Mucus is a vital substance that coats the mucous membranes in the body, including the lining of the lungs and airways. Normally thin, watery and barely noticeable, this liquid -- which is also referred to as phlegm -- changes in consistency and color in response to irritation, airway pollution, inflammation and infection. The appearance of phlegm, notably its color, can provide helpful clues to the nature of the cough and the underlying reason for the excess mucus production.
Lower airway mucus, or phlegm, is the first line of defense for the lungs -- keeping airway membranes moist and functioning properly. In concert with cilia, the tiny hairs that line the mucous membranes, mucus traps airborne substances such as dirt, infectious agents and allergens, while the cilia propels this tainted mucus upwards, so it can be coughed out of the lungs. Mucus also contains immune factors such as white blood cells and antibodies, so it can fight off airborne invaders. This substance is normally clear, but depending on the circumstances, mucus may appear white, yellow, green, red, pink or brown.
Behind the Colors
Airway mucus gets its color from a variety of factors, since phlegm may also contain cells, blood, microorganisms, aspirated stomach contents and other foreign particles. Brown phlegm, for instance, can be a result of inhaling dirt or coal dust, and red or pink mucus suggests the presence of blood in the lungs or airways. Yellow and green phlegm can sometimes, but not always, indicates the presence of infection -- since this hue is linked to the accumulation of myeloperoxidase, a greenish enzyme released by the white blood cells that fight the infection. Phlegm that is white, gray or cloudy is related to thick, slow-moving mucus that has lost some of its moisture.
White mucus may develop when the airways are inflamed due to allergies, asthma or bronchitis, which is the inflammation of the bronchial tubes that carry air from the mouth to the lungs. Phlegm that is white or gray may also occur briefly, at the start of a viral or bacterial infection, when the amount of infection-fighting white blood cells is still small. However, due to the absence of green color, white mucus is more commonly associated with no infection. While the color of phlegm provides a clue to what's going on, an accurate diagnosis is based on the full spectrum of symptoms, including a health history.
Coughing is a normal response to excess mucus in the respiratory system, and is an important reflex which expels irritants, allergens and pollutants from the lungs. But a persistent cough, and ongoing production of excessive mucus -- regardless of color -- should be evaluated by a doctor. If you are coughing up white, frothy mucus and have shortness of breath, wheezing and difficulty breathing, see a doctor right away as these are signs of fluid in the lungs, and may indicate an urgent medical problem.
Reviewed by Kay Peck, MPH RD
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