When defining asthma in epidemiologic studies, the prevalence of asthma can be described in a number of ways. It is important to be aware of this when interpreting results of these studies. Point prevalence asthma at the time of the study survey, although the easiest measurement of prevalence to obtain, will likely underestimate asthma's presence within the population. Lifetime prevalence asthma at any time in a person's life presents the problem of recall bias. In general, the most commonly accepted measurement used is period prevalence, usually over the past 12 months. Some epidemiology of asthma exacerbations, some of which occur sporadically while others appear to be clustered and often associated with seasons of the year. Some of the latter may be truly seasonal, such as those caused by pollens and moulds, while some apparent seasonality may be driven by administrative decisions that coincidentally foster exposure to factors that cause asthma exacerbations.
Understanding of the factors provoking these seasonal cycles may offer significant opportunities for improved disease management. Some factors that can cause worsening of asthma symptoms, such as occupational dust and chemical exposures, are not intrinsically cyclic in their appearance but may be influenced by factors that are. Many exacerbations of asthma may result from a combination of insults, only some of which may be cyclic.
Asthma prevalence appears to differ between certain races and by socioeconomic status. In the U.S., asthma mortality is more common among nonwhites, those living in urban areas, and the poor.
The CDC analysis, mentioned in the Morbidity and Mortality section, found that African Americans more so than Caucasians experienced a higher annual death rate and a higher age adjusted hospital discharge rate for asthma as a primary diagnosis. Mortality rates were found to increase during this time and were highest in census tracts with the highest percentages of low income and minority residents, especially African Americans.