The Clean Air Act outlawed the emission of “smoke nuisances” or “dark smoke” and required new furnaces to emit little or no smoke. In 1968, lawmakers further strengthened the laws.

In 1952 London, 12,000 people died from smog ... government — after much denying any connection between the deaths and pollution — to pass the world’s first Clean Air Act. Christie’s ...

The act — along with its subsequent amendments in both 1968 and 1993 — led to a dramatic improvement in air pollution across the United Kingdom, Prof. Mudway said.