Piccadilly Circus, 1983....

From a time when air quality was very poor, and it was common to have house facades covered with a black crust due to the combustion of fossil fuels.

Black crust....

Until the 1980s, this was a common phenomenon in large cities where entire facades could be covered with a black crust. Since catalytic converters became mandatory on new cars, the quality of the air has improved and the problem has gradually become almost a rarity in European cities.

Black crust....

..was a common view in the cities of Europe due to the combustion of fossil fuels. Today, the air quality is so much better that buildings and statues covered with a black crust are a rarity.