Why does your nose start to run when you come indoors?

If you're on the Northern Hemisphere, chances are it's cold outside.

Before you leave your house, you may need to put on a scarf and hat. After walking outside in the cool air for a while, you come back indoors and it happens - your nose starts to run.

But why does this happen?

The nose is like a gatekeeper

"All the tiny blood vessels in our noses constrict when it's cold," Dr. Wenche Moe Thorstensen says.

She is a senior physician at the Ear-Nose-Throat Clinic at St. Olavs Hospital and associate professor at the Norwegian University of Science and Technology (NTNU) in Trondheim.

When you come back into the warm air, the blood vessels expand again. You have cells in your nose that detect whether it's hot or cold.

"Your nose warms up the air you inhale. Simultaneously, your nose makes sure that the air is clean and moist enough before it enters your lungs," Thorstensen says.

The cells in your nose communicate with each other

The cells that detect that it's getting cold or wet communicate with another type of cell.

Because you also have many cells in your nose that produce mucus.

When it gets colder, the cells produce more mucus to protect the nose from the cold.

"When you come in from cold weather, signals are sent to these mucus cells to produce thin, runny mucus," Thorstensen says.

The mucus cells suddenly start producing even more mucus. It is transparent and very thin, almost like water.

This mucus is different from the snot you produce when you're sick.

Not everyone experiences this

Most people experience this, but not everyone.

Thorstensen explains that it's like being cold: Some people are prone to feeling cold, while others aren't as bothered by it.

This also applies to the amounts of mucus your nose produces.

"For example, older people have slightly different mucus membranes than when they were children and adults. The mucus they produce is often even thinner. You may have seen older men walking around with a transparent drop hanging from the tip of their nose," says Thorstensen.

Your nose cleans itself – and that's completely normal

"For the vast majority of people, the cells in their nose produce thinner mucus when it's cold. It has to do with how your body is made," Thorstensen says.

So treatment is only necessary in certain cases.

"Those who receive treatment often have other problems as well. For some, it can be quite annoying," she says.

For most of us, though, making sure we have a tissue in our pocket is enough.