These masks are NOT ALLOWED to be used on any of our boats.
Research has discovered that people can desaturate within 5 minutes of light exercise on these masks; almost all people experience increased forms of hypercapnia. The problem seems to be that the internal seal in these masks, even in a ‘perfect fit’ situation, does not completely work and allows rebreathing between the oronasal and eyepocket.
Do not bring them with you, and please do not be offended when we ask you not to use them. It is a significant health and safety hazard.
Full-face snorkel masks increase the incidence of hypoxaemia and hypercapnia during simulated snorkelling compared to conventional snorkels
For more information, you can visit the following link on the studies conducted on the subject.