Reflex movement by means of which the air passages keep themselves clear; dirt, mucus and bacteria are forcibly expelled. The air passages are also supplied with hairs which keep up a continuous waving movement to sweep out dirt. Coughing can be a symptom of almost any condition of the air passages and lungs. A distinction should be made between normal coughing, in which mucus (sputum) is coughed up, and a dry, irritable cough. The colour of the mucus is important: white mucus suggests irritation of the air passages by a cold, for example; yellow-green mucus points to a bacterial infection such as bronchitis and blood in the mucus can be a sign of a serious condition such as bronchiectasis, tuberculosis or lung cancer, but this is by no means always the case. Coughing can be acute, in a cold, bronchitis or pneumonia, or chronic, as in chronic bronchitis. Smoker’s cough is also chronic, caused by continual irritation of the air passages by the smoke, but is often not even noticed because it has persisted for so long. A decision to consult a doctor about a cough should be based on the degree of discomfort and the length of time for which the cough has persisted. An irritable nocturnal cough can be most irksome. A cough that persists for coughing blood (haemoptysis) Appearance of blood in the mouth after coughing. This can be the consequence of various disorders, and can vary in severity from a streak of blood in phlegm which is coughed up to the coughing up of large quantities of blood. It is important to establish for certain that the blood was definitely coughed up, and did not come from the mouth (haemorrhage of the mucous membrane), the nose (nosebleed), or the throat. Vomiting blood (haema-temesis) is also easily confused with this condition. The countless causes include many harmless ones; it may be simple bronchitis, bronchiectasis, tuberculosis , a lung embolism or a deficiency in the clotting of the blood. Another cause can be reduced effectiveness of the left-hand side of the heart, causing increased pressure in the blood vessels in the lungs. In the case of cigarette smokers aged over forty lung cancer should not be ruled out. If blood is coughed up a doctor should be consulted; he will produce a diagnosis by means of associated symptoms, physical examination, and possibly an X-ray of the lungs and examination of phlegm which has been coughed up. Sometimes further tests in hospital are desirable, such as bronchioscopy (examination of the lung with a tube). In some cases no cause is discovered and the condition may disappear of its own accord; if it persists the doctor should be consulted again.