Inhaled CO₂ concentration while wearing face masks: a pilot study using capnography

None of the available evaluations of the inhaled air carbon dioxide (CO₂) concentration, while wearing face masks, used professional, real-time capnography with water-removal tubing. We measured the end-tidal CO₂ using professional side-stream capnography, with water-removing tubing (Rad-97™ capnograph), at rest, (1) without masks, (2) wearing a surgical mask, and (3) wearing a FFP2 respirator, in 102 healthy volunteers aged 10-90 years, from the general population of Ferrara province, Italy. The inhaled air CO₂ concentration was then computed as: ((mask volume × end-tidal CO₂) + ((tidal volume - mask volume) × ambient air CO₂)) / tidal volume).

The mean CO₂ concentration was 4965±1047 ppm with surgical masks, and 9396±2254 ppm with FFP2 respirators. The proportion of the sample showing a CO₂ concentration higher than the 5000 ppm acceptable exposure threshold recommended for workers was 40.2% while wearing surgical masks, 99.0% while wearing FFP2 respirators. The mean blood oxygen saturation remained >96%, and the mean end-tidal CO₂ <33 mmHg. Adjusting for age, gender, BMI, and smoking, the inhaled air CO₂ concentration significantly increased with increasing respiratory rate (with a mean of 10,143±2782 ppm among the participants taking 18 or more breaths per minute, while wearing FFP2 respirators), and was higher among the minors, who showed a mean CO₂ concentration of 12,847±2898 ppm, while wearing FFP2 respirators. If these results will be confirmed, the current guidelines on mask-wearing could be updated to integrate recommendations for slow breathing and a more targeted use when contagion risk is low.