Urban upbringing and childhood respiratory and allergic conditions: A multi-country holistic study

Highlights: • Suggested higher microbial load indoors was associated with an increased risk of respiratory infections in early childhood. • The same exposure was linked to a decreased risk for asthma and allergic rhinitis in later childhood. • The assumed biological mechanism might be associated with greater family size. We integratively assessed the effect of different indoor and outdoor environmental exposures early in life on respiratory and allergic health conditions among children from (sub-) urban areas.