Filter media for an active field polarized media air cleaner includes two layers of dielectric material with a higher resistance air permeable screen sandwiched between the lower resistivity electric layers. The filter media may further include a mixed fiber filter layer having fibers from different sides of the triboelectric scale. The filter media may further include a layer of relatively higher density dielectric material followed by a layer of relatively lower density dielectric material.

A method of making a fluorinated fibrous web, which method includes providing a nonwoven web 22 that contains polymeric fibers, creating a plasma that contains fluorine atoms at a first location 14, and contacting the nonwoven web with products from the plasma at a second location 26 remote from the first location 14. The method avoids exposure of the web to the plasma and hence expands the manufacturing processing window. Webs so fluorinated have a different C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio when compared to locally fluorinated webs having similar levels of surface fluorination. The remote fluorinated webs can be subsequently charged electrically to provide a good performing electret filter 40 suitable for use in an air purifying respirator 30. Webs fluorinated in accordance with this invention also may exhibit good performance even after being aged at high temperatures.

A method of making a fluorinated fibrous web, which method includes providing a nonwoven web 22 that contains polymeric fibers, creating a plasma that contains fluorine atoms at a first location 14, and contacting the nonwoven web with products from the plasma at a second location 26 remote from the first location 14. The method avoids exposure of the web to the plasma and hence expands the manufacturing processing window. Webs so fluorinated have a different C.sub.3F.sub.4H.sup.+ to C.sub.2F.sub.5.sup.+ ratio when compared to locally fluorinated webs having similar levels of surface fluorination. The remote fluorinated webs can be subsequently charged electrically to provide a good performing electret filter 40 suitable for use in an air purifying respirator 30. Webs fluorinated in accordance with this invention also may exhibit good performance even after being aged at high temperatures.

A collection unit, an air filtration system, and a method of producing a collection element for a collection unit is herein proposed for adsorbing hazardous or undesired gases as well as collecting airborne particles. The novel collection unit for an air filtration system includes a plurality of collection elements that are configured to provide surfaces to attract airborne particles by electric charge, wherein said surfaces of said collection elements are treated with a chemisorption compound. In the novel method, a collection element is provided and a gas flow is provided over the collection element surface. A chemisorption compound, such as one based contained in a liquid solution, is aerosolised to form an aerosol and, subsequently, the aerosol formed in the gas flow is deposited on the surfaces of said collection element.