phlegm is the mucus which we can cough up from the lungs. In the mouth it mixes with saliva (spit) to become sputum, which is then expectorated: phlegm plus saliva equals sputum, which is commonly studied by doctors to give signs of what is happening in the lungs.

In health the output of phlegm is too small to be measured accurately, but estimates give values of 15–50 ml/day, a minute amount. This is carried up to the larynx by the ‘ciliary escalator’, the wave-like movement of the hairs on the cells lining the trachea and bronchi. Once in the larynx, the phlegm is either coughed out, or more usually swallowed with, at the most, a throat-clearing ‘huff’. In disease, excessive production of mucus in the airways is characteristic of illnesses such as chronic bronchitis, usually diagnosed by the large production of phlegm; the mucus stimulates nerve receptors in the lining of the airways, which excite cough, and this leads to the removal of the phlegm. The commonest causes of phlegm production are airways infections, such as influenza, and cigarette smoking. Smokers' cough is due to the irritation of smoke stimulating mucus output from the glands in the trachea and bronchi. At night this mucus stays in the lungs, and when the smoker gets up in the morning the accumulated mucus is coughed up. The greatest output of phlegm is seen in a rare condition, bronchorrhoea, in which as much as two litres/day of sputum may be produced.

Analysis of sputum can indicate what disease process may be present in the lungs. If it is white or yellow, there may be pus and bacterial infection in the lungs: viral infections usually leave the sputum translucent. Green sputum may point to an infection with a bacterium, Pseudomonas pyocyanea, common in cystic fibrosis. Red colouration indicates lung haemorrhage. Black sputum is a sign of inhalation of particles, usually from cigarettes, but classically from coal dust in miners. Occasionally, jelly-like casts of the bronchi are seen in severe chronic asthma, and even parasitic worms can be coughed up from the lungs. Detailed analysis of the chemistry and types of cells in sputum is increasingly being used to help precise diagnosis of lung diseases.

Hippocrates listed phlegm as one of the four humours, that which was cold and watery. Here is a paradox, because in its Greek origin phlegm means ‘heat’ or ‘burning’, which is consistent with its appearance in lung infections and inflammation; Galen claimed that there was an excess of phlegm in fevers. But phlegm has come to symbolize a cold clamminess and, in its relation to human personality, coldness and dullness of character. It is the humour of the winter, when we have coughs and colds and expectoration. Hippocrates believed that epilepsy was due to an excess of phlegm blocking the airways so that the body became convulsed in an effort to free itself from the obstruction; but we now know that, although too much phlegm may be a sign of infectious lung diseases, and can cause violent coughing, it certainly does not cause epilepsy.

Coughing up phlegm is always a sign to be taken seriously, although it could be due just to a common cold or to a smoky environment.

John Widdicombe

phlegm / flem/ • n. the thick viscous substance secreted by the mucous membranes of the respiratory passages, esp. when produced in excessive or abnormal quantities, e.g., when someone is suffering from a cold. ∎  (in medieval science and medicine) one of the four bodily humors, believed to be associated with a calm, stolid, or apathetic temperament. ∎  calmness of temperament: phlegm and determination carried them through many difficult situations.DERIVATIVES: phlegm·y adj.ORIGIN: Middle English fleem, fleume, from Old French fleume, from late Latin phlegma ‘clammy moisture (of the body),’ from Greek phlegma ‘inflammation,’ from phlegein ‘to burn.’ The spelling change in the 16th cent. was due to association with the Latin and Greek.

mucus The slimy substance secreted by goblet cells onto the surface of a mucous membrane to protect and lubricate it and to trap bacteria, dust particles, etc. Mucus consists of water, various mucoproteins (collectively called mucin), cells, and salts.

mu·cus / ˈmyoōkəs/ • n. a slimy substance, typically not miscible with water, secreted by mucous membranes and glands for lubrication, protection, etc. ∎  a gummy substance found in plants; mucilage.

phlegm mucus, considered as one of the four humours (cold and moist) XIV; as a secretion of membranes XV; coldness or sluggishness supposed to proceed from the predominance of the humour XVI. The present form appears XVI, as the result of assim. to the Gr.-L. original or earlier fle(u)me, fleam(e) — OF. fleume (mod. flegme) — late L. phlegma clammy moisture of the body — Gr. phlégma inflammation, morbid humour as the result of heat, f. phlégein burn, blaze.
So phlegmatic XIV (fleu-).