Answer Geek: How Smoke Alarms Work

-- Q U E S T I O N: The question I have for you is how do smoke alarms work? And why is there a warning label about radioactive material in the alarm itself? What purpose does that serve?

— Andy U.

A N S W E R: Andy, I love this question. Why? Smoke detectors are clever little devices that save thousands of lives each year, and your query gives me a chance to do a little public service by reminding readers to make sure that A) you have at least one smoke detector on every floor in your home, and B) you CHECK THE BATTERIES occasionally to make sure they still have juice!

In simplest terms, a smoke alarm is a device armed with two key components: a sensor detects that there’s smoke in a room, and a loud, shrill alarm is designed to wake you up no matter how soundly you are sleeping.

There are two basic types of detectors: photoelectric sensors and ionization sensors. Let’s start with the photoelectric version.

These babies generally contain a T-shaped chamber with an light-emitting diode (LED) that sends a beam of light across the horizontal bar of the T; a photocell sits at the bottom of the vertical base of the T that generates a current when it is exposed to light. Under normal, smoke-free conditions, the LED beam moves uninterrupted in a nice straight line without striking the photocell, which sits at a right angle below the beam.

But when smoke enters the chamber, the beam is scattered by smoke particles, and some of the light makes its way down the vertical part of the T where it strikes the photocell. When enough light hits the cell, the current triggers the alarm to sound. In my house that usually means I forgot about the leftovers heating up on the stove for lunch.

A Radioactive Warning?

Ionization sensors contain a small amount of radioactive material and something called an ionization chamber, which is basically two metal plates, one of which carries a positive charge and one of which carries a negative charge. Between the two plates, air molecules — made up mostly of oxygen and nitrogen atoms — are ionized when electrons are kicked out of the molecules by alpha particles from the radioactive material. The result is oxygen and nitrogen atoms that are positively charged because they are short one electron; the free electrons are negatively charged. The positive atoms flow toward the negative plate, as the negative electrons flow toward the positive plate. This registers as a small but steady flow of current.

That current is disrupted when smoke enters the ionization chamber because smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the flow of electricity between the two plates in the ionization chamber, and when the electric current drops below a certain threshold, the alarm is triggered.

Safe Amounts

The radioactive material used in smoke detectors is Americium-241, which was discovered by scientists working on the Manhattan Project during World War II. A smoke detector contains about 1/5000th of a gram of Americium-241. Most of the radiation emitted by Americium-241 is in the form of alpha particles, which is absorbed by air once it travels an inch or two, and cannot, in any case, penetrate human skin. According to one source I found, continuous exposure to a smoke detector from a distance of three feet results in gamma particle exposure that is about 1/2000th the average annual exposure to background radiation. Hold that alarm in your hand, and you get the same exposure in about an hour.

What that means is that the amount of radiation exposure you receive from a smoke detector is minimal under most circumstances, and the very, very tiny risk it might possibly pose is far outweighed by the safety benefits. Warning labels are there mostly to discourage the foolhardy from opening a smoke detector up to see what radioactive material looks like.

So back to my public service announcement: Are there enough smoke detectors in your house? Do the batteries work? Are you sure? Tonight would be a good night to check.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.