Filters for Smoke Stacks
Add a Filters upgrade for Smoke Stacks, or a separated building for Filtered Smoke Stacks. It should recude the pollution, but suffer a reduced productivity and an increased manteinance cost.
I've no data to suggest a % pollution reduction by filters and so for productivity, but I find interesting the possibility to reduce the pollution, it could lead to a new production chain to construct filters, even a new mainteinance resource and building dedicated to filters.
There is already an Exhaust Scrubber in the game.