Image copyright GUILLOT/AFP/GETTY Image caption The mayor of Paris wants the city to become 'semi-pedestrianised'

The Mayor of Paris Anne Hidalgo wants to ban diesel cars and the pollution they bring from the streets of the French capital. But not long ago, diesel engines were thought to be environmentally friendly. What could have gone wrong?

Opinion on diesel cars has swung widely over the years.

Diesel is a more efficient fuel than petrol, but in the past diesel engines were often noisy and dirty.

Then, with growing concerns over climate change, car manufacturers were urged to produce cleaner, quieter diesel cars to capitalise on their extra fuel efficiency.

The cars were fitted with a trap to catch the particles of smoke associated with the fuel. Several governments rewarded the manufacturing improvements by incentivising the purchase and use of diesel cars.

But the policy has backfired.

Going into reverse

First, there have been problems with the particle traps - some drivers have removed them because they sometimes don't work properly unless the car is driven hot.

Second, the diesels are still producing nitrogen dioxide (NO2), which irritates the lungs of people with breathing problems. Diesels make several times more NO2 than petrol cars.

Now, in order to meet European air pollution laws, politicians are being forced into an embarrassing U-turn, telling drivers that they've decided they don't much like diesels after all.

MPs in the UK have mooted a scrappage scheme for diesel cars, while the mayor of Paris has called for a ban.

Several European nations are currently in breach of EU clean air laws.

The EU’s NO2 limit was exceeded at 301 sites in 2012, including seven in London. The concentration on Marylebone Road was more than double the limit.

Districts in Athens, Berlin, Brussels, Madrid, Paris, and Rome are also exceeded the ceiling.

Not just carbon: Key pollutants for human health

Particulate matter (PM): Can cause or aggravate cardiovascular and lung diseases, heart attacks and arrhythmias. Can cause cancer. May lead to atherosclerosis, adverse birth outcomes and childhood respiratory disease. The outcome can be premature death.

Can cause or aggravate cardiovascular and lung diseases, heart attacks and arrhythmias. Can cause cancer. May lead to atherosclerosis, adverse birth outcomes and childhood respiratory disease. The outcome can be premature death. Ozone (O3): Can decrease lung function and aggravate asthma and other lung diseases. Can also lead to premature death.

Can decrease lung function and aggravate asthma and other lung diseases. Can also lead to premature death. Nitrogen oxides (NO2): Exposure to NO2 is associated with increased deaths from heart and lung disease, and respiratory illness.

Exposure to NO2 is associated with increased deaths from heart and lung disease, and respiratory illness. Polyaromatic hydrocarbons (PAHs), in particular benzo a-pyrene (BaP): Carcinogenic.

Politicians are now scurrying to persuade the courts that they are obeying an EU demand to clean up the air as soon as possible.

The Paris mayor said at the weekend that she wanted the city to become ‘semi-pedestrianised’, with a ban on diesel cars in the city centre and some neighbourhoods given entirely to residents’ cars, delivery vehicles and emergency vehicles.

"I want diesel cars out of Paris by 2020," she said.

Ms Hidalgo hopes that her plan will improve the quality of the air in a city where, on average, people live six or seven months less than those who are not exposed to the same levels of pollution.

Adding electric vans and putting limits on tourist buses would also help lessen the public health risk, she said.

Premature death

Bikes are expected to become the favoured form of transport, with cycle lanes doubled by 2020 in a $141m (£90m) plan.

The Mayor of London, Boris Johnson has promised to halve pollution, spending around $516m (£330m) to bring 2,400 hybrid buses, zero-emission taxis and 10,000 street trees. The announcement came weeks after he was forced to accept that Oxford Street has some of the highest levels of NO2 in the world.

Central London will also have an 'Ultra Low Emission Zone' in 2020. Mr Johnson has previously faced criticism from health and environment lobby groups complaining that he was dragging his feet in meeting EU targets.

The UK government says it is responding to EU demands by bringing forward new plans. Labour say the government has ignored the issue - they demand low-emissions zones in all of the UK’s major cities.

Image copyright Thinkstock Image caption Shipping is also a major contributor to air pollution

According to the European Environment Agency, air pollution is the top environmental risk factor for premature death in Europe; it increases the incidence of a wide range of diseases.

Particulate matter (PM) and ground-level ozone (O3) are the most harmful pollutants.

Vehicles are by no means the only source of pollutants – some industries are major polluters too, and shipping in some places. But the politicians who run Europe’s biggest cities have protested that they cannot control pollution from industry elsewhere that drifts into their area.

With so many nations failing to meet pollution laws, the EU is under pressure to relax air standards.

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