Asthma
Signs and symptoms
Asthma is characterized by recurrent episodes of wheezing, shortness of breath, chest tightness, and coughing.[16] Sputummay be produced from the lung by coughing but is often hard to bring up.[17] During recovery from an attack it may appearpus like due to high levels of white blood cells called eosinophils.[18] Symptoms are usually worse at night and in the early morning or in response to exercise or cold air.[19] Some people with asthma rarely experience symptoms, usually in response to triggers, whereas others may have marked and persistent symptoms.
Associated conditions
A number of other health conditions occur more frequently in those with asthma including: gastro-esophageal reflux disease (GERD), rhinosinusitis, and obstructive sleep apnea.[21]Psychological disorders are also more common[22] with anxiety disorders occurring in between 16–52% and mood disorders in 14–41%.[23] It however is not known if asthma causes psychological problems or if psychological problems lead to asthma.[24]
Causes
Asthma is caused by a combination of complex and incompletely understood environmental and genetic interactions.[4][25] These factors influence both its severity and its responsiveness to treatment.[26] It is believed that the recent increased rates of asthma are due to changing epigenetics (heritable factors other than those related to the DNA sequence) and a changing living environment.[27]
Environmental
Many environmental factors have been associated with asthma's development and exacerbation including: allergens, air pollution, and other environmental chemicals.[28] Smoking during pregnancy and after delivery is associated with a greater risk of asthma-like symptoms.[29] Low air quality, from traffic pollution or high ozone levels,[30] has been associated with both asthma development and increased asthma severity.[31] Exposure to indoor volatile organic compounds may be a trigger for asthma; formaldehyde exposure, for example, has a positive association.[32] Also, phthalates in PVC are associated with asthma in children and adults[33][34] as are high levels of endotoxin exposure.[35]
Asthma is associated with exposure to indoor allergens.[36] Common indoor allergens include: dust mites, cockroaches, animal dander, and mold.[37][38] Efforts to decrease dust mites have been found to be ineffective.[39] Certain viral respiratory infections may increase the risk of developing asthma when acquired as young children such as:[40] respiratory syncytial virus and rhinovirus.[41] Certain other infections however may decrease the risk.[41]
Hygiene hypothesis
The hygiene hypothesis is a theory which attempts to explain the increased rates of asthma worldwide as a direct and unintended result of reduced exposure, during childhood, to non–pathogenic bacteria and viruses.[42][43] It has been proposed that the reduced exposure to bacteria and viruses is due, in part, to increased cleanliness and decreased family size in modern societies.[44] Evidence supporting the hygiene hypothesis includes lower rates of asthma on farms and in households with pets.[44]
Use of antibiotics in early life has been linked to the development of asthma.[45] Also, delivery via caesarean section is associated with an increased risk (estimated at 20–80%) of asthma—this increased risk is attributed to the lack of healthy bacterial colonization that the newborn would have acquired from passage through the birth canal.[46][47] There is a link between asthma and the degree of affluence.[48]
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