There are many reasons we get a runny nose, technically known as rhinorrhea. When you have a cold or the flu, the mucous membranes that line the cavities in your nose produce a combination of mucus and fluid designed to fight off and wash away the germs.
About 50-90% of people get a runny nose when it’s cold. We call this “cold-induced rhinitis,” or “skier nose.” People with asthma, eczema and hay fever seem to experience it more.
It’s the job of your nose to make the air you breathe in warm and wet so that when it gets to your lungs it does not irritate the cells.
Cold, dry air stimulates the nerves inside your nose, which send a message through your nerves to your brain. Your brain then responds to this impulse by increasing the blood flow to the nose and these dilated blood vessels warm the air passing over them.
Secondly, the nose is triggered to produce more secretions via the mucous glands in order to provide the moisture to humidify the air coming through.
The cold, dry air also stimulates cells of your immune system (called “mast cells”) in your nose. These cells trigger the production of more liquid in your nose to make the air more moist. It is estimated you can lose up to 300-400 mL of fluid daily through your nose as it performs this function.
So when you inhale cold, dry air, the moist tissue inside the nose automatically increases fluid production to do its job of protecting sensitive lung tissue. But when there’s too much fluid, the excess tends to drip out, creating a runny nose.
Winter has other effects that make it more likely you’ll have a runny nose. Cold temperatures can cause the small water droplets inside the moist nose to join together, forming big, heavy drops of water that can also drip from your nostrils. And cold air also speeds up mucus production.
While a runny nose is annoying in just about any season, it’s not harmful. And if you’re worried that you’ll get a cold from being out in the cold, don’t fret.