Is allergen exposure the major primary cause of asthma?

In recent decades a number of authors have argued that allergen exposure is the major primary cause of asthma, and that the global increases in asthma prevalence are due to increases in exposure to aeroallergens. We have asse ssed the epidemiological evidence in support of this hypothesis. No longitu dinal studies were identified in which allergen exposure during infancy in a random population sample has been related to asthma risk after the age of six years. Two studies have been conducted in selected populations chosen on the basis of a family history of asthma or allergy; one study found a no nstatistically significant association whereas the other study found no ass ociation. Many of the identified prevalence studies in children showed nega tive associations between allergen exposure and current asthma, and the wei ghted averages of the population attributable risks in children were 4% for Der p 1, 11% for Fel d 1, -4% for Bla g 2, and 6% for Can f 1. There was l ittle change in these estimates in studies in which children whose parents had adopted allergen avoidance measures were excluded. Furthermore, evidenc e from population studies is equivocal and provides little consistent evide nce that allergen exposure is associated with the prevalence of asthma at t he population level. Population-based cohort studies are clearly required, but currently available evidence does not indicate that allergen exposure i s a major risk factor for the primary causation of asthma in children.