CO₂ Levels Behind and in Front of Different Protective Mask Types

Objectives: 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.

Methods: 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.

Results: 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.

Discussion: 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.

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