Sometimes viruses and bacteria float in the air we breathe. Mucus, the nose's microbes and antibodies together form a front line of defense against infections. 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. The nose is not a sterile place. It has its own community of microbes, called a microbiome.

Mucus is made of water, surface cells, dead white blood cells, mucin and mineral salts. Mucin gets its slipperiness from sugars attached to a protein backbone. How do we know how wet it is? 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.

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. The cilia are tiny, about 5 to 7 micrometers long, and they move a bit like arms swimming the breaststroke. Mucus needs water because cilia need the airways to stay hydrated to work properly. In the nose, cilia clean the nose about five times an hour.

So where does it all come from, and how much is there? Cells in the mouth, throat, nose and sinuses make mucus. 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. That is an open question.

Where does the mucus go? 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. Mucus also coats the stomach in a layer about one millimeter thick, which keeps strong stomach acid from digesting the stomach itself.

What happens when a cold gets into the nose? A cold virus can slow the cilia in the nose, and the effect can last for several weeks. A runny or blocked nose is a main sign of a cold, which 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, and it is also called seasonal allergic rhinitis. When the body reacts to pollen, it releases chemicals such as histamine, and those chemicals cause the allergy symptoms. Hay fever can last weeks or months, unlike a cold. Allergy symptoms usually come on suddenly and last as long as a person is exposed to the allergen.

Then there is the color of snot, and what it can and cannot tell us. 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. A change in mucus color may signal a harmless shift in the body, or it could be a sign of infection. The color of snot alone does not tell you what is going on inside the body.

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 spread when germs from coughs and sneezes 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.

What about the hairs at the front of the nose, and what should we do with them? Hairs in the nose grow only in the front part, the vestibule. Hairs inside the nostrils help block bigger bits of dirt, and mucus-covered hairs help trap larger particles as they enter the nose. Pulling out nose hairs can cause a bad infection, so they are best left alone.

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. 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.

Words to know

• hydrated: full of water
• mucin: gel protein
• microbiome: microbe community
• histamine: allergy chemical
• infectious: able to spread