"Levels in the raw 2S scooter exhaust were as high as 300,000 μg m−3 or 146 p.p.m.(v) from idling. The EU annual mean limit for the protection of human health is 5 μg m−3 (ref. 13), while the US National Institute for Occupational Safety and Health recommends that workers wear special breathing equipment when exposed to benzene at levels exceeding 1 p.p.m. for 15 min. Waiting in traffic behind a 2S scooter, for example, at junctions and while the scooter is idling, may therefore be highly deleterious to health." [0]: http://dx.doi.org/10.1038/ncomms4749
But cities are full of shitty mopeds and old Vespas, for no benefit but cultural cosplay reasons.