Thank you for the thoughtful question about gas exchange in children due to wearing masks. There is no study that I am aware of that has tested children wearing masks for prolonged periods of time to see if there is any change in oxygen (O2) or carbon dioxide (CO2) level. The main concern related to children wearing masks is that if a child puts something in their mouth and starts to choke it is conceivable that we will not be able to tend to them in a timely manner, this is why children less than 2 are not recommended to wear masks. Of course, this would be exceedingly rare.
From adults, we know that there is no risk of wearing a mask except for on rare occasions where an individual has severely compromised lungs.
The molecules of O2 and CO2 will freely flow through masks and around the mask perimeter. It is actually very hard for children to become hypoxic (low oxygen) unless you have lung disease or an infection that affects both sides of your lungs. If there is a small increase in the percent of CO2 that happens to occur in the small reservoir of the mask it is conceivable that the child may have to increase their respiratory rate to compensate for this to blow off more CO2, but this would be a minimal increase in effort (i.e. 32 breaths per minute up from a baseline 30 breaths per minute). I could easily bring a pulse oximeter to school one day and test a bunch of children if the school would like me to, but I am certain I will not find any hypoxia.
If I can add one more of my observations that may be useful. From my experience wearing a mask in the hospital for prolonged periods of time and more recently wearing various masks all day long at work. There are two physical issues I experience with masks.
(1) If they are really thick (two thick tight layers of cloth) and tight fitting (when I breathe the mask moves in and out with each breath) then I feel shortness of breath when I have to speak to patients for longer periods of time. I do not attribute this to lack of O2 or build up of CO2 from poor gas exchange. Rather, I think it is due to the increased effort it takes me to actually pull air into my lungs and thus I am likely taking smaller than average inhalations that in turn increases my CO2 and makes me feel short of breath and work harder to breath.
(2) Aches (head, neck, ear), I have experienced some achy sensations from wearing some types of face masks. Again, I do not attribute this to poor gas exchange but rather to how the mask is held to the head. For me, a mask that has tight ear elastic loops that pull behind my ears are the worst. After wearing those for 3 hours straight the area around my ears start to ache. Cloth loops are better if there is almost no tension. The very thick mask that ties behind my head in two places that makes me struggle to breathe at times gives me no achy issues at all.
The point is you may want to try a couple different masks on your child and have them wear it for a duration 1-2 hours to see which ones are most comfortable before sending them to school.
Jesse Lam, MD
Kapaa Pediatrics, LLC