Can Better Slums Save Lives? Rio Study Links Urban Upgrades to Sharp Drop in Heatwave Deaths

A Rio de Janeiro study finds that slum upgrading can cut excess heatwave mortality by 3.9%–5.5%, offsetting nearly half of the additional deaths linked to extreme heat. The findings position basic urban infrastructure as a cost-effective tool for public health, climate resilience and inclusive development, with major implications for governments, development partners and private investors.

A major study from Rio de Janeiro suggests that improving basic infrastructure in informal settlements can do more than provide better streets, water systems and public spaces, it can also save lives during extreme heat. Researchers Diogo Oscar Borges Prosdocimi of the RAND School of Public Policy and RAND Corporation, Kelly Klima of RAND Corporation and the RAND School of Public Policy, and Maria DeYoreo of RAND Corporation found that slum upgrading was associated with a 3.9% to 5.5% reduction in excess mortality during heatwaves, potentially offsetting nearly half of the additional deaths caused by extreme heat.

The findings are important as more than 1.1 billion people worldwide live in informal settlements, where poor housing, overcrowding, inadequate water and sanitation, limited vegetation and weak public services can increase exposure to rising temperatures. With climate change making heatwaves more frequent and intense, the research suggests governments may already have a practical adaptation tool available: investing in basic urban infrastructure.

Rio's Numbers Show Infrastructure Can Save Lives

The study examined 31 slum-upgrading interventions under Brazil's PAC Assentamentos Precários programme in Rio de Janeiro. Researchers combined mortality records from 806 healthcare facilities with weather, census and government programme data, creating a dataset covering 5,113 days.

The interventions included street paving, drainage, sidewalks, public lighting and improvements to public and green spaces. The Rio projects analysed represented approximately R$2.1 billion in investment.

Researchers mainly defined a heatwave as at least three consecutive days with a heat index above 32°C, or 90°F. Their analysis found that heatwaves were associated with mortality increases ranging from 5.1% to 11.9%, depending on the statistical model. Preferred estimates placed the underlying heatwave mortality penalty at around 5.1% to 8.2%.

A separate comparison found that deaths during heatwaves were roughly 10% higher than during the days immediately before extreme heat began. The danger also increased as heat persisted, with mortality effects building during the first several days before weakening after roughly seven days.

Against this background, the impact of infrastructure upgrading was substantial. Using a 500-metre measure around intervention areas, the estimated reduction in heatwave mortality ranged from 3.9% to 9.8%. The researchers' preferred distance-based models produced reductions of approximately 5.1% to 7.5%.

A Development Investment With a Climate Dividend

The results could change how governments calculate the economic value of urban infrastructure. Slum-upgrading projects are normally justified through better sanitation, housing, mobility and access to services. The study indicates that avoided heat-related deaths and public-health costs should also be considered when assessing their benefits.

The gains may not be limited to climate emergencies. The preferred models found a 2.8% to 3.9% reduction in overall mortality outside heatwave periods, suggesting that upgrading can deliver a double benefit: healthier communities during normal conditions and stronger protection when extreme temperatures arrive.

The cause-of-death results reinforce this conclusion. The strongest reductions were concentrated in cardiovascular and respiratory deaths, which are closely connected with heat stress, dehydration and respiratory strain. Similar benefits were largely absent for causes unrelated to heat, such as accidents and cancer.

The mechanisms are relatively straightforward. Reliable water can reduce dehydration. Better roads and sidewalks can improve access to healthcare. Lighting and safer public spaces can allow residents to leave dangerously hot homes during cooler evening hours. Trees and vegetation can provide shade, while improvements to buildings and neighbourhood design can reduce heat exposure.

Governments and Development Partners Have an Opportunity

For national and municipal governments, the study supports bringing climate, health and urban-development policies together. Instead of treating climate adaptation as a separate spending category, authorities could build heat resilience directly into housing, water, sanitation, drainage and neighbourhood-upgrading programmes.

International development partners, including multilateral development banks, bilateral agencies and climate funds, could also reconsider how they finance informal-settlement improvements. Projects that reduce poverty while improving climate resilience may qualify for broader financing when their health and adaptation benefits are properly measured.

Targeting will matter. The Rio interventions were not primarily selected according to heat vulnerability. Future programmes could use heat-vulnerability maps combining temperature exposure, population characteristics, vegetation, water access and infrastructure conditions to direct scarce funding towards communities facing the greatest risks.

For developing countries with limited fiscal resources, this approach could be particularly valuable because the same investment can contribute to public health, poverty reduction, infrastructure development and climate adaptation.

Private Investment Brings Opportunity, and Risk

The findings also point to growing opportunities for construction companies, engineering firms, water and sanitation providers, building-material manufacturers and climate-technology businesses. Rising temperatures could increase demand for reflective roofing, affordable insulation, passive ventilation, permeable surfaces, reliable water systems and green infrastructure.

But the study does not suggest that every infrastructure project will automatically reduce heat mortality. Researchers could not determine exactly how much of the observed benefit came from water, housing, streets, lighting, vegetation or changes in residents' behaviour.

There is also a social risk. Successful upgrading can raise land values and rents, potentially displacing poorer households. Governments, investors and development partners therefore need affordability safeguards, tenure protection and meaningful community participation so that infrastructure improvements benefit existing residents rather than push them elsewhere.

Future research should examine household-level temperatures, water use, ventilation, healthcare access and behavioural responses to determine which investments deliver the greatest health benefits. Studies across Latin America, Sub-Saharan Africa and South Asia could also test whether Rio's experience can be replicated elsewhere.

The policy message remains powerful: against an estimated 5.1% to 8.2% heatwave mortality penalty, slum upgrading was associated with a 3.9% to 5.5% reduction in excess deaths. For rapidly urbanising countries facing hotter climates and tight budgets, improving informal settlements could therefore become one of the most practical ways to advance development while protecting vulnerable communities from extreme heat.