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  • AutorenbildMichael Mutter

Full face snorkel masks - too risky?

Full face snorkel masks (FFSM) are becoming increasingly popular, especially among inexperienced "snorkeling tourists", because they promise safety and comfort. ("Goodbye water in your snorkel, goodbye difficult breathing!" is how one Swiss online store advertises them). Although they are a simpler alternative to the traditional combination of mask and snorkel, there are concerns about the risk of CO2 rebreathing from the exhaled air. Several snorkeling accidents, including fatal ones, associated with the use of FFSMs have been published, and potential CO2 accumulation in the mask is a possible factor in these fatal incidents.

Image template: Karin Aggeler

How an FFSM works

source: ebay

In an FFSM, the mouth-nose (oronasal) compartment is sealed off from the rest of the face, as in full-face masks for divers or in diving helmets, in order to limit the dead space. (Dead space is the part of the respiratory tract that does not participate in gas exchange). When breathing through an FFSM, inhaled air is sucked through the snorkel, through the eye pocket and then through one-way valves into the oronasal compartment. When exhaling, the air is passed through another one-way valve and expelled upwards through the snorkel via a separate exhalation channel.

If the mask works as intended, the airflow only flows in one direction without mixing inhaled and exhaled air.

The problem of CO2 rebreathing

A normal snorkel has a dead space of around 160 ml. The anatomical dead space consists of the trachea and bronchi and is approx. 150 ml in adults. The dead space in the oronasal compartment of an FFSM is around 250 ml, but can increase to over 1400 ml depending on the mask brand if the seals or valves are not working properly. Because the inhaled air is no longer able to sufficiently ventilate a dead space that greatly enlarges the anatomical space, there is a risk of rebreathing the exhaled air in this situation, which can lead to hypercapnia (excess CO2) or hypoxia (lack of oxygen). Hypercapnia can cause dizziness, shortness of breath, headaches and unconsciousness, while hypoxia can cause confusion and eventually unconsciousness.

Comparison with conventional snorkeling equipment

A recent study investigated the effects of wearing an FFSM compared to conventional snorkeling equipment at rest and during light and moderate exertion. During exercise, an increase in CO2 in the exhaled air to a clinically relevant level (>7 kPa) occurred approximately twice as often (40-45% vs. 20%) when using an FFSM ad with a conventional snorkel. Almost half of the participants showed a drop in oxygen saturation to below 95% under the FFSM, a quarter even to below 90% (with a conventional snorkel only 25% and 5% respectively). The partial pressures of CO2 and O2 in the oronasal compartment were higher (CO2) and lower (O2) in all FFSMs than in the inhaled air, which proves that CO2 is rebreathed in an FFSM.

CO2 rebreathing is common under FFSM

These findings are remarkable, especially when considering that the study was conducted on land and that the work of breathing increases in the water, even when only snorkeling, and therefore no more favourable values are to be expected in the real world setting. But even with a normal snorkel, hypercapnia and peripheral oxygen deficiency can occur, as this study clearly shows. This should not be forgotten either.

If the potential increases in dead space of these masks reported in the study are taken into account, it is almost irony that they are often used by untrained people on the assumption that they are safer than conventional snorkeling equipment.


It would be going too far to dismiss the right of FFSMs to exist. However, anyone using an FFSM would do well to ensure that the oronasal compartment is tightly sealed and that the one-way valves function properly. This applies in particular to people with a small lung volume, especially children, as they are particularly at risk of insufficient ventilation of an FFSM with a leaky oronasal compartment due to their small breathing volume.

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