer Product Safety Commission study found that combination models produced twice as many nuisance alarms as those with only one sensor.

The home-inspectors group cited a Texas A&M University risk analysis that found that in smoldering-ignition fires, the probability of fatality because of alarm failure was 55.8 percent for ionization versus 4.06 percent for photoelectric. In flaming-ignition fires, it was 19.8 percent for ionization, versus 4.06 percent for photoelectric.

Regardless of which type you use, make sure your alarms work, experts said.

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