Researchers have found that snot is crucial for supporting our immune system. Photo / Getty Images
Changes in our nasal secretions and new insight in nasal microbiome offer vital clues to the state of our respiratory system.
No one enjoys struggling with a streaming nose. We’re quick to apologise to colleagues for constantly snivelling and quick to discard our tissues, too.
While you may think it disgusting, researchers looking into our nasal secretions find snot fascinating. The nasal microbiome is becoming increasingly studied, with experts learning more about how our airways work, which protective bacteria dominate and which appear at the site of disease. They’ve also found that snot is crucial for supporting our immune system and that studying it could hold the key to curing respiratory problems such as asthma and sinusitis. So what exactly is snot, and why do we need it?
“Snot is essentially a hydrogel, similar to the water-based gels used in beauty products like moisturisers,” says Professor Rebecca Carrier, a professor of chemical engineering at Northeastern University in the US. “Its chief role is keeping the nasal passageways moist and defending us against infection.” As a result, it contains proteins that control the microbes’ ability to penetrate the mucus wall, which, in turn neutralises dangerous bacteria.
Dr Aran Singanayagam, a respiratory clinician scientist, adds: “It is absolutely critical for protecting the airways by trapping the virus and bacteria that we breathe in. Bacteria binds with snot and the snot changes the way the bacteria function, influencing the severity of a disease,” says Carrier.
Snot colour and consistency can be a good indicator of our health and can often tell us a lot about the state of our respiratory system. So, what might the colours of our snot indicate about our health? Here, Dr Pedro Barata, a professor at the University of Porto, who has worked extensively with colleagues to learn more about the nasal microbiome and identify the role mucus plays in immune health, shares his insight.
Mucus colours, decoded
Clear
Normal, allergies, early viral infection
“Clear snot is mostly water, with a bit of salt, protein and immune cells,” says Dr Barata. “It’s normal, but it can also mean allergies or the very start of a cold. If it lasts more than 10 days with sneezing and itchy eyes, and you have clinical history, it could be hay fever.”
White
Congestion, viral infection
Barata says: “White snot can occur when your nose is congested due to swelling inside the nasal passages. This slows down mucus flow and traps tiny air bubbles, making it cloudy or white. This is common with colds and mild sinus inflammation.”
Yellow
Infection, immune response
“Yellow snot usually means your immune system is fighting something, like a cold or sinus infection. The colour comes from white blood cells (eosinophils) and the debris they leave behind. If it lasts more than a week or gets worse, you should be seen by a doctor.”
Green
More advanced infection
Green snot occurs when your immune system has been fighting an infection for several days already, says Barata. “The green colour comes mainly from enzymes in white blood cells and debris. It can happen with viral or bacterial sinus infections. The smell can also be a characteristic. If symptoms are severe or persist for more than 10 days, see a doctor.”
Red
Irritation, injury, pregnancy
A pink or reddish runny nose usually means a small amount of blood in nasal mucus, says Barata. “The most common causes can be from irritation or a small injury, or pregnancy.” Dry air, mild infection or hay fever can cause pink or red mucus if you sneeze or blow your nose frequently, he adds.
“Pregnancy-related snot can sometimes be a cause of body changes, such as an increased blood volume, hormone shifts and a swollen nasal mucus membrane. It can also be caused by nasal congestion and vascular engorgement due to an increase in oestrogen and progesterone, making it easier for nasal capillaries to rupture.”
Brown
Old blood, dust, dirt
“Brown snot is usually from old blood mixing with mucus, or from inhaling dust or dirt. It can also happen after a nosebleed. If it keeps coming back without a clear reason, see your doctor.”
Black
Heavy pollution, smoking, fungal infection
“Black snot can occur when you inhale large amounts of smoke, dirt or pollution,” says Barata. Rarely, it can also be caused by a serious fungal infection, especially in people with weak immune systems. If you see black mucus and feel unwell, seek medical help.
With all that said, Singanayagam advises that “the colour isn’t always a consistent signifier of health” and that one persistent colour doesn’t always correlate with a specific health concern. “Some people will have green snot all the time and that is completely normal for them. Each individual person is different, and no two healthy people will have the same snot appearance and characteristics. You may find a colour change in your snot secretion depending on what you breathe in,” he says.
However, he adds that changes in mucus consistency can be an indicator of health complications. “In some diseases, such as asthma, hay fever or general allergies, mucus will get thicker. This can be a bad thing because when the mucus quality changes, it will block the airways and impair their normal function, as well as the hairs that line the airways.
