Crowded cities may have a special problem, especially if they have a serious ambient air pollution situation anyway. Those of us who live in the provinces often forget that, in my case the heavy scent of downtown Chicago at rush hour. However, most places including many smaller or medium size cities, with some open areas, I think the idea that people are picking up significant secondhand smoke exposure from passing pedestrians is absurd. Would it work to spray vitamins into the air for public health? No one would buy that. No one would believe benefits were being imparted.