Apparently this was an unpopular opinion so I went back and checked the math. A snorkel is about 18 inches long with a diameter of 0.75 inches. This gives it a

Apparently this was an unpopular opinion so I went back and checked the math. A snorkel is about 18 inches long with a diameter of 0.75 inches. This gives it a volume of 7.95 cubic inches or 130 ml. The poster I replied to suggested, in another comment, that rebreathing volumes as low as 50ml could be problematic. If that were the case you would run into major issues with snorkeling. I am not saying that there aren't people that have difficulty breathing in masks. I am saying that having difficulty wearing a mask is not normal and you should discuss it with your doctor to find out why.