Few urban initiatives have greater potential to promote health equity and to advance the well-being of poor households in the Global South than participatory, multi-objective slum upgrading. Urban informal settlements are communities that are highly diverse and have a range of locally-specific names (such as barrios, bustees, mjondolo, or favelas). Here we use the terms “slums” and “informal settlements” to denote largely self-built urban communities, which are rarely recognized officially and typically are denied life-supporting services and infrastructure. Not only can upgrading these settlements improve health outcomes via enhanced access to shelter, water, or clean energy, but it can also advance the 2030 Agenda for Sustainable Development. The 2030 Agenda has established 17 interrelated Sustainable Development Goals (SDGs), which will require implementing multi-sectoral strategies to support health and well-being and include explicit reference to slum upgrading. Yet, as we will highlight, upgrading projects are rarely designed to explicitly improve nor are they evaluated for how well they have positively shaped the social determinants of slum health.

The social determinants of health (SDOH) are factors outside medical care that shape health outcomes, such as safe housing, food access, political and gender rights, education and employment status [1]. In urban informal settlements, residents are often burdened with multiple and overlapping challenges that can undermine the SDOH, from entrenched poverty, to overcrowded shelter, to inadequate infrastructure and tenure insecurity, all of which can combine to contribute to increased risk of exposures to environmental pathogens that increase infectious and non-communicable diseases in urban poor areas [2,3]. Slum-dwellers also experience spatial, political, and economic exclusion when compared to wealthier urban residents, and these too are recognized as powerful determinants of who gets sick, suffers more and dies early [4].

In this review article, we analyzed slum upgrading projects and published evaluations of these projects or policies for whether and how they captured the influence of upgrading on the SDOH [5]. We analyzed completed slum upgrading projects from a dozen different countries and performed content analyses to determine if these projects included explicit measures of any SDOH and/or specific disease outcomes. We focused our review on upgrading projects and policies that took an “integrated” (i.e., multi-sectoral) approach to upgrading since we determined that these aimed to influence the SDOH through interventions focused on more than one physical, social and/or political issue facing the urban poor [6,7]. We also limited our review to slum upgrading interventions where we could identify at least two published evaluation reports. By incorporating a wide range of peer-reviewed and grey literature, we aimed to distinguish our study from past research that only reviewed the health outcomes, but not necessarily the broad determinants, of slum upgrading projects utilizing randomized studies [8]. As discussed in more detail below, we found that a majority of slum upgrading projects rarely measured how upgrading projects affected health outcomes or social determinants. We suggest that this represents a lost opportunity for health promotion and conclude with some suggestions for how slum upgrading might more explicitly promote the SDOH amongst the urban poor in the Global South [9].

Slum-dwellers increasingly face a “triple threat” of infectious diseases, non-communicable conditions (e.g., diabetes, cardiovascular disease, and mental illness), and injuries due to violence or road traffic accidents. Table 1 offers some definitional characteristics of slums and associated environmental health issues. However, there is no single adequate definition of slums, since they are often heterogeneous in terms of housing types, poverty prevalence, tenure (in)security, levels of infrastructure and service provision, environmental hazards and health risks [10].

Similarly, there is no single definition of “slum upgrading”, as projects and policies differ based on region of the world, political and development histories, and other factors [11]. Beginning in 1972, the World Bank launched urban upgrading projects to improve services, infrastructure and housing in hopes of reducing poverty and meeting basic needs [12]. Yet governments often persisted in slum clearance and neglected to maintain what little infrastructure had been upgraded. By the 1980s–1990s, international development agencies shifted their approach to slum upgrading; rather than construct housing and infrastructure, governments acted as “enablers” by offering financing for mostly non-governmental organizations’ projects, which largely focused on housing [13]. By the early 2000s another shift occurred and a greater emphasis was placed on in-situ upgrading, limiting slum clearance and launching policies that aimed to integrate the urban poor into the larger fabric of growing metropolitan economies [6,13].

