It’s no secret that smoke alarms save lives. Between the mid-1970’s and today the percentage of homes with at least one smoke alarm has gone from about 10% to around 96%, according to a recent survey. Having a working smoke alarm cuts the chances of dying in a reported fire in half, according to the National Fire Protection Association.
But there does seem to be a secret – a very well-kept secret – about the two types of smoke alarms that are available.
About 94% of the smoke alarms in U.S. homes are ionization type smoke alarms, while only about 6% are photoelectric. These different types of alarms react very differently to different types of fires, and this difference can add up to precious minutes that you may need to escape from a fire.
A photoelectric smoke alarm, as indicated by the “P”
A smoldering fire will just create smoke for a long time, as much as two hours or more. Then the fire will flash over into flames and quickly spread. Most fire deaths are from smoke inhalation, so these types of fires are very deadly. A smoldering fire is associated with newer synthetic materials. The other type, a flaming fire more quickly flashes over into flames, and is associated with accelerants and highly combustible materials. In tests, a smoldering fire is usually simulated by the cushions of an easy chair, while flaming fires are simulated by news print in a wicker basket.
In general, ionization alarms respond slightly faster to flaming fires – about 10 to 15 seconds faster than photoelectric alarms. But when dealing with smoldering fires, ionization alarms respond as much as 30 to 60 minutes slower than photoelectric. That’s right, 30 to 60 minutes – not seconds. This drastic lag in responding to smoldering fires makes ionization alarms unacceptable to a growing chorus of fire protection specialists.
Smoldering fires are not only more deadly, they are very common and becoming more so as synthetic materials continue to be used more and more in our homes. And yet the type of smoke alarm present in 94% of U.S. homes may be an hour late in responding to these types of fires. It’s clear that photoelectric smoke alarms are the safer type.
A study at Texas A&M University from 1995 concluded that for a smoldering fire the probability of a fatality due to the failure of a photoelectric alarm was 4%, while the probability of a fatality due to the failure of an ionization alarm was 56%. For flaming fires the probability of a fatality due to the failure of a photoelectric alarm was again 4%, while the probability of a fatality due to the failure of an ionization alarm was 20%.
Underwriters Laboratory, which sets standards for smoke alarm performance, published the Smoke Characterization Study in 2007. They tested both ionization and photoelectric smoke alarms using the two existing test standards and with traditional test materials. They also tested the alarms using a variety of synthetic materials, and toast. Ionization alarms failed the UL tests 20% of the time using the traditional test materials. And remember, this is the same test that these alarms are supposed to pass just to be offered for sale in the U.S. When tested using newer synthetic materials, ionization alarms had a failure rate of 100%. On the other hand, photoelectric alarms has a pass rate of 100%. There was only one material for which ionization alarms outperformed photoelectric – burnt toast. Let me summarize: for the most common type of fire, ionization alarms had a 100% failure rate, while photoelectric alarms had a 100% pass rate.
This leads to the other problem with ionization alarms. They are notoriously susceptible to nuisance tripping, from cooking, showering, and other common activities. So they are commonly disabled. In fact, ionization alarms are up to 8 times more likely to be intentionally disabled. A disabled alarm of any type is of no value at all.
Combination smoke alarm with both ionization and photoelectric sensors
Combination smoke alarms with both photoelectric and ionization detectors are available. But there are no industry standards for these dual detector alarms, and the manufacturers can adjust the alarm points of the two detectors so that the unit behaves no better than a photoelectric-only alarm. The International Association of Fire Fighters recommends against installing combination smoke alarms.
Finally, if you want to know what type you have, here are some guidelines. If the label mentions radioactive material, Americium-241, or if the model number has an “i” in it, then it is almost certainly an ionization alarm. If you’re not sure, then you should assume it’s ionization and replace it with a photoelectric type. The life you may save is precious.
An ionization smoke alarm