face mask; masks; CO2 level; CO2 toxicity; carbon dioxide; respiratory protective devices; personal protective equipment; COVID-19; SARS-CoV-2; coronavirus infections; N95 respirators; N95 mask


Health and Medical Administration | Public Health | Public Health Education and Promotion



Many individuals have difficulty adapting to face mask use and report symptoms while using masks. Our primary objective was to determine whether continuous mask-wearing causes elevated levels of carbon dioxide (CO2) behind the facemasks.


CO2 concentrations were measured behind 3 different types of face masks and were compared to CO2 concentrations at the mask front in 261 subjects who continuously wore masks for at least 5 minutes. These CO2 concentrations were also measured in several randomly selected subjects after a 5-minute walk.


There were significantly higher CO2 concentrations behind the mask (3176 ppm) compared to the front (843 ppm) with an average of 49 minutes of continuous mask use. Of all the subjects, 76.6% had a behind-the-mask CO2 concentration of more than 2000 ppm (the threshold for clinical symptoms), and 12.2% had a CO2 concentration of at least 5000 ppm (occupational health exposure limit). The CO2 level behind the N-95 masks was highest (especially after exertion) and was lowest behind cloth masks. The combination of warm ambient temperature, an N-95 mask, exercise, and young age appeared to induce exceedingly high CO2 levels that should be avoided.


Although masks might be necessary for healthcare workers or to lessen the spread of airborne disease, we found that elevated CO2 concentrations were present while wearing them. Elevated CO2 concentrations have historically caused symptoms of CO2 toxicity. Periodic mask breaks in designated areas may be needed to avoid adverse effects.


The use of masks increased the CO2 concentration in the air behind them to levels historically associated with toxicity.