This story was originally published by the New Republic and is reproduced here as part of the Climate Desk collaboration.

It was known as the Great Pea Soup. In 1952, a thick, greenish-yellow fog smothered London, halting traffic and daily life. At the time, when households burned cheap coal for heat, factories spewed unregulated smoke, and buses burned diesel fuel, Londoners were used to a certain degree of greasy haze. But the Great Smog or Big Smoke, as this 1952 pea-souper was also known, was unprecedented. Bitterly cold air “soaked up the pollution and held it like a blanket over the city” for four days straight, according to the Daily Mail. Twelve thousand people died.

Sixty-five years later, our scientific understanding of air pollution has advanced immeasurably. We now know — because of events like the Great Pea Soup, but also a groundbreaking 1993 Harvard University study of smog-ridden U.S. cities and countless research papers since then — that short-term and long-term exposure to air pollution can kill people, particularly those with pre-existing conditions. “The evidence is so large,” said C. Arden Pope, a professor at Brigham Young University and world-renowned researcher of air pollution’s impacts on human health. “There are very few people conducting this research and publishing it in the peer-reviewed literature who don’t think fine particles pollution can lead to death.”