How the Nose Works
The nose on your face is just the tip of an iceberg. The important functions--breathing and smelling--actually happen mostly inside your head, in your nasal cavity. The air inhaled through your nostrils travels to this cavity on its way to the lungs.
The nasal cavity cleans and conditions the air. Pollutants, viruses and bacteria are trapped in mucus, which then travels to the stomach, where stomach acid neutralizes the germs.
Three structures on the sides of the cavity, called the turbinates, humidify the air. By the time air reaches the lungs, it's almost completely saturated with water vapor.
The water vapor is essential for proper function of the lungs. It's also crucial that air reaching the lungs be neither too cold nor too hot. In winter, the membranes in the nasal cavity raise the temperature of the air you breathe so your lungs never feel the bitter cold.
While scientists understand the nose and breathing, its ability to smell remains somewhat mysterious.
At the rear of the nasal cavity, behind and just above the bridge of the nose, is the olfactory epithelium--the "smelling skin" made of cells that react to chemicals.
When a molecule of a substance hits one of these receptor cells, a nerve impulse is sent to the brain. The impulse--not the smell--travels through the brain for decoding, crossing the area concerned with memory, emotion and motivated behavior. That's why the smell of popcorn may also trigger memories of a favorite movie.
As long as your nose is working, the best advice is to leave well enough alone. But if you have a lot of nosebleeds and allergies or if you suddenly can't smell, you should consult a doctor.