Here's all the settings for polution: pollution= { enabled=true, -- these are values for 60 ticks (1 simulated second) -- -- amount that is diffused to neighboring chunk -- (possibly repeated for other directions as well) diffusion_ratio=0.02, -- this much PUs must be on the chunk to start diffusing...

/c game.map_settings.pollution.diffusion_ratio = 0.02

Also, you can reduce this:
/c game.map_settings.pollution.min_to_diffuse=15
(explanation from game files for this setting is -- this much PUs must be on the chunk to start diffusing)


Enjoy :*