Passive electrostatic filters have polypropylene fibers that pick up a static charge as the air passes over them. The static charge helps to hold fine particles. These filters can be washed and reused. Passive filters work better than furnace filters and fit into the filter slot of most residential air handlers. Pleated filters use a finer mesh, which catches smaller particles.
Pleated filters are one to six inches thick.
One or two inch filters will fit in the filter rack of many furnaces. Thicker filters are more effective, but require duct modifications. HEPA is short for High-Efficiency Particulate Air filters. HEPA filters have hundreds of square feet of filter paper folded into a couple of square feet of space. They remove very small particles, including some viruses.