According to a new study, London’s dirty air is killing up to 9,500 people per year. The report, commissioned by Transport for London and the Greater London Authority (GLA), found dangerous levels of nitrogen dioxide (NO2) gas and fine particulate matter (PM 2.5), tiny particles in the air that can be linked to health conditions. The particles seem mostly to blow in from outside the city or come from natural pollutants, such as dust, but the NO2 comes from the diesel engines of cars, buses and trucks that crawl through London’s streets.

NO2 is the major problem, and just last year, London was accused of having “the worst NO2 pollution on Earth.” The study uses a standard called “life-years lost” to quantify the impact of the pollution:

The mortality burden is expressed as life-years lost across the population as a result of deaths in 2010 (a life year is one year lost for one person). This is the most accurate representation of the mortality burden, as it is when people die rather than whether they die that matters.

These years are then used to estimate the number of people that would have died if NO2 were the sole cause. But early death isn’t the only effect of nitrogen dioxide pollution. Four hundred and twenty hospital admissions were recorded in 2010 (the year of the study’s figures), with another 2,700 admissions due to inhaling the PM 2.5 particles.

One might conclude that motor vehicles and city-dwelling don’t mix.

London is working to reduce its fossil fuel burning transit. A new all-electric bus, for example, will soon be introduced–a zero-emission version of London’s iconic red double-decker buses. Previously, batteries to power double-deckers have been a problem. You can’t put them on the roof because the vehicles are already so tall, and if you put them inside there’s less space for passengers, negating the advantage of the two-floor design. The Chinese automaker BYD has come up with compact batteries that make the buses feasible.

London has almost 9,000 buses, so it’ll take a long time to replace them all, but the real problem is caused by the countless cars and rumbling trucks that clot the city’s arteries each day.