Coughing is a normal reflex that helps keep the lungs clean by forcefully propelling accumulated mucus and debris from the airways. Inhalation of dusty air, secondhand smoke or air pollutants typically triggers a coughing episode. Repetitive or prolonged coughing often indicates an underlying abnormality. Although the possible causes of excessive coughing are numerous, infections, allergies and chronic lung conditions account for most cases of this symptom.
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Upper Respiratory Infections
Upper respiratory infections, including head colds, bronchitis, the flu and sinusitis, are among the most frequent causes of coughing, notes the National Library of Medicine's online encyclopedia, MedlinePlus. Viruses cause most cases of upper respiratory infection. Bacteria and, less frequently, fungi can also cause upper respiratory infection, or URI. With these infections, irritation and inflammation of the moist lining of the nose, throat and upper airways cause increased mucus production, commonly triggering cough. Most URIs resolve within a few weeks.
Allergies prove a frequent cause of cough, notes the American Academy of Allergy, Asthma & Immunology. An allergic reaction in the upper airways may trigger reflex coughing, which clears inhaled pollen, spores and other allergy-provoking substances from the airways. Alternatively, allergy-stimulating particles in the nasal passages may precipitate a post-nasal drip, which can lead to coughing. Treatment for allergies often reduces coughing and other allergy-related symptoms such as nasal congestion, sneezing, itchy eyes and scratchy throat.
Pneumonia is an acute infection of the air sacs and supporting tissue matrix of the lungs caused by bacteria, viruses or fungi. The infection leads to an accumulation of fluid, mucus and debris in the airways, triggering the cough reflex. Pneumococcus, or Streptococcus pneumoniae, remains the most frequent cause of adult pneumonia, reports the National Heart, Lung and Blood Institute.
Other bacterial causes of pneumonia reported by the University of Maryland Medical Center include Streptococcus pyogenes, Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa, Chlamydia pneumoniae, Klebsiella pneumoniae, Mycoplasma pneumoniae, Moraxella catarrhalis and Legionella pneumophila. Significant causes of viral pneumonia include respiratory syncytial virus, influenza viruses and herpes viruses.
Persistent, excessive coughing is a hallmark symptom of chronic obstructive pulmonary disease, or COPD. Chronic irritation of the airways leads to protracted inflammation with excessive production of mucus, loss of the mucus-clearing mechanisms of the lungs, and airway swelling and scarring. The cough associated with COPD is a "wet" cough, with large amounts of mucus expelled.
Cigarette smoking is the principal risk factor for COPD, reports The Merck Manual Online Medical Library. The risk for developing COPD is heightened in smokers with asthma or reactive airway disease. People with the genetic condition alpha-1-antitrypsin deficiency are also at increased risk for COPD.