The mayors of Paris, Mexico City, Madrid and Athens say they will stop the use of all diesel-powered cars and trucks by the middle of the next decade. They are implementing the ban to improve air quality. They plan to give incentives for alternative vehicle use and promote walking and cycling. The commitments were made in Mexico at a biennial meeting of city leaders. 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.