The Airbox 4+ carbon filter has 12" flanges and a CFM of 3500. These units are completely airtight and easy to install, use, and maintain as they fit into any ducting or filtering system. Made with 100% premium virgin coconut charcoal, they feature replaceable high-flow filters which use 100% of the available surface area, for optimum performance. Great for commercial or residential use. (1) pre-filter included. Made in the USA.