We have a walkthrough to clean your unit yourself in two options,
The Longer Version(Venmar/Broan) The Shorter Version(Venmar/Broan)
Constructo 1.0 Constructo 1.0
The exhaust vents that pull the polluted indoor air from inside can be become quite dirty as well.
This video summarizes how to clean a High Efficiency Particulate Air (HEPA) filtered unit 31/4100 by Venmar.