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One of the consequences brought by urbanization is this: such large concentrations of people pose immense environmental problems. In Mexico City, for example, one-quarter of the population has no access to running water, and one in five houses is not connected to the sewerage system. Located in a valley, the city suffers so badly from air pollution from motor vehicles that the city centre must periodically be closed to traffic. Mexico City’s problems are replicated in most other large cities in the developing world, and to a lesser degree in the large cities of industrialized countries as well.