This essay highlights how nineteenth-century epidemic diseases catalyzed environmental thinking in architecture by examining air and vapor bathing technologies and their role in transmitting environmental medical principles into British building culture. Vapor baths were popular medical devices used to fortify a patient’s health by passing hot or cold air charged with steam over the skin, a vital membrane through which impurities were thought to be excreted from the body. This essay discusses vapor bathing’s role in establishing environmental mediation as an underlying principle of modern British architecture. It shows how bathing practices were adapted to deliver localized forms of atmospheric medicine inside buildings, making architectural interiors a key site for the formulation of Victorian ideas of public health. Bathing offers evidence of how the climate-controlled interior emerged out of a confluence of medical ecology, moral demography, and technical innovation.