Unlike countries such as Japan and China that have long since normalized wearing face masks, Americans have had to adjust to a new normal—one in which cloth face coverings are recommended to limit the spread of coronavirus. Having your mouth and nose obstructed, even by a breathable fabric like cotton, has led some to speculate that face masks might impede your oxygen intake or make you breathe in exhaled air—or even lead to carbon dioxide (CO2) poisoning.

Neither is likely to occur. Here’s why.

Both loose-fitting surgical masks and cloth masks are porous. Air can move through the material, but it’s more difficult for a respiratory droplet to pass through, making masks an effective obstacle for infectious germs that would otherwise be released into the air. Wearing a mask might feel like your airflow is reduced, and reduced airflow can lead to hypoxemia (low arterial oxygen supply) or hypoxia (a lack of sufficient oxygen in tissue).

But masks can’t affect that intake level. Instead, they cause a mechanical obstruction that may give the wearer the sensation of having to breathe harder or that less air is being inhaled. The oxygen level is not affected.

The other concern relates to hypercapnia, or too much carbon dioxide in the bloodstream. The condition can cause drowsiness, headache, and, in extreme cases, loss of consciousness. The thinking here is that a mask can prevent exhaled air from dissipating, leading the wearer to rebreathe it. But there’s no evidence that could ever occur. While some CO2 can be inhaled, it’s not in quantities that could pose a threat to healthy mask users. The amount is easily eliminated by a person’s respiratory and metabolic systems. If a mask is worn for a prolonged period, it might be possible to develop a headache, but nothing more.

“There is no risk of hypercapnia in healthy adults who use face coverings, including medical and cloth face masks, as well as N95s,” Robert Glatter, an emergency room physician at Lenox Hill Hospital in New York, told Healthline. “Carbon dioxide molecules freely diffuse through the masks, allowing normal gas exchange while breathing.”

There are exceptions. If a person has lung issues owing to disease or other breathing problems, they should consult with their physician before using a face covering. Masks are also not recommended for anyone under the age of 2.

Additionally, extended wear of N95 masks in a health care setting has been associated with hypoventilation, or a reduction in the frequency and depth of breathing. According to the Centers for Disease Control and Prevention, these masks, which are intended to filter out 95 percent of particles, present more breathing resistance. The CDC advises those in the medical field to take breaks from wearing these masks.

But in healthy adults who wear cloth or surgical masks for limited periods of time, hypoxemia, hypoxia, or hypercapnia is highly unlikely to occur.

[h/t USA Today]