Biofilters are used on mechanically ventilated livestock buildings to treat the ventilation air. A bed of biological material, normally wood chips, is created and the ventilation air flows through the material. Gases are absorbed by cultures of microbes that develop in the bed.This fact sheet is part of the Air Management Practices Assessment Tool (AMPAT) developed at Iowa State University and funded by the National Pork Board. Additional resources can be found on the AMPAT web page at: www.agronext.iastate.edu/ampat