…after several hours of teaching, I feel like I’ve been breathing myself for hours, swallowing my own CO2,

I can tell you that a respirator with an exhaust valve is much cooler. I had to figure out how to filter the exhaust so I wasn’t breathing on everyone. It’s a bit harder to exhale than before, but much cooler than a no-valve mask. And given intake and exhaust valves, there’s very little rebreathing. For me, a major downside with a respirator is that it’s hard to make myself understood. I still wear a P100 when indoors with strangers, even though I have to raise my voice.

I’m a little surprised that someone hasn’t come up with a valved mask with separate filtration for the exhaust. Might be time for me to work on a prototype.

All that said, we need some CO2. On average, cerebral blood flow reduces around 2% for every 1mmHg decrease of carbon dioxide in the blood (PaCO2). I have seen no evidence that slightly elevated levels might be problematic (like from the rebreathing that comes from using a snorkel which effectively reduces unbreathed air getting to the lungs — diver here).