Flooding events are associated with increased death rates for cardiovascular diseases, infectious and parasitic diseases, injuries, and respiratory diseases, according to NIEHS-funded research. Both infectious disease and injury death rates vary by sex and age, with different demographic groups at greater risk for distinct flood events. The study was published Jan. 3 in Nature Medicine.

“Our study shows the sweeping and hidden effects of floods, such as those due to tropical cyclones, heavy rain, snowmelt, or ice jams,” said senior study author Robbie M. Parks, Ph.D., an assistant professor of environmental health sciences at Columbia University. “Currently tropical cyclone warnings for tropical storms and hurricanes are very sophisticated in most places, but potentially other flood types can have more improved warning and resilience planning systems.”

The impressively large scale of the study — more than 35 million deaths over 18 years — allowed Parks and first author Victoria D. Lynch, Ph.D., to dive deeper into the data than most other studies, according to Abee Boyles, Ph.D., a health scientist administrator in the Population Health Branch at NIEHS.

“They looked at impacts on specific types of mortality over longer windows of time after differing sources of flooding events,” Boyles explained. “This allowed them to subdivide the data by these factors, as well as by population demographics, which will allow for more targeted public health warnings because not every flood carries the same risk for all people.”

Frequent flooding

Flooding, the inundation of water onto typically dry land, is an urgent global health concern. Between 2000 and 2015, the global population living in flood-prone areas increased from 58 to 86 million people. Since 1985, human settlements in areas with the highest flood risk have grown by 121%. In addition, scientists project that river, coastal, and flash floods will become more frequent globally.

Flooding has been associated with increased rates of injuries, cardiovascular diseases, infectious diseases, and mental health conditions. However, most previous work has focused on specific flooding case studies related to tropical cyclones, floods over a short period of time, or on fatalities attributed to acute flash floods.

“Flooding greatly endangers public health and is an urgent concern as rapid population growth in flood-prone regions and more extreme weather events will increase the number of people at risk,” Parks said. “Until now, there had been a critical knowledge gap about cause-specific flood mortality risks in the U.S. over time, and how risks may vary among groups within the population.”

Diving deeper

Parks and his collaborators addressed this gap using a validated flood exposure dataset to comprehensively examine the association of large floods with death rates from 2001 to 2018 at the county level across the United States. Using nearly two decades of detailed flood exposure data, the researchers found that flooding events were associated with increased death rates for cardiovascular diseases, infectious and parasitic diseases, injuries and respiratory diseases, with generally greater associations for more severe floods compared with less severe.

Injury death rates were higher for females and older adults following acute exposure to tropical cyclone-related flooding. Infectious disease mortality was elevated for males following exposure to all flood causes, particularly tropical cyclone- and heavy rain-related floods.

According to Parks, more research is needed to examine death rates from specific causes at a finer scale.

“Other important avenues include how evacuation dynamics from floods — tropical cyclone-related and otherwise — impact the long-term health of those who move and those who do not or cannot,” Parks said. “We have a lot of research planned for floods, both in the United States and worldwide.”