Airborne transmission of respiratory infections is a major cause of morbidity with wide spread economic impacts. The influence of the built environment on airborne transmission is not generally appreciated. This presentation will (1) review available data that suggest the airborne route is a frequent and important mode of disease transmission for upper respiratory infections, (2) present data from a recent observational study of outdoor air supply rates and absence rates among corporate employees, (3) present the current status of an experimental study of outdoor air supply and transmission of upper respiratory infections in offices, and (4) develop the argument that for these studies, measurement of outdoor air supply rates is unnecessary and should be replaced by continuous monitoring of CO2 concentrations.