Why Mumbai’s rain is a public health risk

A new study maps deaths during the infamous monsoon flooding in Mumbai and finds that rainfall claimed as many deaths as cancer in the 2006-2015 period

Amrita Rudra

Pic credit: Wikimedia Commons

Rainfall claimed nearly 8% of all monsoon deaths recorded in Mumbai, with women and children most vulnerable, shows a recent study that analysed mortality data from the city’s municipal corporation for the period between 2006 and 2015. 

The findings, which show the poor accounted for most deaths, must guide major coastal cities across the world to invest in adaptive infrastructure of water, sanitation, waste management and natural drainage systems to save lives and “reduce inequality in the face of a changing climate”.

The study, authored by Tom Bearpark, Ashwin Rode and Archana Patankar at the Institute for Climate and Sustainable Growth, University of Chicago, estimates that 2,300 to 2,700 people lost their lives “in each season during the 2006 to 2015 period”. 

The city recorded almost the same number of deaths due to all forms of cancer during the same period, the study notes.

“The health impacts of rainfall are far more severe for people living in slums than for those in other parts of the city—with slum residents accounting for 85 percent of the deaths from heavy rainfall,” it states.

Source: The University of Chicago study.

Proportion of monsoon season total deaths by subpopulation (black bars), and the proportion of monsoon season deaths caused by rainfall by subpopulation (red bars);

Source: University of Chicago study.

Mumbai's monsoon mayhem makes news headlines every year.

If the city doesn't invest in effective adaptation measures, it will risk countless lives and livelihoods, the study notes.

A staggering 85% of deaths caused by rainfall occur among slum residents, with children and women most vulnerable. This is compounded through high sea-levels and tides, worsened by inefficient drainage

The threat isn't limited to Mumbai with studies showing that 1.8 billion people globally are under high flooding risk, with maladaptive urbanization not only outpacing drainage infrastructure development but exacerbating risk as permeable surfaces are paved over for roads, as noted by Katy Daigle and Maanvi Singh of Science News.

The implications for rainfall-related mortality during high tides, as analysed by Bouchra Zellou and Hassane Rahali of the University Med V, are worrying for a city like Mumbai in the context of future sea-level rises and almost half of its population living rough in informal dwellings, argue P. E. Zope, T. I. Eldho, and V. Jothiprakash of IIT Bombay.

A 2019 report by the Municipal Corporation of Greater Mumbai notes that the antiquated drainage system, encroachment and subsequent overflow of the Mithi River, and combination of high tides and high river flow all contribute to regular floods. Built on reclaimed land from the sea, Mumbai’s coastal tract is low-lying. It also houses some of the most populous city spots in the Bandra Worli Sea Link and the Lokhandwala complex. 

A 40 cm sea-level rise is projected to increase rainfall-related excess mortality by 56%, relative to a no sea-level rise baseline, if no adaptive measures are undertaken, according to the University of Chicago study.

These impacts are not just immediate – aside from short-term effects seen in drowning, electrocutions, or traffic disruptions, rainfall also increases deaths in subsequent weeks as flood-related diseases such as dengue, malaria, and diarrhoea take time to develop and spread. 

This August, Mumbai recorded 800mm rainfall in four days, with several fatalities, people relocated from low-lying areas to temporary shelters, and transport delays, reported the BBC. While the city has in recent years invested to improve its drainage infrastructure particularly in low-lying areas that get inundated every year, new waterlogging spots have emerged.

Experts at the World Resources Institute (WRI) India have said that on average, Mumbai experienced 4 extremely heavy rainfall events, with more than 204.5mm rain within 24 hours, every year in the last decade (2010-2020), indicating the urgent need for effective flood mitigation infrastructure.

Delhi University’s S. K. Sinha and Ravi Shekhar, in a recent study, find that Mumbai draws both inter-state and intra-state migrants with most working in the informal sector as domestic helps, cleaners, drivers, construction workers, tea sellers, and vegetable sellers. Many migrants are unskilled. Mumbai's slums offer them the only affordable housing, but it comes with risks: weak drainage systems, poor waste disposal and overall unsanitary living conditions. When it rains, not only do the shanties flood, but the waste spreads, carrying diseases. 

Participants of Sinha and Shekhar’s study complained to the authors that they are never consulted by the government or local bodies during the preparation and implementation of programmes meant for their benefit. Consequently, their priorities are not reflected in the policies. 

Times of India reports that post the 2005 floods, the Chitale Committee recommended upgradation of the drainage system, the phased implementation of which culminated in the current 55mm/hr capacity. However, after August 2025, BMC finds the need to increase capacity further, especially in low-lying areas, to 120mm/hr. An implementation target is yet to be set. 

Experts at WRI India, and a study by Christophe Le Jallé, Denis Désille, and Gilles Burkhardt of the Programme Solidarité Eau (pS-Eau), recommend holistic measures, including dealing with rainfall at every stage, such as through rainwater harvesting in buildings; adding holding ponds on railway land to reduce transport delays; decentralising holding tanks to place them even in upstream areas; and using a mix of grey and blue infrastructure such that the latter also helps foster biodiversity. This goes into the area of both flood mitigation and adaptation – not just minimizing the impacts of rainfall as and when it happens but anticipating its effects especially on vulnerable groups and acting accordingly. 

While these are important, it is important to recognize that a majority of deaths could be prevented by involving slum residents in policy consultations and addressing the main catalysts in their deaths. 

Major flood events in Mumbai

26^th July 2005:  Mumbai recorded 944mm rainfall, over 40% of its annual rainfall, in a single day. Coinciding with a high tide, and with almost 90% of the city’s land surface concretised, the excess water could not drain into the ground. Mumbai’s drainage system, designed to handle 25mm/hr of rain, was inundated. The Mithi River, which could have functioned as a natural drainage channel, was severely encroached upon in its route through populated industrial areas. The ensuing flood submerged 30% of Mumbai, claimed 1000 lives, and caused USD$3 billion in economic losses. 

In 2019, after yet another rainfall event took over 30 lives, as noted by Prevention Web, the Comptroller and Auditor General of India (CAG) produced a report highlighting major deficiencies in Brihanmumbai Municipal Corporation’s (BMC) flood management system, documented by Hindustan Times. 

In August 2025, Mumbai saw 800mm rainfall in four days, with several fatalities, people relocated from low-lying areas to temporary shelters, and transport delays, reported the BBC. Opposition members of the government termed this an “absolute collapse of governance”, alleging that despite red alerts being issued, the government had done no planning, as new waterlogging spots emerged around recently built infrastructure. Citizens further called out Mumbai’s collapsing infrastructure, particularly the ageing drainage system.

Amrita Rudra holds an MSc in Public Policy and Administration from the London School of Economics and Political Science, with interests in climate adaptation and mitigation policy, governance, and social development.