When you hear the words "pea soup", you probably think of, well, pea soup. But pea soup is also the name of a particular type of thick fog, and in the early 20th century London was literally drowning in it.

Also known as black fog or killer fog, it's caused by soot particles and poisonous sulfur dioxide produced by the burning of soft coal in homes and factories. The fog is potentially lethal, especially for the elderly and those with respiratory problems, and as you can see from these vintage photographs, there was no way to escape it. The choking pollution culminated in the Great Smog of 1952, and although the Clean Air Act was implemented just four years later, an estimated 12,000 people are believed to have died as a result of the hazardous air pollution. (h/t: vintage everyday)