Originally Posted by Peaty
Air Flow is from top to bottom,
Honestly, I'm not sure it makes that big a difference which way the filter faces. It seems there is an orientation to the filter and one to the filter holder. It's an easy mistake to make if you ask me (I've done it myself)
I'm running a filter manufacturing business.
Normally we indicate the air flow when the e.g. filter media has a gradient fibre structure or is electrostatically charged on one side.
As to what I see on my filter pad, the structure is even. Can't actually check on electro-charge as to current life-cycle.
So from what I find, You could basically flip it as You like, filter function won't change.
As a pure opinion from the filter's media structure, the efficiency is no more than a MERV 7 or 8 (to stay on US standards), so with a bit of initial dust loading it will hold max. 40% of particulate ≤ 0,4 micron.
So all fine dust like diesel soot, brake dust, etc. will likely pass through without problem.
By the way, my estimation of the manufacturer sales price is ≤ 1,50 Euros/ 2,- USD