Now that we as a global human race are experiencing an airborne viral-pandemic, it is more important than ever to breathe through your nose. Yoga has been teaching us this for millennia. Basic human physiology 101 also teaches us that your nose is on your face to breathe through. If humans were meant to breathe through their mouths, then our noses would not be on our faces. Yes, we smell with our noses, but if you are busying yourself breathing or sighing through your mouth, then you aren't smelling much of anything, anyway. Your nose hair filters large particles and has cilia and mucous to filter and trap allergens and dust. Your nose warms the incoming air in order to prepare it for the lungs. So breathe through it - both in and out.
During a pandemic, if you are mouth-breathing, you are spreading your germs directly into another person's space via aerosol spray in the form of spittle, so pull your mask up over your nose. This happens not only with mouth-breathing, but with talking, singing, yelling, and laughing. All habits take a minimum of two weeks to form, so let's all close our mouths and breathe through our noses for the sake of others.
For your own sake, breathing through the nose also releases nitric oxide into your blood. (Nitrous Oxide is that gas you breathe at the dentist's office when you have odontophobia, or fear of dentists). This is desirable, as it increases carbon dioxide in the blood, which in turn is what releases oxygen. If you have stairs to climb, try climbing them while breathing through your nose. Find a comfortable rhythm that allows you to do so without feeling like you need to open your mouth to breathe. This will gauge your own personal-pace of stair climbing, allowing you to feed your blood that wonderful, life-enriching oxygen.