1. 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. It should be noted that Nest Protect was the only smoke detector that passed all of our detection tests. Half the devices could not detect our test smoke from 10 feet away and only Nest Protect sensed danger from the 15 foot mark. All smoke alarms feature a loud blaring tone to alert you to potential dangers in your home. Nest Protect goes a step further by adding voice alerts that calmly tell you what kind of danger is present, where it is and how to proceed.