An air quality index maintained at King's College London, which started in 2012, found it was at its worst in history at 20 different sites.
Pollution levels were twice the legal hourly limit, leading to the Mayor of London, Sadiq Khan, issuing his first “very high” pollution alert.
“The shameful state of London’s toxic air today has triggered a ‘very high’ air pollution alert under my new air quality warning system,” he said in a tweet.
Mr Khan’s office estimates 9,400 premature deaths a year are caused by poor air quality in the capital.
A text message warning from City Hall sent to people in affected boroughs read: “High air pollution forecast for Thursday. Consider spending less time outdoors, take medication. If unwell contact GP.”
City Hall is taking steps to address the problem. More hybrid buses are set to be used in the capital and if individuals wish to drive high-polluting vehicles there are set to be extra costs to do so.
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