Comment by amluto

Even in a medical context, the difference, when operated like an air purifier, is negligible.

The genuine HEPA filter in a cleanroom [0] is not sitting in front of a fan in the middle of the room. It’s very carefully installed such that all the air coming into the clean area goes through it once. The calculation is entirely different. (A medical or industrial HEPA filter may well be in the exhaust, in which case the considerations are again different.)

[0] There’s none of this “true HEPA” stuff in a cleanroom. There is a filter that meets a specific standard, and that filter will have a gasket that seals with considerable force against the air handling equipment. The “true HEPA” filter in a Wirecutter-approved air purifier achieves nowhere near 99.97% due to the lack of the aforementioned gasket regardless of how amazing the filter media may be.