Cities, urbanization and health

Abstract

It is in cities, and not the countryside, where the human ‘creative flame’1 has burnt most brightly. For millennia they have been the centres and drivers of commercial, scientific, political and cultural life, having major influence upon whole countries and regions. The positive and progressive aspects of cities and urban centres recognized by historians, economists and other social scientists contrast with the more pessimistic tone of much of the epidemiological and public health literature on cities and urban life. In part this derives from the iconic place in our discipline of accounts of poor urban health in the 19th century in countries such as Britain. Almost everyone who has taken a course in epidemiology will have come across John Snow's classic studies of cholera in London in the 1840s, with its attendant images of poor sanitation and contaminated water.2 This history finds a resonance in contemporary concerns about the impact of cities and urbanization on health today. First, there are the health and social problems of inner-city areas of high income countries such as drug addiction, violence and HIV/AIDS.3 Second, there is concern about the health and welfare of the growing number of people in low and middle income countries who are living in urban areas—many of whom are in what are classified as slums.4 Third, as mentioned by Dr Chan the WHO Director General in the 2007 World Health Report,5 increasing global urbanization is classed as a threat to ‘Public Health Security’ as the unprecedented level of population agglomeration may facilitate the spread of epidemic diseases. Finally, there is the plausible connection between urbanization and the rising levels of non-communicable diseases in low and middle income countries. This editorial sets out to challenge the largely negative view of the population health impact of cities and urbanization in the contemporary world. While there are real urban threats to health, on balance cities and urban living today have important health advantages particularly in the developing world. It is desirable that epidemiologists and public health advocates develop a better and more informed understanding of the nature of urbanization and the differences between urban and rural life today. In particular careful study is required of the way in which urban–rural differences in disease risk are heterogeneous and often context-specific. A number of striking statistics about urbanization are frequently cited in reports and research papers. It has been estimated by the United Nations that in 2007, for the first time in history, half of the world's population were living in urban areas.6 This follows decades of continuous increase in the proportion of the global population living in urban areas. This increase has been particularly in low and middle income countries (Figure 1). In sub-Saharan Africa the proportion of population in urban areas is estimated to have risen from just over 10% in 1950 to over 35% today. Most of the growth in the world's population over the next 25 years is anticipated to be in the urban populations of low and middle income countries, particularly those in Africa and Asia. Trends and projections in urban population as a percentage of total population by world region 1950–2030. Source: World Urbanization Prospects: The 2005 Revision Population Database http://esa.un.org/unup/index.asp?panel=1 (accessed 30 November 2007) These headline figures are more uncertain and problematic than is usually appreciated. In a comprehensive paper published in 2004, Cohen gives an informative and critical account of our state of knowledge about urbanization.7 The definitional issue of what constitutes an urban area is his starting point. The UN estimates are necessarily based on data provided by individual countries, despite the reality that there is wide variation between them in the criteria used to define urban and rural. Changes in definitions by individual countries also create problems in interpreting time trends. For example, in the 1980s China changed its criteria that resulted in a substantial jump in the population classified as living in urban areas. Other problems include the absence in many countries of nationwide censuses and the lack of recent data. These data problems, which are often not appreciated by non-specialists, need to be taken more into account when making comparisons across countries and over time. However, Cohen's view, is that despite these problems, the broad picture of an increasingly urbanized world, affecting all countries and continents, is correct. Cohen also takes up several misconceptions about which urban areas are growing fastest and which social processes contribute most to this increase. The rapid pace of global urbanization is sometimes confused with, or reduced to, the growth in the number and size of very large cities (of 10M or more inhabitants). However, most of the growth in urban population observed in the past few deca