The World Health Organisation (WHO) says air pollution causes the death of seven million people a year and accounts for a third of fatalities from stroke, lung cancer and heart disease, with more than 90% of children breathing toxic air every day.

Until we tackle the root causes of air pollution - burning fossil fuels mainly - we can at least learn where and when it is likely to be at its most dangerous.

And this is where the data collected by tens of thousands of air quality sensors and analysed in real time by machine learning algorithms could help.

London was once nicknamed "The Big Smoke" in its coal-burning industrial heyday. Now it is traffic pollution that's smothering the UK's capital with "filthy, toxic air", according to mayor Sadiq Khan. This pollution contributes to more than 9,000 deaths a year, medical experts have estimated.

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