London smog.

In 1952 an estimated 4,000 people died as a result of severe air pollution in London. Today the same number are estimated to die early each year because of poor air quality in the Capital.
Dr Benjamin Barratt, of the Environmental Research Group at King’s College London, measures air quality across the city. He says in some respects London is quite clean, but in others the city could be considered the worst in Europe.