Indoor dampness is associated with respiratory symptoms in studies of homes and their occupants in many nations.1 Specific office building populations have allowed study of incident diagnoses such as building-related asthma and hypersensitivity pneumonitis in relation to damp indoor environments, with incidence density of such diagnoses increased as much as sevenfold after occupancy of a damp office building compared with the prior period during adulthood.2 ,3 Study of large numbers of persons in one building or school is more efficient than studies of occupants of many dwellings and allows increased precision in exposure classification for individuals. Within a single or small number of damp buildings, both measured microbial markers and observational grading of moisture indices are associated with a risk of building-related respiratory symptoms and diagnoses among occupants.4^–7 However, effective regulation in most countries requires demonstration of consistent relationships between environmental measurements and health risk across many buildings, and such evidence is lacking to date. The body of work underway in the HITEA study8 …