Global, regional and national trends and impacts of natural floods, 1990–2022

Abstract Objective To assess global, regional and national trends in the impact of floods from 1990 to 2022 and determine factors influencing flood-related deaths. Methods We used data on flood disasters from the International Disaster Database for 1990–2022 from 168 countries. We calculated the annual percentage change to estimate trends in the rates of people affected and killed by floods by study period, World Health Organization (WHO) region, country income level and flood type. We used multivariable logistic regression analysis to assess the factors associated with death from floods. Findings From 1990 to 2022, 4713 floods were recorded in 168 countries, which affected > 3.2 billion people, caused 218 353 deaths and were responsible for more than 1.3 trillion United States dollars of economic losses. The WHO Western Pacific Region had the most people affected by floods (> 2.0 billion), accounting for 63.19% (2 024 599 380/3 203 944 965) of all affected populations. The South-East Asia Region had the most deaths (71 713, 32.84%). The African and Eastern Mediterranean Regions had the highest number of people affected and killed by floods per 100 000 population in 2022. The odds of floods causing more than 50 deaths were significantly higher in low-income countries (adjusted odds ratio: 14.34; 95% confidence interval: 7.46 to 30.04) compared with high-income countries. Numbers of people affected and mortality due to floods declined over time. Conclusion Despite the decreases in populations affected and deaths, floods still have a serious impact on people and economies globally, particularly in lower-income countries. Action is needed to improve disaster risk management and flood mitigation.


Introduction
Natural disasters are catastrophic events of atmospheric, geological and hydrological origins.These disasters include earthquakes, floods, volcanic eruptions, landslides, tsunamis and droughts. 1From 1998 to 2017, 1.3 million people were reported to have died from climate-related and geophysical disasters, and a further 4.4 billion people were injured, made homeless, displaced or in need of emergency assistance. 2 In the same period, disaster-hit countries also reported direct economic losses valued at 2.9 trillion United States dollars (US$). 2 Disasters are a major contributor to entrenched poverty in low-and middle-income countries; thus, reducing losses from disasters is key to eradicating poverty.The United Nations (UN) Office for Disaster Risk Reduction called for a substantial reduction in direct disaster losses as well as associated infectious diseases, and social and economic losses, and noted that the responsibility should be shared by the stakeholders. 294][5] A 2013 study showed that from 1980 to 2009, floods were responsible for more than 500 000 deaths and more than 350 000 injuries, and affected nearly 3 billion people across the world. 3The World Health Organization (WHO) South-East Asia and Western Pacific Regions were the most flood-affected regions and were estimated to account for nearly 50% of flood-related deaths in the last 25 years of the 20th century. 6,7Additionally, an estimated 1.81 billion people were directly exposed to 1-in-100-year floods, 1.24 billion of whom lived in the WHO South-East Asia Region. 8In the WHO European Region, about 400 floods caused the deaths of more than 2000 people, affected 8.7 million others, and resulted in at least 72 billion euros in losses during 2000-2014. 9However, a survey conducted by WHO and Public Health England highlighted the gaps in strategies to prevent health effects of floods and in the availability of a coordinated and systematic health response to floods. 9 recent systematic review identified various factors that can influence flood-related deaths, which could be classified into five categories, namely hazard-related, individual, environmental, socioeconomic and managerial factors. 10However, which of these factors has a significant causal relationship with flood-related deaths is still unclear.Furthermore, the trends in the global impact of floods in the past three decades is not well understood.In this study, we aimed to summarize the trends in the global, regional and national impact of floods from 1990 to 2022, and to assess the factors associated with deaths from floods.The findings could provide a comprehensive understanding of the trends in the impact of floods and the factors influencing flood-related deaths.Such information is important to help strengthen disaster risk management and enhance preparedness for and effective responses to floods.

Study design and data sources
We conducted a worldwide observational study which covered all countries and territories that reported flood Bull World Health Organ 2024;102:410-420| doi: http://dx.doi.org/10.2471/BLT.23 The income level and number of populations of various countries in each year from 1990 to 2022 were extracted from the UN open database, UNdata. 28o ethics approval was required for this analysis of publicly available data.

