The unit uses a split spectrum sensor, a special type of photoelectric sensor, which is designed to detect both smoky and flaming fires. The Nest Protect also has a built in a carbon monoxide sensor and will sound an alarm if it detects dangerous levels of the invisible, odorless gas which can be lethal if you breathe it for too long. To help cut down on false alarms from steam and burnt toast, the Nest Protect also uses humidity and heat sensors. We tested the detection capabilities of each smoke detector in our comparison by spraying test smoke from an aerosol can directly at each one from 5, 10 and 15 feet away. When we tested the 5 and 10 foot mark, the Nest Protect sounded its alarm after 18 seconds. At 15 feet, the alarm activated at 28 seconds.