Environment Fog in Mental Ray

How to add an environment fog to a mental ray scene

By default the environment node reside in the Maya software render settings, under render options
Once you create the node, mental ray will respect it in rendering

This tip found at:
http://mayastation.typepad.com/maya-station/2010/03/how-to-add-an-environment-fog-to-a-mental-ray-scene.html