I'm no scientist but for years ICE cars have had a special pipe to send the noxious fumes out of the back of the car, well away from the occupants, which with a bit of hose has sometimes been used in successful suicides.
So I can't imagine that exhaust gases are anything but bad. Maybe a reduction in car use will enable them to measure their impact more accurately.
I also think wood burning stoves are a contributor but I remember the pea soupers from well before they became fashionable in cities. I suppose there was coal then though.
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