A cursory Google search will bring up several fear-inducing articles that discuss the dangers of full-face snorkels, and they all relate to CO2. If a full face mask doesn’t fit well, forming a tight seal around the nose and mouth with exit vents for exhalation, you risk breathing potentially dangerous levels of CO2 back into the mask, which you could then inhale with your next breath. Inhaling CO2 can lead to dizziness and even losing consciousness, making poorly made or inappropriately used full-face masks dangerous.   Luckily, there are a few things you can do to make sure your mask is safe. Don’t buy the cheapest mask you can find; when it comes to your safety, it’s worth spending a few extra dollars to ensure the model you use was made thoughtfully and follows regulations and guidelines. Try on the mask before jumping into the water, making sure the seals are tight to your face. Stay on the surface, don’t dive deep underwater, and try to avoid submerging the mask at all. Don’t engage in strenuous swimming while wearing your mask: leisurely drift, or float through the water.