The small hairs inside the nose, called cilia, as well as nasal mucus carry out this cleaning process. The nose not only acts as nature's vacuum cleaner, but it is also a humidifier and temperature regulator. Think about when you go outside on a very cold day. If that sub-zero air got through to your lungs, it would cause serious damage. Your nose and respiratory system warm the air to body temperature, protecting the sensitive lining of the lungs and bronchial tubes.
Most people breathe through their nose when they can.
When your nose is blocked, you have to switch to breathing through your mouth. This is because the nose humidifies the air you breathe, keeping your throat from getting too dry. Sometimes people snore because they are unable to breathe through their nose. Newborns breathe exclusively through their noses. This becomes important during eating, when the mouth is full.