A report published Monday by the U.K. think tank Policy Exchange has answers—some predictable, some unexpected. First off, it’s fair to note that London has made headway in cutting some emissions. The city’s particulate-matter (PM) pollution has dropped below EU safe levels, and sulfur-dioxide levels have plummeted. With NO2, however, there’s been little or no progress. Culprit number one is the huge uptick in the number of diesel-fueled vehicles on U.K. roads. As the report notes, in 1994 they made up just 7 percent of the entire U.K. fleet. Today, they represent 36 percent.

Ironically, environmental concerns have in part caused this spike. Included in a lower car-tax band, diesel vehicles have been favored by the U.K. government due to their lower CO2 emissions and greater fuel efficiency. This favoritism unfortunately overlooked their considerably higher NO2 and PM emissions. Diesel cars have also lagged behind gas-run cars in emission improvements and, the report states, performed worse on the roads than they have in tests:

The tests are currently performed in laboratory conditions, and it has been argued that they do not adequately represent real-world driving conditions—particularly urban driving conditions. The test cycle is “unrealistic and undemanding,” with cars able to accelerate slowly under relatively low engine loads, and therefore fails to represent real-world driving.

The pollution such vehicles create can be clearly linked to London’s high NO2 level, 45 percent of which comes from vehicles. Gas combustion (separated into domestic and non-domestic categories), non-road mobile machinery, industry, rail, and aviation also play a role, but none of these sectors’ pollution portions exceed 13 percent of the total. If London is going to clean up its air, it’s going to have to push for a root-and-branch low-emissions overhaul of its vehicle fleet.

Thankfully, London already has a plan to do just that, at least in its most polluted sections, and albeit quite far into the future. In September 2020, London’s congestion-charge zone will also become an Ultra-Low Emissions Zone where high-emissions vehicles will have to pay a charge to enter. The scheme is expected to cut central London’s NOx (the combined term for the pollutants nitric oxide and nitrogen dioxide) and PM levels by half. If London’s current pollution problem seems bleak, its pall should lighten considerably in the future.

Within this proposed zone, however, the pollution picture is a little more complex than in the city as a whole. At 48 percent of the total, NO2 emissions from vehicles here are even higher than elsewhere, but there’s also another major player in the pollution problem: Non-domestic gas combustion now contributes a hardly inconsiderable 33 percent of central London’s NO2 emissions.