Full face snorkel mask had been well recommended for snorkeling and the best thing about the use of this mask is that it allows you to breathe both from your nose and mouth underwater so you need not have separate pieces for it.
These types of snorkel are less intrusive, more hygienic, and you will be more relaxed with one of these on, rather than constantly clenching your mouth, and they are a bit less constrictive for people with respiratory issues.
The concerns that are raised with the use of full face snorkel mask are due to the misuse of such equipment when it is supposed to be used according to its intended purpose such as recreational use or just floating on the surface and comfortably looking down onto the fishies.
One of the issues that also raised concern with the use of full face snorkel mask is the CO2 buildup from breathing, but then the reputable manufacturers of these masks have already made innovative designs on it in order to address the issue and to make the equipment more efficient and reliable underwater that does not compromise safety.
So when you choose a full face snorkel mask, remember that it is intended for relaxed snorkeling only and not made for freediving or extensive open water swimming and to get the best quality you need to try on the mask and breath in it on land, and make sure only the breathing part should fog up.
Also one of the most important factors is to ensure that the mask fits well about your face and that its breathing part also fits neatly over your mouth and nose, because if this is not looked into properly, then you will have issues with the use of the mask.
The quality of the full face snorkel mask must be the top one and you have to be very wary of those that are out in the market, even on known online websites that are cheaper/knockoff models and imitations that have been the most cause of many safety-compromising issues about full face snorkel mask.
Nothing will beat an informed purchaser, and it is more advisable that you research about the product that you will be purchasing especially from where it came from and manufactured, and all the same get as much info about its features so you can have all that you need to know, above all never compromise the quality over the price as it will be more convenient to buy an expensive item where you are assured of quality and its safety hen you use it according to its purpose.