Furnace filters have various rating systems, but the most common one is MERV, which stands for Maximum Efficiency Reporting Value. MERV ratings vary from 1 to 16. The higher the MERV, the denser the filter. But the denser the filter, the more the furnace has to work to pull air through it, so you don’t want to get a filter with a MERV rating that’s too high. Some manufacturers add other features to filters that are supposed to attract dust, thus increasing the capability of the filter without increasing the MERV rating.