This entity also exists in Deathmatch Classic but works in a different way than Counter-Strike.
For the DMC version of this entity, please consult this page instead.
Attributes
Fog density (density) - Density/strength of the fog. The value should be less than 0.01, otherwise the the fog will not appear. 0.009 is the thickest and lesser value means thinner fog.
Fog color (R G B) (rendercolor) - The colour of the fog.
Notes
env_fog was never used in official Counter-Strike maps but was used a lot in Condition Zero. Since both games share the same basic code, the entity still functions well in Counter-Strike. Most Counter-Strike FGDs do not come with this entity listed so you can manually add it with Notepad as follows: