I still don’t understand why this sort of masks should be so dangerous they have to forbid them...
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Well that's the usual reaction for problems that cost lives and that no one has a complete understanding of the mechanism.
That being said, would I not let my wife use one. The reason is that she is just this side of panic when snorkeling, she won't unless I'm there holding her hand. Over time she will let me freedive as long as I get right back quickly. If she does panic, she rips off the mask and snorkel, and calms down enough to be a decent swimmer. I don't get it, but...
The FFM Snorkles are made for folks like her, and even ones that can't swim. The problem is that if the mask fits wrong, and/or made poorly, the chance of CO2 increases, and with it the increased chance of panic. Add a mask that may not come off quick and easy, because no one thinks they will have any problem, or need any training, with a child's toy while floating. Then toss in a old, overweight, out of shape tourist that's primed for a cardiac event, while panicked and thinks they are going to die because they can't get the mask off, and the perfect storm is a brewing.
Basicly, they would be out of business if they were to ban old, overweight, out of shape tourists that may be waiting for their first, or most recent, cardiac event. So no FFM Snorkels. It's easier than checking the masks for fit, type, and emergency removal procedures.
It gets down to their shop, their rules.
Bob