The Emergence of Megacities

human history. The first cities were built on the plains of Mesopotamia near the Tigris and Euphrates rivers between the years 4000 and 3000 BC. The first city to reach a population of one million was Rome, in about 5 BC. Not until about 1800 AD had another city, London, reached the one million mark. Urban populations grew rapidly throughout the 19th century, fueled more by migration from rural areas to manufacturing centers than by absolute population growth. Throughout the 20th century, the number and sizes of cities grew, along with the percentage of the total population living in cities. In the past several decades we have seen the emergence of megacities--cities with populations greater than 10 million. Most of these are now in the developing world. Size confers advantages on cities, at least up to a point. Public works infrastructures and systems to deliver social services can achieve measurable efficiencies of scale, and larger cities can have increased capacities to stimulate creativity, innovation, and economic development. There is reason to question, however, whether megacities will continue to have these advantages. One of the first clear indicators of adverse health effects associated with megacities has been reported by the World Resources Institute (WRI). Children in cities with populations of more than 9 million, according to the WRI, are at greater risk for health problems due to air pollution than are children in smaller cities [1]. An analysis of the forces that drive the growth of megacities--and their public health and environmental impacts--could influence critical decisions that will determine the social and environmental outcomes of that growth.