Use of Radioactivity in Smoke Alarms

History of Smoke Alarms

• The first U.S. patent for a fire alarm came in 1890; two men invented a system to sound an alarm when the temperature in a building reached a certain level. From there, the fire alarm developed in various ways. In 1930, a Swiss physicist named Walter Jaeger invented a sensor that could detect smoke particles; later inventions and developments led to the introduction of the household smoke alarm in the 1960s. Although they were relatively slow to catch on, smoke alarms are now found in almost every home in America.

History of Americium-241

• Americium-241 (Am-241), the material found in smoke alarms, was first discovered in World War II. A man-made compound of radioactive metal, it was initially part of the Manhattan project, the project that eventually produced atomic weapons. Am-241 decays very slowly; its half-life is about 432 years, so it is still emitting the same amount of radiation after 10 years of use as it did at the beginning of its use.

Types of Alarms

• There are two types of smoke alarms: ionization and photoelectric smoke alarms. Photoelectric alarms shine a light into the sensor chamber; whenever smoke disrupts the flow of light, it causes light to shine on the sensor and sounds the alarm. These work well for detecting smoldering fires. Ionization alarms, however, have a small bit of radiation situated between two metal discs. The radiation ionizes the air, and when smoke passes through that area, it disrupts the ionization, causing the alarm to sound.

Amount of Radioactivity

• In every ionization smoke alarm, there is a minuscule amount of radioactivity. The whole source in the detector chamber is very small; it weighs only about .5 gram. However, the actual radioactive material amounts to just a millionth of a curie. In other words, one gram of Am-241 radioactive material is enough to supply 3 million household smoke alarms.

Risk

• There is little to no danger from household smoke alarms. The radiation level does not present a health hazard; even if someone swallowed the material, it would be too minuscule to do any damage. If you do not tamper with the smoke detector by disassembling it, there is no cause for concern. Background radiation levels present in the world emit higher volumes of radioactive material.