What are tea bags made out of?

My roommate thinks I’m weird because I put tea bags in the garbage disposal after I’m done with them. He seems to think that the material that the bag is made out of isn’t biodegradable. I always thought it was just cotton or some other natural fiber.

So does anyone know what the bag is made out of, or why I shouldn’t throw them in the disposal?

At least some newer teabags are made of polypropylene or polyethylene cloth. http://mitsuichemicals.com/spep/fybrel/