It has a CO2 sensor. The description here in the shop is obviously incomplete. The display of the air quality is based on the CO2 content of the air in ppm. 5/5 stars means <500 ppm. 0/5 >2451 ppm.

I have no way of verifying this exactly. But when I stop the air and exhale into the ventilation slots of the unit after about a minute, the quality drops drastically for a short time. But it quickly returns to normal again.