Various filter media can be used to remove contamination in a number of applications.  Filter media may be designed to have different performance characteristics, depending on their desired use.  For example, relatively lower efficiency filter media may be used for heating, ventilating, refrigerating, air conditioning applications.  For applications that demand different performance characteristics (e.g., very high efficiency), such as for clean rooms or biomedical applications, high efficiency particulate air (HEPA) or ultra-low penetration air (ULPA) filters may be used. 
Filter media can be formed of one or more fiber webs.  A fiber web provides a porous structure that permits fluid (e.g., gas, air) to flow through the filter media.  Contaminant particles contained within the fluid may be trapped on or within the fibrous web.  Filter media characteristics, such as surface area and basis weight, affect filter performance including filter efficiency, pressure drop, and resistance to fluid flow through the filter media.  In general, higher filter efficiencies may result in a higher resistance to fluid flow which leads to higher pressure drops for a given flow rate across the filter media. 
There is a need for filter media that can be used in a variety of applications which have a desirable balance of properties including a high efficiency and a low resistance to fluid flow across the filter media, leading to high gamma values.