MERV 1-4 (Greater than 10 microns): pollen, dust mites, sanding dust, carpet fibers
MERV 5-8 (3-10 microns): mold spores, hair spray, cement dust, powdered milk
MERV 9-12 (1-3 microns): Legionella, lead dust, coal dust, auto emissions, welding fumes
MERV 13-16 (0.3-1 microns): bacteria, tobacco smoke, insecticide dust, copier toner
One micron equals one thousandth of a millimeter. To put it into perspective what particle sizes these filters will remove from the air, you need to consider that the thickness of human hair is between 50 and 100 microns! As shown above, a MERV 11 rated filter will remove particles that are down in the 1 micron size range!