This filter is 8x20x4 and comes 4 in a pack. It has a MERV 8 rating. Electrostatic charge captures airborne allergens SMALLER THAN 1 micron including fine dust, smoke, dander, mold, pet dander and dust mites. The filter also traps most large allergens including pollen and lint. It is up to 20-times more effective at capturing micro particles than ordinary fiberglass filters. Designed for uniform air flow. Lasts up to 3 months. The filter will not support the growth of bacteria, mold, mildew or fungi in normal use.