As the world’s population expands past the six billion mark, it becomes more and more unevenly distributed. The great concentrations do not always occur on the most productive land; and people tend to gravitate towards what are often already large cities. In 1950 the largest metropolitan areas were all in the developed world- New York, London, Tokyo and Paris. Now those have been overtaken dramatically by Shanghai, Calcutta, Bombay and Singapore rising rapidly on the list. The developing world’s urban population now larger than the total population of Europe, North America and Japan combined. Many of the cities have grown beyond the control of planners and include illegal slums that pack millions of people together and concentrate population and disease. An estimated 600 million people in the cities of the developing world lack clean water, sanitation and secure homes. Even if living standards improve, cities seem set to expand putting more land under concrete and producing more fumes from industry and vehicles in congested streets.