The London Fog, one of the attractions of London in the early 20th century, was actually a deadly smog. It covered the city for 90 days a year. Here are incredible images of this beautiful but menacing haze.
In the 1950s the "fog" became stronger than ever. The worst days were known as The Great Smog of '52 or Big Smoke, and lasted for only five days between 5th and 9th of December 1952. But the thick yellow fog caused the death of 12,000 people, according to research done in 2004. The Clean Air Act was approved by some countries, but air pollution is still a serious problem.