Most fatal fires occur when the family is asleep. Working fire alarms properly placed and tested regularly most often give victims the precious moments they need to make a swift escape.
The average battery/line voltage operated alarm detects smoke using an ionization process. A radioactive material creates a current between two electrodes. Even a minute amount of smoke entering the sensing chamber will interrupt the current which triggers the alarm. Another common type of smoke detector uses a photoelectric beam, which when dispersed by smoke, will sound it's alarm.
These devices are based on detecting smoke and serve their purpose when placed above kitchen, bedrooms and basement doors. Areas where smoke from a fire will accumulate rapidly.
