In the early decades of the twentieth century, the rapidly growing air-conditioning industry in the United States promised a new form of architectural environment: An odorless, germ-free, and well-tempered interior that aimed at improving the efficiency of production and human conduct. These spaces became pivotal in the conception of modern architecture and were in turn, as this essay suggests, based upon a new understanding of air itself. Perceived no longer as a natural resource but as a compound product, air transformed into an actively configurable material mediating between the human organism and the architectural environment. The various case studies in this essay focus on the specific ways in which scientific knowledge, often backed by the commercial interests of industrial corporations, was employed to dislocate air from the realm of nature and place it in the hands of ventilation engineers. As this modern air became an essential material for the maintenance of modern buildings, the architectural interior gradually became the domain of air-control.