Snot is one of the mundane realities of life you probably don't think much about, until you or someone you know gets sick.
- Green snot is most often caused by a viral common cold, which antibiotics cannot treat
- Healthy snot (mucus) is made from water, proteins called mucins and salt
- It becomes green after gathering dead viruses or bacteria and white blood cells, which oxide and change colour with time
Then suddenly it is everywhere — often in oozing, vibrant green abundance.
But does green or yellow snot mean you need antibiotics? Probably not, according to the experts.
Research has found that doctors are more likely to prescribe antibiotics to patients with green or yellow nasal discharge.
But the same study showed they were often doing so unnecessarily.
Dr Michael Tam, from the UNSW School of Public Health and Community Medicine, said colourful snot was not a good reason to go rushing to the family doctor.
"The presence of green snot … does not indicate that you need antibiotics," Dr Tam said. "Green nasal discharge is most commonly due to a viral infection of the nasal mucosa — basically, the common cold."
Antibiotics will not help treat a viral illness. So if your snot turns green as the result of a common cold (which is caused by a virus) there's no point taking them, Dr Tam said.
The longer it hangs around, the more discoloured it might get.
Dr David King
He said green or yellow nasal discharge could be caused by a bacterial infection, but even then, unless the infection is severe, you are better off without antibiotics.
Using antibiotics when you don't need them can contribute to antibiotic resistance in the microorganisms in your own body and within the broader community.
Dr Tam said it was important to remember antibiotics could also have unpleasant side effects, such as diarrhoea and allergic reactions.
What makes snot green?
It might gross you out, but snot has an important bodily function. Snot, or mucus, is a mixture of water, salt, and proteins from your own body called mucins, which give it its sticky, stretchy qualities.
The role of mucus is to maintain the integrity of the lining of the nose, mouth, throat, and airways, Dr Tam said.
"It traps foreign material, it keeps the airways moist … it's quite a natural part of the body. The reason you produce more mucus when you get something like a cold, is because it is in part … flushing out some the foreign material," he said.
Dr David King, a general practitioner and senior lecturer from the UQ School of Medicine, said when you notice a change in the colour of your mucus, it is usually for a number of reasons.
"Once you get inflammation, it changes consistency," he said. "It will be picking up foreign things like viruses and bacteria along the way, as well as inflammatory white cells that have leaked out of the blood to fight infection."
Your body also produces more mucus when you have an infection or allergy, which causes it to clump in your nose and throat.
And if you have an infection, it will sit and gather the debris from the battle between your white blood cells — which fight disease — and the virus or bacteria.
While the mucus sits there, components of the white blood cells will oxidise in the presence of oxygen, and this is the primary reason for the colour change.
A similar chemical reaction occurs when you cut an apple and the white flesh goes brown when exposed to the air.
"The longer it hangs around, the more discoloured it might get," Dr King said. "It's a product of both infection of some sort, inflammation, and the time that it's there."
Dr King said brown snot colour usually meant there was actually blood present, as old blood goes brown. But he said this was not generally something to be worried about.