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"here is little reason that the exhaust shouldn't be cleaner than the air it is taking in."

And it isn't anyway. Just stand next to a running engine in a badly ventilated area for a while and you'll know that engine doesn't make things better.

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Actually, if you run a modern engine in a closed garage now, your eyes won't burn, you won't choke and it won't kill you. While this is a stupid thing to do, the bad stuff is only coming out before it's warmed up (meaning closed loop and catalyst warm enough to function) and if you back your car into the garage you can tell the difference the first time you do a cold start.

Better and Cleaner are 2 different things; No matter what, an ICE is going to output CO2 and H20; and depending on the fuel a few extra Cs.

CO2 or H2O concentrations are enough to make things worse for an engine or a human, but are not necessarily dirty.

NOX and CHs however are not as "naturally" in the air and producing them definitely makes the air "dirtier"

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