This is very harmful.

We should always use great care to sit erect and to draw the shoulders well back.

Then, if we take pains to fill the lungs well a great many times every day, we shall form the habit of expanding the lungs, and shall breathe deeper, even when we are not thinking about doing so.
~11.

Breathing through the Nose.~--In breathing, we should always take care to draw the air in through the nose, and not through the mouth.

The nose acts as a strainer, to remove particles of dust which might do harm if allowed to enter the lungs.