In 1996, the United Nations held its "Habitat" or "City" world conference in Istanbul. A major issue was the contributions of migrant workers to cities, and the problems caused by increasing migration to the world cities. Some believe that migrant workers make great contributions to the prosperity of cities, and others claim that more migrants to cities destroy the ability of cities to sustain their own prosperity.
Mexico City is now the world's largest city, with a population of eighteen million. Much of the growing migration to the Mexico City and other world cities increases their own individual prosperity, but places serious and almost impossible demands on the city. Since many migrants to cities are poor, they often require greater social and welfare support by the city, and the city often finds that it is almost impossible to provide all of the health, shelter, anti-pollution, and environmental requirements that this rapid addition of migrants causes the city itself. Thus, while migration to cities may have benefits individually and to the city, it is just as possible for there to be more disadvantages than positive contributions.