Take a tour of the nose, starting at the front door. What stands guard there? Hairs in the nose grow only in the front part, the vestibule. Hairs inside the nostrils help block bigger bits of dirt from getting into the nose. Mucus-covered hairs help trap larger particles as they enter the nose.

Next, let's look at the tiny hairs that move the mucus, and how small they are. Now zoom in on the cilia. Cilia are 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. They wave back and forth all the time. They line the airways and push mucus along, like an escalator, and they sweep mucus, with the stuff it has caught, toward the throat. In the nose, cilia clean the nose about five times an hour.

Now let's look at what the stuff is made of, and how we know what we know about it. What is mucus made of? Mucus is made of water, surface cells, dead white blood cells, mucin and mineral salts. Mucin is the main part, and it gives mucus its gel-like feel. Mucin gets its slipperiness from sugars attached to a protein backbone. How do we know it is so wet? In MIT lab tests, the coating mucin formed on a surface was 90 percent water. The ability of mucin to hold large amounts of water is critical to keeping the cells that line the body healthy. As of November 2026, the nose and sinuses make about a quart, or about 1 liter, of mucus every day, but experts say nobody knows the exact amount.

What does all of that do for the body? Mucus works as part of the immune system. It blocks germs and harmful particles and moves trapped things out of the body. Almost all the dust and germs we breathe in get trapped in mucus. Mucus also holds antibodies, which are proteins that help the body find germs.

What else is inside the nose? Mucus keeps the inside of the nose moist. The nose is not a sterile place. It has its own community of microbes, called a microbiome.

What about the color of snot, and what can it tell us? Now the colors. Yellow mucus often shows up at the start of an infection, when white blood cells are working hard. 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. Smoking or toxins in the air can cause brown mucus, and breathing in dust, smoke or chemicals can make it black. Once an illness is over, mucus slowly returns to its normal color. The color of snot alone does not tell you what is going on inside the body.

What happens when a cold gets into the nose? A runny or blocked nose is a main sign of a cold. A cold usually gets better on its own in about one to two weeks. A person with a cold stays infectious until the symptoms are gone, and can spread it a few days before they begin.

What about allergies? Hay fever 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. Hay fever can last weeks or months, unlike a cold that usually clears in one to two weeks. When pollen sets off hay fever, the body releases a chemical called histamine, which causes the runny nose, sneezing and swelling.

Now, what about boogers, and what is the best thing to do about them? A booger is dried mucus mixed with bits of dust, pollen and dirt. Nose picking has a long formal name, rhinotillexomania, and other primates do it too. Picking the nose can make it bleed, and boogers contain lots of germs, so a tissue is a better tool. Blowing your nose into a tissue is a good way to clear out boogers, and a tissue traps germs.

How do colds spread, and what can we do about it? 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. Washing hands with soap for about 20 seconds is a good length. If soap and water are not handy, the CDC says to use a hand sanitizer with at least 60 percent alcohol, as of November 2026.

Last, let's look at a puzzle that has to do with the way we breathe. 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, and the busy side switches about every two hours.

Words to know

• micrometers: tiny lengths
• mucin: gel protein
• microbiome: microbe community
• vestibule: nose front
• sinuses: head spaces