Urbanization--an emerging humanitarian disaster.

I 2008, the proportion of the world’s population living in urban areas crossed the 50% mark. The current rates of urbanization suggest that in China, 870 million people — more than half the population — will be living in cities within less than a decade, and the capital of Botswana, Gaborone, will grow from 186,000 to 500,000 inhabitants by 2020.1 Most observers believe that essentially all population growth from now on will be in cities: the urban population is projected to grow to 4.9 billion by 2030, increasing by 1.6 billion while the rural population shrinks by 28 million.1 This transition is happening chaotically, resulting in a disorganized urban landscape. Although many expect urbanization to mean an improved quality of life, this rising tide does not lift all boats, and many poor people are rapidly being absorbed into urban slums. Urbanization, in fact, is a health hazard for certain vulnerable populations, and this demographic shift threatens to create a humanitarian disaster. The threat comes both in the form of rising rates of endemic disease and a greater potential for epidemics and even pandemics. To protect global health, governments and international agencies need to make commensurate shifts in planning and programs, basing all changes on solid epidemiologic and operational research. Although natural disasters and armed conflicts cause migration into urban centers, most people relocate to cities in search of employment. When they arrive, many find only one affordable housing option: illegal and unplanned dense settlements lacking basic public infrastructure, where they must live in lodgings made from tenuous materials, such as used plastic sheets, discarded scrap metal, and mud. The United Nations Human Settlements Program (UN-Habitat) reports that 43% of urban residents in developing countries such as Kenya, Brazil, and India and 78% of those in the leastdeveloped countries such as Bangladesh, Haiti, and Ethiopia live in such slums.2 These slums, which are making up an increasing proportion of growing cities, lack not only most basic government services but also political recognition; as a result, so do their inhabitants. These residents are usually tolerated and their presence tacitly accepted, but the local government generally ignores them, accepting no responsibility for accounting for them in planning or the provision of services. The current public health paradigm delineates urban health hazards as comprising injuries, pollution, and chronic diseases, such as diabetes and hypertension. Although these hazards are indeed more specific to urban than to rural areas, urbanization also exacerbates long-standing hazards specific to populations that have not undergone the epidemiologic transition from a predominance of infectious diseases. Increasing the population density in cities without proper water supplies and sanitation increases the risk of transmission of communicable diseases. Mortality among children under 5 years of age and among infants is higher in urban slums than in rural settings (see table).3 Though in most countries health care is more limited in rural than in urban areas, the urban environment may lack health support often provided in rural settings while also posing new risks. For example, for women and children, the rural environ-