Face masks are used to reduce the transmission of airborne diseases. Nevertheless, for certain people, extended use of an N95 mask may be problematic, and that is what Chinese researchers set out to study in a randomized clinical trial published this year in JAMA Network Open.

For 14 hours in a controlled setting, 30 healthy participants were randomly assigned to receive various interventions with and without the N95 mask, including two 30-minute, light-exercise sessions. Biometric measurements and blood samples were taken before, during and after each testing period.

The scientists found that wearing an N95 mask resulted in reduced respiration rate and oxygen saturation within one hour, and an elevated heart rate two hours later, which continued until the mask was taken off. During exercise, mask-induced cardiopulmonary stress was further exacerbated, as heart rate and blood pressure increased, and respiration rate and oxygen saturation decreased. Energy expenditure and fat oxidation were elevated. 

After the 14-hour masked period, blood pH markers decreased, which may cause drowsiness, stupor and even coma in extreme cases, according to the National Institutes of Health. The study authors note that chronic cardiopulmonary stress may increase cardiovascular diseases and overall mortality in elderly individuals, children and those with cardiopulmonary diseases. Further investigation is needed to explore the effects of different masks on various populations.