For coughing up phlegm, water is key | Science News for Students

For coughing up phlegm, water is key

To thin out dry, sticky lung mucus, just add water
Feb 20, 2019 — 6:45 am EST
a girl coughing into her hand

Coughing up mucus after a cold or flu is gross, but it’s also a sign your body is doing its job. 


Most people are familiar with phlegm. It’s the gunk you might cough up when you have a cold or the flu. This mucus is not pretty. But as long as it keeps moving, it does a body good. With certain diseases, though, phlegm becomes too thick to be easily cleared. It can end up clogging the lungs, making it hard to get oxygen in and out. In some diseases, such as cystic fibrosis, people risk suffocating from a phlegm overload. The question has been why their mucus sticks so firmly to their lungs. A new study finds that it all comes down to water.

The new data might help doctors better gauge which treatments work, and what new medicines might help patients breathe more freely.

“We wanted to know how mucus is cleared by cough,” says Brian Button. He studies biophysics — the physics of living things — at the University of North Carolina in Chapel Hill. Phlegm gunks up the lungs in two ways. The first is adhesion, the tendency of mucus to stick to the surface of the lungs. The second is cohesion, the tendency of mucus to stick to itself.

To measure these things, Button uses a device called a peel tester. It gauges how sticky something is by measuring how much force it takes to peel it off a surface. Normal peel testers are designed for products like tape. Button and his colleagues had to invent a new type for phlegm.

a picture of mucus being slowly lifted off of a film
This is Button’s peel tester at work, slowly lifting mucus off a film to gauge its stickiness.
B. Button

Next, they had to collect a lot of mucus. They got some samples from living patients. But they also got some from donated lung tissue. Some came from healthy patients who had donated their organs to science after death. Others came from people with cystic fibrosis or other lung diseases who had received a lung transplant (leaving their unhealthy tissue available for science).

From those donor lungs, Button says, “We scrape off the airway cells and grow them [up in a dish].” As the lung cells multiply, they make mucus. They even wave their tiny cilia around, pushing the mucus in slow circles. In a way, his team became “mucus farmers,” Button says.

The mucus in the lungs of people with cystic fibrosis is especially gloppy. But scientists did not know how this viscous stuff interacted with the lungs, or what would help a patient cough it up. Some researchers thought they could thin out mucus by making it more acidic — like lemon juice.

Button’s team thought maybe the thick mucus was just too dry. Normal mucus is 98 percent water. In people with cystic fibrosis, that mucus is only 79 percent water. Using the peel tester, the team showed that the force of a cough could easily tear normal phlegm off a surface. But at 79 percent water, mucus clung too tightly. A cough would not be strong enough to propel it out of the lung’s tiny airways.

The more watery a mucus, the more easily it tore away. And when Button and his colleagues added water to samples from patients with cystic fibrosis, their mucus now peeled away just fine.

These researchers shared their findings in the December 4, 2018 issue of the Proceedings of the National Academy of Sciences.

Just add water?

“There are so many people we could help if we could figure out how to make their mucus thinner,” says Stephanie Christenson. She’s a pulmonologist, or lung expert, at the University of California, San Francisco. What she finds especially cool about the new mucus study: “They were looking … at not just what it looks like, but how it interacts with the airways and the body.”

a young woman sitting at a table using a nebulizer
Nebulizers push salt into the lungs. Water from the blood rushes in towards it. This makes mucus in the lungs flow better, making it easier to cough up.

Drinking more water won’t solve the problem. But there is already one method to make phlegm a little more watery. Patients can inhale concentrated salt through a device called a nebulizer (NEH-beu-lye-zur). Salt in the lungs yanks water out of the blood and delivers it to the dry mucus. The wetter mucus can then get hacked out. Now, Button notes, scientists understand just why the nebulizer works so well.

But its effects don’t last long. “As soon as it’s turned off, all the fluid is reabsorbed,” Button says.

Button’s group is now exploring ways to make fluid stick around to keep the phlegm flowing. They also are looking into other treatments that might break up mucus chains or make thick mucus more slippery.

Power Words

(more about Power Words)

adhesion   (syn. adhesiveness) The ability of a substance to adhere, or stick, to some other material. (in medicine) A bit of tissue that has inappropriately attached itself to another.

bacteria    (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

biophysics    The study of physical forces as they relate to living things. People who work in this field are known as biophysicists.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cilia     (singular cilium) Small hairlike features that occur on the surface of some cells and larger tissue structures. They can move and their wavelike motion can propel liquids to move in a particular direction. Cilia play an important role in many biological functions throughout the body.

cohesion     (syn. cohesiveness) The ability of a substance to adhere, or stick, to itself.

colleague     Someone who works with another; a co-worker or team member.

cystic fibrosis     A life-threatening disease that causes a buildup of sticky mucus in the lungs, digestive tract and other regions of the body. A defective gene leads to the production of extra-thick mucus, which can foster lung infections and digestive problems. Although millions of people carry the CF gene, only people who inherit a copy from each of their parents will actually go on to develop cystic fibrosis.

defense     (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

gel     A gooey or viscous material that can flow like a thick liquid.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

immune    (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

immune system     The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

infection     A disease that can spread from one organism to another. It’s usually caused by some type of germ.

Influenza  (also known as flu) A highly contagious viral infection of the respiratory passages causing fever and severe aching. It often occurs as an epidemic.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

link     A connection between two people or things.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

mucus     A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water but also includes salt and proteins such as mucins. Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.

organ     (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

oxygen     A gas that makes up about 21 percent of Earth’s atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

nebulizer    A medical device that produces a fine spray of liquid. It can be used to administer medicines to people as they inhale the spray into their lungs.  

phlegm    A thick, sticky mucus produced by the lungs. The body uses it to help carry germs and other unwanted materials out of the lungs.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

Proceedings of the National Academy of Sciences     A prestigious journal publishing original scientific research, begun in 1914. The journal’s content spans the biological, physical and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

pulmonology  Someone who studies the lungs and treatment of lung disease. A doctor who works in this field is known as a pulmonologist.

salt     A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.

suffocate     To be unable to breathe, or to cause a person or other organism to be unable to breathe.

transplant     (in medicine) The replacement of a tissue or an organ with that from another organism. It is also a term for the material that will be transplanted.

viscous     (n. viscosity) Adjective referring to something that is thick, sticky and hard to pour. Molasses and maple syrup are two examples of viscous liquids.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


Journal:​ ​​B. Button et al. Roles of mucus adhesion and cohesion in cough clearance. Proceedings of the National Academy of Sciences. Vol. 115, December 4, 2018, p. 12501. doi: 10.1073/pnas.1811787115.