Primarily devices for removing particles from aerosols. Filters utilize a variety of microscopic forms and a variety of mechanisms to accomplish this. Most common are fibrous filters, in which the fibers are of cellulose (paper filters), but almost any fibrous material, including glass fiber, wool, asbestos, and finely spun polymers, has been used. Microscopically, these fibers collect fine particles because fine particles vibrate around their average position due to collision with air molecules (Brownian motion). These vibrations are likely to cause them to collide with the fibers as they pass through the filter. Larger particles are removed because, as the air stream carrying them passes through the filter, some of the particles are intercepted as they pass close to the fibers and touch them. Other particles are in air streams that would cause them to miss the fibers, but when the air stream bends to go around the...