Proper ventilation helps improve indoor air quality. Ventilation can control indoor humidity and airborne contaminants, both of which either contribute to or act as health hazards. High indoor humidity can spur mold growth.
Ventilation: It is one of the most important factors determining indoor air quality. Poorly aired offices and schools, for instance can affect health and work or academic performance. Controlled ventilation is especially needed in heavily insulated buildings that allow little air exchange with the outside.
(GreenFacts- Facts on Health and the Environment)