Walking in the woods vs walking by a busy road?

This question is probably self explanatory; but I 'll explain what I mean. I have asthma; live in a fairly polluted town; I struggle with my Asthma especially when I don't go for walks in more "green" areas, I have today been for a 20 minute walk to pick the Niece & Nephews up from their respective Nursery & School; and took a short cut through the Valley area; the air always seems to smell sweeter amongst the trees than it does in paved areas. I have been horribly pale; pasty and ill looking all through this harsh winter. Now after that little 20 minute walk through the green wooded valley area; I feel rosier already!