$maxfogdensityscalar
The $maxfogdensityscalar when specified in a VertexLitGeneric material modulates the amount of fog the material receives.

Usage
$maxfogdensityscalar "0.97"
Caveats
Only tested in L4D2, support in other games not verified. Not known if its cheap or expensive.
Does not work in Team Fortress 2.