Hello! =)
I lived in Porto for some years, and although air quality isn't the same as in a rural area, it is quite satisfying. I was never bothered just by breathing in air and never noticed pollution except for the black dirt that got deposited over the curtains after some months. What is terrible tough, is humidity. Porto is near the ocean, thus Winters are rainy and foggy. My books would get soaked even if they never left their shelves! On the other hand, the smell of the Atlantic ocean is frequently felt at the inner city and is rich in iodine, great for your health 
All the best!
Raquel
