Pack of 3x 5-micron Carbon Block Filters Activated carbon is a form of carbon that has been treated with oxygen to make it very porous, allowing for a larger surface area for chemical reactions to occur. The carbon absorbs organic materials and Chlorine from the water it comes into contact with. The Activated Carbon Block is enclosed in a 5-micron filter to catch tiny carbon dust particles that may break off during the filtration. What's in the box 3x 20 inch Big Blue (20x4.5" size) Carbon Block Filters