No one has a good word to say about air pollution as it can cause much misery, illness and even death – or, perhaps. more accurately, to be a significant contributor to all these.
The Mayor of London's Office is won't to say that air pollution in the capital causes "4000 premature deaths a year". The figure comes from an Imperial College environmental research group study and it's statistics such as these that justify the expansion of the Low Emission Zone into the outer London suburbs.
In marketing terms, a statement such as "4000 premature deaths a year" seem so pellucidly clear that even a child would comprehend it. We're surely entitled to conclude that 4000 Londoners die as result of air pollution each year who would not otherwise have done so.
But it's not what the Imperial study said; rather, it's an officially approved interpretation of what they said. The Times reports that the study concluded that between 61,800 and 70.200 'life years' are lost each year by Londoners because of air pollution. This is the equivalent (on average) of every person (in London) losing between 2.5 and 2.8 days of life expectancy each year: around a month every 10 years.
The conversion from this to the 4000 deaths figure comes through the use of a government algorithm which was created, it's said, to make the idea of life years more understandable to the public. Clearly this has not been a roaring success.
For a bit of context, the ONS says that life expectance will likely grow by about 49 days a year over the next 25 years.
The Times article examines all this and related matters in some detail. I recommend it, especially for its account of other research and of FoI data showing attempts by London politicians to get academics to change their inconvenient research reporting.