Four of the world's largest and most polluted cities have decided to ban diesel cars and trucks from their streets by 2025.

The mayors of Paris, Mexico City, Madrid and Athens announced the commitment in Mexico City on Friday at the C40 Mayors' Summit, a meeting of city leaders.

Mayors of large American cities pledged climate leadership at the same summit, despite president-elect Donald Trump's position that there is too much regulation to protect the environment.

Nitrogen oxide and respiratory problems

The diesel ban is meant to address concerns about air quality, as diesel engines are responsible for emitting particulates into the air at ground level, as well as emitting nitrogen oxides.

Nitrogen oxides can help form ground-level ozone, which can lead to breathing difficulties for some people with respiratory problems.

The World Health Organization says seven million deaths every year are linked to exposure to outdoor air pollution.

The cities plan to promote walking and cycling, and encourage alternative fuels for diesel-using trucks, taxis and delivery vehicles. It's not known if the bans would be only in city centres or would affect wider areas.

The scandal involving Volkswagen's cheating on emissions tests for its diesel vehicles has focused attention on the role of the fuel in air pollution.

Plans for low emissions zones

London Mayor Sadiq Khan has proposed an ultra-low emission zone in central London by 2019, an area within which all cars, motorcycles, vans, minibuses, buses, coaches and heavy goods vehicles would need to meet stringent exhaust emission standards or pay a fee. Any fee would be added on top of London's existing congestion charge.

He has also proposed expanding the zone London-wide for trucks, buses and coaches. The proposals will go to London council sometime in 2017.

Such a zone started in Paris in July 2015. All vehicles must have an air quality certificate before they enter the zone.

"Mayors have already stood up to say that the climate change is one of the greatest challenges we face," said Paris Mayor Anne Hidalgo, now chair of the C40 Cities Climate Leadership Group.

An independent commercial test house in Britain tests on-road emissions. London has proposed a fee for cars entering the city that don't have a low-emissions certificate. (Hannah McKay/Reuters) "Today, we also stand up to say we no longer tolerate air pollution and the health problems and deaths it causes — particularly for our most vulnerable citizens."

Mexico City, with more than 25 million inhabitants and more than four million private vehicles, is extremely concerned with air quality. The city's policy of restricting which cars can enter the city by the last number of their licence plate exempts low emission vehicles, but it is looking to new ways to control pollution.

Cities want nations to help regulate

Mexico City Mayor Miguel Ángel Mancera said he has planned increased investments in public transport.

Athens also restricts vehicles from the city centre by plate number, but now Mayor Giorgos Kaminis says his goal was to remove all cars from the city centre.

Madrid has discussed a low emissions zone, but has yet to move forward on the plan.

The cities said they planned to work with federal regulators and manufacturers to promote electric vehicles and other clean alternatives, and move toward taking diesel vehicles off all roads.