Lots and lots of carburetor equipped trucks, with no catalysts, because leaded fuel is still in use, in Africa... where lots of monkeys live. Probably better for them I guess. 1960s levels of pollution? Wife's friend was just down there, she rode around in a Land Rover that (her words) smelled like 20 lawnmowers running at once.
Seriously, when does it end?
Ban them all! Make people walk! Or maybe monkey-drawn carriages? How many monkeys would it take? Would there be a poo-flinging guard?
NASA also used monkeys to do tests for the space program in the '60s.