Earlier quoted context omitted.
I had a moment like this recently with, of all things, a snorkeling mask. It was one of those full face masks with a big snorkel sticking out the top like a unicorn's horn that popped on the market a few years ago. It is a marvel. It took all the downsides of the old masks and ingeniously fixed them. Full mouth and nose breathing so you can breathe naturally and comfortably, an airflow pattern that pulls dry air over…
Be careful with those. Diver's Alert Network states that full face snorkeling masks can cause dangerous hypercapnia unless they have tight seals and working one-way valves. https://blog.daneurope.org/en_US/blog/are-full-face-snorkeli...
> So how to know a mask is safe? Check whether the mask has a one-way breathing system, verify that one-way valves are in place both in the snorkel as well as in the orinasal mask section, and last but not least check if the orinasal mask makes a good seal on your face. If these checks are positive, then it is a good indication the mask is safe to use.