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."
Interesting, my comment was based on the assumption that the tidal volume of human breath would be much larger than the tube/mask, but it seems I was wrong, it's just 500ml for an average human breath, opposed to 6 liters of lung volume.
As long as they are working properly. The problem is that the valves can fail (or not be properly designed in the first place), and then they become dangerous.
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.
