Another study.

A significant rise in carbon dioxide occurring while wearing a mask is scientifically proven in many studies, especially for N95-masks (Table 2) due to their higher deadspace and breathing resistance [[48]].

Possible toxicity of chronic carbon dioxide exposure associated with face mask use, particularly in pregnant women, children and

adolescents “Fresh air has around 0.04% CO2, while wearing masks more than 5 min bears a possible chronic exposure to carbon dioxide of 1.41% to 3.2% of

the inhaled air. US Navy toxicity experts set the exposure limits for submarines carrying a female crew to 0.8% CO2 based on animal studies which indicated an increased risk for stillbirths. Additionally, mammals who were chronically exposed to 0.3% CO2 the experimental data demonstrate a teratogenicity with irreversible neuron damage in

the offspring. There is circumstantial evidence that popular mask use may be related to current observations of a significant rise of 28% to 33% in stillbirths worldwide and a reduced verbal, motor, and overall cognitive performance of two full standard deviations in scores in children born during the pandemic.”