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Mucus figures drawn like robots

Mucus, Our Body’s Silent Defender

Boogers get a bad rap. Yes, they are slimy and can pop up (or out) at the worst of times, but that isn’t their only job. The gel-like substance you’re used to finding in your nose—mucus—is also in your mouth, throat, lungs, intestines and eyes. It serves an important role in keeping those parts of your body hydrated and protected.

We did a deep dive into the fascinating world of mucus with Richard Boucher, MD, director of the UNC Marsico Lung Institute/UNC Cystic Fibrosis Center, and Dana Neutze, MD, PhD, associate medical director of the UNC Family Medicine Center.

Mucus: The Protector

Mucus can vary slightly depending on what part of the body produces it, but typically it is made up of 98 percent water, 1 percent salt and 1 percent biopolymers—very long molecules that interact with one another and give mucus that gel-like quality.

“It’s a great substance,” says Dr. Boucher, a James C. Moeser eminent distinguished professor of medicine. “It’s like a cross between Jell-O and glue that lines many surfaces of your body that typically interface with the outside world. It’s designed to both keep those surfaces moist and healthy and to trap foreign materials like infectious agents.”

The biopolymers in mucus have bristles that stick out from a central (protein) thread, like a bottle brush. Those bristles are responsible for trapping potentially harmful things we breathe in. The mucus is then cleared by leaving through the nose or mouth or by being swallowed into the stomach, where digestive acids kill most anything that could make us sick.

Those same bristles are also responsible for retaining water, which is crucial for mucus to be able to hydrate surfaces while trapping the bad stuff.

“The difference between a 98 percent water mucus and a 92 percent water mucus can be fatal,” Dr. Boucher says. “That small difference means the mucus can’t move or coat surfaces as it should. Dry mucus could cause something as innocuous as a dry mouth or itchy eyes or something as serious as lung damage.”

Many factors lead to dry mucus, including side effects caused by medication and diseases such as cystic fibrosis. The simplest cause is not drinking enough water, so stay hydrated to keep your mucus healthy.

Mucus: The Warrior

If your body was a nightclub, mucus would be the bouncer—located at all entrances and ready to kick out anyone causing trouble. When a sickness-causing agent like a virus or bacteria enters your body, the cells that produce mucus kick into a higher gear and pump out more of the slimy stuff, which then picks up the germs. Mucus usually clears itself out of the body as we’ve discussed, but sometimes it needs a little help. Coughing and blowing your nose are the best ways to help mucus fight the good fight.

“Coughing is good,” Dr. Boucher says. “When you cough up mucus when you are sick, you are essentially clearing the bad guys—viruses or bacteria—from your body.”

For that reason, Dr. Boucher does not recommend taking a cough suppressant medication. If your mucus is dry and you are having trouble coughing it up, you can do things like take a steamy shower or use a humidifier to wet and loosen the mucus. When you do cough up phlegm (another word for mucus) from your chest, Dr. Boucher says it really doesn’t matter if you spit it out or swallow it.

Mucus: The Messenger

When you are blowing your nose or coughing, pay attention to your mucus. When it starts to change consistency and color, it’s telling you something about what’s happening on a deeper level.

When you’re healthy, mucus looks clear or “pearly white,” as Dr. Boucher puts it. If you get dehydrated or the body has been introduced to something harmful, it turns yellow and gets thicker. You also might experience a runny nose or phlegm in your chest and throat, which is a symptom of increased mucus production. This situation is most likely the result of a common cold. When you start coughing up darker, greenish-looking mucus, that’s a sign of bacterial or viral infection.

“One of the reasons mucus changes color is because of what’s in it,” Dr. Neutze says.

Mucus not only traps infectious agents, but it also picks up cells produced by your body. Inflammatory cells created by the body in response to a sickness or irritation get absorbed by mucus, giving it that green hue. But color alone isn’t a foolproof indication of infection, because environmental factors like pollution and smoke inhalation can also produce dark mucus. Keep tabs on any other symptoms you might have in conjunction with discolored mucus.

Dr. Neutze says: “If you have a fever, difficulty breathing or just keep getting sicker, that would be an indication of infection and a good reason to see your doctor.”

The most concerning color to watch out for in mucus is red. If you cough up blood, that means there could be damage to your lungs, and you should see a doctor. This is different from a hint of blood in mucus from your nose, which most likely means a small blood vessel has ruptured from the stress of coughing, sneezing and blowing your nose. If you can’t tell if the blood is coming from your mouth or nose, see a doctor.

Mucus: The Infector

While we’ve discussed the many ways mucus helps your body, someone else’s mucus can cause harm to you. That’s why it’s important to keep our mucus to ourselves, because bacteria and viruses can be present.

“Viruses can live for up to 24 hours outside of the body,” Dr. Neutze says. “Mucus can definitely transmit infection, even on used tissues or door handles.”

That means sneeze and cough into your inner elbow, not your hand, and wash your hands frequently, especially after you’ve touched your face or blown your nose. Don’t share food or drinks, don’t loan someone your pen, and avoid shaking hands.


Dealing with sinus problems, a common cold or another illness? Find a doctor near you.