Are full-face snorkel masks actually safe?

This is the single biggest dispute in our source mix. Specialist subreddits including r/snorkeling and r/VisitingHawaii repeatedly flag CO2 buildup risk and note that several Hawaii tour operators have banned them; manufacturer and mainstream review coverage tends to downplay these concerns. If you do choose a full-face mask, stick to a reputable brand with a documented dry-top and separate inhale/exhale channels, use it only for calm surface snorkeling, and never for freediving.