An air filter including a visually uniform filter life indicator (30), the air filter (1) comprising: a conformable, unframed air filter media (10) that is installable on at least a portion of an upstream face (11) of a filter-support layer of an air-handling system, wherein the conformable, unframed air filter media comprises at least one passivated area (20) that provides a visually uniform filter life indicator of the air filter. Filter life indicators are used in order that aspects of the filtration performance of an air filter can be monitored.