Anything that compromises respiration while in the water is very concerning to me and I wouldn't use them, though it seems to be resolved in newer models. I prefer a basic but high quality J snorkel with a comfortable mouthpiece but I understand why these types of masks are appealing too.

Your thought that we're naturally armed against CO2 build up is generally true, but what we aren't armed against is a lack of O2 in the presence of a lack of CO2. Our urge to breathe occurs not because we're losing O2, but because we have too much CO2 in our bodies. I think this is critical to understand in the context of snorkelling.

If you're in a room and CO2 is gradually built up, you will likely experience symptoms of the build up occurring in your body, absolutely. When it's more acute though, you often don't experience symptoms in a time frame in which you'll be able to respond properly. In the case with this mask that's probably not a concern at all.

Another concern, far more applicable here, is hypoxia. This kills snorkellers and divers frequently. Typically they deplete CO2 levels in their body via over-exertion and/or hyperventilation (intentionally or not) then go under water for some period waiting for their warning signals to return to the surface to breathe. Unfortunately the signals never occur because CO2 levels haven't reached a level which causes their nervous system to respond by causing an urge to breathe. Instead, oxygen is depleted causing a blackout to occur either under water or near the surface. The person isn't able to protect themselves while unconscious, so they often drown.

I wanted to point this out because in the context of water sports, more people need to be aware of this. Your body won't always let you know you're in danger. It's often why people experience it and/or die from it - they simply didn't know. We expect our bodies to tell us when we need to breathe. This is because our bodies are typically in conditions which allow for this to happen and we're very accustomed to that - we take it for granted. Once you skew the O2 and CO2 levels in your body, things don't occur as you'd expect at all. Much like any other situation where homeostasis is compromised.

Hopefully I'm not coming across as lecturing or anything. I'm genuinely intending to be helpful.

Some key tips when in the water, regardless of what mask you use:

- Breathe normally, don't hyperventilate

- Only dive if your breathing is at a normal rate and you feel relaxed

- Say you dive down for 30 seconds - spend at least 1 minute (2x your dive time) recovering oxygen, preferably 3x

- Always, always try to go with other people - accidents happen, and you'll need each other

- If it's your first time spending time under water, gradually build up your time under there. Feel out your comfort zone before testing yourself.

- Spit out your snorkel when you go under water. If something goes wrong, it becomes an easy entry point for water to get to your lungs.

It's not like they're execution devices. But the symptoms include dizziness and disorientation which isn't ideal in the ocean. Plus it can exacerbate pre-existing conditions where you might skip the mild symptoms and go straight to serious ones.

I started to write this out but realized a more polished explanation might be more useful:

"Snorkels constitute respiratory dead space. When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is inhaled again, reducing the amount of fresh air in the inhaled volume, and increasing the risk of a buildup of carbon dioxide in the blood, which can result in hypercapnia. The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated. Including the internal volume of the mask in the breathing circuit greatly expands the dead space."

https://en.wikipedia.org/wiki/Snorkel_(swimming)

(The face mask snorkels are relevant to the "greater the volume of the tube" part.)

I don't know if it actually applies to a facemask with unidirectional flow (on at the top, out at the bottom). But the CO2 buildup mechanism is sometimes described as a reason for medical facemasks to be close-fitting. When you exhale into a mask with a big space, and then inhale again, you (very roughly) first inhale everything that was in the mask before inhaling new air.

The extreme example is breathing from a long skinny tube. If the volume of the tube is bigger than that of your lungs, you never inhale new air.

There will be some mixing of fresh air through the snorkel but that's a small opening and a long tube. It will still mostly be the air you exhaled. For a traditional snorkel that's a small volume relative to a breath but not so much for those masks. Plus, if you're diving under the water a traditional snorkel will be completely purged while these masks will retain the air.