> I would not want to live in a city where I have to be careful letting in outside air or going outside because there's too much air pollution...

1. Not living in a city (polluted or otherwise) still does not solve the problem of letting out cooled air and letting in hot-humid air in the summer, and letting out warmed air and letting in cold air in the winter. If your CO2 is high are you going to crack open a window when it's -10 outside? Or in the middle of a heat wave (esp. if you have AC and paid to run it to cool your house).

2. Not-city living also has pollen and other allergen leakage. You're also more likely to get wild fire particulates in less urban areas.

Building tight and ventilating right is applicable in all locations and all climates.

And in the extreme case, if you believe the outside is the healthiest environment, live in a tent or under a tarp. :) Buildings were invented to have a separate outside and inside, and leaky houses reduce the effectiveness of that separation.