On a clear day you can see forever.

Th e culprit in London’s killer fog wasn’t the fog itself. It was thousands of tons of tiny particles that clung to the stagnant fog and fi lled residents’ lungs. Th ick soot from the city’s coal-burning home hearths, diesel buses, and factories hung near the ground, trapped by a slow-moving temperature inversion. Black smoke concentrations measured during those fi ve days reached more than  times normal levels.