Mucus v. Phlegm: What’s The Difference?

Most people are all too familiar with the icky, uncomfortable feeling of having mucus build up in the nasal passageways while suffering through a cold or bad allergy flare-up.

Or is it phlegm, rather than mucus? What exactly is the difference between these two words? Do they describe the same slimy annoyance or separate substances? Below, experts break it down.

“Mucus is a slippery, gel-like substance your body naturally produces to keep your nose, throat, lungs and other parts of your respiratory system moist and protected,” Dr. Anitha Srinivasan, chief medical officer at NYC Health + Hospitals/Metropolitan, told HuffPost. “It traps dust, germs and other particles so they don’t irritate your airways or cause infection.”

Your body constantly produces mucus as part of your everyday functioning immune system.

“It is kind of like saliva for the nose! Its main job is to keep those areas moist and to trap dirt, bacteria and viruses before they can cause harm,” said Dr. Nick Rowan, an endoscopic sinus and skull base surgeon with Johns Hopkins Medicine. “One of my favorite ENT jokes is, ‘you think it’s mucus, but it’s snot.’”

It’s a thin substance produced by mucous glands in mucous membranes throughout the body.

“Mucins are the key component that give mucus its characteristic viscosity and elasticity,” said Dr. Nora Tossounian, an internal medicine specialist with Hackensack University Medical Center.

“Mucus is mostly water, but also contains proteins, enzymes, antibodies and salts,” said Dr. Phillip Kadaj, an internal medicine specialist and medical expert with JustAnswer. “Patients will often use mucus and phlegm interchangeably, but they are not quite the same thing.”

“Phlegm is a type of mucus, but specifically, it’s the thicker mucus that’s produced in your lungs and lower respiratory tract, especially when you’re sick,” Srinivasan said.

Regular mucus tends to be thin and slippery, and it is generally a constant presence on a day-to-day basis. Meanwhile, phlegm is thick and sticky and it appears when there’s airway inflammation, hyperreactivity or an infection.

“It shows up when you are sick, such as with a cold, bronchitis or pneumonia,” Kadaj said.

Allergies, sinus infection, asthma and other conditions can also lead to phlegm production.

“While all phlegm is mucus, not all mucus is phlegm,” Srinivasan noted. “People often use these words interchangeably, but it’s more accurate to think of phlegm as the thicker, usually colored mucus that you cough up during a cold or respiratory illness.”

Tossounian compared the difference between phlegm and mucus to squares and rectangles.

“All squares are rectangles, but not all rectangles are squares,” she said. “While technically phlegm is mucus, using ‘mucus’ to describe what’s coughed up from the lungs isn’t quite right. It would be like calling a square a rectangle — technically correct, but not precise.”

As noted, mucus and phlegm are an important part of the body’s defense system against pathogens and irritants.

“The extra mucus helps trap and flush out the germs causing the illness,” Srinivasan said. “This can make your throat feel full or clogged, and you might notice more mucus or phlegm collecting there. Inflammation and irritation from the infection can also make mucus thicker and harder to clear, which adds to that clogged feeling.”

If you’re feeling uncomfortable, there are ways to clear out excess mucus and phlegm, which can help foster a quicker recovery.

“Especially when you’re sick, staying well-hydrated is key ― water helps thin the mucus, making it easier to clear,” Rowan said. “Saline nasal rinses, like with a neti pot or squeeze bottle, can flush mucus from the nose and sinuses and are something that I routinely recommend to patients, whether they have a cold or long-term sinus symptoms.”

Warm fluids can also help loosen mucus, so drink herbal teas and try steam and warm showers. There are medication options as well, like antihistamines or nasal steroids, if allergies are the culprit.

“Be careful with over-the-counter decongestant nasal sprays like oxymetazoline,” Rowan warned. “While they can offer quick relief, using them for more than a few days in a row can actually backfire. Your nose can become ‘addicted,’ leading to rebound congestion — a condition called rhinitis medicamentosa ― which makes the problem worse.”

In general, you can boost your immune system with regular exercise, a healthy diet, proper hygiene and adequate sleep.

“If you ever notice mucus that’s unusually thick or discolored ― like green or yellow ― or if it’s paired with symptoms like fever, chest pain or shortness of breath, it’s important to follow up with a healthcare provider,” Srinivasan said. “And of course, keeping your hands clean and avoiding irritants like smoke can go a long way in keeping mucus production and your immune system at healthy levels.”

Phlegm that is brown, pink, red or otherwise bloody in appearance could indicate bleeding in the respiratory tract that you’ll want to investigate. Green or yellow mucus typically indicates that your body is fighting off an infection. Pay attention to color changes, especially if you notice excessive or persistent phlegm production.

“Cold and cough symptoms shouldn’t be regular occurrences,” Srinivasan said. “If they are, they may signal underlying issues such as environmental triggers, lung infections or a predisposition to respiratory problems. In some cases, frequent mucus or phlegm can be a sign of a certain congenital condition, like cystic fibrosis or primary ciliary dyskinesia. These scenarios should always be managed by a specialist.”