So here's the scoop. Straight out of the box, the 80/20 blend filters (the Wynn 9L300BL or the Camfil Farr 211736-1 HemiPleat) only capture about 50% of the 1 micron dust, and less than 30% of the .5 micron dust. This link on the Wynn site
http://www.wynnenv.com/filter_efficiency.htm shows the actual results when the filters are tested when brand new, and I have received information from Camfil Farr about their filters which show very similar results. This means that until the filters are used or pre-seasoned, they pass a LOT of the very dangerous dust. The rating given to them is MERV 10 (despite what the Camfil Farr brochure says about them being MERV 12 - that is old data). This is a good link that helps to understand what a MERV rating means:
http://allergyclean.com/article-understandingmerv.htm
So then, why do they claim 99.99% efficiency at 0.5 microns as Wynn does on the link above and Camfil Farr does here
http://www.farrapc.com/literature/products/hemipleat.pdf ? This last link gives the answer in the added word "
by weight". This is part of the old ASHRAE 52.1 test. I quote from a Camfil Farr Technical Service Bulletin: "A 5-micron size particle has a relative weight of 125. 125 1-micron size particles have a relative weight of 125. If 126 particles (one 5-micron & 125 1-micron particles) are fed to this filter, the one 5-micron size particle is captured and the 125 1-micron size particles pass through. This filter may be deemed efficient at removing 50% contaminant
by weight. However, the efficiency of this filter
by particle count is 1/126 of 0.088% or less than 1%." (Italics added) So the newer test checks actual particles while the older one tests weight. So when using this test, it can look like most dust is being caught when actually most of the smallest (more worrisome) dust is passing through but almost all of the larger, heavier particles are being caught. This is still a very common way of describing the capacity of filters in the industry, but then people in the industry should know what it really means. The rest of us are mislead because we don't understand what it means and expect it to mean 99.99% of the particles down to 0.5 microns (and the Wynn site doesn't even use the words "by weight").
Now this is BRAND NEW. The Wynn links shows (and Camfil Farr has sent me the testing on theirs) that as the filters are loaded their efficiency increases. The test puts 30 gms of material into the filter (doesn't seem to depend on size of filter and they don't say what size particles are in that 30 gms) initially, and then repeats that 4 more times without cleaning the filters. As there is more material in the filters, they work better. Neither give data on the increase in resistance, and therefore the decrease in air flow, due to the loading. But this is what is happening in practice in our shops, and I know that our old filters did keep the air really clean as tested by the Dylos.
In fact, these companies (at least Camfil Farr) actually sell "precoat material" to season your filters with before you use them. This dust has an average size of 40 microns, and the instructions say: "If the material to be collected is small in particle size (predominantly under 5 microns), oily, sticky or otherwise unfriendly to the cartridge, longer cartridge life and higher initial efficiency can be achieved by pre-coating the cartridges." The idea is, as Don01 stated, to coat the filters with larger particles that trap the smaller ones and, from the sounds of it, make it easier to clean the smaller particles out.
It finally got cold enough that we had to start using the filters. Within a couple of hours using the grinder the number of 1 micron particles coming out of the filter was between 3500 and 4500 as opposed to the 12,000 right at the beginning. Still far from good, but a whole pile better. And they will continue to get better as we get more dust into them. We don't have the ability to test to see increase in resistance or decrease in air flow due to the loading, but lots of air is still moving for sure. It will be interesting to see what comes through after we clean them the first time - if they stay relatively good or go back to passing more fine dust again.
So the practical application of all this? Brand new filters of this type still spew a lot of fine dust for a little while. So wear your masks for a while when starting new filters and be prepared to do a good shop clean after they get seasoned enough to trap the smaller particles. Or go the extra bucks and buy the filters labelled MERV 15 (Wynn 9L300NANO) or MERV 16 (Camfil Farr PTU-Poly-Tech Ultra High Efficiency) that really do trap 95% plus of the 1 micron sized dust when they are right out of the box. And look forward to spring when you can vent outside again.