The claim: On some days Oxford Street in London is the world's most polluted street.
Reality Check verdict: This was true of Nitrogen Dioxide pollution in 2013. But on average in 2014, Brixton Road in Lambeth was worse than Oxford Street.
He unveiled a plan to deal with London's pollution problems last month.
His claim comes from research by King's College London, and it only refers to one pollutant: Nitrogen Dioxide (NO2).
NO2 pollution is mainly caused by diesel traffic and most of Oxford Street is only open to buses and taxis, which means a very high proportion of the traffic is using diesel.
Also, researchers at King's explain that a high proportion of the bus fleet on Oxford Street uses technology that burns off sooty particles but at the cost of increasing NO2 emissions.
In 2014, which is the latest full year for which data is available, Oxford Street had an average of 143 micrograms of NO2 per cubic metre and there were 1,532 hours during the year when that figure was above 200, although there were gaps in the data collected from Oxford Street.
The EU limit based on World Health Organisation guidelines is that the average NO2 should be below 40 micrograms per cubic metre and there should be no more than 18 occasions when that figure is above 200.
In 2013, researchers at King's said that average NO2 levels on Oxford Street were clearly the highest in Europe and that data available in the rest of the world suggested there were unlikely to be higher levels anywhere else, particularly because the heightened levels of NO2 were being caused by particular types of diesel engines.
But in 2014, Brixton Road in Lambeth was even worse, with an average of 153 micrograms per cubic metre, and 2001 hours above the 200 microgram limit.
So there may have been days when Oxford Street had the highest level of NO2 in the world, but overall, in 2014 Brixton Road in Lambeth was worse.
The final report for 2015 is not yet out, but from the raw data it seems that Oxford Street was back in top spot, although this time there are gaps in the data from Brixton Road.
Mayor of London Sadiq Khan told Radio 4's Today Programme on Friday: "Oxford Street on some days of the year is the most polluted street in the world."