Biofilters work by using a porous solid medium to support microorganisms and give them access to the contaminants in the air flow. The media used vary tremendously. The particles may be organic materials, natural inorganic solids, or entirely synthetic. They vary in size, which affects crucial medium characteristics such as resistance to air flow and total biofilm surface area. The particles may be smooth, with a low surface-to-volume ratio, or they may have complex shapes and internal micropores which create a huge surface area for adsorption. Some media come with a culture of microorganisms, while others must be inoculated. Costs vary tremendously, from a few dollars per m3 for composted yard waste to many hundreds of dollars per m3 for manufactured media or coated polyurethane foam. The nature of the biofilter medium, or packing material, is a fundamental factor for successful application of biofilters. It will affect the frequency at which the medium is replaced and will have a major impact on key factors such as bacterial activity and pressure drop across the reactor. Consequently, the nature of the medium will influence both the removal performance and the operational costs.