We have a small cleanroom at our company, so we wanted the surrounding AC system to be cleaner as well. We use a particle counter to check our cleanroom, and these filters are actually doing what they say. We check 0.3 um, 0.5 um and 5.0 um. All of them are tremendously reduced.The two images show the filtered reading and the reading outdoors at the same time. The AC system at our company brings in enough outside air that really you are limited by that every time through the filter. Even on a clear day, it's far dirtier than inside.I wish more companies would produce filters like these. The pressure drop isn't too bad due to the high density of pleats.We've used 8 filters so far, and of these 8, one of them was labeled differently as to the air direction. So we couldn't be confident about the correct direction. Probably it wouldn't matter. the test result was great either way.