Originally Posted by MCW

Question 1: How come your nose runs when you go out in the cold? Is this how you catch a cold?

Dr. Mitchell answers:
It's been cold enough outside that I, too, am grabbing for the tissues when I first come in. I asked Thomas Kidder, MD, exactly why this happens. Dr. Kidder, Associate Professor and Chief of the Division of Adult and Pediatric General Otolaryngology at the Medical College of Wisconsin, started his explanation with a discussion of the normal functions of the nose.

While the obvious job of the nose is the sense of smell, the nose's other big job is to ready the inhaled air for the throat, voice box and lungs. The nose warms up and humidifies the air, and it filters out dust, germs, smoke and other particulate matter. Inhaled air picks up heat and moisture as it contacts the inside of the nose, which has grooves and ridges to make a large surface area.

The lining of the nose has glands that secrete water and mucus and, just under the surface, there are hundreds of yards of blood vessels that supply a constant source of heat. (You can see why it's easy to get a bloody nose.)

Under ordinary circumstances, the nose and sinuses produce as much as four cups of mucus every 24 hours. This mucus is constantly being swept back into the throat and subsequently swallowed. On a very cold day, when both the temperature and relative humidity are low, the nasal blood vessels dilate so more blood reaches the nose and thus its mucus and heat machine. Because more liquid is being produced in the nose than can be swept back into the throat, the nose "runs."

This normal function of the nose is not how you catch a cold. A cold is an infection of the lining of the nose, sinuses and throat, caused by a virus that managed to penetrate the body's defenses. The cold virus also causes an expansion of blood vessels and so you get the same symptoms of congestion and runny nose.