Those Full-face Snorkeling Masks – More to the Story

Undercurrent needs to go further in explaining the problems of full-face masks, says Max Weinman MD (Atlanta, GA), since there are inherent dangers that we didn't describe in full in our January article.

He wrote, "In physiological terms, any area not involved in gas exchange, that is, the uptake of oxygen and the removal of CO2, is a 'dead-space.' So [danger] is not merely a function of mask fit, but also the volume of dead-space in relation to the relative removal of CO2 by breathing, ventilation, and its production, metabolism. Should one of these factors alter while dead-space is constant, the latter may become increasingly important.

"Specifically, if [breathing] effort increases, such as a snorkeler finning against a current, CO2 production will rise dramatically, and dead-space will become increasingly meaningful in terms of CO2 build-up. Firefighter masks (upon which these devices were apparently based), like scuba regulators, are fitted with one-way exhalation valves designed to minimize dead-space and CO2 buildup. These are absent in [some of] these full-face masks, which have the additional volume of the snorkel. All this additional dead-space acts as a potential CO2 reservoir. A healthy fit person may be able to deal with this physiological challenge, but this is not always the case."

In the Italian-made masks (Ocean Reef, Mares, Cressi or Seacsub), fresh air is drawn down through the snorkel over the faceplate and through two butterfly valves in the internal bib. CO2-laden exhaled air is vented through both a valve at the front of the mask as well as back up the snorkel tube. However, cheaper masks allow the exhalations back into the dead-space, mixing with the fresh air, thereby encouraging hypercapnia. An ill-fitting bib will tend to do the same.

These masks are for use while floating relaxed in the water. If you need to swim hard and thereby increase your breathing rate, you should be prepared to pull the mask off.