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A vanilla electric furnace has a pollution output of 1/m. A tier one productivity module gives +5% pollution. Two would be +10%. Unless there's hidden matrix math, 10% added to 1 is 1.1, not 1.98 as the game claims. Tested with mods and vanilla. To reproduce, simply place two electric furnaces and put two tier one productivity modules in one of them, then compare the pollution.

You also have to factor in the increase in energy consumption which also adds pollution.

It's (misleadingly) not the full story, because there are two factors that affect an entity's pollution:

1. Changes to its power consumption
2. Changes to its (displayed) pollution attribute

The actual pollution is the relative energy consumption multiplied by the pollution value.

Prod module 1's have +40% energy consumption and +5% pollution. Two modules would thus give the total pollution of:

(100% + 80%) energy consumption * (100% + 10%) pollution = 198% = 1.98 pollution