Phat HEPA Intake Filter, 4"The tightly woven synthetic HEPA fibers in this filter will help prevent mold, bacteria, pollen, insects and plant pests from entering your growroom or greenhouse. The HEPA fibers block out most airborne particles down to . 3 microns in diameter. That is 300 times smaller than the diameter of a human hair. This is a serious insurance filter, without it you are leaving your grow room susceptible to the elements. The filter is also washable for extended