MERV is a rating based on the percentage of particles a filter can collect in a single pass. Because of physics, smaller and larger particles are easier to trap and most filters are least effective at 0.3 microns. MERV-13 is recommended because it blocks most particles in the 0.5 to 1 micron size. Many HVAC systems are built to run MERV-8 filters. As a result, allowing air to flow faster with less resistance but trapping only 40% of particles in the 1 micron size.