the filter is absolutely *NOT* a HEPA filter.. In fact, no cabin filters for *any* make of vehicle are HEPA.. The small size of the filter, the high air flow requirements and the extremely variable air quality (especially since vehicle interior ventilation systems don't normally have a pre-filter) make a true HEPA filter unsuitable for the application.. They may be labeled as "High efficiency" or "High efficiency particulate" (I've just finished some equipment filtration specifications at work and common HVAC industry use of these terms means a filtration of 90% or better at a certain particle size (typically 2 microns), which is a typical "good" filter for most HVAC applications), but that does *not* make them HEPA (guaranteed 99.97% at 0.3 micron)- that last "A" in the acronym makes a huge difference in the actual filtration level!! Filter manufacturers will claim all sorts of efficiency numbers and acronyms, but they are all very careful not to claim "HEPA" when they are not, as they must be *certified* to bear the HEPA label.

A true HEPA filter of the small size in the Prius would clog up in just a few days of typical outdoor city air, and would introduce a significant amount of flow restriction even when brand-new and clean.. Other signs that this is NOT a HEPA filter include the fact that there are no polymer seals around the filter element or the filter tray- the seals are a requirement for true HEPA rating as just the small amount of air making it around filter at the edges would cause it to fail to meet HEPA filtration requirements; additionally, if you hold the OEM element up to the light, there are visible paths through to the other side- they're small to be sure, but on a true HEPA filter, the media is much thicker and the holes are so small that you can never directly see the light source..

The filter is simply a miniature high-efficiency filter of similar material to the better grade of household furnace filters.. I estimate that the filter is the equivalent of somewhere in the range of ASHRAE MERV 8-10.. I tried a piece of the high-end 3M Filtrete MERV 14 filter in my Prius and the restriction caused by the filter was much too high- the ventilation system sounded like a vacuum with a plugged hose (yet another reason why the OEM filter cannot possibly be a HEPA- if a MERV 14, which is very good, but still a lower filtration than HEPA is restrictive enough that it won't work in the car, there's no way that a true HEPA, with an even higher restriction level, would).. I went down to a MERV 10-11 filter with an activate charcoal layer to reduce odors which sounds close to the OEM filter when new but you can definitely hear the fans start to labor when the filter starts to get clogged, which supports my estimate of an equivalent MERV 8-10 rating on the OEM filter.