Many governments have made nose and mouth covering or face masks compulsory for schoolchildren. The evidence base for this is weak. The question whether nose and mouth covering increases carbon dioxide in inhaled air is crucial. A large-scale survey in Germany of adverse effects in parents and children using data of 25 930 children has shown that 68% of the participating children had problems when wearing nose and mouth coverings.
The normal content of carbon dioxide in the open is about 0.04% by volume (ie, 400 ppm). A level of 0.2% by volume or 2000 ppm is the limit for closed rooms according to the German Federal Environmental Office, and everything beyond this level is unacceptable.
https://jamanetwork.com/journals/jamapediatrics/fullarticle/2781743