via city-journal.org:
Evidence continues to mount that mask mandates were perhaps the worst public-health intervention in modern American history. While concluding that wearing masks “probably makes little or no difference” in preventing the spread of viruses, a recent Cochrane review also emphasized that “more attention should be paid to describing and quantifying the harms” that may come from wearing masks. A new study from Germany does just that, and it suggests that the excess carbon dioxide breathed in by mask-wearers may have substantial ill-effects on their health—and, in the case of pregnant women, their unborn children’s.
Mask-wearers breathe in greater amounts of air that should have been expelled from their bodies and released out into the open. “[A] significant rise in carbon dioxide occurring while wearing a mask is scientifically proven in many studies,” write the German authors. “Fresh air has around 0.04% CO2,” they observe, while chronic exposure at CO2 levels of 0.3 percent is “toxic.” How much CO2 do mask-wearers breathe in? The authors write that “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.”
In other words, while eight times the normal level of carbon dioxide is toxic, research suggests that mask-wearers (specifically those who wear masks for more than 5 minutes at a time) are breathing in 35 to 80 times normal levels.