Numerous scientific studies have measured oxygen levels and found no decrease when wearing an N95 mask or any other mask, with or without valves and with or without exercise. In fact, they found a slight increase in oxygen levels. Nurses who wore an N95 mask for an entire twelve-hour shift had no decrease in oxygen levels but did have elevations in CO2 levels. This was also true for pregnant women during exercise. The rise in CO2 was minimal (3 percent) and stabilized after fifteen minutes. The rise was statistically significant, but was it clinically significant? Did it cause any impairment in performance or cognitive functions? These studies didn’t address that, but a 2020 study showed that masks had no effect on performance during vigorous exercise. And here’s another study that showed no impairment.

According to experts, “There is no solid scientific evidence that wearing masks causes hypoxia or cognitive impairment. Doctors and healthcare workers who work in operating rooms wear them for hours a day and there has been no evidence that these people have had cognitive decline.”

A 2021 study on the effects of wearing masks on oxygen and ventilation at rest and during physical activity found that “The risk of pathologic gas exchange impairment with cloth masks and surgical masks is near-zero in the general adult population.”