Linked to higher mortality rates, large floods emerge as ‘an urgent public health concern’

Key takeaways:

• The all-cause mortality rate increased each month for 3 months after a large flood caused by heavy rain.
• Some populations, like women and older adults, appear more vulnerable to the effects of flooding.

Large floods over the last 2 decades corresponded with increased rates of death from injuries and other causes vs. normal conditions in the U.S., a retrospective analysis published in Nature Medicine revealed.

The findings are consistent with recent studies showing the significant effect that increasingly prevalent changes in climate, such as rising temperatures and extreme precipitation, have on mortality.

“Flooding is an urgent public health concern as sea level rise, rapid snowpack melting, and increased storm severity will lead to more destructive and frequent events,” Victoria Lynch, PhD, a post-doctoral research fellow at Columbia Mailman School of Public Health, said in a press release. “Our results show that floods were associated with higher death rates for most major causes of death, even for rain- and snow-related floods that are less likely to generate rapid emergency responses.”

According to data from the National Weather Service, 168 fatalities related to flooding occurred in the U.S. in 2024, the highest total since 2015 and up from 79 in 2023.

In the study, Lynch and colleagues assessed U.S. death records from 2001 to 2018 to determine how flooding affected the death rates from multiple types of causes over a 3-month period following the flood.

They also looked at how the associations varied based on the cause of the flooding, such as a tropical cyclone, ice jams, snowmelt or heavy rain, and flood severity.

The analysis showed 35,613,398 deaths from six causes of death — including injuries, cancers, CVD, neuropsychiatric conditions, infectious and parasitic diseases and respiratory diseases — in counties that experienced floods during the 18-year period, with 93 large flood events.

Heavy rain caused most of the floods, with tropical cyclones serving as the second-most common cause.

Lynch and colleagues found that very severe heavy rain-related flooding corresponded with a higher rate of death from all causes during the month of flooding (1.5%; 95% CI, 1%-2%) as well as 2 months (1%; 95% CI, 0.4%-1.3%) and 3 months (0.6%; 95% CI, 0.2%-1%) after the flooding.

This type of flooding also increased the rates of death from infectious and parasitic diseases (3.2%; 95% CI, 0.1%-6.2%) and CVD (2.1%; 95% CI, 1.3%-3%) during the month of flooding.

Meanwhile, tropical cyclone-related flooding may be tied to an increased rate of death from injuries during the month of flooding for severe (4.7%; 95% CI, 1.8%-7.6%) and very severe (15.3%; 95% CI, 12.4%-18.1%) floods.

The researchers noted that increases in rates of death from injuries during the month of tropical cyclone-related flooding appeared greater among:

• adults aged 65 years or older (24.9%; 95% CI, 20%-29.8%) vs. adults under 65 years of age (10.2%; 95% CI, 6.6%-13.8%); and
• women (21.2%; 95% CI, 16.3%-26.1%) vs. men (12.6%; 95% CI, 9.1%-16.1%).

Snowmelt-flooding corresponded with higher rates of death from respiratory diseases (22.3%; 95% CI, 19%-25.5%), neuropsychiatric conditions (15.9%; 95% CI, 12.5%-19.3%) and CVD (8.9%; 95% CI, 6.9%-11%).

Lynch and colleagues explained that disruptions to drinking water and sewage infrastructure from flooding can lead to waterborne disease transmission, while socioeconomic factors may impact mortality from causes like CVD in areas that experience frequent flooding.

“In the U.S., floods have a devastating effect on society, yet a comprehensive assessment of their continuing health impacts had been lacking,” Robbie M. Parks, PhD, an assistant professor in environmental health sciences at the Columbia Mailman School of Public Health and study senior author, said in the release. “Our study is a first major step in better understanding how floods may affect deaths, which provides an essential foundation for improving resilience to climate-related disasters across the days, weeks, months, and years after they wreak destruction.”

References:

• Floods linked to rise in U.S. deaths from several major causes. Available at: https://www.publichealth.columbia.edu/news/floods-linked-rise-u-s-deaths-several-major-causes. Published Jan. 6, 2025. Accessed Jan. 15, 2025.
• Lynch V, et al. Nat Med. 2025;doi:10.1038/s41591-024-03358-z.
• NWS preliminary US flood fatality statistics. Available at: https://www.weather.gov/arx/usflood. Accessed Jan. 15, 2025.