What Is an Ionization Smoke Detector?

Function

• Ionization smoke alarms contain a tiny amount of radioactive material, typically americium 241, that constantly gives off alpha particles, which ionize the air within a sensing chamber in the unit. This ionized air is conductive and allows an electric current to flow between two electrodes. When anything other than air enters the chamber, including smoke, the conductivity of the ionized air decreases. Electronics within the unit detect this change. If the conductivity drops below a certain level, the unit sounds an alarm.

Form

• Ionization smoke detectors are similar in form to other types of smoke detectors. The electronics and sensing mechanism of the device usually is housed within a circular plastic container, with mounting holes or brackets on the back and vents on the front. Some units run on batteries, while others are hard-wired to the electrical system within a building. Because ionization smoke detectors contain radioactive material, they are labeled as such, with terms including "Mercurie," "Americium 241," "Ionization" or "Radioactive Material." They also may bear the three-pronged international emblem for radioactivity.

Safety Concerns

• While the risk of exposure to radioactivity from an ionization smoke alarm is small, the presence of these substances has led many countries in Europe to ban use of these devices, and some states are either recommending or passing legislation to facilitate phasing them out. The sensitivity of photoelectric smoke detectors has improved to the point that they are competitive with ionization detectors.

Disposal

• If you plan to replace your ionization smoke detector with a photoelectric model, remove the detector intact from the wall or ceiling. Do not attempt to open it or tamper with it in any way. Most manufacturers will accept the return of their ionization smoke detectors. Alternately, check with your local fire department or the Environmental Protection Agency for places in your area where you can dispose of the detector properly.