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Vince's avatar

As the article says, submariners are more fit by standards of the general population, just by virtue of being in the military - but passing a PRT twice a year is no great feat, and the submarine community is notorious for fudging the PRT and letting sailors who fail it stay in anyway. Compared to most of the military, submariners are not that fit. Not that this invalidates your point, but still, thought it was funny.

The other thing is that being on a submarine you definitely do notice a cognitive decline that lifts when you open hatches, but that’s probably more from lower oxygen levels rather than higher CO2 levels. Not to mention lack of sleep and high stress.

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petulantskeptic's avatar

I have a very high degree of confidence that we do not need more research on whether "high" ambient levels of CO2 affect cognition and I'm fairly sure I know where the disconnect is.

As background, at risk of being extremely too basic, all humans have a significantly higher amount of CO2 in their body and blood than is in the ambient air as a product of their metabolism. I suspect a big part of the reason people studying human physiology and people studying environmental effects of substances don't intuit one another's knowledge on this is that they are discussed in fundamentally different ways.

In physiology the amount of CO2 is discussed in terms of its partial pressure as a dissolved gas in one's blood. In the US we talk about dissolved blood gasses (mainly oxygen and carbon dioxide) using mmHg as units. Other countries often use SI units were the units are kPa. My medical training is American but I'll try and include kPas parenthetically. A normal partial pressure of CO2 in a healthy individual's blood is 40 mmHg +/- 5 (5.4 kPa +/- 0.7). In venous blood the amount of CO2 is slightly higher owing to this blood being the downstream recipient of metabolic products. The amount of CO2 in our blood (ie its partial pressure) is one of the most tightly regulated concentrations in our body and it is adjusted from moment to moment by our body subconsciously shifting our respiratory rate or the volume of our breaths.

Our body is able to adjust this so readily precisely because of a couple things alluded to in your piece. First, atmospheric/ambient CO2 is orders of magnitude lower than our physiologic CO2. Per Dalton's law we know the partial pressure of CO2 in well mixed atmospheric air at sea level is ~0.3 mmHg (0.04 kPa). Second, CO2 is a small nonpolar molecule that easily diffuses across the semipermiable membranes in our body (eg alveolar capillary endothelium, the blood brain barrier, and many others that are key to its many regulatory functions). Third, it's been shown across a variety of mammals and a variety of conditions that as the amount of CO2 rises in a subject's blood their minute ventilation (respiratory rate x tidal volume) will increase in order to maintain the blood CO2 concentration in a normal range. Fourth, in a number of disease states we know exactly how subjects appear and what the signs and symptoms of being unable to excrete enough CO2 to remain at a normal level.

The fourth point is largely tangential because it is well demontrated from the third point that mammal physiology (and humans in general) have a huge amount of respiratory reserve, which is to say we know that humans exercising can product many times the amount of CO2 that they do at rest but maintain their blood CO2 concentrations at baseline.

Since this has already gone on a long time I'll also point out this EPA study (https://www.epa.gov/sites/default/files/2015-06/documents/co2appendixb.pdf) showing that even at CO2 concentrations of 1% (10,300 ppm; which is to say, 25x atmospheric concentrations) the physiologic tolerance of this is indefinite though it induces physiologic changes which are predictable from what I wrote out above.

Given that our brain is continually exposed to CO2 levels approximately a hundred times higher than atmospheric CO2 and that its concentration is so tightly regulated with a lot of reserve it seems extremely unlikely that small (relative to the concentrations in the human body) increases in the ambient air would result in cognitive changes (it would also be easy to disprove CO2 as the causative agent in these analyses since we can easily test for the blood concentrations in humans to see if they are altered by the small [again, relatively speaking] changes in ambient CO2).

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