The Asian country’s road to cleaner air has been gradual, with lead-free gasoline only becoming a requirement in 2000, almost three decades behind the U.S. This month, China imposed new curbs on the amount of sulfur in vehicle fuels to about a fifth of the previous standard, putting it on par with Europe, which has the world’s strictest emissions controls. While the change helps China battle the smog that’s choking residents from Beijing to Xian, many of its refiners still produce the dirtier fuel.