CO2 sensors do not overlap in purpose with smoke detectors OR carbon monoxide detectors.
The purpose of the CO2 sensor is 2-fold.
To detect, by the rate of drop and average level, how well the ventilation system is clearing the air once the room becomes unoccupied.
To determine if respiration by humans or pets, in a room that may not be well ventilated, may pose either a nuisance or a hazard, as elevated CO2 in a room with ventilation causes issues ranging from headaches to neurological impairment. In an non-ventilated room it can kill you.
The trends and levels of the sensor can also be used for extremely accurate presence detection, and the more of those sensors in the house the more accurate it can be.
Using a combination VOC/PM/CO2 sensor, I have been able to determine some unusual things, like being able to detect when someone pours a can of coke into a cup, microwaves bacon, drinks alcohol, or even puts lotion on their skin, as they all have slightly different combinations of readings, and they are all surprisingly consistent.
What I have found for actual air quality and human comfort is as follows:
Humidity: very important in the winter/dry season, if its too low you get static cling/shocks, bloody noses, cracked skin, etc. If it drops too low you can be notified to check the water level on a dumb humidifier.
Temperature: sudden increases in certain rooms at certain times of day due to sun exposure that is not quickly detected by the main thermostat, the additional sensor can trigger aircon earlier than the thermostat for comfort reasons. Can also alert to potential failure of heating and aircon systems if the house average starts to trend for an extended period at a high rate while outside of the comfort threshold range.
VOC: Not worthless, but there are many benign things that trigger it, like food and drinks. Can detect chemical leaks if it goes up but then does not go down quickly, extremely sensitive to ethanol fumes. A very small chem leak in my garage made the livingroom sensor go from 100pb to 60000ppb in the course of a few hours.
PM: has not been helpful in general for quality since I am in a more rural area, but on occasion if I have the window open and a neighbor is burning something ( or fireworks ) will give useful data. Some foods, especially greasy foods that sizzle in a frying pan or microwave will make it go crazy. It can also detect food burning in a way that a smoke detector may not, especially ceiling mounted detectors that are designed to reduce nuisance alarms, since the dense oil type smoke from frying food tends to stay lower to the ground.
CO2: Notes as above, has been the most useful as consistently high levels indicate poor room ventilation or house air exchange with fresh air.
