DAN Europe published a highly relevant and interesting article regarding Full Face Snorkel Masks (FFSM), since accidents involving FFSM around the world have led to several posts and articles suggesting that FFSM are dangerous to use - is this really the case?
Back to basics
The air we breathe contains about 21% oxygen (O2) and up to around 0,04% carbon dioxide (CO2). It is the CO2 that is primary responsible for our breathing rate & depth, and not O2. CO2 is therefore a very important component of the air in our lungs and increased CO2 levels could lead to unconsciousness which, if happening while in the water, can result in drowning.
O2 is consumed - metabolized - and CO2 is produced by our body, resulting in an increased level of CO2, around 4%, and a decreased O2 level, around 16%, in the air we exhale. As we breathe out, we do not empty our airways completely, since a small amount of air enriched with CO2 remains in the airways. This volume of a breath that does not participate in gas exchange is called the dead space. Therefore, when we inhale again, we are actually breathing in a mix of new/fresh air and CO2 enriched air.
When breathing through a snorkel we are increasing the volume of the dead space (or the length of our airways) Should this total volume become too big, we would end up with a high concentration of CO2 in the inhaled air, leading to the risk as described earlier. This is also the reason why snorkels are limited in length and diameter. If we now add the inner volume of the mask to this, we could easily double the volume of dead space and this can lead to a dangerous increase in CO2 levels.
The Facts
To reduce the dead space, a FFSM should have an internal orinasal pocket, which seals the mouth and nose from the rest of the mask. If this orinasal pocket is not present or does not assure a good seal, then the internal volume could become too big, leading to increased CO2 levels.
But, even if the internal pocket has a volume of 200ml and the snorkel has the same volume (200ml), the dead space is doubled and could have CO2 levels that are unacceptable. To avoid this, the mask should have a one-way breathing circulation, which means that the snorkel is divided in into an inhalation and an exhalation part (with one-way valves that prevent exhaled air from mixing with inhaled air).
When breathing in you would thus only inhale “fresh air”, while the exhaled air is removed from the mask through a separate channel. This is exactly where the problem lies with some of these masks, since some companies copied the original mask, but did not take all these technical issues into account.
Conclusion
FFSMs are safe to use if the mask design is correct. If not, it could lead to potential fatalities. It is however wrong to state that all FFSM are dangerous or that all of them are safe. So be aware if using a FFSM, or just play it safe and use a (low-volume/freediving) mask with separate snorkel....!