Snot plays a powerful role in protecting us from disease – and its colour alone can provide insights into what's going on in our bodies.

In Ancient Greece, snot was thought to be one of the four bodily fluids responsible for balancing human health and personality. The physician Hippocrates developed a theory stating that phlegm, blood, yellow bile and black bile were the four "humours". A person's balance of these humours could dictate their temperament; an excess of any of them could cause illness. For instance, phlegm was thought to be made in the brain and lungs, and during the cold and wet seasons, it could become too abundant and even cause epilepsy. Somebody with a phlegmatic personality would have a cold, damp and aloof character.

Of course, we now know that snot doesn't affect people's personalities or cause diseases – rather, it helps to protect us from them.

And though nobody likes a runny nose or flinging snot across the room in a sneeze, the mucus in our nasal passages is arguably one of the wonders of the human body. It protects us from intruders, and it has a unique composition that can reveal profound insights into what is going on inside us. Now scientists are hoping to hone the powers of snot to better diagnose and treat everything from Covid-19 to chronic lung conditions.

The gooey substance shields the insides of our nose, moisturising the nasal passages, and trapping any bacteria, viruses, pollens, dirt, dust and pollution trying to get into our body through our airways. Aided by hundreds of tiny hairs, snot is a barrier between the outside world and our inner one.

The adult body produces over 100 millitres of snot over the course of a day but children tend to be much snottier than adults because their bodies are learning to deal with being exposed to all of the world's molecules for the first time, says Daniela Ferreira, a professor of respiratory infection and vaccinology from the University of Oxford in the UK.

With a simple glance, our snot's colour and consistency can already help us glean a little bit about what's going on: snot can be like a visual thermometer. A runny schnozzle with clear mucus suggests the body is likely expelling something that's irritating its sinuses, like pollen or dust. White mucus means a virus may have entered the premises, as the white is caused by the white blood cells called up to fight off intruders. When mucus turns denser and yellowish-green, it's just a lot of dead white blood cells accumulating after having gathered in great numbers and flushing out. If your snot is reddish or pink, it may be a little bloody: maybe you've blown your nose too much and irritated its insides.

But looking at snot is just the first step.

While the gut microbiome – the ecosystem of bacteria, viruses, fungi and other microorganisms that inhabit our bodies – is very much in the public consciousness, scientists think that the microbiome in our snot is equally important. In fact, scientists now believe that it's intricately linked to human health and the proper functioning of the immune system.

Why snot really should gross us out

Technically, Hippocrates wasn't all wrong when he theorised that mucus makes people sick. Mucus is a protective barrier for the nose, but it does help bacteria and viruses spread when the nose gets runny, says Ferreira. We wipe our faces, we touch things, we sneeze and inadvertently fling snot across to the other side of the room. When we're infected with a respiratory pathogen, snot is hijacked as a vehicle for bacteria and viruses to multiply and travel........

© BBC