The Pettenkofer number is a guideline for indoor air quality, set at 1000 parts per million (ppm) of carbon dioxide (CO2). Levels above this number can lead to physical symptoms like headaches, indicating poor air quality. Above 1000 ppm: First symptoms like headaches may occur. Good outdoor air quality: Approximately 350 ppm. Typical urban air quality: Around 500 ppm. Intensive ventilation Limit combustion Reduce chemical cleaners