The question about how safe one is by using full face masks emerged after news hit the media concerning full mask drowning in a place called Hawaii. The response is that full face snorkel masks are dangerous. We breathe in oxygen and exhale carbon dioxide. When you are actively engaged in an activity, your body demands more oxygen and increases the need for faster and shallower breaths. In case this happens in an enclosed space as it is the case with a full face mask you wear on your head, you are not in a position to push full face CO2 buildup from the snorkel tube. This causes you to start breathing in the bad air with high contents of carbon II oxide. Breathing in air that is full of carbon II oxide causes you to experience headaches, unconsciousness, and dizziness.
This may sound scaring but you should not worry. If the snorkel equipment has been designed well and tested, it can be safe for use. It is therefore always recommendable that you buy snorkel gear of good quality from popular and trusted brands. Luckily enough, producers of full-face snorkel masks are taking this into account to help them come up with masks that are designed accordingly. They have separated breathing part from that of looking with the breathing part sealing the mouth and the nose of the viewing part. On the top of the breathing part, there are little valves which only allow fresh air to enter. If the design is right with silicone sealing perfectly, carbon II oxide is pushed out of the snorkel through full face safe mask's side channels.
Full face snorkel masks came to the snorkel gear market some few years ago but they are among the best selling snorkel masks. The market changes faster and companies heed to customer needs in coming up with continuous product upgrades. Initially, Easybreath full mask was made by Decathlon but the latest version called the Tribord Easybreath Full Face Snorkel Mask is under Subea brand. The main aim of this innovation was to revolutionize snorkeling through developing the first full face snorkeling mask in the world to enable you to see and breathe easily while in water as it is on land. Easybreath full face mask became the first full face snorkel mask whose design was patented. Decathlon Easybreath mask underwent many alterations, simplification and upgrade. Of late, the name has even been changed with the whole Decathlon underwater product variety running under Subea.
Read more here: https://en.wikipedia.org/wiki/Snorkeling.