Clear, white and green: why snot changes colour

Mucous and phlegm contain cells and chemical compounds that help us power through a cold.

Ever wondered why our mucous changes colour and consistency when we’re sick?
The video above by the American Chemical Society’s Reactions crew explains the science.

Yellow and white mucous forms when we’re congested – they’re thickened by our immune cells. If it’s green, your body is probably trying to fight off a viral infection. The green hue is due to the chemical myeloperoxidase, which helps produce powerful immune cells.

Traces of red, due to blood, is normal – but a sign that we should stop blowing or picking at our nose.

A runny nose when we’re sick is likely due to histamine, which increases blood flow to mucous-producing tissues to help fight off germs.

Some runny noses have nothing to do with being sick. In cold weather, a runny nose can form when cold air condenses in our warm nostrils.

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Vishnu Varma R Vejayan is a physics student from Queen Mary University of London with an interest in scientific writing and research in physics. He interned at Cosmos in early 2017.
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