Who knows when "mouth-breather" became an insult synonymous with "dork" or "dweeb," like a character from "Napoleon Dynamite." But listen to the heavy breathers that are likely surrounding you: It's January, when more of us are prone to the sniffles, which means more of us have stuffy noses, which means -- we've entered the month of the mouth-breather.

But the term isn't just a barb that tracksuited TV villain Sue Sylvester loves to hurl like a slushie at the "Glee" kids; experts say that breathing while slack-jawed can actually lead to some surprisingly unpleasant health problems. If you're fighting a cold right now, you know your inability to suck any air through your congested nostrils can cause dry mouth, dragon breath and lack of sleep, for starters. You'll be fine when you kick the crud; the big problems start when you've become a chronic mouth-breather -- children and adults with allergies, for example.

As Dr. Yosh Jefferson, a New Jersey functional orthodontist, explains, "Mouth-breathing also irritates the tonsils and adenoids, so you have a double whammy where the sinuses are congested, which causes further blockage of the upper airway." Now you really can't breathe out of that nose. What's more, when you take in oxygen through your nose, it passes over the mucous membrane and into the sinuses, which produces nitric oxide, which your body needs for all the smooth muscles, like your heart and your blood vessels. So when you're not breathing through your nose, your blood actually isn't getting all the oxygen it needs to function properly.

Jefferson believes breathing though the mouth is often an overlooked root cause of many health and behavioral problems, particularly in school-age kids. ("Just think of the child," he says. "How do you think they’re doing in school? These kids are tired, they’re irritable, they can’t concentrate in school. And a lot of these kids (may be) diagnosed with ADD and hyperactivity.")

But here's the absolute weirdest thing that mouth-breathing can cause: It can actually change the shape of kids' faces, according to a report Jefferson published last year in the journal General Dentistry. "Severe mouth breathers develop what they call long face syndrome -- long, narrow faces, very unattractive facial features. Also if their tonsils are swollen, they sometimes position their jaw in weird ways in order to get more oxygen into their bodies. It can happen in adults as well ... but it’s more prominent in children," Jefferson says. "People think they grew to this face because of genetics –- it’s not, it’s because they're mouth-breathers." It's reversible in children if it's caught early -- an orthodontist might use a device to expand the jaw, which will widen the mouth and open the sinuses, helping the child breathe through the nose again. (This can be done in adults, too, but it's more difficult.)

"It's best to treat them early," Jefferson says. "It drives me crazy that there are so many kids who are mouth breathers and no one is doing anything about it."

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