It's time to pull out the ionization detectors and go photoelectric.

These kinds of newspaper headlines are depressingly common. The copy in the article says, “There were no working smoke alarms.” This is a significant difference in nuance; smoke detectors have been legally required in all housing for decades. But they are often not maintained, and in fact they are often disabled because of false alarms, usually from cooking.

This is obviously a serious problem for everyone, but also for those interested in green and healthy building as we try to get rid of flame retardants. And like everything I complain about, it is also a design problem; people don't do these things randomly but respond to their conditions, their environment. If people are disabling their smoke detectors then there is something fundamentally wrong with their design.

Photoelectric vs ionization

NFPA Public service download/Public Domain

Ninety percent of installed smoke detectors are ionization detectors, which run on batteries, which is why they are so common; they are easier to install. There is a tiny bit of Americium 241 which emits alpha radiation, which ionizes the air between two plates connected to the battery. When a particle of smoke enters the detector it disrupts the ionization and sets off the alarm. It doesn’t take a lot of electricity to keep this going so it can run a long time on batteries. Some, like the ones I have in my cabin, have lithium batteries that can last as long as the smoke detector; most cheaper ones need batteries to be changed.

Ionization alarms are supposed to be better for “fast flame” fires that come from something like cooking oil or grease, which is why they tend to go off when people cook. And no doubt, the ongoing trend to open kitchens just exacerbates the problem with them.

NFPA Public service download/Public Domain

Photoelectric smoke detectors have a light emitting diode that shines a light into a photocell; the smoke particles physically block the beam of light. Keeping that light going takes more energy, so they are usually hard-wired (although there are battery powered units available). But photoelectric detectors are better with smouldering fires, which often produce smoke before there is fire. In a provocative post by Skip Walker of Walker Property Evaluation Services, he notes:

The smoldering fire tests standards were developed when most home furnishings were natural materials, cotton, wool, etc. Today, virtually all furnishings and a large percentage of the building materials are synthetic and engineered materials. The behavior of natural and synthetic materials in a fire is radically different. Yet the UL standards have not been adjusted to account for this shift.



Green and healthy homes need photoelectric smoke detectors.

Of course, TreeHugger promotes the return to natural materials made without synthetic foam and without halogenated flame retardants. So in a green and healthy home, where stuff smoulders instead of flaming up, you want photoelectric detectors. Skip Walker says they are also far more effective.