Variables
Flood was defined as the overflow of water from a stream channel onto normally dry land in the floodplain (riverine flooding), higher-than-normal levels along the coast and by lakes or reservoirs (coastal flooding), as well as ponding of water at or near the point where rain fell (flash flooding). 11In the EM-DAT, flood disasters are one of the hydrological disasters (including flood, landslide and wave action), which are defined as hazards caused by the occurrence, movement and distribution of surface and subsurface freshwater and saltwater.We used the following variables from the database: country (countries in which the flood occurred); date (when the flood occurred and ended); flood type (riverine flooding, coastal flooding or flash flooding); deaths (number of people who died because of the flood); affected individuals (number of people who were injured or made homeless by the flood or who required immediate assistance during a period of emergency, i.e. basic survival needs such as food, water, shelter, sanitation and immediate medical assistance); and adjusted total damages in US$ (all damages and economic losses directly or indirectly related to the flood, adjusted for inflation using the consumer price index).

Statistical analysis
In the descriptive analysis, the number of affected people, the number of deaths and the total economic damages in each year were summed from 1990 to 2022 by country.The number of floods in each WHO region every month was also summed from 1990 to 2022.Given differences in population size and fluctuations of population size in different years and regions, we calculated the rates of people affected by flood per 100 000 population (affected rate) and the rates of flood-related deaths per 100 000 population (mortality) using the total number of people affected by the floods, the number of deaths caused by the floods, and the total population of each region in each year.We calculated the estimated annual percentage change to show trends.This indicator is widely used to show the trend and annual change over a specified time period. 12,13We fitted a regression line to the natural logarithm of rates using the formula y = α + βx + ε, where y is the rate, x is the calendar year, α is the expected value of the dependent variable when all explanatory variables are 0, β is regression coefficient and ε is the error term (the part of the y variable that the model cannot explain).We calculated the estimated annual percentage change, with its 95% confidence intervals (CIs) to assess the time trend in the rates, as 100 × (e β -1), where e is the Euler number.If the estimated annual percentage change and its 95% CI were both > 0, we considered the trend in rates to be increasing in the given time interval.Conversely, if the change and 95% CI were both < 0, we considered the trend in rates to be decreasing.Additionally, we divided all the flood events into four groups based on the number of deaths they caused: (i) zero deaths or no data; (ii) 1-9 deaths; (iii) 10-49 deaths; and (iv) ≥ 50 deaths.We used the χ 2 test to assess differences between the four groups by study period, WHO region, country income level and subtype of flood.We used multivariable logistic regression analysis to estimate the associations of factors (study period, WHO region, income level and flood type) with the risk of death at the group level compared with floods with 0 deaths or no data, expressed as adjusted odds ratios (aOR).We used R, version 4.1.1(R Foundation, Vienna, Austria) for all analyses.

Global and national impact
From 1990 to 2022, 4713 floods were recorded in 168 countries and territories, which affected more than 3.2 billion people, resulted in 218 353 deaths and caused more than US$ 1.3 trillion in economic damages.Of all the countries where floods occurred, China was the most affected, with the largest cumulative population affected (1.9 billion people), the most economic damage (US$ 442 billion) and the second largest number of deaths due to the floods (30 890 deaths).India had the second largest population affected (629 million people), the third most economic damages (US$ 115 billion) and the largest number of deaths (46 506 deaths).In Bangladesh, 159 million people were affected by floods, the third largest in the world.In the United States of America, floods caused about US$ 135 billion in economic damages, second only to China; and Bolivarian Republic of Venezuela recorded the third highest number of deaths (30 342 deaths) from floods (Fig. 1).(17 million).In Western Pacific Region, there were 24 years when more than 10 million people were affected by floods yearly and, among those years, 10 years when more than 100 million people were affected.In the South-East Asia Region, there were 25 years when more than 10 million people were affected by floods yearly, and in 1993, 146 million people were affected.More than 10 million people were affected by floods in 4 years in the Eastern Mediterranean Region: 34 million in 2022, 21 million in 2010, 13 million in 1992 and 12 million in 2019.The highest number of people affected by floods in the Region of the Americas was 16 million in 2008 (Table 1).Globally, the largest number of people affected was in 1998 (243 million people), followed by 212 million people in 1991 and 157 million people in 1996.

