Why does my nose run in the cold?

Nobody knows for sure, but one reason is that the nose has to "work overtime" when the inspired air is cold, said Dr. Ralph Metson , a sinus surgeon at the Massachusetts Eye and Ear Infirmary.

The nose is a kind of "fancy air conditioner" whose job is to warm and humidify the air we inhale, Metson wrote in an e-mail. When the air is unusually cold, the nose kicks into high gear to warm and humidify it. Blood vessels dilate, mucosal tissue swells, and glands secrete extra mucus. This extra mucus results in a runny nose.

In addition, as the newly heated, newly moist air is exhaled, the moisture in it condenses when it hits the cooler outside temperature and then drips out as fluid, said Dr. Andrew Lane, director of the division of rhinology at the Johns Hopkins University School of Medicine.

For people who have a major problem with this kind of runny nose, technically called "vasomotor rhinitis," there is a prescription nasal spray called ipratropium bromide that might damp down this reaction.

JUDY FOREMAN

E-mail health questions to Foreman@globe.com.