Almost five decades have elapsed since the effect of the so-called “urban factor” on lung cancer has been suggested.^1–3 Air pollution has always been an attractive explanation for the 10–40% increase in lung cancer mortality observed in urban versus rural areas, but confounding from smoking and other factors has been a great limitation in interpreting geographical comparisons. Several attempts have been made to specifically evaluate the role of air pollution on lung cancer aetiology during the 1960s and 1970s.⁴ In 1976, in a review for the International Agency for Research on Cancer (IARC), Higgins⁵ concluded that the studies available:

“provide support for the view that air pollution is a factor in this disease. But the effect of pollution cannot be large. It is likely to be a small fraction (possibly a tenth) of the effect of cigarette smoking.”

When the results of the two large American cohort studies on air pollution, the Six Cities Study⁶ and the American Cancer Society (ACS),⁷ were published, a strong association between particulate matter (PM) concentrations and cardiorespiratory mortality was reported, and …