Hi John, Thanks for the info and the links. (Only the first link works)
I do agree fit is most important. Without a proper fit, no filter can help.

I understand there are both gaseous pollutants (e.g. CO, Ozone) and particulate pollutants (e.g. PM10, PM 2.5) within traffic pollution, do you know which is more damaging to human body?

Reason I ask is that masks containing both types of filters seems to have significant higher breathing resistance.