The incidence of new cases of asthma is highest in those younger than five years. However, after age 10 and throughout adulthood, the incidence remains at approximately 0.2 to 0.4 new cases per 100 person-years at risk. Still, the number of cases that develop in persons who have never smoked cigarettes is sufficiently large that true adult-onset asthma cannot be considered rare. This disparity disappears in teenagers and even reverses in young adults. The prevalence of asthma is higher in black children than in white children, though the difference has decreased in recent years.
Large differences exist in the reported prevalence of asthma among different countries.
For example, the reported prevalence of asthma in the United States is higher than that in Japanese and Eskimo populations but lower than that in New Zealand. It is not known whether these differences reflect genetic or environmental factors or result from variable diagnostic criteria. However, data suggest that there are genetic markers on chromosomes 5 and 11 that relate to bronchial hyperresponsiveness and atopy. The prevalence of bronchial hyperreactivity far exceeds that of clinically manifest asthma. In large population surveys, many of the persons with hyperresponsive test results are free of respiratory symptoms.