REUTERS/Paul Hackett

A big part of the answer appears to be diesel fuel.

When it was rolled out in 2008, London’s Low Emission Zone might have looked like a pretty radical move. Slapping a charge of between £100 and £200 (roughly to $150 to $300) per day on any heavily polluting vehicle, this zone covering almost all of Greater London was presented as a broadside against some of Europe’s worst traffic pollution. A new study published last month that charts the zone’s first three years finally shows the consequences that the move has had for citizens’ health. The answer is a shock—the zone has had absolutely no effect whatsoever.

A cautionary tale for cities worldwide, London’s failure shows that simply putting up barriers to access for dirtier vehicles isn’t in itself enough to guarantee a boost in air quality. In fact, pollution continues to harm London’s children at exactly the same rate it did before the zone was introduced.

Bringing kids into the issue straight off the bat might seem sensationalist, but it was exploring their conditions that formed the study’s basis. Conducted by researchers at Kings College London, the investigation measured the respiratory health of more than 1,800 children attending elementary schools near pollution hot spots in the city’s East, from 2008 to 2011. The researchers expected to see a drop in traffic pollution-related respiratory and allergic symptoms. When the figures came in, however, they found that the “LEZ has had no beneficial effect on these symptoms, up to three years after its implementation.” The most immediate explanation for this was pretty obvious: Conditions were still bad simply because air quality had not improved. According to data gathered by the London Air Quality Network, there was no evidence of a reduction in pollution at any of the sites.

But why was the air still so dirty? Looking at both the study’s conclusions and at the findings of British clean air campaigners, there’s a singular unanimity as to what is behind the problem. It’s diesel fuel. While noting that the Zone’s staggered introduction may have had an effect, the study’s authors lay the blame largely with:

“…the increasing proportion of diesel cars within the fleet…and evidence that NOx emissions from newer diesel engines…have not fallen as predicted by the current emission inventories.”

In reacting to the study, Simon Birkett, director of Clean Air in London, is blunter in pointing the finger at growing diesel use:

"We know that there are almost limitless problems with diesel vehicles, with the technology even separately to its health effects. We know, for example, that real world emissions are much higher than we would assume from looking at E.U. approval tests. We know that diesel vehicles produce from 90 to 95 percent of the fine particles and N02 in exhaust. We cannot reduce traffic related air pollution unless we get rid of diesel. It is a technology that has failed.”

Diesel advocates might well demur at this, given that the E.U. has introduced successively more stringent tests on diesel vehicles. A study published by the International Council on Clean Transportation last year nonetheless suggests that in themselves these are far from enough. It found a huge discrepancy between diesel vehicle emissions during regulatory tests and their actual performance on the road. Analyzing 15 new diesel passenger cars over a total of 140 driving hours, the study found that on average their emissions were seven times higher than the limit set by E.U. controls.

If cleaner diesel vehicles are possible in the future (one car in the study did manage to fall within E.U. emissions limits), current guidelines clearly aren’t doing enough to make this happen. Some clean air advocates even suggest that manufacturers are willfully trying to pass tests without making significant changes. As Ralph Smyth, Transport Campaign Manager at the Campaign to Protect Rural England, puts it to Citylab: “Vehicle manufacturers have cynically tweaked their engines to fool E.U. tests, preferring to risk people's lives rather than acceleration and top speed scores.”

If other cities want to beat their own pollution problems, they’ll need to learn from London’s mistake. Indeed, just across the channel, it seems they’re doing just that. On Tuesday, Paris introduced its own version of the Low Emission Zone, one that bans vehicles of over 3.5 tons from entering the city between 8 a.m. and 8 p.m.

On its own, Paris’s move might not do much beyond leveling out daytime pollution spikes, as trucks retreat to nights and early mornings to beat the ban. It’s been given more muscle, however, by another city objective. Mayor Hidalgo has also promised to eradicate all diesel from Paris by 2020. If the plan works and Parisians—unlike Londoners—find themselves breathing a little easier, the arguments for vehicle bans, rather than just fines and controls, will be increasingly hard to ignore.

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