Well most routers have vents for the heat to escape, but usually they don't have fans built into them.
If you can, either get a can of compressed air (or air duster as they are also called) and blow through the unit.
You'll be suprised how much dust these things can suck up.
I took my Playstation 3 to my local garage and used their airline with a blowgun and did exactly that.
the main mechanic and I were amazed at how much dust came flying out