What your snot can reveal about your health

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.

Everybody has a unique snot microbiome. It is affected by sex, age, location, diet – and even whether you vape. The microbiome's makeup is what helps it fend off intruders, and some of these interactions are subtle. Research from 2024, for instance, found that whether potentially harmful Staphylococcus bacteria survive in the nose and infect a person, causing fever and pus-filled boils, depends on how the snot microbiome's bacteria hold onto iron.

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Ferreira is working to figure out exactly what a healthy snot microbiome looks like so that it can be put in an everyday nasal spray to boost snot health, like taking probiotics for gut health. "Imagine if you could alter what we have in our nose with lots of very good-guy species that stay there and colonise, and do not allow for the bad guys to come in and cause us to get sick," says Ferreira. 

Ferreira's colleagues have selected the bacteria they think make up the perfect schnozzle microbiome, and they're testing them to see if these bacteria can take over people's airways and last long enough to impact and improve their health.

Since the snot's microbiome is so tightly linked with the immune system, says Ferreira, they are also studying it to fine-tune how to boost the immune system and even make it more receptive to vaccines. Research suggests that how a body reacts to a vaccine is altered by the type of microbiome a person has. Studies on the Covid-19 vaccine, for example, suggest it affected the snot's microbiome, and in turn, the microbiome affected how efficient the vaccine was.

"The Covid-19 vaccines were great at stopping us from getting sick, but we continued to transmit the virus," says Ferreira. "We could actually develop much better vaccines [so] the next generation people don't even get sick, whether that is Covid-19 or flu or any other respiratory viruses – and it's all there in that snot immunity." 

While Ferreira's work pinpointing the exact formula for the perfect snot microbiome might take a couple of years, in Sweden, scientists have had a head start by transplanting healthy people's snot into those who are sick with a chronically blocked nose and hay fever, everyday symptoms of rhinosinusitis.

The researchers asked 22 adults to shoot themselves up the nose with a syringe full of snot from healthy friends and partners each day for five days. They discovered that symptoms like cough and facial pain, for instance, dropped by almost 40% for up to three months in at least 16 of the patients. "That was great news to us, and no one reported any negative side effects," says Anders Martensson, a senior consultant in otorhinolaryngology and head and neck surgery from Helsingborg Hospital in Sweden, who led the study. These trials were inspired by work done in other laboratories about gut microbiomes, with faecal transplants, he says.

That first pilot programme, however didn't gather much data about how these people's snot microbiomes changed and what happened to the specific bacteria in their nose, whether they increased, decreased, and so forth. So another larger and more precise trial is underway.

In fact, snot can be a great barrier to chronic nose and lung diseases.

Jennifer Mulligan, an otolaryngologist at the University of Florida, uses snot to study people with chronic rhinosinusitis and nasal polyps – a condition that affects about 5 to 12% of the global population. In the first years of her career, she needed to surgically extract nose tissue from rhinosinusitis patients, but that was invasive and limiting. Now, her research has shown that snot can be an accurate proxy to more closely examine what's happening inside the body when someone develops rhinosinusitis. "We're using it to whittle down who are really the guilty culprits here, who's really driving this condition?" says Mulligan, adding that every patient has a slightly different profile for what's causing their rhinosinusitis.

Similarly, while treatment before was mostly trial and error – varying greatly from patient to patient, and sometimes costing tens of thousands of dollars for treatments lasting months – Mulligan suggests a snot analysis can quickly help identify the right treatment or surgery needed. 

Several clinical trials for Mulligan's technique are underway worldwide. Emerging health-teach companies, such as Diag-Nose, launched by engineers at Stanford University, are developing snot-analysing AI systems and patenting devices for nasal microsampling: in 2025, they launched the first FDA-approved nasal microsampling device – a sampling device that collects precise volumes of nasal fluid – to reduce research variability by standardising sampling methods.

"We have learned so much that we could have never learned with just tissue biopsies. It's completely changed what we know about the disease, and it's going to change the way patients are diagnosed in the future and how they receive treatment," says Mulligan.

Mulligan uses the same snot tools to study what causes people to lose their sense of smell, too. Her team has already found that a vitamin-D nasal spray could potentially help restore a sense of smell in people who have lost it due to inflammation from smoking.

Plus, Mulligan says, what happens in the lungs happens in the nose and vice versa. So these diagnostic tools and therapies can be used for lung diseases too. New research suggests that by simply analysing how much of the IL-26 protein is present in a patient's snot, doctors can tell whether somebody is more or less susceptible to developing chronic obstructive pulmonary disease – a common smoker's disease, and the fourth most widespread cause of death in the world. With snot analyses, patients can be diagnosed early and treated rapidly.

Similarly, research teams around the world are developing analogous tools and methods to use snot to detect asthma, lung cancer, Alzheimer's disease, and Parkinson's disease. Snot can also be used to measure radiation exposure and several recent studies suggest that the gooey nasal fluid can pinpoint how much somebody is exposed to pollution, such as heavy metals and microparticles in the air.

"Snot is the future of personalised medicine. I wholeheartedly believe that," says Mulligan.

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