California law requires smoke detectors to be installed in all "dwelling units intended for human habitation." The specific requirements may vary depending on the type of property, the number of units and the number of stories of the property.
"Where there is smoke there is flame" as the saying goes, and it is true. Unlike CO, smoke is actually small particles that are transported upward on a column of hot air. Smoke detectors detect the smoke particles, and sound the alarm
When CO builds up, it spreads throughout a structure like water spilled on a table. Since CO is heavier than air, it spreads across the floor. For this reason structures need only a few CO detectors.
Smoke rises to the ceiling on the other hand and fills a room until it finds a doorway to spill under to fill other spaces.
For proper safety, multiple fire detector should be installed on every level of a building and its stairways as well as hallways outside sleeping areas. Smoke detectors that work off of household current must have backup batteries in case of a power outage.
All smoke detectors should be tested and their batteries changed every six months. The ideal way to remember is to do it the same time you change your clocks for daylight savings time.