This testimony considered the activities of NIOSH in the area of indoor air quality. Energy conservation concerned in the 1970s forced the construction of buildings with the key element being preventing infiltration of untempered outside air. Many buildings were effectively sealed against air entry. Requests for health hazard evaluations due to a suspected poor quality of indoor air have increased dramatically in recent years. Indoor air quality problems may arise from a variety of sources including human metabolic activity, smoking, structural components of the building and contents, biological contamination, office and mechanical equipment, and outside air pollutants that enter the building. Many times the symptoms and health complaints reported by workers were diverse and not specific enough to readily identify the causative agent. The results from the health hazard evaluations have enabled NIOSH to classify the findings by primary type of problem: contamination from the building materials, 4 percent; microbial contamination, 5 percent; other contamination from inside the building, 15 percent; contamination from outside the building, 10 percent; inadequate ventilation, 53 percent; and unknown, 13 percent. Sources inside the building include duplicators, signature machines, blueprint copiers, pesticides, boiler additives, cleaning agents, tobacco smoke, combustion gases, construction glues and adhesives, lined ventilation ducts, and organic solvents. Ergonomic and psychosocial issues often complicated the findings.