Smoke Alarms and Fires

Fire kills more Americans than all natural disasters combined and over 80 percent of all fire deaths occur in residences. Direct property loss due to fires in the United States is estimated at $8.6 billion per year. Cooking and smoking are the leading causes of residential fires, followed by heating fixtures. A smoke alarm is a battery operated or electrically connected device that senses the presence of visible or invisible particles produced by combustion and is designed to sound an alarm within the room or suite within which it is located. There are two types of household smoke alarms in common use: ionization and photoelectric smoke alarms. An ionization alarm uses a small amount of radioactive material to ionize air in the sensing chamber. As a result, the air chamber becomes conductive, permitting current to flow between two charged electrodes. When smoke particles enter the chamber, the conductivity of the chamber air decreases. When this reduction in conductivity is reduced to a predetermined level, the alarm is set off. Most smoke alarms in use are this type. A photoelectric smoke alarm consists of a light emitting diode and a light sensitive sensor in the sensing chamber. The presence of suspended smoke particles in the chamber scatters the light beam. This scattered light is detected and sets off the alarm. Smoke alarms should be maintained in accordance with the manufacturers’ instructions. Occasional light vacuuming will keep the air vents clean. Long life smoke alarms have been designed to use lithium batteries where the battery life is predicted to last 10 years with the normal low battery drain of ionization smoke alarms. The smoke alarms are still designed to provide a low battery audible signal as the battery charge is reduced to a level that may make the smoke alarm inoperable. Although these batteries are designed to last 10 years, ongoing testing and maintenance is required as per manufacturers’ instructions.


Inside Smoke Alarms and Fires