Let me take it to an extreme. You could have an impermeable plastic film - that will give you 100% efficiency but 0 flow rate. We certainly do not want that. 13/

So, based on the pollutant size of concern, we decide on a filter type. There will be a range of filters in this group. They may have different flow restrictions. The compromise we need to strike is the bet value of efficiency times flow rate. 14/