Seems like the key thing would be to measure the concentration under mask during inhaling vs exhaling. Exhaled breath has many % CO2. The article references a whole table of articles measuring CO2 under masks. I haven't taken the time to check if they separate exhale vs inhale but would be curious to know.

Just thinking about the numbers. Exhaled air has 4% CO2, while fresh air has 0.04%. But if you inhale air with 3% CO2 that should add to those 4% yielding 7% exhale.

Now, in order to get 3% CO2 in your inhale this would mean that you would have to breathe in 42% stale air (with 7% CO2) mixed with 58% fresh air (~0 CO2).

I wonder if this 42-58 mix is plausible.

That would mean that the volume of gas trapped in a mask is 42% of lung capacity? Seems extremely implausible to me.

A typical breath is much less than full lung capacity, but even then I don't think it adds up.

I checked Roberge et al 2010 and it seems they don't separate inhalation from exhalation in the gas measurements under the mask.

I checked Sinkule et al 2012 and it's quite hard to tell how the review got those numbers from that paper.

Might check more when I have time later.

Since the volume of gas trapped by a mask is miniscule compared to the lungs, I wonder if it even matters. Might be better to just try to measure blood concentrations.

Yeah I was surprised they didn't measure blood concentrations--feels like the actual metric we should care about. I remember like, a lot of mic-drop social media posts about people being like "I wore a mask for 16 straight hours and my spo2 is 100% (or whatever), quit whining"

Yep, at 0.1-0.2% (1000-2000ppm) people start to get drowsy. at 0.2-0.5% (2000-5000ppm) people get heachaches, poor concentration, elevated heart rates, etc. It's quite unpleasant. 3% is 30,000 ppm.

While wearing a mask isn't fun, I've never experienced anything to that level while wearing mine.

Not buying it either.

I'd caution you against this idea that you can "know the feeling" of acute CO2 exposure.

For my PhD work (gas geochemistry) I spent hundreds of hours working in volcanic ice caves with CO2 levels up to 2% (we measured up to 3% but had a rule against being in above 2%). I always had a gas monitor with me so it was interesting to try and guess the levels. Neither I, nor anyone I was with, could guess reliably.

More than once I was with others in a cave, and they would freak out and say they felt high CO2 and we'd check the readings and nope. I actually think people (including myself) were responding to high humidity and heat and misinterpreting it as high CO2.

Of course, high CO2 is dangerous, all I'm saying is that it's surprisingly hard to disentangle it from all the other factors affecting how well your feel as a human, when you actually test yourself.

I'm reasonably certain I notice levels above 800ppm. It's subtle, but everyone in the house starts getting a little edgy (kids and adults). Sure enough, the CO2 meter is reporting > 800.

I doubt I could differentiate between 800 and 1600 though.

I'm not saying you're wrong, but remember 1600 ppm is only 0.16%, so that's impressive sensitivity. 4x atmospheric.

We've been discussing 1% to 3%, which is 10,000 to 30,000ppm.

In the early days of COVID, before we knew anything, I would go for runs outside and wear a mask (N95). I live in a densely populated city so this seemed like the appropriate thing to do.

While it was not pleasant, at no point did I start having issues you described so I have trouble buying the original claim.