The reason for this is that in much of the northern hemisphere, the prevailing winds are westerlies – blowing from west to east. The massive, unchecked pollution from these early industries would therefore drift eastward, making the air quality much lower in the east end of cities, lowering the desirability (and price) of the housing. Middle classes preferred the cleaner west ends.
In many cities, this will have been compounded – or confused – by the direction of the main river in the environment, which would have been relied on for many uses, including sewerage. London, as an example, displays a massive east/west divide, caused in large part by both early industry and the west-to-east flow of the River Thames.Of course, as polluting industries are moved out of affluent countries to eastern Europe, China and the developing world, the old industrial areas, positioned close to the centres of cities as they were before widespread car ownership, are being cleaned up and gentrified, and the rich/poor divide is turning into a boring-rich/exciting-rich divide.
In Australia (well, at least in Melbourne and Sydney), the relationship is reversed, with the western suburbs being poorer and more industrial, and the eastern suburbs being more affluent. One would imagine that this would suggest that the prevailing winds in Australia are easterlies, but the wind map linked seems to contradict this; if anything, Melbourne seems in the path of westerlies, and northeasterlies don't start until somewhere around the Tropic of Capricorn.
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