There are a lot of cases where old cars that have gotten very good maintenance pass the emmissions exams in flying colors.
One of the biggest sources of air pollution in the Mexico City basin are the old "beater" cars that see no maintenance. At one point the air in Mexico City was like a 3 pack a day habit. Makes me wonder how the 1 hour cycling records at the Mexico City velodrome were even possible.
Mexico City had a smog problem long before the invention of the automobile. It looks bad when you visit and see the traffic, but it would still be bad even without the cars.
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