Pollution

Pollution attracts Biters to your factory. Pollution is represented as an abstract "cloud" visible on the minimap when Extended info is on (default Alt). It is produced by many buildings involved in processing items and spreads outwards at a steady rate.

Biters who find themselves in a polluted area will attempt to reach the source of pollution and destroy it. Pollution dissipates slowly (0.6 units) at every chunk (32x32) of map it covers. Trees also absorb some pollution.

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.