In relative comparison, the air quality is surely much better in the east than in the west as heavy industries are focussed there. I used to be dealing with air purifiers and there were higher demands of it in the west than the east. My personal experience in the west was after parking my car there for a while, it was coated with a layer of soot.
As for seepage issue at the highest floor, building materials deteriorates with time. Hence it can happen not just to highest floor. I've a case just recently that even in the mid-floor unit with bad seepage issue after heavy rain.
Maybe if you are keen with highest floor units, it will be better to look out for signs.
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