Smoke alarms are your early warning system that a fire is present. A properly maintained smoke alarm alerts you to the present of smoke and fire and alerts you to leave the building.
Batteries in smoke alarm should be changed at least twice a year. A good thought process is to change the batteries when you change your clocks at daylight savings time and when switching back. A well charged battery ensures that the smoke alarm will give you that early warning to leave the building and to safety.
Ionization Smoke Alarms
This style of smoke alarm features a harmless radioactive source within a dual detection chamber. Ionization alarms respond to invisible by products of combustion. They operate by sensing for a change in the electrical conductivity across the detection chamber. The advantage of the ionization detector is that the smoke can be invisible to the human eye, while remaining very much visible to the ionization detector.
Photoelectric Smoke Alarms
This type of smoke alarm works on a different principle then that of the ionization style. Photoelectric alarms look for the presence of visible by-products of combustion in the detection chamber. When a sufficient density of visible combustibles fill the detection chamber, the alarm is activated alerting you to the condition.