Health care facilities have traditionally used two air changes per hour (ACH) for most spaces. This number can be traced back to the architectural literature of the 1870s. It has been carried forward in codes and standards for nearly 140 years and currently is in ASHRAE Standard 170. This paper proposes an alternative rationale for outdoor air ventilation (fresh air ventilation) in health care spaces, based on the contemporary methodologies of ASHRAE Standard 62.1 and other international indoor air quality standards.