As used here, “slum upgrading” denotes initiatives seeking to improve housing quality, infrastructure provision, social services, livelihoods, and official recognition for residents. Slum upgrading is also a process that meaningfully includes the urban poor in project and policy design and implementation, aims to integrate slums into the larger fabric of the city, is attentive to emerging challenges of climate change adaptation, and is often incremental—meaning that residents remain in place while improvements occur to prevent displacement (cf.: http://www.citiesalliance.org/About-slum-upgrading#Why_is_slum_upgrading_important). Yet, the scope of upgrading can vary from small-scale sector-specific projects (i.e., water-taps, paved roads, street lighting) to comprehensive housing and infrastructure projects (i.e., piped water and sewers into improved housing) to integrated projects that combine built-environment interventions with social programs and political empowerment [13,14]. Many upgrading programs have also focused on providing residents with legal rights to land and housing tenure [15]. Importantly, slum upgrading requires state recognition of the urban poor, their settlements, and the need to deliver basic services. By granting official legitimacy to slum-dwellers and their “right to remain”, upgrading represents an important alternative to slum removal and may be a process that in itself may offer health benefits [8]. However, as we explore below, upgrading projects are rarely evaluated on their ability to positively influence the social determinants of health.

Our analysis of slum upgrading evaluations incorporated both grey literature and published peer-reviewed studies, with a focus on integrated interventions. We started with a scan of published literature between 1997 and 2016 through on-line databases using general search terms, including “informal settlement”, “slum”, “upgrading” and “health”. Based on this review, we found 182 possible evaluations but narrowed them down to thirty-two by selecting those that had at least one additional published report on the project. We also narrowed our search by focusing upon “integrated” or multi-sectoral interventions, which aimed to address physical, social and urban governance concerns in informal settlements [26,27]. We focused our search on multi-sectoral interventions because we hypothesized that these might offer reviews and/or evaluations of several environmental and social impacts. Further, global slum upgrading programs, such as UN-Habitat’s Participatory Slum Upgrading Program, call for integrated or “blended” projects that aim to achieve multiple objectives [28]. We also limited our search to find projects that might act as examples of the type of cross-sector interventions called for under the Sustainable Development Goals [29]. We utilized the WHO definition of SDOH to sharpen our search terms and refined our findings into three categories under which upgrading initiatives may influence health in informal settlements: national and municipal policies, structural determinants, and living and work conditions [1,9]. By “national and municipal policies”, we mean any legislation or government-sponsored initiatives that may positively influence the SDOH, including a slum upgrading policy or financing scheme. This category thus underscores that governmental decisions and priorities—often outside the medical and public health sectors—are key drivers of population health. By “structural determinants”, we focus on the specific population drivers of health inequities frequently identified in the SDOH literature, including income and wealth, educational access, employment, early childhood development, food security, and social exclusion. By “living and working conditions”, we focus on the physical characteristics of neighborhoods and urban places, such as access to quality and affordable housing, water, sanitation, electricity, transport, etc.

We selected nineteen interventions from Latin American, African and Asian cities that met our screening criteria; while not exhaustive, this list is meant to illustrate the range of slum upgrading projects globally and the potential of participatory, integrated projects for improving human health. We summarize select characteristics of each slum upgrading project and whether and how they were evaluated for impacts on human health in Table 2.

Of our nineteen selected projects, thirteen evaluations directly discussed potential SDOH, such as reduced water collection times; enhanced community pride and social cohesion; greater levels of micro-savings, loans, or economic stability; and increased women’s empowerment. Community participation—residents’ and/or community-based organizations’ involvement in the project—was present in all but four of the upgrading evaluations we reviewed. However, these issues were rarely discussed as having potential benefits for population health. Eleven of the reviewed projects explicitly mentioned health impacts, using self-reported data or state collected data such as cause of death. Only three slum upgrading projects were evaluated for their influence on specific disease outcomes. In sum, our scan revealed the following:

Health outcome measures were very limited and tended to focus on childhood mortality or morbidity (particularly due to diarrheal or other communicable illnesses);

Including health and social determinants criteria into slum upgrading projects is clearly challenging. Our scan of the literature found a limited number of integrated evaluations, and some reasons might be that there are few incentives from donors and national governments for such studies, that there are methodological challenges that make impact evaluations challenging, and resources may not exist after project implementation to conduct detailed studies of often highly mobile urban populations [71]. Further, as Cameroon et al. note from a systematic review of impact evaluations between 1981 and 2014, very little impact evaluation evidence exists of interventions in urban development, including informal settlements [71]. The World Bank’s Independent Evaluation Group (IEG) reports that only 17% of completed projects included an impact evaluation in their required Implementation Completion and Result Reports (ICRs) [72]. Thus, we suggest that the lack of evaluations for urban slum upgrading is a lost opportunity for improving understanding of what can work for global health. In this final section, we explore practices that might better encourage the political processes and information- gathering needed for measuring the broad health impacts of urban slum upgrading.

