Ask Smithsonian: Why Does My Nose Run When It’s Cold?

The nose knows that runny flows are necessary in the cold

The nose is more than just a cosmetic appendage. It is responsible for filtering out the bad stuff that could make us sick and in the face of cold, dry air, ensuring that the lungs stay moist and supple.

When the temperature dips—say below 40 degrees—and the air becomes crackly with static, the nose begins working overtime to add humidity. The reaction—called rhinorrhea—can be almost instantaneous. Step outside and a river will start to flow.

“The compensatory action of the nose is to produce more mucous and more liquid to keep the nasal tissue hydrated and moist,” says Mitchell H. Grayson, a pediatric allergist and immunologist at the Medical College of Wisconsin in Milwaukee.

It’s essentially a nervous system response, mediated by chemical neurotransmitters that control the glands that produce mucous, says Grayson. It’s the same system that causes the mouth to water in anticipation of a meal. 

The second cause of a cold-air-induced runny nose is simple condensation. The air inside the nose is kept at about 96 degrees F, says Grayson. When that warm air runs smack up against cold air, the reaction is moisture. That condensation—essentially water—joins with the mucous and adds to the outward flow.

It may be prodigious, but it will be generally a thin flow. “It’s not the stuff you see when you have a cold,” says Grayson. The fluid has to keep nasal tissue, and hence, the lungs moist, and “thick goopy mucous isn’t going to do that,” because that will merely just stop up the lung tissue, he says.

Heating the air and mucous production are among the nose’s primary activities. Usually, only one nostril is wide open at any given time, with the closed side doing the work. Every four hours or so, the two nostrils switch, with the closed side opening and the open side closing. The warm, humid air is sent to the lungs. Without moisture, the lungs will constrict and become damaged, says Grayson.

Mouth breathers pay the price of bypassing the nose—a sore throat brought on by the dry air.

Breathing through a scarf or bandana in cold air will help the nose do its job—and cut down on the outward nasal flow.

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