There is a common misconception out there that the more you breathe, the better. The more oxygenated we are, the better... right? Wrong. At a basic level we need food, water, and air. We know too much food is bad for us. We also know too much water is bad for us. So it only makes sense that too much air is bad for us. Let's look at why.
Carbon dioxide is not simply a waste product. We require a certain amount of CO2 in our bloodstream (about 6%) for proper oxygen transfer. Oxygen is transported in the blood by hemoglobin molecules. They deliver oxygen to all the tissue cells. However, hemoglobin molecules cannot release their oxygen unless carbon dioxide levels are sufficient. Over-breathing depletes carbon dioxide levels creating the paradoxical situation where we have all this oxygen in the blood, but we can't use it.
Mouth breathers typically suffer from over-breathing. When not engaged in intense activity we are meant to breathe through our nose. The nose contains mucous membranes that infuse incoming air with nitric oxide which promotes vasodilation of the blood vessels in the lungs. The nitric oxide also has a sterilizing effect on the air. Breathing through the nose engages the diaphragm which draws air into the deepest part of the lungs where there is greater capacity. The result is a more efficient exchange system that naturally promotes a slower breathing pattern.
Check in with yourself throughout the day to see if you're breathing with your nose - not your mouth. Spend a couple minutes a day making a conscious effort to slow your breathing. Meditation is great for this. Being a more efficient breather at rest will directly translate into better performance during exercise when oxygen transfer is crucial.