Almost 9,500 people in London had their lives cut short by air pollution in 2010, accounting for a fifth of all deaths in the city that year, and the effect was deadlier where traffic was heaviest, according to a new report from King's College London.

The research was commissioned by Transport for London and the Greater London Authority and looked at the number of deaths associated with nitrogen dioxide (NO2) tied to vehicles' diesel fumes, in addition to particulate matter — such as dust, dirt, soot, smoke, and microscopic liquid droplets— in the air in order to provide a more complete understanding of London's air quality.

A past report from 2008 only studied particulate mater, to which it attributed 4,267 premature deaths. The King's College London study, which pooled results from multiple studies on particulate matter and NO2 exposure,puts the new death toll twice as high, at 9,416, and found that pollution associated with NO2 was the main killer: 5,879 early deaths were related to NO2 emissions, compared to 3,537 from particulates.

Heather Walton, an author of the study, said that because the researchers broadened their lens, looking at NO2, the researchers were able to compare air quality in different regions within a single city. Most research on urban air pollution looks only at particulate matter, which is more evenly spread in the atmosphere, providing a good measure of average pollution over a large area, but not for determining variation within an urban area. N02 concentrations, however, remain high close to where they are emitted but dissipate rapidly the farther one is from the source of pollution, such as a vehicle tailpipe.

The London study notes, however, that the link between mortality and NO2 was less clear than it was for particulate matter, acknowledging that deaths associated with NO2 may also be due to other traffic pollutants — making NO2 more useful as a proxy for deaths resulting from car emissions, especially diesel engines, which emit around 20 times more NO2 than petrol cars.

"While [the World Health Organization] considers that it was plausible that long-term exposure to nitrogen dioxide was associated with mortality, this does not necessarily mean that it is responsible for the whole of the effect," Heather Walton, another study author, told VICE News. "It is unknown how much of the effect is due to NO2 and how much to other traffic pollutants."

Still, by comparison, the study's death toll is higher than the 8,550 annual deaths in London resulting from smoking from 2008 to 2010. Figures also suggest that air pollution killed six times as many people in London as the number killed in car accidents each year throughout all of England, said Penny Woods, Chief Executive of the British Lung Foundation.

"Exposure to air pollution increases the risk of lung cancer, impairs child lung development, and increases the risk of hospitalization among people with a pre-existing lung condition," Woods told VICE News. "It is time we stop talking and take immediate action to prevent more people being needlessly killed by the air that they breathe."

Traditionally, particulate matter is the standard way air pollution is measured, and is the preferred yardstick used by organizations like WHO. Last year, the agency released a report on outdoor particulate air pollution for 1600 major cities across the globe, using data collected locally.

The WHO found that London's levels of PM2.5, which are very small particles of air pollution, was16 units per cubic meter of air, which was on par with New York's (14) and Paris (17), but lower than that found in Los Angeles (20). Levels of PM2.5 in South American cities were typically twice as high: Brazil's Rio de Janeiro came in at 36 units, while Santiago, Chile was 26 units of PM2.5 per cubic meter of air.

Beijing, a city widely maligned for its air quality, came in at 56 units of PM2.5 — roughly half of what the WHO found in five major cities in India.

The WHO estimated that outdoor air pollution killed about 3.7 million people in 2012, roughly 6.7 percent of all global deaths.

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