CO₂ Levels Behind and in Front of Different Protective Mask Types

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.