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.
With the raising concerns about full face snorkel mask, there is a need to ensure that such mask is used accordingly as to its purpose or as to what it is intended for, that is for recreational use or just floating on the surface and comfortably looking down onto the fishies.
The very common concern with full face snorkel mask is the CO2 build up, but the manufacturers now have already addressed this concern and are able to produce innovative solutions to make this equipment more reliable underwater with all its safety.
So then, when you choose to purchase full face snorkel mask be sure to try on the mask and breath in it on land, and make sure only the breathing part should fog up when you try to breathe on it, and bear in mind the purpose of buying it that it is intended for relaxed snorkeling only and not made for freediving or extensive open water swimming.
One important thing that you will also remember when purchasing a full face snorkel mask in order for you not to encounter any issues or problems when using it, is to make sure that it fits well about your face and that its breathing part also fits neatly over your mouth and nose.
You should never forget to always go for the best quality and brand of a full face snorkel mask and never settle for cheaper/knockoff models and imitations that are widely available in the market as that kind of brands have the most safety-compromising result.
It would be preferable that you make a good research and gather the most detailed info about the features of the mask, where it came from or manufactured and do not settle quality over the price, it would even be more advisable to purchase those expensive types especially if you are going to use this often as you can guarantee about its safety too.