The leaders of four major global cities say they will stop the use of all diesel powered cars and trucks by the middle of the next decade.
The mayors of Paris, Mexico City, Madrid and Athens say they are implementing the ban to improve air quality.
They say they will give incentives for alternative vehicle use and promote walking and cycling.
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.
As the evidence has mounted, environmental groups have used the courts to try and enforce clear air standards and regulations. In the UK, campaigners have recently had success in forcing the government to act more quickly.
Now, mayors from a number of major cities with well known air quality problems have decided to use their authority to clamp down on the use of diesel.
The peak is due to the combination of emissions from vehicles and from domestic wood fires, as well as near windless conditions that mean pollutants have not been dispersed, the agency said.
Paris authorities have imposed a “half traffic” measure for the second day running, in which only vehicles with odd-number licence plates are allowed in the city and surrounding suburbs. (On Tuesday, it was even numbers). Public transport is free for the second day running.
