Extreme rainfall puts people’s lives at risk

Intense rainfall appears to contribute to a higher risk of death from all causes and cardiovascular or respiratory causes, suggests a study. This association varies with local climate and urban infrastructure.

“The significant effects of extreme events were modified by climate types and were most pronounced in locations characterized by low variability in precipitation or scarce vegetation coverage,” the investigators said.

More than 600 locations across 34 countries or regions were included in this two-stage time series analysis. Using daily mortality data consisting of 109,954,744 all-cause, 31,164,161 cardiovascular, and 11,817,278 respiratory deaths from 1980 to 2020, the investigators examined the association between daily mortality and rainfall events with return periods of 1, 2, and 5 years, with a 14-day lag period.

Return period was defined as the expected average time between occurrences of an extreme event of a certain magnitude. A continuous relative intensity index was applied to create intensity-response curves that estimated mortality risks on a global scale.

There were 50,913 rainfall events with a 1-year, 8,362 events with a 2-year, and 3,301 events with a 5-year return period identified during the study period. [BMJ 2024;387:e080944]

A day of extreme rainfall with a 5-year return period significantly correlated with an elevated daily all-cause (cumulative relative risk [RR] across 0–14 lag days, 1.08, 95 percent confidence interval [CI], 1.05–1.11), cardiovascular (RR, 1.05, 95 percent CI, 1.02–1.08), and respiratory (RR, 1.29, 95 percent CI, 1.19–1.39) mortality.

On the other hand, rainfall events with a 2-year return period correlated with respiratory deaths only, while no significant associations were observed for rainfall events with a 1-year return period.

In nonlinear analysis, the protective effects (RR <1) seen with moderate-to-heavy rainfall turned into adverse effects (RR >1) when rainfall intensity became extreme.

Notably, climate type, baseline variability in rainfall, and vegetation coverage appeared to modify the mortality risks associated with extreme rainfall events. On the contrary, population density and income level showed nonsignificant moderating effects. Additionally, locations with lower variability of baseline rainfall or scarce vegetation were at increased risks.

Mechanism

“Although the biological mechanisms linking rainfall intensity with health outcomes are not fully elucidated, several plausible explanations may be helpful to clarify the complex association observed in our study,” the investigators said. 

For instance, moderate-to-high rainfall intensity may have protective effects because of the improvement in air quality (rainfall can reduce concentrations of PM_2.5 particles in the atmosphere) and changes in the behaviour (rainfall may alter people’s daily pattern, leading to more time spent indoors). [Earth Space Sci 2019;6:1915-1925; PLoS One 2013;8:e81153]

“This reduction in direct exposure to outdoor air pollution and nonoptimal temperatures could explain the protective effect observed on various health outcomes when intensity is not extreme,” the investigators said. 

Moreover, as rainfall intensity increases, the initial protective effects may be influenced by several negative impacts, namely critical disruptions to resources, physiological impacts, and indirect effects.

“These findings underscore the need for comprehensive public health strategies, developed through collaboration among meteorological, public health, and urban planning sectors,” the investigators said.

“Such strategies are crucial to mitigate the broad health effects of extreme rainfall. This is especially important considering the well-established trend of increasing short-term rainfall intensity as a result of climate change,” they added.