“It’s all about finding the right balance,” he says. “Mucus needs to be thick enough to fulfil its role – not too thick and not too thin, otherwise its protective function will be lost.”
Don’t confuse snot for phlegm, though, Singanayagam warns. “The nose is the upper respiratory tract and the lungs are the lower respiratory tract, but there is constant communication between the two.
“Phlegm is a lung secretion. However, you may get post-nasal drip (the drainage of snot from your nose to your throat), in the lungs, which could be coughed up in phlegm.”
Phlegm is a symptom of a cold, flu, or other respiratory infection, which is slightly thicker than mucus produced in the nose.
It helps clear irritants and infections from the lungs, but coughing up phlegm when you aren’t ill might be a sign of a more serious health condition, such as lung disease or heart failure.
Ten per cent of the global population suffer from chronic rhinosinusitis, says Jennifer Mulligan, an otolaryngologist at the University of Florida.
Rhinosinusitis is a chronic condition where the nasal passages remain inflamed and swollen. It can cause a permanently blocked or runny nose, facial pain, and a decreased sense of smell or sense of taste.
“We don’t know why some people get a perpetually runny nose and others don’t,” she says. “It was originally thought to be driven by allergies, because 80% of patients with the condition also suffer from hay fever, but we’ve seen that treating allergies doesn’t make it better.
“Patients who suffer from respiratory issues like asthma and rhinosinusitis have an immune system that constantly thinks it’s fighting a parasite that is not there,” she adds.
One of the most popular theories about the mystery behind chronic rhinosinusitis is what researchers call the “hygiene theory”, which is the idea that a person’s health is due to the exposure to different microorganisms in childhood.
“This has been around since the 1950s,” says Mulligan. “We know that children who live in the country tend to be healthier than children who live in the city. This goes back to the fact that those living in the city have clean water, clean food and clean air, which is great.
“The problem is your immune system hasn’t had the opportunity to get primed early in life, so as fewer of us live out in the country and grow up working in fields with exposure to bacteria, we are seeing an increase in those living in industrialised countries developing asthma and sinusitis and allergies.”
In response to the increase in respiratory disease, Mulligan and her team use snot to closely examine these conditions and work on ways to improve symptoms by targeting the nasal microbiome.
While the gut microbiome has been a popular buzzword among researchers for quite some time, the nasal microbiome has only recently gained traction. “Just like the intestinal microbiome, it’s very important for our health,” says Carrier. “We have a very large population of bacterial species and other types of microorganisms, like fungal species. They exist there and play a role in communicating with underlying tissues and regulating immune response.
“We know that there are certain bacteria which tend to be there in a healthy state, and we know that those bacterial populations can shift when in a disease state. There are changes in the bacteria in the microbiome depending on the disease. Unfortunately, the nasal microbiome has been much less studied than the intestinal microbiome, so more research is needed.”
Anders Mårtensson, a senior consultant in otorhinolaryngology who treats nasal issues at the Helsingborg Hospital in Sweden, views the microbiome as “a meadow with flowers and plants – there’s usually a balance. Suddenly, something might disrupt that balance, and then there’s one plant that’s growing more”.
Indeed, the balance of the nasal microbiome varies from person to person, and the strains can vary depending on gender, age and diet. In total, 150 nasal bacterial species have been identified, with 19 seen as dominant. Perhaps more importantly, these bacterial differences are known to influence the development of respiratory conditions, such as sinusitis or chronic rhinosinusitis – a permanent blocked or runny nose, often paired with extreme tiredness.
Now, research has discovered that by transplanting the snot of healthy noses into the noses of patients suffering from chronic rhinosinusitis, symptoms can be improved.
Mårtensson led the study and recruited 22 patients with the condition to have the snot of a relative or trusted person rinsed into their own, in an attempt to change the microbiome composition and ease symptoms.
“I explained to the patients how it would work: they would find a friend or family member to donate their snot and, if they didn’t have a donor of their own, I would try to recruit a nurse to help. The donor would rinse their nose, we’d collect the snot and then the patient would rinse their nose with the donor’s snot.
“If people said that it sounded disgusting, then I would know that they weren’t willing to do anything to get rid of the disease,” he says.
The study found that the microbiome diversity went up after the snot transplant treatment, and symptoms decreased. Out of 22 patients, 16 reported significant improvements in their condition. Despite the positive results, the transplant has not yet become mainstream treatment on the NHS or in private practices.
But one thing is clear: snot isn’t just an annoying sticky liquid that seeps out of our nose in the winter months – it’s the cornerstone of our immune system.