Question:
The above prompted something that's been at the back of my mind for some time. In a local (Australian) electronics magazine (Electronics Australia) a few weeks ago, a correspondant to the editor remarked at the design of most (if not all) smoke detectors in this country and how they used radioactive Americium (sp? - atomic number in my textbook at home) seemingly as the detecting element. His (the correspondant) concern lay with the fact that he had a faulty unit which needed to be disposed of. The upshot was that none of the local authorities (councils, fire dept, etc) wanted to deal with it, and it required a call to a hazardous waste disposal company who arrived complete with white overalls and masks at the ready (no mention of geiger counters!) to collect the faulty smoke detector. My questions are: Is the Americium contained in these detectors a cause for concern in terms of safety? What about disposal? What about the house that has a fire where one of these is installed - sure the occupants may get out safely, but if the Americium is incinerated and carried away with the smoke, does the whole community suffer?

Being an electrical engineer (though no specialist on sensors) I was wondering is there a better way to detect smoke than this (that's also why I posted to this forum - Should I ask elsewhere?)?.

Hmmm. Lots of '?'. Comments, anyone?

Answer: Most home smoke detectors are made the way you describe. These are called "ionization" type detectors. I am looking at a defunct one now. It contains 0.9 microcurie of americium 241. This is an alpha particle emitter which ionizes the air within the sensor chamber. This ionization allows a miniscule electrical current to flow which keeps the alarm silent. When certain combustion products (not specifically smoke particles) enter the chamber, they trap the alpha particles preventing them from ionizing the air. This causes the current to drop and sounds the alarm. Other smoke detectors use an LED and photosensor to detect when smoke enters and darkens the test chamber.

Yet other detectors sense a rapid rise in temperature.

Most residential detectors are the ionization type. They tend to be the cheapest. They can be had for 10 to 15 dollars. A few are the photoelectric type. The best for residential purposes are the "combination" type which include both ionization and photoelectric sensors in the same package. These are hard to find since they cost two to three times as much as an ionization unit and therefore do not sell as well. Commercial detectors commonly combine all three types of sensors.

The reason that the combination sensor is the best for residential uses lies in the differing mechanisms on which they operate.

Open flames produce many of the products of combustion that trigger the ionization detectors but produce very little dark smoke. Thus, an ionization detector will sense an open fire faster than a photoelectric detector will.

A smoldering fire will produce a large quantity of smoke but very little of the other products. This will trigger a photoelectric sensor long before the ionization unit.

Because of this, a combination detector will respond the fastest to any type of fire.

By the way, when you here any smoke detector, get out FAST! I have seen training videos of a house set on fire intentionally. From the time that the first smoke detector sounded until smoke and heat made the upstairs hallway an instant death trap was only 2 1/2 minutes!

This may be more than you were looking for but it's kind of an important thing with me. I'm an Electronic Engineer by trade but my hobby is Firefighting.