Chimney from Great Smog of London

Origin
The Great Smog of 1952 was a severe air-pollution event that affected the British capital of London. A period of cold weather, combined with an anticyclone and windless conditions collected the airborne pollutants from the use of coal into the air. This formed a thick layer of smog over the city that lasted for six days, dispersing when the weather changed. The thick smog moved across the city like it was alive, reducing visibility and even penetrating indoor areas. Medical reports explained that twelve thousand people died during those six days and one hundred thousand more were made ill by the smog's effect on the human respiratory tract. This was the worse air-pollution event in the history of the United Kingdom.

Effects
If touched by fog unleashes sulfur dioxide into the air.