Paris, Mexico City will lead diesel-vehicle ban by 2025

The mayors made the announcement at the C40 Mayors Summit in Mexico City last week. They also said they'd spur investment in alternative forms of transportation such as bus, rail, and bicycles.

As many as 6.5 million annual deaths are caused by air pollution, hence the effort. Specifically, diesel exhaust contributes to lung damage thanks to its particulate matter as well as ground-level ozone. As a united front, the four mayors said they would push automakers to stop making diesel vehicles within the next decade.

They're not just talking, either. This summer, Paris imposed a ban on all vehicles built before 1997 – diesel or otherwise – from entering the city. Paris is looking to cut its greenhouse-gas emissions to 25 percent of current levels by 2050. The mandate was slated to impact about 500,000 vehicle owners in the French capital.

Meanwhile, Mexico City recently imposed one-time bans of vehicles in the city after smog checks and other pollution-reducing initiatives were relaxed last year. Specifically, Mexico City enacted a vehicle ban of about 1 million vehicles the city this past March on particularly bad air days, though drivers of about a fifth of those vehicles ignored the ban, extending that bad-air warning to four days. Beijing, Bogota, New Delhi, Milan, and Rome are among the other larger global cities that have also had to enact temporary bans on vehicles in an effort to combat oppressive pollution levels.