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 ...

/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 :*