Rethinking urban environments and health.

All urban environments have characteristics that both promote and harm health.1 Multiple factors interacting to shape health in urban environments give rise to a continuously evolving balance between the so-called urban health advantage (i.e., the health benefits of economic opportunities, infrastructure, social support, and access to health care in urban areas) and the urban health penalty (i.e., the concentration of disadvantaged individuals and exposure to environmental hazards leading to worse health among urban compared to nonurban residents).1,2 Previous frameworks have been proposed for understanding the multiplicity of factors operating at different scales to shape health in urban contexts.2,3 In this commentary, I discuss topics that have not been sufficiently addressed by these existing frameworks, including: (1) the value of descriptive epidemiology in providing the broader context for etiological research on the health effects of urban environmental hazards; (2) the important role that exposure science and the exposome approach can play in bridging descriptive epidemiology and etiological research; and (3) the opportunities to promote the urban health advantage in rapidly urbanizing areas through climate change adaptation and mitigation. THE VALUE OF DESCRIPTIVE EPIDEMIOLOGY Whereas other disciplines have focused primarily on the economic and other advantages of urban living, epidemiologists, and environmental epidemiologists in particular, have maintained a more pessimistic view, focusing primarily on the concentration of health hazards in urban areas.2,4 A large body of evidence from etiologic studies documents positive associations between environmental exposures concentrated in urban areas (e.g., air, noise, and light pollution, and lack of green space) and cardiometabolic disease,5–8 a leading cause of mortality and morbidity globally. However, the conclusion that urban residents are worse off than nonurban residents is not necessarily supported by the descriptive epidemiology characterizing disease distributions according to urbanicity. In a recent large systematic review of urban-rural differences in hypertension prevalence in population-based studies in low- and middle-income countries (LMICs), the urban-rural prevalence difference was modest: 2.45% (95% CI = 1.57, 3.33) and decreased from 5.75% (95% CI = 4.02, 7.48) in 1990–2004 to just 1.38% (95% CI = 0.40, 2.37) in 2005–2020.9 The urban-rural difference in prevalence decreased with increasing development until a point of convergence, after which rural areas had higher hypertension prevalence in the most developed LMICs.9 Studies including data from high-income countries (HICs) showed higher hypertension prevalence in rural compared to urban areas in HICs.10 Similarly, pooled analysis of trends in urban-rural differences in body mass index from the Noncommunicable disease Risk Factor Collaboration showed that the fastest increase in obesity occurred in rural, rather than urban areas, and that body mass index was persistently higher in rural compared to urban areas in HICs.11 Urban-rural differences in health outcomes are highly context-dependent, with urban environments overall having generally better cardiometabolic health in HIC settings despite higher levels of many harmful environmental exposures. BRIDGING DESCRIPTIVE EPIDEMIOLOGY AND ETIOLOGICAL RESEARCH Multiple factors interact to generate urban-rural differences in health; however, the most relevant mechanisms and how the dominant mechanisms vary across contexts are not well understood.2,4 Improved exposure science and more holistic research approaches are critical to better characterizing urban environments and unpacking the black box they represent to identify the main drivers of urban-rural differences in health. While it is clear that urban areas are rapidly expanding across the globe, there remains no consensus on what is urban.12 Most available data, including studies of urban-rural health differences,9 rely on widely varying definitions of urban adopted by individual countries which can refer to settlements ranging from a few hundred to tens of thousands of people.12,13 Similarly, cities are often defined based on administrative boundaries that rarely coincide with the extent of urbanized territory.13 Remote sensing has been particularly valuable in characterizing and mapping urban areas over time and space in a consistent fashion.12,14 Recent developments provide globally consistent definitions including the degree of urbanization,15 which classifies a country’s territory along the urban-rural continuum at 1 km2 resolution, and urban-rural catchment areas reflecting the interconnection between urban centers and their surrounding rural areas.16 Adequately capturing environmental changes associated with urban expansion for epidemiological studies requires time series data on the scale of decades. Examples where remote sensing data spanning more than a decade have been used to understand how land use change due to urban expansion influences health include studies investigating associations between built-up land use change over 14 years and cardio-metabolic risk factors in peri-urban India17 and changes in normalized difference vegetation index over 14 years and all-cause mortality in China.18 Aside from demographics, urban-rural health differences reflect the totality of multiple environmental exposures over the lifespan lived in urban environments, or in other words, the urban exposome.19 An exposome research approach is particularly well-suited to elucidate the mechanisms underpinning observed urban-rural differences in health across global contexts. However, far more emphasis should be given in urban exposome-based research on established drivers of the urban health advantage such as access to goods and services (e.g., health services), and improved education and employment opportunities.1 Urban exposome-based research spanning multiple regional and national contexts has unique potential to identify the marginal