Individual and Interactive Effects of Floods and Tropical Cyclones on Mortality: A Multicountry Time-Series Analysis

Floods and tropical cyclones (TCs) are two of the most common and costliest climate disasters worldwide, yet a consistent evaluation of their individual and interactive health effects across events and countries remains lacking. We aimed to quantify the individual and joint all-cause and cardiopulmonary mortality risks associated with flood and TC exposure across countries and regions. Daily all-cause, cardiovascular, and respiratory mortality data between Jan 1, 2000, and Dec 31, 2019, were obtained from 2049 locations in 69 countries or territories globally. Historical flood and TC events were sourced from the Dartmouth Flood Observatory and the International Best Track Archive for Climate Stewardship, respectively. For each location, we applied quasi-Poisson regression with distributed lag nonlinear models to estimate cumulative mortality risks over 14, 30, 60, and 180 days following flood-only, TC-only, and joint flood-TC exposures. Location-specific estimates were then pooled using multivariate random effects meta-analysis to derive overall risk patterns. A total of 24.9 million all-cause deaths, 6.9 million cardiovascular deaths, and 3.4 million respiratory deaths from 114 locations with ≥1 joint exposure day were analyzed. Relative to hazard-free days, TC-only exposure days were associated with substantially higher and more prolonged mortality risks than flood-only days. Overall, we observed a 3% (95% confidence interval [CI]: 0–7%) increase in mortality over 60 days following flood-only days, compared to 19% (8–32%) for TC-only days. In the modification analysis of flood effects by TCs (using nonflood days as risk reference), flood days co-occurring with TCs exhibited considerably higher mortality risks than flood days without TCs, particularly for cardiovascular deaths at 30 days following the event (1.9% [95% CI: 0.9–3.0%] versus 0.1% [0.0–0.2%]). However, additive interactions between flood and TC exposures were generally not statistically significant. Strong effect modification was observed for economic development and institutional quality, as locations having lower GDP per capita, government effectiveness, and regulatory quality exhibited particularly elevated mortality risks, especially for mortality related to TC-only exposure days. In the current study, TC-only exposure days were found to be associated with greater short-to-medium-term mortality risks than flood-only days, and floods co-occurring with TCs could exhibit higher mortality risks than flood-only events, particularly from cardiovascular causes. Strengthening economic and institutional capacity and directing targeted postevent clinical support (particularly cardiovascular care) to socioeconomically and institutionally vulnerable locations may help reduce disaster-related deaths, especially after TCs.