Snot is mucus, a sticky, slimy stuff that the nose makes, and a surprising amount about it is unsettled. The nose and sinuses make about a quart (about 1 liter) of mucus every day, but as of November 2026 experts say nobody knows the exact amount. One scientist says the textbook view is that mucus is a barrier to infection, but that it is not at all clear how it does so. Researchers say it is still unclear why the nose swaps its busy side back and forth, even though total airflow stays almost the same (as of November 2026); both questions are about the nose and its mucus, and neither has a settled answer.

Start with the recipe. Mucus is made of water, surface cells, dead white blood cells, mucin and mineral salts, and a second description lists water, mucin proteins, defense proteins and salt. In healthy airway mucus, water makes up about 98 percent of the mass and mucins about 0.7 percent, with salts and a few other large molecules making up the rest. Water and mucins together are about 98.7 percent of the mass, so everything else fits into a little over one percent. Cells in the mouth, throat, nose and sinuses make it, and clear, thin mucus is normal and healthy. Everybody makes it; mucus keeps the inside of the nose moist, and in the nose it catches dust and germs so they stay out of the lungs.

The protein that does the holding is mucin, the main part of mucus, which gives it its gel-like feel. Goblet cells, shaped a bit like a fancy drinking glass, make it; mucin gets its slipperiness from sugars attached to a protein backbone, and sugar chains make up more than 70 percent of the mass of the mucins in airway mucus; the sugars give mucins a brush-like shape, and without them the protein folds into a ball and has less space to hold water; in MIT friction tests, a solution containing mucins was several orders of magnitude slipperier than a solution that held only salts. Mucin is only about 0.7 percent of the mucus, yet sugar chains make up more than 70 percent of the mass of the mucins, so the sugar does a great deal of the work.

Water is the whole point of the brush. Mucins are packed dry inside goblet cells, and after release they must soak up more than 100 times their own mass of water to turn into the right kind of gel for the cilia to move, which means a released mucin ends up holding far more water than its own weight. In MIT lab tests, the coating mucin formed on a surface was 90 percent water, a little below the roughly 98 percent found in healthy airway mucus; mucus needs water for a second reason as well: cilia need the airways to stay hydrated to work properly.

Mucus does not sit in one lump. Airway mucus has two main layers: a sticky gel layer on top, and a watery layer underneath that surrounds the cilia and lets them beat; a thin layer of surfactants sits between the gel and watery layers, so the cilia beat in the watery layer while the sticky gel rides on top; the lining of the nose has two main kinds of cells, ciliated cells that sweep and secreting cells that make mucus; each ciliated cell in the nose carries about 100 to 250 cilia, and each cilium is about 5 to 7 micrometers long, roughly a tenth the width of a human hair.

The cilia wave back and forth all the time, and they move a bit like arms swimming the breaststroke. Each cilium has a power stroke that pushes the mucus toward the mouth and a recovery stroke that brings it back to rest, ready to grab the next bit of mucus. Cilia line the airways and push mucus along, like an escalator; in the nose they clean the nose about five times an hour, which works out to one full cleaning about every twelve minutes. Scientists once tracked nasal cilia by putting a tiny drop of mildly radioactive fluid in the nose and following it with a scanner, a test since replaced by others.

Where does the mucus go? Almost all the dust and germs we breathe in get trapped in mucus, and the cilia carry that mucus toward the throat. A booger is dried mucus mixed with bits of dust, pollen and dirt; mucus drips down the back of the throat, carrying the dust and germs it caught; when we swallow mucus, it meets stomach acid, which helps destroy the harmful material in it.

What changes how fast mucus clears? In the lung, cilia and airflow, including coughing, clear mucus, and how fast mucus moves depends heavily on how concentrated the mucus is. In the nose, a cold virus can slow the cilia, and the effect can last for several weeks. A runny or blocked nose is a main sign of a cold, and a cold usually gets better on its own in about one to two weeks; colds are spread by germs from coughs and sneezes, which can land on hands and surfaces, and washing hands helps stop respiratory infections from spreading.

Nasal mucus carries antibodies and enzymes, so it works as a chemical shield as well as a trap. The nose is not a sterile place; it has its own community of microbes, called a microbiome. Mucus-covered hairs help trap larger particles as they enter the nose. In a dish, MIT scientists found that mucins stopped bacteria from sticking and clumping together, which kept them from doing harm. One scientist says the textbook view is that mucus is a barrier to infection, but that it is not at all clear how it does so.

How do scientists measure the nasal cycle? A 1977 Mayo Clinic study measured nose airflow every 15 minutes for about 7 hours in 50 people, which is about 28 readings for each person, and found that 72 percent, or 36 of the 50, had a clear nasal cycle, which leaves 14 of the 50 without a clear one. A 2016 study recorded airflow in each nostril for 24 hours in 33 healthy people, a longer window with fewer people. The 24 hours of 2016 are more than three times the 7 hours of 1977, though the later study covered 33 people instead of 50.

The nasal cycle is when airflow shifts back and forth between the two nostrils, so one side is more open than the other. At any moment you breathe mostly through one nostril while the other rests. One description says the busy side switches about every two hours; another description gives a mean length of about two and a half hours. The 2016 study found about 2 hours when people were awake and about 4.5 hours in sleep, which is more than twice as long. People usually do not notice any of it, because the total resistance to airflow through the nose stays fairly constant even as the two sides trade places.

What drives the swap? Researchers say nose airflow differs between nostrils because erectile tissue in the nasal passage swells and shrinks, and that the cycle is related to the body's autonomic arousal. The nasal cycle is controlled by a central regulator in the hypothalamus; body position changes it: the same 2016 study found that lying on one side led to more airflow through the nostril on the opposite side. Gravity matters too. If someone lies on their right side, gravity makes the lower nostril more congested, and if the nasal cycle has already made the other side stuffy, breathing may feel hard and they may wake up.

Hay fever is one common cause of a runny nose. It is an allergy to pollen, the fine powder plants make; it is also called seasonal allergic rhinitis, and it is caused by pollen from trees, grasses and weeds. It tends to come back at the same time every year, and in the UK it is usually worse between late March and September; when pollen sets off hay fever, the body releases a chemical called histamine, which causes the runny nose, sneezing and swelling. In a study of twelve people with birch-pollen hay fever, those with more histamine in the nose lining during pollen season had stronger nose symptoms, so more histamine went with worse symptoms. Hay fever can last weeks or months, and allergy symptoms usually come on suddenly and last as long as a person is exposed to the allergen.

Color gives hints but not answers. The color of snot alone does not tell you what is going on inside the body; yellow mucus often shows up at the start of an infection, when white blood cells are working hard, and green mucus shows up when the body fights a more severe or longer-lasting infection, but it is only one clue. Blood can make mucus pink-tinged or streaked with red, so color varies for several different reasons. Once an illness is over, mucus slowly returns to its normal color. Each color is only a clue and not an answer.

Mucus lines the inside of the nose and also the lungs and the digestive tract, and it coats the stomach in a layer about one millimeter thick, which keeps strong stomach acid from digesting the stomach itself. Mucus therefore turns up in more places than the nose. Scientists are studying how to design fully synthetic mucins that copy the real ones.

Words to know

• mucins: mucus gel proteins
• surfactants: slippery film makers
• microbiome: community of microbes
• autonomic: automatic body control
• histamine: allergy chemical