evidence: Nor is it the first time the authors of the review have claimed face masks could have harmful effects. Several of the authors speculated in a review published in March that masks might have caused stillbirths and other health issues. But this was an extrapolation based on studies exposing animals to carbon dioxide — and ignored the fact that masks don’t increase levels of the gas in humans in any clinically meaningful way. The Daily Mail nevertheless covered the paper in a story with an alarming headline, screenshots of which have been widely shared on social media. [...] There are a couple of theoretical ways a mask could impact the respiratory system, Hopkins said. Masks slightly increase something called dead space, equal to the volume of air that doesn’t make it out of a person’s airway when they breathe out. When a person breathes in again, the first air they take is a little more than a cup of old air from this dead space. Masks very slightly increase the amount of this old air people take in. To overcome this, a person just has to take a very slightly bigger breath. [...] Kisielinski, Kai et al. "Physio-Metabolic and Clinical Consequences of Wearing Face Masks—Systematic Review with Meta-Analysis and Comprehensive Evaluation." Frontiers in Public Health. 5 Apr 2023.