Replacement Anti-Microbial 2.5 PM filter for the Winix T1 air purifier is designed to remove particulates of less than 2.5 micrometers (less then 20 times the width of a human hair) from the environment and Eradicate allergens and bacteria as they pass through the unit. The Winix 118460 Replacement Filter M comes with 1 Anti-Microbial 2.5 PM filter and for optimal performance should be replaced once every 12 months.