Air pollution 'as bad as smoking in increasing risk of miscarriage'

This is somewhat interesting, I find it equally interesting that since the 1900s we have exponentially decreased the pollution , especially such particulates such as magnetite with the obiteration of the steel and manufacturing industries. Certainly we need to remain vigilant making certain that we are being responsible.

The most interesting fact in which I came across after following the links and the looking at some miscarriage statistics is that the US (at least) performs more abortions than the reported number of miscarriage.

Note that they found a correlation with the contamination of NO2, not just any contamination. [I guess NO2 is absorbed in the lungs and enters the blood stream.]

Also, a ~50% increase of the concentration of the NO2 increase the miscarriage rate in a 16%. I don't understand exactly how they compare it to the smoking risk. How many cigarettes should someone smoke in a day to increase the miscarriage rate a 16%?