Air changes per hour are a useful metric if you have a momentary release of a pollutant as it enables easy calculation of time to remove 95% of pollutant. It is less useful when there is a constant release of a pollutant - for example an infector releasing virus into the air 1/4

Because volume flux of outside air entering to dilute a pollutant in ach are dependent on room size. Stating x ach should be the minimum to reduce far field viral transmission is not helpful eg 6 ach in a 200m3 space is 333 l/s, whilst in a 1000m3 space it is 1666l/s 2/4

so if 6ach is ok in 200m3 space, then 333l/s should also be ok in a 1000m3 space which is 1.2ach.

we have the same issue when describing flow rates in terms of per person per second

10l/s/p for 5 person office is 50l/s
10l/s/p for 20 person office is 200l/s

3/4

If we want to dilute a constant pollutant source in an indoor space with outside air, we should be considering the volume flux of outside air in litres/second 4/4