Just wanting to clarify the actual mechanics of this: is the 'smelting time' of an air filter the period of pollution reduction, or is completion of the 'smelting' when the pollution is reduced as an 'end result' of the process? Is there any chance of manually including the filter efficacy in the machine description (knowing that you can't get it to display negative pollution generation otherwise.) Thanks!

the mechanics is just like any other entity producing pollution just with a negative value, except when the pollution in the current pollution square is 0, then its just wasting filters and electricity.