Floods by WHO region
Of the 218 353 deaths due to floods, the South-East Asia Region had the highest number (71 713; 32.84%) followed by the Region of the Americas (48 630; 22.27%).The Western Pacific Region had 42 721 deaths (19.57% of all deaths).In the South-East Asia Region, the greatest number of deaths (6989 deaths) occurred in 2013, followed by 4635 deaths in 2007.In the Region of the Americas, the greatest number of deaths (30 868 deaths) occurred in 1999, followed by 3603 deaths in 2004.In the Western Pacific Region, the greatest

No. of deaths
In 1990, the Western Pacific Region had the highest affected rate of floods (1120.50 per 100 000 population), followed by 335.13 per 100 000 in the African Region and 236.64 per 100 000 in the Eastern Mediterranean Region.The affected rates in the South-East Asia, American and European Regions in 1990 were all lower than 100.00 per 100 000 population.In 2022, the Eastern Mediterranean Region had the highest affected rate (3817.94per 100 000 population), followed by 383.21 per 100 000 in the African Region.Between 1990 and 2022, the affected rates decreased significantly in the South-East Asia (estimated annual percentage change -5.75%; 95% CI: -9.00% to -2.38%) and Western Pacific (estimated annual percentage change -6.76%; 95% CI: -12.33% to -0.83%) Regions (Table 2).In certain years the affected rates of floods were particularly high in some regions, such as in 1991 in the Western Pacific and African Regions, with 5420.13 per 100 000 and 4407.24 per 100 000 population affected, respectively (Fig. 3).
In 1990, the African Region had the highest mortality caused by floods (1.72 per 100 000 population), followed by 0.58 per 100 000 in the Eastern Mediterranean Region (Table 2).In 2022, we estimated the highest mortality in the Eastern Mediterranean Region (2.66 deaths per 100 000 population), followed by 1.10 deaths per 100 000 in the African Region.Between 1990 and 2022, mortality decreased significantly in all WHO regions except the Region of the Americas.The greatest decrease was in the Western Pacific Region (estimated annual percentage change −6.99%; 95% CI: −9.45% to −4.47%), followed by the Eastern Mediterranean Region (estimated annual percentage change: −3.74%; 95% CI: −7.31% to −0.03%).Mortality was particularly high in 1999 in the Region of the Americas (27.24 per 100 000) and in 1991 in the African Region (10.68 per 100 000; Fig. 4).

