I just started the breast stroke in a swimming class. I have heard something like, "It is bad to breathe in using your mouth because there are greater chances of inhaling things that go directly inside lungs. In the case of nostrils, it automatically filters the air we inhale in." Even elders advise us to inhale air through nose.
Today my swim coach said to me that while your head comes out of water in breast stroke, you should not use your nose alone to inhale but to use mouth to inhale air.
Of course I am following it while swimming, but why is there a contradiction between what I have heard?