The mayors of four major global cities -- Paris, Mexico City, Madrid and Athens -- announced plans to stop the use of all diesel-powered cars and trucks by 2025. The leaders made their commitments in Mexico at a biennial meeting of city leaders. BBC reports: At the C40 meeting of urban leaders in Mexico, the four mayors declared that they would ban all diesel vehicles by 2025 and "commit to doing everything in their power to incentivize the use of electric, hydrogen and hybrid vehicles." "It is no secret that in Mexico City, we grapple with the twin problems of air pollution and traffic," said the city's mayor, Miguel Angel Mancera. "By expanding alternative transportation options like our Bus Rapid Transport and subway systems, while also investing in cycling infrastructure, we are working to ease congestion in our roadways and our lungs." Paris has already taken a series of steps to cut the impact of diesel cars and trucks. Vehicles registered before 1997 have already been banned from entering the city, with restrictions increasing each year until 2020. The use of diesel in transport has come under increasing scrutiny in recent years, as concerns about its impact on air quality have grown. The World Health Organization (WHO) says that around three million deaths every year are linked to exposure to outdoor air pollution. Diesel engines contribute to the problem in two key ways -- through the production of particulate matter (PM) and nitrogen oxides (NOx). Very fine soot PM can penetrate the lungs and can contribute to cardiovascular illness and death. Nitrogen oxides can help form ground level ozone and this can exacerbate breathing difficulties, even for people without a history of respiratory problems. The diesel ban is hugely significant. Carmakers will look at this decision and know it's just a matter of time before other city mayors follow suit.