Factors affecting deaths
As shown in Table 3, the proportion of floods with more than 50 deaths decreased from 22.53% (194/861) in 1990-1999 to 12.28% (188/1531) in 2010-2022.From 1990 to 2022, the South-East Asia and Western Pacific Regions had the greatest proportion of floods with more than 50 deaths, 36.20%(236/652) and 23.01%(150/652), respectively.In the South-East Asia Region, 30.37% (236/777) of floods caused more than 50 deaths.Income level was also associated with flood-related deaths.Floods in low-and lower-middleincome countries caused significantly more deaths than in high-and uppermiddle-income countries (P < 0.001).
The odds of floods with more than 50 deaths decreased significantly over time compared with floods with no deaths or no data.Compared with the African Region, floods in the Eastern Mediterranean Region were more likely to cause 10-49 deaths (aOR: 2.83; 95% CI: 2.04 to 3.94) or > 50 deaths (aOR: 3.85; 95% CI: 2.55 to 5.83).Similar findings were observed in the South-East Asia Region (aOR: 1.87; 95% CI: 1.40 to 2.49 for 10-49 deaths; aOR: 6.72; 95% CI: 4.74 to 9.62 for > 50 deaths) and the Western Pacific Region (aOR: 1.79; 95% CI: 1.33 to 2.41 for 10-49 deaths; aOR: 4.33; 95% CI: 2.94 to 6.44 for > 50 deaths).Compared with high-income countries, the odds of more than 50 deaths from floods were significantly higher for all other incomes levels, particularly for low-income countries, (aOR: 14.34; 95% CI: 7.46 to 30.04).Compared with coastal floods, the odds of causing 10-49 deaths were higher for flash floods (aOR: 3.16; 95% CI: 1.59 to 6.41) and riverine floods (aOR: 2.16; 95% CI: 1.12 to 4.26).No significant differences were seen between flood type in causing > 50 deaths (Table 4).China was the country most affected by natural floods, with the highest number of people affected and second-highest number of deaths.Despite significant investments in flood defences following the 1998 floods, urban flooding remains a serious concern, with more than 157 cities affected since 2006. 14,157][18][19] In the United States, floods also pose a great threat to lives and property, and impose a substantial financial burden on the National Flood Insurance Program, which has accrued more than US$ 20 billion in debt since 2005. 20,21limate change, implicated in escalating precipitation levels, contributes to growing flood-induced damages. 22imilarly, Bangladesh and India have faced increasing flood incidences due to climate change, with hasty developmental projects exacerbating the impacts. 23herefore, governments need to use scientific evidence to devise robust flood mitigation strategies, including successful so-called room for the river approaches, which involve enlarging the capacity of rivers and improving overall floodplain management. 24,25Collaboration across state and international borders with relevant stakeholders is critical.
Regionally, large numbers of people in the Western Pacific and South-East Asia Regions are affected by floods.The South-East Asia, American and Western Pacific Regions, in that order of size, accounted for 74.68% (163 064/218 353) of all deaths due to floods.Of note, our study also found that although the Eastern Mediterranean and African Regions had fewer people affected, the population affected and the mortality due to floods in these regions were the highest in 2022.This situation reflects that in low-income countries, although the disaster-affected population might be smaller, the magnitude of the disaster is greater than in other areas.Thus, areas where populations are substantially affected require comprehensive flood reduction strategies that are well planned and executed to mitigate the effects of floods.Regions with high mortality need to gauge flood risks more accurately and improve emergency management of floods and other disasters to reduce fatalities from floods.To achieve this goal might require greater investment in and redistribution of resources for better flood prevention and emergency response systems.
The risk of death from floods was higher in lower-income countries than in high-income countries.Although economic development can increase disaster-related economic losses, improvements in emergency preparedness, response and coping capacity can reduce vulnerability to disasters, thereby lowering the number of affected individuals and disasterrelated deaths. 7Therefore, economic assistance and policy support for lowincome countries are important.Helping these countries to build disaster risk analysis systems could guide the development of strategies for managing disasters, and the identification of vulnerable countries and regions and the factors contributing to social vulnerability. 18,27ur analysis showed notable time trends in the population affected and mortality due to floods, as well as reduced odds of floods resulting in more than 50 deaths.The decreasing trends suggest an improvement in our global, regional and national responses to such disasters, potentially owing to advances in predictive technology, infrastructure and disaster management strategies.However, it is important to remember that, while these declines are promising, flood events are dependent on a variety of fluctuating factors, such as changing climate patterns.

Research
rant further in-depth investigations and discussion to fully appreciate their complex implications.
Our study had some limitations.Our findings depended on the data source.In several events, the reported number of deaths, injuries and people affected might be lower than the actual situation, and hence the impact of floods might be underestimated.Our study did not focus on personal factors, and future studies that have access to individuallevel data might want to consider exploring these dimensions, given the potential influence of personal factors on the impact of floods.This information could provide a more nuanced understanding of how personal factors might modulate the effects of floods.
Our findings highlight the continued serious impact of f loods on countries and populations and hence the urgent need for stronger disaster risk governance, effective flood mitigation strategies, improved emergency systems and international aid, especially for low-income countries.■     El número de personas afectadas y la mortalidad por inundaciones disminuyeron con el tiempo.Conclusión A pesar de la disminución de las poblaciones afectadas y de las muertes, las inundaciones siguen teniendo un grave impacto en las personas y las economías de todo el mundo, en especial en los países con ingresos más bajos.Es preciso actuar para mejorar la gestión del riesgo de catástrofes y la mitigación de las inundaciones.
WHO: World Health Organization.a

Table 4 . Multivariable logistic regression analysis of factors associated with natural flood-related deaths, 1990-2022
: adjusted odds ratio; CI: confidence interval; WHO: World Health Organization.Note: The comparison is 0 deaths or no data during a flooding event for all the three death groups.For example, low-income countries have a higher likelihood of experiencing more than 50 deaths instead of 0 deaths than high-income countries.Bull World Health Organ 2024;102:410-420| doi: http://dx.doi.org/10.2471/BLT.23.290243 aOR