By TIERNEY PLUMB

Fredericksburg resident Denise Culberson thinks she knows why she's gotten the flu only once in the past 10 years--she's a frequent hand-washer.

She's less conscientious about dressing for winter weather. The upstate New York native walks her dog in 45-degree weather, wearing no socks and a sheer jacket despite the popular belief that cold air can make you sick.

"I don't dress properly," said Culberson, a former nurse anesthetist. "But I'm a nut about washing my hands."

And that's more important than bundling up.

The cold, hard fact is this: Cold air alone can't make people sick.

But when cold air is combined with an existing virus, it can exacerbate the sickness, said Dr. Peter Smith of Allergy & Asthma Associates of Fredericksburg.

"If there is any barometric change, it can result in quite significant changes in both the upper and lower airways," Smith said.

Problems linked to cold air include congestion, swelling in sinus and nasal areas, and worsening of asthma.

Cold air changes the consistency of the mucous tract lining the respiratory system. That tract traps harmful particles before they can reach the lungs, and it moves them out of the system. But exposure to cold air causes the mucus to thicken, meaning inhaled particles exit less easily. Hence, a cough.

In addition, when warm blood rises to heat the cool air, tissues lining the nose swell as capillaries dilate. This causes nasal congestion from an increase in blood.

And winter's low humidity dries out the lining of the nasal passages, making them more susceptible to infection.

So it's not a stretch for people to think their ills have something to do with cold air.

But it's the interaction indoors--not outdoors--that actually makes us sick.

"During the holiday season, when we go to more parties and there's close physical contact, there is more spread of viral infections, which triggers sinuses and bronchitis," Smith said.

Claudia Emerson, associate professor of English at the University of Mary Washington, says the bronchitis she contracted last winter was due to close proximity to students in her classroom--not because of her daily outdoor run.

"Students seem to come back from Thanksgiving break with viruses," Emerson said.