Why is snot green?

We have had a recent bout of coughs and colds in this house. You know how it is once you become parents… there is suddenly no subject too gory or vile for conversation. Serious discussions about nappy content, vomit content and the like are par for the course in rearing these little organic beings. Hubby and I had one of these conversation in bed the other night…. yes, 13 years together and romance is obviously still very strong between us!

The chat started out about what level of a cold each child had and ended with himself asking…

“So, why is snot green then!”

Under normal, healthy circumstances we produce mucus in the lining of our nose, sinuses, mouth, airways, lungs and digestive tract.  This mucus is deliberately made by the body and acts to lubricate the airways and also to trap any invading viruses or bacteria, or any dust or other particles. Mucus is made by the cells lining the airways. Our bodies typically make about one to one and a half litres of mucus a day.

Most of the time the mucus in our airways is fairly clear and stringy. However, when the airways come under attack by infection the body sends in its army which includes cells called nutrophils. These nutrophils attack the infecting bacteria or viruses, killing them with their chemical warfare.  They secrete an enzyme that contains iron and it is the iron that gives the mucus its wonderful array of colours from yellow, orange brown and the most common… green!

photo credit: anna gutermuth via photopin cc


Science blogger and writer; Owner of Dr. How's Science Wows; Mother of three junior scientists who have taught me that to be a great scientist you need to look at life through the eyes of a child!

2 thoughts on “Why is snot green?

  • December 5, 2013 at 9:41 am

    We have had a lot of ‘snot’ conversations this week in our house of the common cold…. I can’t believe I didn’t get asked that question, I’ll be ready for it though because it will happen!

    • December 5, 2013 at 11:33 am

      Yes I was surprised I hadn’t been asked it before too Joanna, at least now you are well ready for the question when it does arise ;0)

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