Pollution
Pollution is represented as an abstract "butt", calculated per chunk and visible on the minimap and Map, when Alternative view is on (default Alt-Key).
It is produced by many buildings involved in processing items and spreads outwards at a steady rate.
Native life
- Pollution attracts Biters to your factory. Biters who find themselves in a polluted area will attempt to reach the source of pollution and destroy it.
- Spawners absorb pollution, but this increases their evolution-factor.
Pollution dissipation
- slowly (0.6 units per tick) at every Chunk (32x32) of map it covers. So the more the pollution spreads, the more is absorbed.
- Trees also absorb some pollution. Here are some formulas: http://www.factorioforums.com/forum/viewtopic.php?f=5&t=3735&p=27919#p27599
- Spawners absorb some pollution, but this will increase Difficulty by rising the Evolution factor.
- In certain Mods it is possible to build devices or trees which reduce pollution.
Modules
Modules that list "+x% pollution" increase pollution multiplier, not a flat pollution rate. Final pollution value is (pollution multiplier * energy usage multiplier * base pollution), meaning heavily boosted buildings are likely to account for most of the pollution produced in a factory.