First study is an all-Italian study still in preprint: CO2 levels whilst wearing a face mask go beyond the safety threshold. 40% of subjects with surgical masks inhale CO2 over the threshold. 99% of subjects wearing an FFP2 inhale CO2 over the threshold. The safety threshold is 5000ppm. The CO2 concentration with the surgical masks was measured to be 4965 ± 1047ppm. With FFP2, it was 9396 ± 2254ppm. It gets worse the younger the subject is. Up to 3 times the threshold in children wearing an FFP2, with an average of more than 2 times (12,847 ± 2898 ppm).
Peer-reviewed study: Carbon dioxide rises beyond acceptable safety levels in children under nose and mouth covering: Results of an experimental measurement study in healthy children.