This is an interesting question. I thought the first issue to look into is how efficient standard paper filters are, and I found the info below on the Web. The authoritative AC Filter/Detroit Diesel SAE test determined that particles in the 2-22 micron range cause most wear in engines, so that's the particle range we're concerned about -- particles below that range shouldn't be an issue since the smallest bearing clearances in operating engines were measured at 2-3.8 microns.
Perhaps some of you engineering types can take the info below and translate it into useful analysis as to a Yes/No/Maybe answer to this "HEPA" question.
"As a result, most PAPER air filters won't catch anything smaller than about 20 to 40 microns with any real efficiency. In most cases, the more expensive the filter, the lower the micron level of filtration - and the lower the better, of course. Because of the depth-type nature of FOAM air filters and their added tack oil, they will remove most particles larger than 5 to 10 microns."
http://www.autoeducation.com/autoshop101/oil-change-4.htm
"AMSOIL Air Filters can filter out dirt smaller than ONE MICRON (conventional average 20 to 30 microns!)"
http://home.earthlink.net/~ztrem/AirFilters.htm
"Another KOHLER Aegis advantage is its heavy-duty air cleaner system that has been designed with a maximum filtering area (838 square inches) and fine-mesh main filter element (5 micron). Each system comes standard on the engine and features a reliable two-stage filtration system that covers a greater area than standard air filters. The main filter traps the dirty air, while the inner filter (8 micron) protects the carburetor from contamination during main filter servicing."
http://www.kohlerengines.com/press/pr_aegis27.html
(The SAE J726 air filter test code): "The OEM minimum specification is 96% with ISO (International Organization for Standardization) test dust which consists of a majority of 0 to 5 micron particles but some up to 20 microns in size. Test results showed that a paper filter has a 99.29% cumulative efficiency and a 96.47% initial pass. A K&N filter, with far less restriction than paper, has a 99.05% cumulative efficiency and a 97.11% initial pass."
http://www.brandsport.com/knfaqs.html
"We were surprised to see some disposable paper air filters with an overall filtration efficiency as low as 93%. The content of the test dust used in accordance with the testing procedure follows:
Particle Size in Microns: % by Volume (+/- 3%)
5.5 to 11: 11%
11 to 22: 13%
22 to 44: 19%
44 to 88: 28%
88 to 176: 16%
http://www.knfilters.com/faq.htm
"Engine air filters begin to restrict airflow into an engine at about 500mm of water (50 mbar)."
http://www.gud.co.za/GUD/gud.nsf/pgTechIS10P1?OpenPage
For what it's worth, I also discovered that CABIN air filters (vehicle interior) filter down to the 0.1-1 micron level, with about 100% filtration achieved at the 3 micron level. Some cabin filters also have a layer of activated charcoal that captures in-bound gases and odors.
Gasoline and diesel engine FUEL filters typically filter down to the 2-10 micron level. Lawnmower engine fuel filters down to 75-80 microns, with racing versions of same for go-karts at about 150 microns apparently.
