The significance of conditions when growing up on the prevalence of atopy in adult Danes

INTRODUCTION: The prevalence of atopy in adults depends on both hereditary and environmental factors. The influence of environmental factors is explained in the "hygiene hypothesis" by maturational changes in the immunological system in response to limited microbiologic challenges in early life. Several studies have indicated protective effects of varying conditions in early life, e.g., living on a farm and in particular with livestock. This study analyses this problem in a Danish population. METHODS: This cross-sectional study used the European Community Respiratory Health Survey (ECRHS) protocol. A screening questionnaire concerning asthma, asthma-related symptoms and nasal allergy supplemented by questions about environmental factors was distributed to 10,000 randomly chosen individuals, 20 to 44 years of age, in five Danish counties. RESULTS: The questionnaire was answered by 7,271 individuals (72.7%). The prevalence of asthma and nasal allergy in adults was independent of growing up in the city or the country, but a significant inverse association was seen when growing up in the country in proximity to livestock. This effect remained after controlling for different hereditary and environmental factors. DISCUSSION: The results of this population-based study are consistent with the results from similar international studies. This study demonstrates that growing up in the country with contact with livestock protects against the development of asthma and nasal allergy in adults.