Sadiq Khan's figure on pollution deaths is a "zombie statistic and it's simply not true," according to a respiratory physician.
Figures have been released claiming pollution causes almost 40,000 premature deaths a year in the UK. Air pollution is also said to cause a total 340,000 years of lost life in the UK.
Figures originated from a Royal College of Physicians report and Mr Khan has cited the figures in announcing measures to cut pollution in London.
But Tony Frew, respiratory physician at the University of Brighton, told Julia Hartley-Brewer this is merely an example of a "zombie statistic", meaning "however much you try to kill it it comes back and it's simply not true."
He explained that the 340,000 life years figure doesn't equate to real life, and in reality each person loses only about three days from their lifespan as a result of pollution.
Frew also said that the 40,000 deaths a year figure is "a guess" using information about two pollutants which overlap.
He added that pollution levels are "illegal because we made it illegal, not because it's dangerous."
Listen to the full interview above