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What is Phlegm?

By Caitlin Shih
Updated Mar 03, 2024
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Phlegm is a sticky, gel-like liquid secreted by the mucous membranes in the respiratory tract of humans and mammals in general. It is formed mainly of lipids, glycoproteins and immunoglobins, as well as other substances. Its function is generally to trap foreign agents that might enter the body through the respiratory tract. Excess phlegm is produced as a sign that the body is fighting some sort of infection. The composition and color of phlegm can vary greatly, from watery to thick or clear to brownish, depending on the environment and the state of the body's immune system at any given time.

Working not only to lubricate the respiratory and nasal passages, phlegm can also trap dust, allergens, viruses and bacteria that enter the body from the outside. These foreign bodies are trapped and neutralized by the mucus and then expelled from the body through coughing or sneezing. This is why, while cough suppressants may provide relief, they may also suppress necessary elimination of immune system waste.

The color of the mucus being coughed out is generally an indicator as to what kind of infection a person has. Normal mucus from a healthy body is typically, but not always, clear in color and generally thin in composition. An irritation of the nasal passageway, such as one caused by allergies or asthma, may result in a whiter, thicker mucus. White or clear mucus may still be present during the initial stage of the flu but will still be infectious during that time. Consistently coughing up white or clear mucus could indicate a mild viral infection, most of which heal independently within five to seven days.

Yellow or greenish yellow mucus generally indicates an active infection. During this time, the mucus will consist more of dead white blood cells and dead bacteria or viruses. This is because the dead cells that had been fighting the infection are discarded from the body through phlegm, resulting in a yellowish color.

Green mucus still indicates an infection, but an increasingly inactive one. The green color is caused essentially by yellow, active phlegm that has become stagnant and turned green. More rarely, coughing up green mucus can be a symptom of cystic fibrosis.

Red mucus, or redness inside otherwise healthy-looking mucus, is usually a sign of bleeding. This may have a benign cause, such as a nosebleed or a cut in the nostril from scratching or rubbing. A high concentration of blood in the mucus, or small spots or streaks of blood over a long period of time, may indicate something more severe, such as bleeding in the lungs, bronchitis or pneumonia. Coughing up blood may be a symptom of a major internal injury or illness, such as tuberculosis, and should be checked with a physician as soon as possible.

Brown mucus is typically caused by old and stagnant blood. Smokers may also have brown mucus, and many will often cough it up as a response to the chronic bronchial inflammation caused by smoking. The mucus of a smoker will typically, if examined, be grainy in texture. This is because the cilia that would normally trap foreign bodies such as dust and dirt have been damaged by the smoking. Increasingly brownish mucus in a smoker may indicate underlying respiratory problems.

Pink mucus can sometimes be an indication of asthma due to a specific kind of white blood cell present during the condition. Mucus that is pink and frothy, however, is a classic sign of pulmonary edema, a severe illness that, if left untreated, could lead to coma or death. Froth in mucus generally comes from an accumulation of fluid in the lungs. Any case of frothy mucus should be checked with a physician as soon as possible.

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Discussion Comments

By browncoat — On Oct 18, 2013

@clintflint - In some ways it hasn't changed, to be honest. Doctors will still sometimes prescribe treatment that won't help at all and may actually hurt the person taking it.

Antibiotics are prescribed whenever it seems like a person has the slightest possibility of infection. The doctor might know the causes of phlegm and other symptoms are viral which means that antibiotics won't help at all, but they still prescribe them so it looks like they are doing something.

By clintflint — On Oct 17, 2013

@irontoenail - My grandfather told me he quit cigarettes when he realized that he was coughing up green phlegm on a boat trip. It was actually quite a smart decision back then, because I think it was in the days when smoking was considered to be a healthy thing to do.

I've even heard that doctors would actually prescribe cigarettes to people who had a cough because they thought the smoke would help to clear the infection. It's amazing how times have changed.

By irontoenail — On Oct 16, 2013
After I stopped smoking, it took a while before my phlegm returned to normal. I was up to about two packs a day at my peak, which took a huge toll on my lungs. They have never returned completely to normal, even six years from my last cigarette.

But I stopped coughing up darker phlegm about a couple of months after I quit. It probably varies quite a bit depending on how much you used to smoke and how much exercise you do afterwards. I made an effort to get my lungs pumping after I gave up the cigarettes and I think that helped to clear my lungs a little bit.

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