European cities are usually worse than American cities for many measures of air pollution because the streets are more tightly packed, which means less ability for the pollutants to disperse. Housing and pavements for walking also tend to be closer to roads. On the other hand, the highest levels of pollution are usually experienced by drivers, because they’re sitting closest to the sources of pollution, in the flow of traffic and in effect just behind the exhaust of the preceding car. So it may be that pollution levels averaged over a city are not an accurate reflection of the health impacts on individuals, or on certain subsets of individuals. You’re much more likely to be walking in London compared to most American cities, rather than driving, but you’re also more likely to be living right next to a road with a lot of traffic. Either way, air pollution in London is a problem that has to be tackled, but it’s not going to be that easy to directly compare different cities.

That "article" is beyond worthless.