A smart consideration. I have taken graduate coursework in environmental health and based on what I learned this is how I would prioritize things:
- Smog - cities have a lot of benefits, but their air quality is usually poor due to the high concentration of vehicles, living spaces, and businesses. If you want to live in or near a particular city, try to see if you can find out more about the overall air quality. Good indicators might be how many moderate to serious smog alerts they have in the summer time, or how many industrial plants are located within the city limits.
- Industrial sites/factory farms/plants/quarries/mines - Generally the closer one lives to these places, the worse the air quality (and the lower the home values). Ground water and soil are also notoriously dangerous within a few miles of very active sites. In particular, you want to steer well clear of fracking sites and CAFOs (factory farms). Both pose extreme hazards to the water supply in addition to their emissions.
- Highway/roadway pollution - If you are more of a rural or suburban type person, check out the highways. Ideally you don't want to be living right next to one, but a distance of even a quarter mile to half mile can make a big difference. I live a little over a quarter mile from a highway. I'd prefer to be further afield, but overall I'm not too concerned about it.
If it were me, I'd rather live closer to a highway than closer to an industrial site or in a big city if I was trying to avoid excess pollution. Some people also factor in the jet stream, but really if you are that neurotic about it, that will essentially prevent you from living anywhere on the entire East coast. Many new englanders live long lives despite being in the "tail pipe" of the country, so I wouldn't worry too much about this one.
A separate big consideration is you home's own indoor air quality. Tightly insulated homes are great for energy efficiency, but they lack circulation. Paint, furniture, chemicals, etc all off-gas into the air. Clothes and carpets release tons of particulate dust. Best way to reduce airborne dust is to reduce the amount of cloth/material in your home (rugs, carpets, furniture) and routinely clean with damp cloths to catch the particles. Indoor air purifiers can also be a wise investment if you have pets, or family members who suffer from allergies.