London smog/fog was legendary until the 1950s. In 1952 a cloud of yellow fog settled on London for five days. By the time it was clear, more than 12,000 people were dead and 150,000 hospitalized.
It triggered a lot of changes - but scientists were never sure quite why it was so bad. Until now. It turns out that London's fog was key to the situation.
The Thames valley makes fog, and morning mist is common in London (although even more common in the Trent valley where I grew up, where not being able to see one's own feet happened several times a year). The fog combined with the pollutants from everyone burning coal to make...sulphuric acid. Ow.
Which is why burning coal is, you know, a bad idea. Hello, China...