contribution of environmental risk factors in shaping health beyond the influence of upstream factors including level of economic development and resource access. Insights from urban exposome research are particularly needed in LMIC settings, where urbanization is occurring most rapidly. SHIFTING THE BALANCE TOWARDS THE URBAN HEALTH ADVANTAGE THROUGH CLIMATE CHANGE ACTION The world’s urban population is expected to grow by 2.5 billion people by 2050, with 90% of that growth occurring in Asia and Africa.20 Population growth in urban areas will result in larger numbers of people most vulnerable to climate change, specifically residents of informal settlements. An estimated 1 billion people currently live in slums or informal settlements, primarily in Asia and sub-Saharan Africa, equivalent to 24% of the urban population.21 Residents of informal settlements are particularly vulnerable to climate change due to a range of individual-level factors (e.g., occupation, existing health conditions, and poverty) as well as socio-economic and political factors (e.g., lack of housing rights and security, political and social exclusion, government neglect, and lack of access to services).22 Recent reviews have identified associations between extreme weather events such as floods, extreme temperatures, and drought with health among residents of informal settlements including increased infectious diseases due to food- and water-borne disease, worse mental health, and increased domestic violence.22,23 Extreme weather events also indirectly affect health through poorer sanitation and hygiene, changes in food consumption, increased food insecurity, and loss of income and livelihoods.22 Well-planned adaptation actions can reduce the likelihood of adverse health impacts due to climate change in urban areas, particularly for those facing poverty, housing informality, and lack of basic services.24 Improved physical infrastructure in informal settlements such as increased access to piped water (<60% of urban populations in LMICs have access to piped water),24 sanitation, and drainage can improve health, reduce inequalities, and improve resilience in the face of flooding or high temperatures. The process for developing robust adaptation plans is critical; the most successful plans are those that are inclusive and developed through consultation and co-production with diverse and marginalized communities.24 Informal settlements have low carbon footprints, with very low per capita greenhouse gas emissions. They are generally high density with mixed land use; most residents rely on nonmotorized, active, and public transport; they are more likely to reuse materials and minimize solid waste; and homes are often constructed from locally available recycled material.25 The high density promotes resource efficiency and minimizes land cover change. There are important opportunities for reducing vulnerability to climate change through upgrading while improving health without following the high carbon footprint development pathways of formal settlements.25 For example, energy efficiency gains from solar energy combined with housing designs that promote natural ventilation and lighting can reduce household energy expenditure and energy poverty in low-carbon ways that benefit health by reducing air pollution emissions and the risk of fires from kerosene-based lighting.25 A guiding principle recommended for pro-poor, pro-health climate action is to address development deficits with health and climate change action in mind and to address climate change action with health and development in mind.25 Complex tradeoffs are pervasive and need to be identified through research to be avoided in the policy. The Intergovernmental Panel on Climate Change 6th Assessment Report provides an urban climate-resilient development framework for assessing synergies and tradeoffs between climate change adaptation in urban areas, mitigation, human health, and other dimensions of sustainable development.24 The framework’s systematic mapping across multiple dimensions reveals several important research gaps regarding the co-benefits and harms of climate action on human health and other sustainable development priorities such as equity and healthy ecosystems.24 Policy action and research that advance synergies and health co-benefits of climate change adaptation, mitigation, and urban development are occurring too slowly and unevenly.24 Environmental epidemiologists have an important role to play in the critical need for inter- and trans-disciplinary research to better understand the nexus between health, urban development, and climate change action. Specific areas where environmental epidemiologists can contribute include: - identifying the contribution of modifiable environmental risk factors to urban-rural health differences. These efforts will be greatly facilitated by novel approaches for measuring urban expansion in a globally consistent way on decadal time scales and integrative research approaches such as the urban exposome that comprehensively measure and interrogate the multiple interacting drivers of urban-rural health differences. -generating new evidence on the co-benefits and potential harms of climate change adaptation and mitigation on the health of urban residents, while simultaneously considering other dimensions of sustainable development such as reducing poverty and marginalization. Evidence in these areas is needed to guide policy and plans that maximize synergies and avoid negative tradeoffs to tip the balance in favor of the urban health advantage, particularly in areas with the most rapid urban growth. ABOUT THE AUTHOR Cathryn Tonne is an environmental epidemiologist focusing on the health effects of air pollution from outdoor and household sources and their intersection with sustainable development. Her research has investigated exposure patterns and health effects of air pollution in high- as well as low- and middle-income countries. One of her main research interests is in the health co-benefits of climate change mitigation.