Air changes per hour (ACH)

Under ideal conditions – in which droplet nuclei are evenly distributed and room air is uniformly mixed – the proportion of infectious particles eliminated with each air change or one “equivalent air change” is 63%. A second air change removes 63% of what remains, and so on. One air change has occurred when the volume of air entering or exiting a room is equal to the volume of the room. Subsequent increases in air change leads to an exponential reduction in droplet nuclei in the room.