Why does my nose run when I get cold?

Question

It seems like my nose always starts leaking when I'm in a cold room or outside in winter. What biological purpose does a runny nose serve?

Answer 1

The biological purpose that a runny nose serves is that it creates a sticky trap for germs, dust, pollen etc. and tries to stop such foreign bodies from entering your lungs. Instead, a lot of the mucous runs out of your nose or down your throat instead. It's annoying, but it's meant to keep us from catching viruses.
When you are surrounded by cold air, the air you are breathing in condenses in your nose (since the temperature in your sinuses is around a constant 98F/38C so that the air going to your lungs is warm. The condensed water mixes with the mucous in your nose and will cause your nose to run, especially if, like me, you are predisposed to produce more mucous that average (due to allergies, bronchitis, sinusitis etc.) For such chronic problems, the solution is not medication, but instead to improve one's immune system (healthy diet, regular cardio exercise, sufficient sleep, clean water, clean air, lower stress environment) so that your immune system is less reactive/defensive. When the immune system is stronger, it will produce less excess phlegm.
Many medications for sinus conditions tend to make phlegm thicker (greener) and therefore, harder to remove from the body than clear, thin mucous. If you find yourself getting "stuffier" with medication, you are probably better off going a more natural route in the long term.
Hope this helps.

Answer 2

your immune system is trying to kick out all the germs in your body. That's why it's bad for you to swallow your snot.