There seems to be unnecessary/undue/excessive criticism in my homemade air filter thread, so I wanted to discuss filters in general.
Whether it be oil, fuel or air filters, in all cases they are a compromise between removing unwanted/harmful particles, and restricting flow to the point that they are doing more harm than good.
Two general statements about filters are:
* Large particles are harmful, and
* Small/minute particles are virtually harmless
Also, the mechanics of filter are such that a brand new filter actually filter less than an older/dirty/clogging filter. The reason: older dirt in a filter actually assists the filter media in trapping new dirt. You can see this by holding a brand new filter compared to a dirty filter up to a light and seeing how much light gets through each.
The bottom line is that a filter's precise micron capability is NOT all that critical; you just need to make sure that you have enough filtering capability that you are removing large to medium size particles, without removing such small/minute particles that you are restricting flow, or that the filter will clog in 10 minutes of use and have to cleaned/replaced before continued use, and I am confident that my homemade filter design fits the bill.