The so-called pendulum breathing occurs when you use a snorkel that is too long. Then a part of the air you breathed out remains in the snorkel and you breathe it in again (pendulum air). This can present an earnest health hazard. Because the content of carbon dioxide in your snorkel increases more and more, until it reaches a critical value. Then you can lose consciousness.
How long should a snorkel be in order to aviod Pendulum Breathing?
Some diving schools indicate up to 50 cm as ideal length. However, due to the danger of pendulum air, the snorkel should only be about 30 – 35 cm long.
Find the best Dive Professionals, Dive Centers and Liveaboards on rateyourdive.com