In the previous post, I listed some journal articles that examine a few physiological effects of mask-wearing. I also advanced a few questions that need to be answered. I’ve now read the articles. Here are some of my observations about the first question.
Question 1. To what extent, if at all, does wearing a mask that is thought to intercept infectious particles effectively cause blood hypoxia?
At least two primary mechanisms are in play here. First, upon exhaling, masks “trap” some of the exhaled gases in front of the mouth and nostrils; perhaps better to say that the flux of exhaled gases away from the face is reduced, so (ceteris paribus) a human has to push harder to move those gases away. If the human intentionally does not exert the extra effort to do so, the concentration (aka, “partial pressure”) of exhaled gases in front of the mouth and nostrils is increased. Simply put, some of the exhaled carbon dioxide (CO2) effectively displaces some of the oxygen that is in our ambient air so that the next inhaling stroke brings in oxygen-depleted air. It’s not total depletion; if the ambient air has 20% oxygen, which is what our bodies are tuned to expect, the prior, mask-impaired exhale might reduce the oxygen content to 17-18%, perhaps more, perhaps less. (One author noted that we don’t have good research on the degree of O2 displacement.) But the claim here is plausible from both aerodynamics (pressure-gas flux relationships) and chemistry (gas partial pressures) points of view.
Our bodies naturally react to this mechanism’s outcome. First, we don’t like breathing in oxygen-depleted air – our bodies notice it even if we’re not consciously aware – so we reflexively start to push harder, or we tug at our mask, or we stop what we’re doing to reduce our body’s oxygen demand. We see evidence of all three of those reflexive actions all around us; retail clerks with their masks just below their nostrils, for example.
Note that if we push harder, the total gas pressure between face and mask has to increase, so the mask itself will move away from the face, reducing the mask’s effectiveness in retarding aerosol escape. If the mask is tight enough, or if its permeability is low – smaller pore sizes – we can exhaust ourselves trying to exhale well enough to ensure our next breath is 20% oxygen.
Perhaps it goes without saying, but the less permeable the mask, the more plausible is this first mechanism. It is clearly a trade-off. The better the mask for intercepting aerosol particles, the more pronounced this effect is likely to be.
The second mechanism shows up on the inhaling stroke. Here, the tighter or less permeable the mask, the greater the vacuum the lungs/diaphragm have to create to ensure enough air is inhaled, and if the oxygen demand is great enough, and the diaphragm/lung system is weak enough, the body may not get enough of that ambient air in each breath. Here, even if the exhale was 100% effective at expelling excess CO2 from in front of the mouth and nostrils, the body is simply not getting enough total air inhaled on the next stroke. The body instinctively knows what’s happening – sinus or nasal congestion creates the same effect, fundamentally – and pulls harder.
Alas, there’s a limit to how hard the body can pull. People with e. g. chronic obstructive pulmonary disease, or COPD, simply can’t generate enough vacuum to get an adequate breath. Masks that have any effectiveness at all trapping tiny aerosol particles will only add to that problem.
Both of those mechanisms are entirely plausible, and both induce either conscious or reflexive behaviors that we can readily observe in ourselves and others. Both mechanisms are also sensitive to how tight or impermeable the mask is. Both would result in lower oxygen intake. It is only a small, logical step to conclude that both would induce hypoxia in the blood – again, all other things being equal, and in rough proportion to how tight or impermeable the mask is.
The human body is marvelously clever, and if it’s healthy, it has ways of adapting. One of the articles looked at surgeons who are wearing masks for long hours during surgery; their airways end up being constricted over time and to a measurable degree. And there’s the rub. Young, healthy people might have plenty of adaptive capacity to overcome those two mechanisms. But older, weaker, or health-impaired people will be less able to overcome it.
The link between mask effectiveness (as an aerosol interceptor) and hypoxemia, to me, is clear and unambiguous. Are there complicating factors? No doubt. Does the human body have means of responding? Yes, and nearly all of them involve compromising the effectiveness of the mask at intercepting aerosol particles.
So, again, the tradeoff affects both the individual and his neighbors: is a person better off in a slightly hypoxic state, or is he better off having his airways wide open to all of the aerosols swirling around him? Is his neighbors’ reduced exposure to his expired aerosols of enough risk-reduction benefit to justify keeping him in a slightly hypoxic state?