Italy has a permitted limit for fine particle pollution (PM10) of 50 micrograms per cubic metre. Some areas of Rome have exceeded that level — above which air quality is considered a potential health hazard — in 10 of the past 12 days.
Pollution levels have also risen sharply in other cities, including Milan, Turin, Florence, Piacenza, Parma, Reggio Emilia and Modena, forcing local authorities there to restrict drivers.
However, Rome was the only city to outlaw all diesel vehicles, drawing the ire of the Italian association of oil companies.
“This appears to be a totally unjustified decision from a scientific point of view, which offers no environmental advantage and therefore unnecessarily penalises a wide range of citizens," it said in a statement.
By contrast, environmental group Legambiente accused Rome of dragging its feet.
“This decision was incredibly delayed. The city council is only banning diesel following 10 days of poisoned air," it said in a statement.