I've lived in Pittsburgh my whole life; this 'ranking' sounds strange to me, as do plenty of other good (crime stats, etc.) and bad (median income, etc.) rankings for this city.
City ratings such as these are inherently inaccurate simply because of Pittsburgh's size (meaning, area). The city of Pittsburgh area only encompasses 55.5 square miles: tiny! The 'city' only has 350,000 people, but closer to 1.2 live in the area and call Pittsburgh home.
Jacksonville, an area with a similar overall population, is 874.3 square miles. Columbus (I chose another similar-sized city at random) is 210.3. Since traffic congestion and industry is typically concentrated in the city center (and it certainly is in Pittsburgh), if one were to take air samples in different parts of the 'City' of Pittsburgh, the farthest one would get from the central area is just a few miles.
Few of the suggestions as to you should live are in the city center area. I am a runner, and I've never had an issue with the air quality except when running on the Eliza Furnace Trail downtown, which goes right along the 376 Parkway.