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...

Even if you live in an air quality paradise, it’s not ideal for your indoor air to be the air that manages to sneak through all the little cracks in your structure. Especially if you have cold outdoor temperatures, indoor humidity such that the outdoor temperatures are below the indoor dew point, and airflow through the walls that can lead to condensation and possibly mold in those walls.

Your indoor air should enter through windows or intentional intakes, not incidental gaps.

> 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.

Living in the countryside won't save you. I spent my childhood in a rural area and our house had the misfortune of being situated on a steep hill, so at all hours of the day and night you'd have cars and motorcycles and tractor trailers revving their engine to get up that hill. Every year we'd have to powerwash that road-facing side of the house to clean off the accumulated black grime, and sleeping with my window open, which faced that same road, always caused me to wake up raspy and hacking. Cars are a problem no matter where you live.