Electrostatic Air Filters clean the air by using static electricity, a safe, naturally occurring phenomenon. An electrostatic Charge is generated by air flowing through a maze of static prone fibers. Airborne particles are attracted and held by the static charge until released by washing. BREATH EASY Electrostatic Filters contain an EPA registered Anti-Microbial material to [...]

That is what is called Dirty Sock Smell.  It is from a colony of micro organisms that can inhabit the evaporator coil.  In the AC mode there is a medium for colonization (the metal coil surface), Moisture (condensation from the air being conditioned), food (dust, debris, various airborne materials) all in a dark space (the [...]