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India's financial capital and largest city, Mumbai, is extremely vulnerable to climate change-induced floods. Every year, extreme rainfall during the monsoon season, which begins in June and ends in September, results in approximately 8% of the coastal megacity's deaths, according to the findings of a recent study. And 80% of the people who die during frequent flooding events, especially during high tides, live in the city's sprawling slum areas.
"Children, particularly those under the age of five, face the biggest increase in mortality risk from rainfall, and women face a greater risk than men," the researchers explained in their study that was published in the journal Nature on November 12, 2025. "The coincidence of rainfall and high tides is known to overwhelm drainage systems and increase flood hazard in Mumbai and cities worldwide, but its health impacts have not yet been studied."
They found that a single day of 150mm of rainfall results in a 5.3% increase in five-week mortality among children younger than 5 years old, compared to a 1.6% increase among Mumbai residents in the 5 to 64 years age range. The mortality percentage increased to 2.3% among those who were older than 65 years.
Whereas a 150-mm day of rainfall caused an increase in five-week mortality of 1.5% for men and a more than double mortality percentage of 3.1% for women.
"Unplanned urbanization, particularly in the global south, has led to rapid growth in the built environment and population, outpacing the development of basic infrastructure such as drainage, sanitation, and waste management. As a consequence, rainfall routinely exposes large, densely concentrated populations to damaging and life-threatening hazards. Slum inhabitants, who number over one billion globally, are especially vulnerable," the researchers added.
"Future sea-level rise will further amplify flooding hazards, making rainwater and urban flood management a defining challenge of twenty-first-century public health, economic development, and urban planning policies," they further warned.
Lead author of the study, Tom Bearpark from Princeton University's School of Public and International Affairs, and colleagues analyzed close to a decade's worth of rainfall data and death records in Mumbai from 2006 to 2015. Interestingly, they uncovered that simply residing in low-lying flood-prone areas in the city is not the driving factor behind why slum residents are at a greater risk of dying during heavy rainfall events. "Slum populations suffer significantly higher excess mortality from rainfall than non-slum populations within each elevation group," the researchers noted. "Within individual PIN codes, slums are not systematically situated in lower elevation areas."
"The ability of water, sanitation, waste management, and built and natural drainage systems to cope with rainfall is the key challenge common to cities throughout the developing world. Investments in safe water and sanitation have been found to drive historical declines in urban child mortality in today's developed countries. Our results suggest that similar declines could be achieved in today's developing urban areas, particularly in slums, which bear the vast majority of rainfall's mortality burden," the researchers highlighted. "As we enter an era in which most of the urban population growth occurs in the developing world, and much of that in slums, understanding the returns to such investments is a critical area for future research."