What causes a runny nose?

A runny nose may not cause any physical discomfort, except for the rawness the results from blowing your nose, but it certainly is one of the most annoying cold symptoms. Even so, it's an important infection-fighting mechanism. Mucus produced by the nose fights infection, humidifies air to moisten the nasal membranes and traps foreign matter so it's not inhaled into the lungs. In fact, the nose produces between one and two quarts of mucus a day, which we normally swallow unconsciously.

Certain factors can cause the nose to go into mucus production overdrive, however, causing a runny nose. When you have a cold or the flu, your nose produces excess mucus to fight the germs and wash out the nasal passages. The same thing happens if you have allergies because your nose treats the allergens like germs. If you live in a dry climate or the air in your home gets especially dry in the winter, your nose again reacts by producing more mucus. And you may have noticed that your nose runs more when you're exposed to cold temperatures. The air in your nose is warmer than the outside air and a steam-like affect happens inside your nose. Water drops condense, mix with mucus and run out your nose.

Increased nasal secretions may be thin or thick; they may be clear, yellow or green. It's normal for mucus to change in consistency and color as it moves through the process of fighting infection. Most of the time a runny nose caused by cold or flu doesn't need to be treated and should be allowed to run its course. If you're uncomfortable, however, a cool mist vaporizer and over-the-counter decongestants can help. Occasionally, thick and green mucus can signal the development of a bacterial sinus infection, which should be treated with antibiotics prescribed by your doctor.

Resources:

The Kids Health Web site has even more info on runny noses.

The American Academy of Otolaryngology provides articles dealing with the nose and its idiosyncrasies.