The impact of megacities on health: preparing for a resilient future
Authors: Ashley Jowell, Bright Zhou, Michele Barry
Journal: The Lancet Planetary Health (2017)

Transitions from rural to urban living are often prompted by promises of increased economic opportunities and cheaper health care, education, and transportation. However, these rural-to-urban transitions are often chaotic. Megacities, defined as rapidly developing urban centres with populations of 10 million or more, present a unique opportunity to study the future of planetary health and how to build a more resilient globe (figure). By 2030, two-thirds of the world's population will reside in urban areas and there will be 41 megacities, of which over 80% will be in low-income and middle-income countries. Megacities are characterised by substantial cultural, socioeconomic, and racial diversity, as well as environmental heterogeneity.2Patel RB Burke TF Urbanization—an emerging humanitarian disaster.N Engl J Med. 2009; 361: 741-743Crossref PubMed Scopus (109) Google Scholar We propose that future comprehensive studies of megacities should move beyond commonly identified single issues to consider the complex health sequelae of the natural and built environmental landscapes. For example, urban-built environments are often poorly equipped to provide stable sources of food for sustaining massive populations. In low-income and middle-income megacities, such as Dhaka, Bangladesh, food shortages and poverty result in increasing food prices and malnutrition.3Keck M Etzold B Bohle H Zingel W Food security in Dhaka: between global risks and local vulnerabilities.in: Kraas F Aggarwal S Coy M Mertins G Megacities: our global urban future. Springer, Dordrecht: Netherlands2014: 59-73Crossref Scopus (3) Google Scholar Additionally, many megacity inhabitants exhibit a shift towards more sedentary lifestyles and a western diet, leading to increased body-mass indices (BMIs) and instances of obesity and diabetes.4Saquib J Saquib Barry M Cullen M et al.Sex differences in obesity, dietary habits, and physical activity among urban middle-class Bangladeshis.Int J Health Sci (Qassim). 2016; 10: 363-372PubMed Google Scholar As urban residents transition from diets sustained on home gardens to rely on cheaper, processed foods, they are at a greater risk of developing obesity and other non-communicable diseases. Studies in China and Egypt have shown that breast cancer incidences—controlled for screening biases—were substantially higher among urban women than among rural women. This difference in breast cancer incidences is postulated to be caused by increased exposure to xeno-oestrogens from processed foods, and higher BMIs and alcohol consumption in urban areas.5Fei X Wu J Kong Z Christakos G Urban-rural disparity of breast cancer and socioeconomic risk factors in China.PLoS One. 2015; 10: e0117572Google Scholar, 6Dey S Soliman AS Hablas A et al.Urban-rural differences in breast cancer incidence by hormone receptor status across 6 years in Egypt.Breast Cancer Res Treat. 2010; 120: 149-160Crossref PubMed Scopus (49) Google Scholar Megacities are also linked with increased instances of violent victimisation and gender-based violence. Urban women, in many instances the victims of violence, are at a higher risk of contracting sexually transmitted infections and HIV than their rural counterparts.7WHO World Health Organisation, Switzerland2010http://www.who.int/kobe_centre/publications/hiddencities_media/who_un_habitat_hidden_cities_web.pdf?ua=1Google Scholar The increased instances of violence in megacities can also contribute to deteriorating mental health within the population. A mental-health survey of the residents in the metropolitan area of São Paulo, Brazil, found a higher prevalence of mental disorders than that found with similar surveys of residents in non-megacities.8Andrade L Walters E Gentil V Laurenti R Prevalence of ICD-10 mental disorders in a catchment area in the city of São Paulo, Brazil.Soc Psychiatry Psychiatr Epidemiol. 2002; 37: 316-325Crossref PubMed Scopus (201) Google Scholar Anxiety, mood, impulse-control, and substance-use disorders were all associated with crime-related traumatic events, particularly in women. Augmented violence in megacities results in increased imprisonments, leading to crowded epicentres for disease proliferation that pose a threat to adjacent populations. A study from 20159Sacchi FP Praça RM Tatara MB et al.Prisons as reservoir for community transmission of tuberculosis, Brazil.Emerg Infect Dis. 2015; 21: 452-455Crossref PubMed Scopus (68) Google Scholar reported that urban centres near prisons are at increased risk of contracting tuberculosis strains developed within those prisons.9Sacchi FP Praça RM Tatara MB et al.Prisons as reservoir for community transmission of tuberculosis, Brazil.Emerg Infect Dis. 2015; 21: 452-455Crossref PubMed Scopus (68) Google Scholar The indirect influence of megacity violence highlights the impact of megacities not only on their inhabitants, but also on the surrounding populations. Air pollution also has a substantial effect on the wellbeing of people worldwide. This pollution can consist of sulfur dioxide, ozone, nitrogen dioxide, and fine-particulate matter of less than 2·5 μm in diameter (PM2·5). 92% of the global population is exposed to higher-than-recommended concentrations of PM2·5, resulting in 3 million premature deaths per year. PM2·5 exposure is particularly high in megacities and can cause fatal diseases including stroke, chronic obstructive pulmonary disease, and lung cancer.10WHOAmbient air pollution: a global assessment of exposure and burden of disease. World Health Organisation, 2016http://who.int/phe/publications/air-pollution-global-assessment/en/Google Scholar Another atmospheric phenomenon is called the urban-heat-island effect, with megacities often being 5–11°C warmer than surrounding areas, and it is associated with increased incidences of dehydration, heat stroke, and cardiovascular and respiratory diseases.11Campbell-Lendrum D Corvalán C Climate change and developing-country cities: implications for environmental health and equity.J Urban Health. 2007; 84: 109-117Crossref Scopus (181) Google Scholar These higher temperatures can also lead to increased use of cooling systems that release hydrofluorocarbons and other greenhouse gases into the atmosphere, further exacerbating the heat-island effect. Other challenges that are also associated with urban-built environments include overcrowding, a scarcity of open space, poor sanitation, and insufficient groundwater management. And challenges that are associated with urban-natural environments, in addition to air pollution, are extreme-weather events, such as floods and heat waves, and increased instances of urban-arboviral diseases with changing vector patterns that are caused by climate change. As human populations become increasingly concentrated in cities, understanding successful social and natural interventions in present-day megacities can help global-health practitioners to better manage our rapidly evolving planetary health for the future. To address food insecurity and the inability of many evolving megacities to be agriculturally independent, cities such as Singapore and New York City have trialled vertical-farming techniques. At the forefront of this technology is aeroponics—a method for growing plants in stacks without sunlight or water.12Despommier D The vertical farm: feeding the world in the 21st century. Macmillan Press, London2010Google Scholar Using green technologies and urban designs, wastewater and rainwater can be recycled and stored to manage water safely in the age of megacities. Finally, megacity interventions should consider the heterogeneity of the city's population. By examining and disaggregating large datasets obtained from so-called smart cities, one can interpret the information to generate optimum methods of energy, food, and wastewater management to prevent so-called splintered cities, wherein goods are maldistributed to only the wealthier sections (Levitt R, Stanford University, CA, USA, personal communication). Considering the complex health sequalae of the natural and built environmental landscapes of megacities, planning to maintain the health of the populations of those cities and prepare them for a resilient future will rely on collaborations between many disciplines including social scientists, health professionals, and environmentalists. We declare no competing interests.