When you see a vacuum cleaner or air purifier with a HEPA designation, that should mean that it has a HEPA filter that removes 99.97% of all particles as small as 0.3 microns in size from the air that passes through it.
But sometimes you’ll see machines with HEPA filtration that filters particles down to 0.3 microns.
There is a difference between the two, and it’s an important one.
“Down to” means that if all the particles being filtered were 0.3 microns – and no larger – the filter will trap 99.97% of them.
“As small as” means that 99.97% of all the particles going through the filter will be trapped, whether they are 0.3 inches or 0.3 microns in size. But that also means that some of the particles as small as 0.3 microns, as well as some of the larger ones, will get through the filter to air you breathe.
Something to keep in mind when you compare air purifiers.