The MERV System tells you what size particles each type of filter can trap. These sizes are referred to as microns. To help put this system into perspective, a hair from your head is about 60-75 microns in diameter. Particles under 35 microns are not visible to the naked eye. You can't see most of the contaminants in the air. Pollen particles can range from 10 to 100 microns in size; dust from 0.5 to 5 microns; human dander, 10 microns or higher; etc. The majority of harmful particles are less than 5 microns in size.