What Is The Cabin Air Filter Actually Filtering?

After a rain storm, my new-used 2012 ES interior smelled a tiny bit like wet dog.

Taking the cabin air filter out I found a significant amount of fine hair (dog hair?).

I was able to clean it out, but my question is, where did it come from? It was my understanding that the cabin air filter was to eliminate pollen and other external pollutants.

Does this mean that the hair was likely picked up from inside the cabin when the climate control was in the recirculate position?

If anyone else has examined their cabin air filter I was hoping they could report what types of debris that they found.

The filter filters the air coming into the A/C and/or heater wherever it come from - Inside air or outside air, depending on whether the LED in the bottom knob is lit or not. In our car, since we don't often use the heater, but use the A/C a lot, 90% of the air it filters comes from inside the car

We've never found anything the eye could see in either of our filters, but we don't own a dog - The previous owner of your car obviously did though

Don