I don't think power use is the issue. I have this cheap CO2 sensor: https://www.domadoo.fr/en/devices/5882-heiman-zigbee-air-qua... which draws 0.5W. This includes thermometer and humidity sensor, Zigbee transmission, and acting as a Zigbee router, but it gives us an upper bound. It also measures continuously (picks up someone breathing on it within 10s), which is overkill. A phone could measure CO2 levels once every 10 minutes which would average under 0.01W, so that would work.

However, this assumes the sensor would fit in a smartphone, which is not a given. And these things need air flow. And they also wouldn't work while the phone is in a bag or a pocket.

>A phone could measure CO2 levels once every 10 minutes which would average under 0.01W, so that would work.

Not sure about that, at least NDIR sensors have to be at certain elevated temperature to work and they do some preheating when you turn them on from standby.

So it's not possible to just measure less often as then energy would have to be spent on heating the sensor.