By Mark Duff
BBC News, Milan
Milan is taking drastic measures to combat its chronic pollution problems.
The controversial ban aims to help rid the city of its smog
The Italian city has banned 170,000 older cars and scooters that do not pass strict environmental benchmarks, for a trial period.
Doctors say fumes contribute to the death of hundreds of Milanese annually.
The decision by Milan's first woman mayor, Letizia Moratti, to sign an emergency decree banning the vehicles on weekdays has been described as a "bolt from the blue" by critics.
Italy's love affair with the car has, for decades, frustrated any attempt to tackle the city's chronic smog.
'Most polluted cities'
Ms Moratti hopes the ban, expected to last until mid-April, will halve poisonous emissions.
She also wants to introduce what is being called a "pollution charge" for drivers who enter the central area of the city, as well as better public transport, more cycle lanes and the creation of more green spaces to absorb pollution.
Milan's shopkeepers are not happy about the ban saying they were not properly consulted. Environmentalists, on the other hand, say it does not go far enough.
Italy has one of the highest levels of car ownership in the world.
Milan regularly features on satellite projections as one of the most polluted cities in Europe.