​Appliance Science: How smoke alarms alert you to fires

Smoke detectors save lives. By warning the occupants of a house when a fire starts, they give them more time to either deal with the fire or evacuate. But there is an increasing debate about how they are best used, and what type of smoke alarm is the best type to use. Let's look at the science behind these early warning devices, and what this means for you.

There are two types of smoke detectors: ionizing and photosensitive. Ionizing models -- which are the most common -- are usually cheaper, smaller and easier to install than the latter type. If your smoke alarm is a small, round disk the size of a hockey puck, it is probably one of these. Photosensitive ones are often larger and more sophisticated, requiring bigger batteries or a permanent power connection. If your smoke detector looks more like a wireless router, it is probably a photoelectric type. If you aren't sure, check with the manufacturer or whomever installed the detector in your living space (like a landlord).

Ionizing

Ionizing smoke detectors use a neat bit of science to detect the smoke. Inside they have two parallel metal plates, with a very small amount of a radioactive element source on one plate. This radioactive material gives off very low power alpha radiation, which knocks electrons off the air molecules it bumps into. These ionized molecules (hence the name) carry an electric charge, meaning that an electric current flows between the two plates. However, if larger, heavier smoke particles are also present in the air between the plates, they absorb the radiation but don't become ionized, so the current can't flow between the two plates. This interruption of the current between the plates trips the alarm.

The upside of this type of alarm is that it is pretty simple to build. Apart from the radioactive material, only a few components are required. A single battery will can power it for at least a year. The downside is that it is rather prone to false alarms, particularly from the small, invisible smoke particles that come from cooking food. So, many people disable their ionizing smoke alarms out of frustration, which means they may not be alerted if a real fire breaks out.

[Science side note: The radioactive material used in most smoke detectors is called Americium Dioxide, which is produced by the decay of plutonium in nuclear reactors. This element was first isolated and identified in 1944 by Glenn Seaborg at the University of Chicago, as part of the Manhattan Project, which created the first atomic bombs. It is kind of neat to think that in most homes there is a tiny amount of something that wasn't known until about 70 years ago. Not much of it, though -- smoke alarms usually only contain about three-millionths of a gram of this material.]

Photoelectric

Photoelectric smoke detectors like the First Alert OneLink WiFi use a different approach. Inside these, there is a chamber that an LED shines light through. At a right angle to this is a photodiode, an electronic component that detects light. Most of the time this won't be triggered, as the beam of light from the LED isn't falling on it. If there is smoke in the air, though, the photodiode will detect the light that is reflected by the smoke particles. It's like dust particles in a sunbeam: You don't see the sunbeam unless there is dust in the air to deflect some of the light. When there are smoke particles in the beam, the photodiode sees the light they reflect and triggers the alarm.

This type of smoke detector is more complex than their ionizing cousins, requiring additional circuitry and power to run the LED as well as to detect the reflected light. That makes them more expensive, and the batteries that run them don't usually last as long. Many of this type are run from hardwired power, using batteries only as a backup solution. They are, however, less prone to false alarms, as the deflected light isn't triggered by the smaller particles in the air from things like cooking or someone smoking a cigarette on the balcony.

Some modern models take this even further. The Nest Protect, for instance, has what the company calls a Split Spectrum Sensor, which uses two LED light sources. One is an IR light source while the other uses blue light. Nest claims that this makes it more sensitive to smoldering fires that produce smaller smoke particles.

So, both types of alarms have their pros and cons. Ionization ones are cheap and use little power, but are prone to false alarms. Photoelectric ones are usually more expensive to install and run, but have fewer false alarms.

The best of both worlds

In the end, the best option is to have both. Many modern smoke alarms now include both types of sensors, with more circuitry that only triggers the alarm if both are tripped. That's the type recommended by the National Fire Protection Association among others since it can cover both major ways of detecting potential flames.