The term “smog”—a combination of smoke and fog—was invented by a British doctor a century ago. In 21st century Europe, air pollution has greatly improved by most measures but is still a substantial health problem, responsible for the early deaths of hundreds of thousands of people each year.1 Estimates of mortality attributable to long term exposure to fine particles are now widely accepted as a key policy indicator of the effect of air pollution. A draft UK government report, written by the Committee on the Medical Effects of Air Pollution and currently open for public review, re-examines the scientific evidence underpinning these estimates.2

The most directly applicable evidence is provided by follow-up of large populations exposed to different long term average levels of air pollution. Findings of the first cohort studies by the American Cancer Society have been confirmed and extended by additional years of follow-up3 and extensive reanalyses,4 and with cohort …