At any moment you breathe mostly through one nostril while the other rests, and the busy side switches about every two hours. This is the nasal cycle; it is the nose's own shift change, and it goes on whether or not anyone is paying attention. In fact, people usually do not notice the nasal cycle, because the total resistance to airflow through the nose stays fairly constant even as the two sides trade places. One side gets a little easier and the other a little harder, while the sum stays about the same.

How do scientists know this is real? In 1977 a Mayo Clinic study measured nose airflow every 15 minutes for about 7 hours in 50 people, and it found that 72 percent of them had a clear nasal cycle. That means a little more than seven in ten, which leaves a good number of people in whom the pattern was not clear; sources give slightly different lengths. A different source, separate from the Mayo study, puts the mean length of the cycle at about two and a half hours, while a second description says the busy side switches about every two hours.

Why does the nose do this at all? Researchers say it is still unclear why the nasal cycle exists, even though total airflow stays almost the same (as of November 2026). What is known is who runs it; the nasal cycle is controlled by a central regulator in the hypothalamus. Position matters too. If someone lies on their right side, gravity makes the lower nostril more congested; if the nasal cycle has already made the other side stuffy, breathing may feel hard and they may wake up.

The nose is not only a pair of doorways. Its lining 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 tiny, about 5 to 7 micrometers long, roughly a tenth the width of a human hair. Cilia move a bit like arms swimming the breaststroke. Each one 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.

Together the cilia work like an escalator: they line the airways and push mucus along, and in the nose they clean the nose about five times an hour. Almost all the dust and germs we breathe in get trapped in mucus, and the cilia carry that mucus toward the throat; there it drips down the back of the throat, carrying what it caught. In the lung, cilia and airflow, including coughing, clear the mucus, and how fast mucus moves depends heavily on how concentrated the mucus is.

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. In healthy airway mucus, water makes up about 98 percent of the mass, and mucins about 0.7 percent; salts and a few other large molecules make up the rest. A thin layer of surfactants sits between the gel and watery layers. So the cilia are not dragging a heavy paste; they beat in a watery layer and move the gel that rides above it.

Mucus is more than a conveyor belt: nasal mucus carries antibodies and enzymes, so it works as a chemical shield as well as a trap. In a dish, MIT scientists put mucins together with bacteria and found that the mucins stopped the bacteria from sticking and clumping together, which kept them from doing harm; mucus works as part of the immune system: it blocks germs and harmful particles and moves trapped things out of the body.

What makes mucin slippery and watery? Sugars. 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. Without them, the protein folds into a ball and has less space to hold water. After release from goblet cells, mucins 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.

Another test measured how slippery mucins really are. In MIT friction tests, a solution containing mucins was several orders of magnitude slipperier than a solution that held only salts. In other MIT lab tests, the coating mucin formed on a surface was 90 percent water. Scientists once tracked nasal cilia in a different way, by putting a tiny drop of mildly radioactive fluid in the nose and following it with a scanner. That test has since been replaced by others.

Hay fever is one common cause of a runny nose. Hay fever is an allergy to pollen, the fine powder plants make, and it can bring sneezing and a runny nose. In hay fever, IgE antibodies sit on mast cells. When enough pollen binds to them, the mast cells release granules of histamine and other chemicals. Histamine causes the runny nose, sneezing and swelling; hay fever tends to come back at the same time every year, and in the UK it is usually worse between late March and September.

A cold and an allergy can feel alike, but their patterns differ. 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; hay fever can last weeks or months. Allergy symptoms usually come on suddenly and last as long as a person is exposed to the allergen. A person with a cold stays infectious until the symptoms are gone, and can spread it a few days before they begin.

Color is a clue, not a verdict. 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. Blood can make mucus pink-tinged or streaked with red. Once an illness is over, mucus slowly returns to its normal color.

Mucus does not stop at the nose. It lines the inside of the nose and also the lungs and the digestive tract; it also coats the stomach in a layer about one millimeter thick, which keeps strong stomach acid from digesting the stomach itself. 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.

Words to know

• hypothalamus: brain control center
• histamine: allergy chemical
• mucins: mucus gel proteins
• surfactants: slippery film makers
• allergen: allergy trigger