The news that Oxford Street is the world's most polluted place comes as no surprise to those people who work in or visit central London. What will surprise them, however, is the scale of death caused by London's air quality.

Back in 2000 when I became mayor, the government's medical advisers estimated that each year 1,200 Londoners would die prematurely – by an average of eight months. The main cause was the nitrous gases (NOx) and particulate matter (PM10s) which pass directly into the blood stream, triggering heart attacks, strokes and cancer.

When we introduced the congestion charge in central London, both NOxs and PM10s were cut by more than 10%. With this example to guide us, we set out to turn the whole of greater London into a low emission zone (LEZ) by making sure that heavy goods vehicles filtered their vehicle exhausts.

When the scheme came into force, my intention was to tighten the criteria and gradually extend it to all goods vehicles, but Boris Johnson deferred this process as one of his first acts as mayor. Two years later, in 2010, the government's medical advisers updated their analysis of London's air quality with the shocking conclusion that 4,300 Londoners died prematurely each year – by an average of 11 and a half years.

Johnson also faced the prospect that London would be fined £300m by the EU for breaching air quality standards. His response was to lobby the EU to defer the fine while employing attractive young women to hand out leaflets urging people not to leave their car engines running while stationary. I tried to make this a central issue in the 2012 mayoral election, but Johnson's response was always to assure the audience he had the matter in hand and it gained little media attention.

One of the worst causes of London's fatal air quality is diesel engines, and the LEZ should now be adapted to discourage all users from driving diesel engines in the city. Cabs and buses remain a big source of diesel pollution, but electric buses are now available. In the Chinese city of Shenzen, 3,000 electric buses are already working on their streets. London does not have one. The charging mechanism for the buses could also be available to our black cabs.

Before the last election, I met the manufacturers producing electric buses. In exchange for a contract to replace London's 8,000 buses over a five-year period, we would have been able to get a deal in which they were manufactured in Britain, thus saving lives and creating good jobs. Johnson's current policy is that this is an issue that cannot be tackled before 2020, thus dooming thousands more Londoners to an early death. We need to make sure it is an issue at the 2016 mayoral election.