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 assessed the epidemiological evidence in support of this hypothesis. No longitudinal 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 non-statistically significant association whereas the other study found no association. Many of the identified prevalence studies in children showed negative associations between
allergen exposure and current
asthma, and the weighted 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 little change in these estimates in studies in which children whose parents had adopted
allergen avoidance measures were excluded. Furthermore, evidence from population studies is equivocal and provides little consistent evidence that
allergen exposure is associated with the prevalence of
asthma at the population level. Population-based cohort studies are clearly required, but currently available evidence does not indicate that
allergen exposure is a major risk factor for the primary causation of
asthma in children.