The symptoms of drowsiness and poor air start when the CO2 level reaches from 1,000 to 2,000ppm. We, humans, are the main indoor source of carbon dioxide in most buildings.
To better understand what stays behind the numbers:
1. Outdoor CO2 usual levels are 350–450 ppm. The indoor concentrations are always higher due to the lack of ventilation and tighter built areas with two or more people inside.
2. Ppm is abbreviation for parts per million, often used to express the dilute concentrations of substances.