Ionising smoke detectors (the most widely used type) use a film coated with Americium 241, a radioactive by-product of the nuclear power industry, to detect danger.

Americium has a half life of 458 years and is a very active alpha radiation emitter. A scintillation meter held an inch away from a smoke detector containing it will detect alpha radiation twelve times the background environmental level. Although hazardous to assembly workers, the devices are probably safe to use unless burned in a house fire. Burning would release radioactive particles which could be inhaled by either occupants or fire-fighters. The Fire Brigade Union is concerned about the risk to firemen attending fires in factories and offices where up to 200 detectors may be installed.

Another type of smoke detector poses a risk to air crew and travellers. Thousands of smoke detectors containing a gamma-emitting fission product have been installed in the metal ceilings of aircraft toilet cabins just inches from the toilet user's head. When standing in front of the mirror, people receive a 400-500 counts per second dose of gamma radiation. This is in addition to the increased background radioactivity always present at high altitudes. Airline managements were not informed about radioactivity when purchasing these gamma smoke detectors and are unwilling to replace them.

There is an equally effective (but slightly more expensive) alternative: photo-electric or 'optical' smoke detectors.