Originally Posted by charliekilothree

The filter actually keeps the dirt and other airborne particles from passing completely through the ventilation system. When I cleaned my stock filter last year, there was a bunch of dirt and pinenuts in between the pleats. I remember having dirt from previous cars blow into my eyes sometimes when I turned the fan on high speed. While cars have been doing fine in the past w/o filters, they are better IMHO with in cabin filters.