Look at how snobby your lungs look! I don't think we can be friends anymore.
First off, breathing through your nose is much actually healthier if you consider the contaminants in the air. One very special area that your nose has and your mouth doesn't, is the nostrils/sinuses. Through the use of the nostril hair, mucus, and nitric oxide, foreign objects and bacteria are mainly prevented from entering the body. What this means is that if you happen to inhale at the same time a large mosquito flies across your face, there's a smaller chance that it'll follow down your respiratory system. (The hairs in your nostril will likely prevent it from entering) In a manner of speaking, you can think of your nostrils as life's natural gas masks.
However, if your town becomes the target of a biological cluster-bomb, you may want to put one of these on.