Runny Nose

On an average day, a typical person’s nose will produce about one quart of mucus/fluid (just under one liter). Most all of this snot generally gets passed back into your throat and swallowed, often without you even really being too conscious of it. When you’re breathing cold air though, the rate of mucus production goes up significantly, causing some of that snot to come out the front of your nose, rather than back in your throat.

What’s going on here is the blood supply to your nose actually increases as a response to the cold air, via tiny blood vessels in your nose dilating to increase the blood flow. This helps keep your nose warm as you breathe, as well as begins to warm the cold air you’re breathing before it enters your lungs.

This increased blood flow doesn’t just help warm the air though, it also has a side effect of providing a lot more blood than normal to the glands which produce the mucus in your nose. This, in turn, causes them to start producing snot at a much higher rate than normal, which causes your nose to run when you’re breathing the cold air.

Once you’re back in a warm air environment, the blood vessels in your nose will constrict and the glands that produce the mucus/liquid mix will go back to their normal rate of around four cups of snot per day.

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