> but how does that translate to reality when 1/3 of their population lives in cities? Population density correlates with exposure, specially if we consider ma

> but how does that translate to reality when 1/3 of their population lives in cities? Population density correlates with exposure, specially if we consider mass transit systems. To put things in perspective: * Stockholm: population 910k, density 4200/km2, * Madrid: population 6617k, density 5400/km2, * Milan: population 3140k, density 7550/km2.