Objective: Our goals were to analyze some of the similarities and differences in the increase in asthma, hay fever, and atopic sensitization between Europe and the United States and attempt to explain "inner-city asthma" within the framework of the hygiene hypothesis.
Data sources: We reviewed historical descriptions of hay fever and asthma as well as the currently available related literature.
Study selection: The authors' judgment was used in the selection of historical and epidemiologic evidence.
Results: Analyses of patterns of risk factors for allergic rhinoconjunctivitis and asthma in Europe led to a causal theory of the epidemic: the hygiene hypothesis. This theory claims that hygiene removed a protective influence against atopy and asthma that was once provided by exposure to infections in early life. This hypothesis has been questioned in the United States, where allergic asthma since the 1970s has increased among minorities living in poverty and with suboptimal hygienic conditions (inner-city asthma). When seen from a historical perspective, the recent increasing trend in respiratory allergies among the less advantaged in the United States may be explained as the consequence of several epiphenomena linked to westernization (including declining exposure to foodborne and orofecal infections) that first affected the richest socioeconomic strata during the 19th century, expanded among the middle classes during the first half of the 20th century, and eventually cascaded down to affect the least-advantaged Americans.
Conclusion: Inner-city asthma may be the final stage of a class-driven urbanization and westernization that began 2 centuries ago in the United States and that is now coming full circle.