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The short answer is that we're great at compensating for CO2 variation, but only if it's constantly elevated for days or more. The long answer: I think the clarifying reference here is 'respiratory acidosis', which is the general name for what blood acidification that comes from CO2 exchange problems instead of internal sources. It's diagnosed as a disease when people exceed a threshold pH, but the body's behavior is the same before that. Acidosis is diagnosed in two forms, acute and chronic. Predictably, both represent the same problem of too-low blood pH, but acute is more extreme and shorter term. What's relevant here is the difference in subsequent outcomes. Regardless of what's driving the acidification, the body responds first by making more bicarbonate, then by excreting more carbonic acid and reabsorbing more bicarbonate. The production response happens on the order of an hour, but heavily undercompensates for acidification. The reabsorption response happens over several days, and multiplies the effect of the first response for full compensation. This makes the problem with acute acidosis obvious: you suffer symptoms much faster than you adjust. A weaker version of the same effect is what happens when you go into a high-CO2 space. Chronic acidosis, meanwhile, creates two different concerns: First, can the body compensate for CO2 levels? If bicarbonate production is disrupted or acidification is very severe (e.g. late-stage emphysema), it's possible to exceed the bounds of compensation and end up with an extended form of the acute symptoms. For our purposes, though, this isn't a concern; at sub-disease levels, the only people who can't compensate probably can't produce bicarbonate at current levels either. Second, does the compensation have health consequences? Chemically, we see slightly elevated serum levels of Ca and K, but even serious acidosis rarely requires any response. Experimentally, submarine atmospheres are substantially higher in CO2 than poorly-ventilated houses or global warming projections, but animal experiments show basically no effect and submariners don't show obvious problems. |