Uh, funny coincidence - to see this when I spent my day there.

"We observed that most of the metals detected came from vehicle and industrial emissions," says the researcher.

No surprise - it's an extremely car-centric city, and it has been like this for decades. And it shows that high demographic concentration isn't enough if people want less cars on the streets, you need to actually design it with pedestrians in account. Add the fact it's a strong industrial pole and here you go.