Particulate matter types
Here is a small example of different particle sizes of common household particles if you are susceptible to certain airborne irritants.
A word on manufacturer’s rating systems
There are different methods of comparing the ability of how effective filters to do their job on the packaging literature, but I will just focus on the two most common ones you may run into.
MERV
which was developed by ASHRAE (ASHRAE 52.1, 52.2). Most filters use this method to describe their products filter efficiency effectiveness. Basically the higher the number, the higher the effectiveness for the cartridge (filter) to filter out smaller particles. This numerical system is the method required for filters in LEED buildings to measure particulate matter during construction and renovations when particulate matter is higher than when the building is occupied.
Particle sizes measured are 0.3-10 microns, and broken down into subcategories for effectiveness at different particle sizes. of 0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns.
3M Filtrete’s system (MPR)
The only reason why 3M Filtrete system is mentioned at all is because “filtrete”, easily the most readily available premium and arguably the most recognizable filter brand. Filtrete uses a “microparticle performance rating” system (MPR). This system only applies to 3M’s filters. The packaging states that 0.3 – 1 micron diameter particles is what it uses for its rating. Filters range from a rating of 300-2200. This MPR is mixed with a lot of marketing lingo in the explanation and not very descriptive on how it arrived at its numbers and what they really mean. The numbers appear to be arbitrary and I don’t think they really have much relevance to your purchase without a comparison to other well known rating systems. Good luck trying to find a MERV rating for Filtrete, you wont find it anywhere on 3M’s product literature, but from what I know, the most common one they sell (1000 MRP). Information I have gathered estimates these filters at around MERV 11 – 12.