A cabin air filter is like a filter on a home furnace or air conditioner. It removes dirt, dust and pollen from air that enters the passenger compartment through the vehicle's HVAC (heating ventilation and air conditioning) system. The filter also helps keep the A/C evaporator clean. This allows the A/C system to cool at peak efficiency and reduces the buildup of contaminants on the evaporator that contribute to the growth of microbes that can cause musty odors.
Some cabin air filters are also "combination" filters that also remove odors, diesel fumes and other pollutants. Combination filters typically have a layer of activated carbon that reacts with airborne chemicals and traps them before they can enter the cabin. The fibers in the filter media may also be electrostatically charged so they will attract and hold dirt and contaminants more efficiently.