Car pollution killing millions?

Looks like another politically motivated green study. It is a pity that high impact journals get invoved in these.

Digging down from the news article to the actual research is difficult because it is even hard to find out what did they actually study. Titles are alarming, of course, but is there any substance.

First of all, what is the correlation between air pollution and diseases and deaths. What about causality? Did they measure the composition of air or something else?