Dr. Margareta Griesz-Brisson MD, PhD, a world-renowned neurologist from Germany, warned that face masks cause oxygen deprivation and permanent neurological damage, especially in the developing brains of children.
https://archive.org/details/Carbon-Dioxide-Face-Masks
This is the long version of a study published previously as a short research letter by JAMA Pediatrics. (Walach, H., Weikl, R., Prentice, J., Diemer, A., Traindl, H., Kappes, A., & Hockertz, S. (2021)
https://old.reddit.com/r/conspiracy/comments/l8dab9/multiple_studies_show_the_dangers_of_face_masks/
https://archive.org/download/hiv-aids-fauci-documentary/oxygen-deprivation-torture.pdf
There is also a more basic reason masks might make you stupid: decreasing oxygen flow to the brain.
Face veils reduce ventilatory function in the long-term (Alghadir, Aly & Zafar, 2012), and surgical masks may reduce blood oxygenation among surgeons (Beder et al., 2008): believe it or not, covering your mouth makes it harder to breathe.
Reviewing the N95 face mask, a 2010 study (Roberge et al.) concluded that “carbon dioxide and oxygen levels were significantly above and below, respectively, the ambient workplace standards” inside the mask.
A post-COVID study found that 81% of 128 previously-fit healthcare workers developed headache.