Massive slums have become major features of cities in many low-income and middle-income countries. Here, in the first in a Series of two papers, we discuss why slums are unhealthy places with especially high risks of infection and injury. We show that children are especially vulnerable, and that the combination of malnutrition and recurrent diarrhoea leads to stunted growth and longer-term effects on cognitive development. We find that the scientific literature on slum health is underdeveloped in comparison to urban health, and poverty and health. This shortcoming is important because health is affected by factors arising from the shared physical and social environment, which have effects beyond those of poverty alone. In the second paper we will consider what can be done to improve health and make recommendations for the development of slum health as a field of study.

In the first paper in this Series we assessed theoretical and empirical evidence and concluded that the health of people living in slums is a function not only of poverty but of intimately shared physical and social environments. In this paper we extend the theory of so-called neighbourhood effects. Slums offer high returns on investment because beneficial effects are shared across many people in densely populated neighbourhoods. Neighbourhood effects also help explain how and why the benefits of interventions vary between slum and non-slum spaces and between slums. We build on this spatial concept of slums to argue that, in all low-income and-middle-income countries, census tracts should henceforth be designated slum or non-slum both to inform local policy and as the basis for research surveys that build on censuses. We argue that slum health should be promoted as a topic of enquiry alongside poverty and health.

The concept of planetary health acknowledges the links between ecosystems, biodiversity and human health and well-being. Soil, the critical component of the interconnected ecosystem, is the most biodiverse habitat on Earth, and soil microbiomes play a major role in human health and well-being through ecosystem services such as nutrient cycling, pollutant remediation and synthesis of bioactive compounds such as antimicrobials. Soil is also a natural source of antimicrobial resistance, which is often termed intrinsic resistance. However, increasing use and misuse of antimicrobials in humans and animals in recent decades has increased both the diversity and prevalence of antimicrobial resistance in soils, particularly in areas affected by human and animal wastes, such as organic manures and reclaimed wastewater, and also by air transmission. Antimicrobials and antimicrobial resistance are two sides of the sword, while antimicrobials are essential in health care; globally, antimicrobial resistance is jeopardizing the effectiveness of antimicrobial drugs, thus threatening human health. Soil is a crucial pathway through which humans are exposed to antimicrobial resistance determinants, including those harbored by human pathogens. In this review, we use the nexus of antimicrobials and antimicrobial resistance as a focus to discuss the role of soil in planetary health and illustrate the impacts of soil microbiomes on human health and well-being. This review examines the sources and dynamics of antimicrobial resistance in soils and uses the perspective of planetary health to track the movement of antimicrobial-resistance genes between environmental compartments, including soil, water, food and air.

With rapid global urbanization, the importance of understanding relationships between the changing urban environment and human health and wellbeing is being increasingly recognized. However, the science underlying the complexity of linkages is poorly developed. In this paper, we explore the different types of health and wellbeing risks in the urban environment, their dynamic, ever evolving nature, and define both their spatial and socioeconomic dimensions. We also present the new innovative program on Health and Wellbeing in the Changing Urban Environment that was launched recently by the International Council for Science (ICSU) wherein an integrated systems approach is recommended to address this complexity. We conclude by identifying key elements needed for the success of the new science initiative