London Mayor's smog deaths claim comes with a health warning

While the Saharan dust cloud that saw air quality across much of England deteriorate last week has now largely dispersed, the political storm left in its wake continues to rumble on.

Writing in his Daily Telegraph column this morning, London Mayor Boris Johnson claimed that it was his ambition for the capital's air to be "alpine in its freshness." According to the Mayor, this isn't currently the case, as he pointed out that:

"We know that there are 4,300 lives in London alone that are brought to a premature close by this pollution."

We took a look at the link between air pollution and mortality on Friday, and found that while it's often difficult to trace a direct link between air quality and premature deaths, one study had estimated that the equivalent of approximately 29,000 deaths could be attributed to pollution.

Boris Johnson is referencing another study, conducted for the Greater London Authority in 2008, which found that the equivalent of 4,267 lives were lost in London that year because of poor air quality.

However there are a couple of reasons to be cautious with this figure. The first is that it is now six years old, and as the Mayor himself claimed in his article, over the same period air quality has improved (although these claims have been disputed). If pollution is now lower on London's streets, we might expect the number of deaths linked to air pollution to begin to fall too.

Equally importantly, the estimate comes with its own health warning, with the researcher responsible saying that the true impact of air pollution on mortality could actually lie within a range of 756 to 7,965 deaths per year.