Cities and industrial sites are the exception with bad air quality, not the default.
That said, I live in SF and my AQI is usually <50. Not as great as 5, but we sometimes get down to single-digits. Cities don't have to have bad air.
Haven't heard of them from grand-designs and those kind of youtubers)
just some ideas what some (public)/most (nice home) japanese toilets offer, which might be hard to come by anywhere else