Enough ventilation to keep the air smelling sweet
In 1850, Florence Nightingale visited the military hospitals of the Crimean war and found them dirty and dangerous. The famous saying is that British soldiers have more chances dying in the hospitals than on the battlefield. She made changes in patient crowding, sanitation and ventilation. At that point many hospitals were not ventilated. She noted down: “Enough ventilation to keep the air smelling sweet.”
Current requirement of 2 ACH of outdoor air ventilation
She has also greatly influenced the architecture as hospitals designed from about 1820 to 1920 in Europe and USA were often designed as Pavilion Hospitals or Nightingale wards. Her idea was to separate wards with two-sided access to natural ventilation. The current requirement of 2 ACH (air changes per hour) of outdoor air ventilation can be tracked back to this period. At that time it was common to refer to ventilation space and volume per bed (9 m2/bed, 28-43 m3/bed and 57-85 m3/h per bed) which can be shorten to 2 ACH. These volumetric principles were commonly used into the early 1900s.
Source: ASHRAE Journal June 2016 by Travis R. English