There are two types of smoke alarms designed for homes. One type is called an ionization alarm because it uses “ions,” or electrically charged particles, to detect smoke in the air. Smoke particles entering the sensing chamber change the electrical balance of the air. The greater the amount of smoke, the higher the electrical imbalance. The horn will sound when the electrical imbalance reaches a preset level.

The other type of alarm is called photoelectric because its sensing chamber uses a beam of light and a light sensor. The sensing chamber is designed so that the light beam does not strike the sensor, but smoke particles entering the chamber deflect the light onto the sensor. The greater the amount of smoke entering the chamber, the more light will be deflected onto the sensor. The alarm sounds when the amount of light hitting the sensor reaches a preset level.

Is one type better than the other?

Both types can meet the test standards of Underwriters Laboratories, but each has its own advantages. The ionization alarm responds faster to small smoke particles, while the photoelectric responds faster to large smoke particles. Flaming fires produce more small smoke particles and smoldering fires produce more large particles.

Fire researchers have learned that a fire that generates a lot of small smoke particles will cause an ionization smoke alarm to sound sooner than a photoelectric. The time delay between the two is relatively small, but these types of fires will make the room untenable to life more quickly, so time is of the essence. On the other hand, a fire with a lot of large smoke particles will cause a photoelectric smoke alarm to sound sooner than an ionization. In this case the time delay between the two can be relatively long, but these types of fires take longer to make the room untenable to life. If you want the advantages of both, you can install one of each everywhere that a smoke alarm is required or recommended, or you can buy “combination” units that have both sensors.

It is true that an ionization alarm responds marginally faster to open fires than a photoelectric smoke alarm. On average, ionization alarms will react about 30 to 90 seconds faster to this type of fire. However, nearly all residential fire fatalities are from smoke inhalation and not from the actual fire. Also most deadly fires occur at night while you sleep. On average, ionization alarms respond about 30 to 90 minutes slower to smoldering fires than photoelectric alarm. In these fatal fires, a photoelectric alarm will alert occupants in time to allow a safe exit. In the interest of your safety, do make sure you have photoelectric smoke alarms.