Face masks, CO₂, and cognitive impairment

This post may be difficult to comprehend while wearing a face mask

Jan 29, 2022

The CDC recommends, and many places require, face masks, in an attempt to reduce the spread of COVID-19. Multiple studies find significant declines in cognitive performance at 1,000 to 1,500 ppm CO₂, a small fraction of the approximately 10,000 ppm associated with breathing through a face mask.

The interior air of KN95 masks contains about 2.5% or 25,000 ppm CO₂ [1] and the inhaled average is about one half or one third of this time average [2].

A substantial volume of exhaled air with high CO₂ (about 36,000 ppm from the previous exhalation) is re-inhaled while wearing a face mask. For each adult breath, a volume of 500 ml is typically inhaled and exhaled. During inhalation, the mask-interior air volume of 100 ml [3] with high CO₂ mixes in a roughly 1-to-4 ratio with fresher air, leading to an inhaled concentration of approximately 7,500 ppm.

Going beyond this simple estimate, a detailed computational fluid dynamics study [4] modeled CO₂ in an N95 mask and found an inhaled average of 10,555 ppm. Cloth and surgical masks have the same basic geometry, and therefore similar CO₂ concentrations. Their filtration is less effective than N95 masks due to larger pore size and edge leakage, but they trap a similar exhaled volume for re-inhalation.

CO₂ concentrations above 1,000 ppm were seen to correlate with reduced cognitive performance [5] (in the following figure, up is worse).

A 2012 study [6] also found improved student performance with fresher air and describes a preferred CO₂ concentration of 1,000 ppm or below.

Finally, a double-blind study of twenty-four test subjects [7] repeatably found far higher scores for cognitive tasks at low levels CO₂.

We found statistically significant declines in cognitive function scores when CO₂ concentrations were increased to levels that are common in indoor spaces (approximately 950 ppm). In fact, this level of CO₂ is considered acceptable because it would satisfy ASHRAE’s ventilation rate guidance for acceptable indoor air quality. Larger differences were seen when CO₂ was raised to 1,400 ppm.

Update for Feb. 8, 2022: Contrary to the above studies, the US Navy, who routinely operate submarines with elevated CO₂, find no significant cognitive changes, even at 15,000 ppm.

A NASA study also found no effect at 5,000 ppm. These results conflict with references 5-7, which do find significant declines for 1,000 to 1,500 ppm.

Reference 7, the main study supporting CO₂ danger, has many authors from Harvard, a deeply anti-CO₂ institution. On the other hand, both the US Navy and NASA would face a huge burden to reduce CO₂ in their vessels. Institutional preferences correlate with study results.