Asthma is a condition that likely results from complex interactions between multiple environmental and genetic influences. Numerous risk factors for asthma have been identified. The best studied risk factors, including gender, airway hyperreactivity, atopy, allergens, infections, tobacco smoke, obesity, and perinatal factors, are discussed in this topic review.
The epidemiology and natural history of asthma are reviewed separately. (See "Epidemiology of asthma" and "Natural history of asthma".)
There are clear-cut gender differences in the prevalence of asthma. Childhood asthma tends to be a predominantly male disease, with the relative male predominance being maximal at puberty . After age 20, the prevalence remains approximately equal until age 40, when the disease becomes more common in females. Reasons for gender differences are unclear and largely unexplored. Possible explanations include:
●The greater prevalence of atopy (ie, evidence of IgE sensitization to allergens) in young boys.
●Reduced relative airway size in boys compared to girls . Smaller airway size may also contribute to the increased risk of wheezing after viral respiratory infections in young boys compared to girls. (See "Virus-induced wheezing and asthma: An overview".)