Two existing practices, Health Impact Assessment (HIA) and Health in All Policies (HiAP), might help more explicitly incorporate social and environmental determinants of health into urban slum upgrading. Both HIA and HiAP aim to promote rigorous, comprehensive planning processes and to mainstream analyses of health equity throughout public policy-making. HIA seeks to systematically judge the potential (and sometimes unintended) effects of a policy, project, or other intervention upon population health as well as the distribution of those effects. Both a process and a method of analysis, HIA aims for transparent decision-making, integrating multiple health determinants, and assessing the short- and long-term health impacts upon specific groups and general population health [73]. Some local and national governments have formally institutionalized HIA as a distinct practice in public health and other agencies; other governments integrate HIA into existing health analyses and public decision-making analyses. However, to our knowledge, there is no instance where HIA has been used in slum upgrading.

HIA is also a strategy for implementing HiAP, a second integrated approach to decision-making that holds promise for slum upgrading [74]. HiAP recognizes that most public policies can influence health and health equity (either positively or negatively), but policymakers beyond the health sector may not routinely consider these consequences and thus miss opportunities to advance health equity [75]. The impetus for HiAP emerged, in part, from the WHO’s Alma-Ata Declaration in 1978 and reflects numerous international calls for policy action to address the social determinants of health. The European Union and numerous governments globally have endorsed HiAP, but we have not seen it applied to either international aid or within national policies to support slum upgrading [76].

Both HIA and HiAP are rooted in relational analyses of place and population health, a framework that may help further elucidate how slum upgrading projects can be designed and evaluated for their potential health impacts [77,78]. For example, in the “relational” view of place and health, interventions aim to avoid the use of traditional, single exposure, disease or risk factor focused strategies and instead seek co-benefits of mutually reinforcing interventions. Similarly, in a relational view, a set of integrated strategies for slum upgrading would need to be implemented at multiple scales, from the national to the local, to have widespread population health impacts. As we discovered in our review, there are slum upgrading projects and policies that aim to embody the relational approach. The examples from Pakistani and Thai cities, as noted above, suggest that a relational approach to urban slum upgrading is not only possible, but can deliver tangible health gains to the urban poor.

A relational approach also demands multiple sources of data to inform project design, implementation and evaluation. For example, oral histories that capture local meanings and a “sense of place”, are combined with quantitative measures of exposures and geography in a relational view of place. Again, we found that some integrated slum upgrading projects in Brazil, among others, valued resident experiences, knowledge and oral histories, along-side other forms of measurement data.

A relational approach to upgrading would also analyze the shifts, if any, in governance (rules, norms, laws and institutional procedures or practices) that often create slums and might act to stymie opportunities for the urban poor to make healthy decisions [4,5,9]. Thus, a relational approach would examine any changes in power between slum-dwellers and other institutions, particularly the state. The projects we reviewed rarely explored these types of governance impacts, although studies in Brazil, Colombia and Thailand did discuss changes in state-community relations. In Table 3, we detail the potential pathways between slum upgrading and key determinants of health equity.

Urban slum upgrading is a process and set of outcomes that can positively influence multiple determinants of health and potentially reduce health inequities experienced by the urban poor. The review presented here suggests that more work is needed to integrate clear social determinants criteria into slum upgrading projects’ design and evaluations. When health is included in urban slum upgrading, it is often limited to a single disease, exposure or risk factor, rather than the multiple criteria for health recognized by the WHO findings on the social determinants of health. The global community’s failure to recognize urban slum upgrading as an environmental health equity intervention is a lost opportunity and one that, if it continues, may hinder our ability to meet the Sustainable Development Goals.

The review presented here was limited by focusing only on published evaluations of past slum upgrading projects and policies, all of which were published in English and Spanish. Important projects with only one evaluation and those published in other languages may have been missed. We also acknowledge that slum upgrading projects and related evaluations may be intentionally narrow, in order to respond to financing and/or donor requirements. Development agencies and foundations that typically finance slum upgrading increasingly require quantifiable outputs and might measure a narrow set of health outcomes, rather than the subtler, often qualitative and harder to measure environmental and social determinants discussed here. However, the use of mixed-methods evaluations, such as in-depth interviews, focus group discussions, before-and-after spatial mapping, and longitudinal cohort surveys, could all deepen understanding of how slum upgrading can influence the multiple drivers of human health in cities. Further research can fine-tune these evaluation techniques, and slum upgrading remains an important health promotion strategy urgently requiring the attention of public health practitioners.