Thank you for your improvements to this mod, I love it. I noticed a small problem — not sure if you could fix it easily.

It seems like pollution only travels one-way: from inside factories to outside. This works fine for all the vanilla machines, where pollution is only produced. However, several mods add buildings with negative pollution (e.g. air filters, greenhouses). It seems like placing these inside a factory has no impact on the pollution outside the factory. Is this intentional, because it might be an overpowered way to clean pollution? Or just a side-effect of the pollution spreading implementation? Thanks.

This is a side effect of how pollution spreading works in the mod.
I don't think it's feasible to make pollution balance between factories for two reasons.

Firstly, there is a balance issue. It's very easy to create undefended territory inside factory buildings. If pollution is stored inside these buildings then there would be almost none left on the outside surface.

Secondly, based the way pollution is implemented, there is no mechanism that prevents pollution spreading into the "void chunks" between factories. If pollution was allowed to remain inside factory buildings for an extended period, it would eventually dissipate into void chunks.

The use of pollution scrubbers is still definitely possible, it's just only really useful on the outside surface.

Definitely an understandable state of affairs. It sounds like the mod looks for pollution in the factory and simply moves it outside. How possible would it be to instead simply obliterate pollution that is created inside the factory entirely, and then each factory sums the total pollution produced (positive or negative) and applies that as a pollution production or consumption on the factory building entity in the overworld surface? TBH, I'm not super familiar with how pollution works at an API level, so this may not be feasible, or it may be hilariously UPS-hungry, but figured I'd throw that thought into the ring.

That said, may be a bit useless of effort. Pollution consumers work best when spread out anyway, due to the mechanics of pollution spread between chunks. A single pollution consumer is usually capable of completely consuming all of the pollution in its chunk, and then it is limited by the "inflow" rate from nearby chunks, so putting a bunch in a factory probably wouldn't be any more effective than a single one, even if all of the issues above were solved. I've actually swapped to using Better Air Filtering as my pollution management mode specifically because its higher-tier buildings don't just consume pollution from their chunk, but from a radial area of chunks around the machine, making them much more effective as pollution cleaners due to the larger "inflow" surface area.

Making pollution teleport to the outside surface is a pretty good idea
Only issue is "outside surface" is pretty hard to define. Lots of edge cases where the outside surface doesn't even exist

For example if you mine a building it will still exist in item form, and all inside machine will still run. Since you mined it there is no outside surface

It's also possible to do a factory recursion where the inside factory is the same as the outside factory (infinite loop). This is another case where there is no outside surface.