Why open a window? The immediate stimulus is sensory: a need to rid a room of stuffiness,to purge it of malodors, or to cool it. As C. P. Yaglou and colleagues (l) remarked in1936: "According to the latest views the air of occupied rooms should give a favorableimpression on entering, taking into consideration such factors as odors, freshness, temperature,humidity, drafts and other factors affecting the senses" (p. 136).

For about two centuries, physiologists have sought to specify the relative roles of chemicaland thermal factors in the need to ventilate. A theory that stressed the importance ofoxygen content in the air achieved early prominence, replaced during the early 19th centuryby an equally untenable theory that highlighted carbon dioxide. Depletion of oxygen andaccumulation of carbon dioxide actually pose hazards only in atypical environments, such assubmarines or mines. Nevertheless, even large changes in the concentration of these gasesgo unregistered by the senses.

Just after the American Society of Heating and Ventilating Engineers mounted a nationwidecampaign to require 30 cfm per student in classrooms, it began to become apparent that thermalrather than chemical factors may actually determine the need to open a window or, more generally, to supply ventilation air. Various experiments revealed that a comfortably coolroom generally seems adequately ventilated to its occupants even when infused with no freshair over long periods. Conversely, a hot, humid room commonly seems stuffy and in need ofventilation even when infused with large amounts of fresh air (3).