How Extreme Rainfall Amplifies Health Risks

Climate change is intensifying the variability of precipitation due to extreme daily and overall rainfall events. Awareness of the effects of these events is crucial for understanding the complex health consequences of climate change. The connections between health and climate have been recognized by physicians and patients even when climatology did not have the status of an exact science. Physicians have often advised their patients to move to a better climate, and when they did, the recommendation was rarely based on precise scientific knowledge. However, the benefits of changing environments were often so evident that they were indisputable.

Today, advanced models, satellite imagery, and biological approaches such as environmental epigenetics are enhancing our understanding of health risks related to climate change.

Extreme Rainfall and Health

The increase in precipitation variability is linked to climate warming, which leads to higher atmospheric humidity and extreme rainfall events. These manifestations can cause rapid weather changes, increasing interactions with harmful aerosols and raising the risk for various cardiovascular and respiratory conditions. However, a full understanding of the association between rain and health has been hindered by conflicting results and methodological issues (limited geographical locations and short observation durations) in studies.

The association between rainfall intensity and health effects is likely nonlinear. Moderate precipitation can mitigate summer heat and help reduce air pollution, an effect that may lower some environmental health risks. Conversely, intense, low-frequency, short-duration rainfall events can have particularly harmful effects on health, as such events can trigger rapid weather changes, increased proliferation of pathogens, and a rise in the risk of various pollutants, potentially exacerbating health conditions.

Rain and Mortality

Using an intensity-duration-frequency model of three rainfall indices (high intensity, low frequency, short duration), a study published in October 2024 combined these with mortality data from 34 countries or regions. Researchers estimated associations between mortality (all cause, cardiovascular, and respiratory) and rainfall events with different return periods (the average time expected before an extreme event of a certain magnitude occurs again) and crucial effect modifiers, including climatic, socioeconomic, and urban environmental conditions.

The analysis included 109,954,744 deaths from all causes; 31,164,161 cardiovascular deaths; and 11,817,278 respiratory deaths. During the study period, from 1980 to 2020, a total of 50,913 rainfall events with a 1-year return period, 8362 events with a 2-year return period, and 3301 events with a 5-year return period were identified.

The most significant finding was a global positive association between all-cause mortality and extreme rainfall events with a 5-year return period. One day of extreme rainfall with a 5-year return period was associated with a cumulative relative risk (RRc) of 1.08 (95% CI, 1.05-1.11) for daily mortality from all causes. Rainfall events with a 2-year return period were associated with increased daily respiratory mortality (RRc, 1.14), while no significant effect was observed for cardiovascular mortality during the same period. Rainfall events with a 5-year return period were associated with an increased risk for both cardiovascular mortality (RRc, 1.05) and respiratory mortality (RRc, 1.29), with the respiratory mortality being significantly higher.

Points of Concern

According to the authors, moderate to high rainfall can exert protective effects through two main mechanisms: Improving air quality (rainfall can reduce the concentration of PM_2.5 particles in the atmosphere) and behavioral changes in people (more time spent in enclosed environments, reducing direct exposure to outdoor air pollution and nonoptimal temperatures). As rainfall intensity increases, the initial protective effects may be overshadowed by a cascade of negative impacts including:

• Critical resource disruptions: Intense rainfall can cause severe disruptions to access to healthcare, infrastructure damage including power outages, and compromised water and food quality.
• Physiological effects: Increased humidity levels facilitate the growth of airborne pathogens, potentially triggering allergic reactions and respiratory issues, particularly in vulnerable individuals. Rapid shifts in atmospheric pressure and temperature fluctuations can lead to cardiovascular and respiratory complications.
• Indirect effects: Extreme rainfall can have profound effects on mental health, inducing stress and anxiety that may exacerbate pre-existing mental health conditions and indirectly contribute to increased overall mortality from non-external causes.

The intensity-response curves for the health effects of heavy rainfall showed a nonlinear trend, transitioning from a protective effect at moderate levels of rainfall to a risk for severe harm when rainfall intensity became extreme. Additionally, the significant effects of extreme events were modified by various types of climate and were more pronounced in areas characterized by low variability in precipitation or sparse vegetation cover.

The study demonstrated that various local factors, such as climatic conditions, climate type, and vegetation cover, can potentially influence cardiovascular and respiratory mortality and all-cause mortality related to precipitation. The findings may help physicians convey to their patients the impact of climate change on their health.

This story was translated from Univadis Italy using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.