Possible toxicity of chronic carbon dioxide exposure associated with face mask use, particularly in pregnant women, children and adolescents – A scoping review

Abstract
Introduction
During the SARS-CoV-2-pandemic, face masks have become one of the most important ubiquitous factors affecting human breathing. It increases the resistance and dead space volume leading to a re-breathing of CO2. So far, this phenomenon and possible implications on early life has not been evaluated in depth.

Conclusions
A significant rise in carbon dioxide occurring while wearing a mask is scientifically proven in many studies, especially for N95-masks due to their higher deadspace and breathing resistance.
Fresh air has around 0.04% CO2 while masks bear a possible chronic exposure to low level carbon dioxide of 1.41–3.2% CO2 of the inhaled air in reliable human experiments.
Animal experimental data shows deleterious proven effects of elevated CO2 of inhaled air in the long term with threshold values of above 0.3%, 0.5% and 0.8% (Neuron destruction, impaired memory and learning, increased anxiety, destruction of cells in testes, stillbirth, and birth defects). The risk for children's mental development starts at levels of above 0.3%, to adolescent male sexual development at levels of above 0.5%, as well as to unborn life at levels of above 0.8% resulting in reduced cognitive performance, reduced fertility and stillbirths.
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.
According to the data found, wearing face masks also has the potential to exceed acute (3% CO2 for 15 min) and chronic (0.5% CO2 for 8 h) NIOSH limits for CO2 respiration. Even if these are not exceeded, assuming that time is a toxicological variable equivalent to dose (Haber's rule of inhalation toxicology, also known as cn × tm = K) long term everyday mask use should be further examined, as chronic (repeated) exposure to smaller daily doses (even subliminally) may not differ significantly in its effect on the organism from exposure to acute/occasional higher (threshold) doses. Instead of only worrying about the potential risks of a future harmful long-term CO2 increase in the atmosphere with impact on human health the focus of research should also be on the current mask-related CO2 increase in breathing air (Table 1) with its numerous effects. Face mask experiments with appropriately long (and variable) exposure times and measurements of e.g., electrolyte, acid-base and renal excretion haemostasis are needed to investigate toxicological risks of carbon dioxide rebreathing for the most vulnerable groups.

https://www.cell.com/heliyon/fulltext/S2405-8440(23)01324-5