This chapter provides a brief overview of building design characteristics and their influence on indoor air quality. It discusses the following topics: site pollution characteristics, general building design, and ventilation and climate control. The quality of the ambient air is a crucial factor in maintaining a good indoor environment. The quality of ambient air is dependent on site location, surrounding population, and meteorological conditions. The Environmental Protection Agencies (EPA’s) “National Primary Ambient-Air Quality Standards” provides guidelines to evaluate the ambient air. The EPA also regularly conducts air quality monitoring at various sites throughout the United States. Indoor air quality concerns should include primary uses of the building, activities, or processes that generate contaminants, the presence of “special interest” groups such as day care centers, and any time-related demands on the ventilation system. High humidity, levels in excess of 60%, can promote microbial growth that contributes to poor indoor air quality.