Air Pollution in Cities

Air pollution varies substantially among cities, seasons, and days. High PM2.5 levels have been measured in Cairo and many other megacities, but “a day equals a pack of cigarettes” is a model-based analogy, not a medical equivalence or a precise estimate for every day. Such equivalents depend on concentration, exposure duration, and the chosen risk model; ambient pollution and tobacco smoke have different mixtures. [1–3]