Also called a pollen filter or cabin filter, the cabin air filter keeps the vehicle's interior air free from dust, pollen, and other unwanted materials. The most basic versions are just a cotton barrier that the air has to pass through and debris will get stuck in, while more advanced filters are electrostatically charged (which is just using static to more effectively grab unwanted particles) or filled with activated charcoal (which cuts down on odors). A cabin filter can protect the blower motor and resistor from debris that could burn up on the resistor or get stuck in the blower fan blades.
Not every vehicle is equipped with a cabin air filter, but some vehicles have an unusued area where a filter can be added if the manufacturer later decides to include one.
Changing Cabin Filters
Cabin filters need to be changed fairly regularly in order to keep the vent system working properly and in some cases, odor build-up. They tend to be located near the firewall, often on the passenger side near the glove box. The following video shows how to change a cabin filter in a second generation (2009-2015) Honda Pilot.
The next video shows how to replace a second generation Dodge Avenger cabin filter, which is also located behind the glove box:
Cabin Filter Replacement
HEPA Cabin Air Filters
A step beyond basic cabin Filters, High Efficiency Particulate Air filters can filter out finer materials to achieve even cleaner air quality.