Factorio Forums

if you hover on an assembly machine, it says: pollution 3

if you hover on an assembly machine 3, it says: polluion 1.8

I guess this is balanced by the used energy, but seems off neverthless

It's an incentive to get better machines early on instead of staying with assemblies 1 till modules. In normal games it's not a big deal, but with high biter settings minimizing pollution really matters.
It also represents the noise low tech machines tend to cause.