Hi Helen. The Pursed Lip Breathing pattern establishes "Smell the flowers, Blow out the candles" for reason. Breathing through the nose triggers sensors to the brain that everything is Okay. Exhaling (more forcefully) from the chest muscles help expel the old air more completely. IF one is under "Flare-up" conditions of a lung infection, it is very difficult to complete the inhale/exhale cycles effectively with mouth breathing only. I noticed this pattern for myself after becoming a non-smoker recently. It would behoove you to begin breaking the mouth-breathing pattern - sooner than later.
be Well.

It doesn't really matter. Pursed Lip Breathing works whether you are a mouth breather when you exercise or not. Breathing through the nose offers some protection from "outside stuff" but that's about all it provides. Many of us with COPD are mouth breathers during exercise.....I am!

Thank you for the answer. I’m normally a in through the nose and out through the mouth. I do feel I can expel more air this way