Spatio-temporal clustering of drowning mortality in Iran, from 2005 to 2022

Abstract

Background

Drowning is a serious and neglected public health threat, and prevention of drowning has a multisectoral nature and requires multidimensional research. Therefore, this study aimed to evaluate the spatio-temporal variation in fatal unintentional drowning rates among the Iranian population from 2005 to 2022.

Methods

In this repeated cross-sectional study, registry data were extracted from legal medicine organisations during 2005–2022. The mortality rate per 1 million population was calculated by gender and province. The joinpoint regression model was fitted to estimate average annual percentage changes and an annual percentage change in the drowning mortality rate. We used spatial scan statistics to detect high-risk clusters of drowning deaths at the provincial level.

Results

Over 17 years 19 547 people died due to unintentional drowning. The highest yearly drowning rate was 15.58 per 1 000 000, and men had the highest rates of death (25.91) compared with women (4.98) in 2019. The overall mortality rate has decreased from 18.69 in 2005 to 12.87 in 2022. In the spatio-temporal analysis, four statistically significant high-risk clusters were detected in the north, southeast and centre of Iran.

Conclusion

The overall mortality rate in 2022 decreased compared with the 17-year period. In the spatial analysis, several high-risk clusters were identified in different locations, which highlights the importance of targeted and more comprehensive interventions. It seems that the prevention of drowning requires the effective participation of all responsible organisations and risk reduction plans in the field of environmental and individual risk factors.

WHAT IS ALREADY KNOWN ON THIS TOPIC.

WHAT THIS STUDY ADDS

• From 2005 to 2022, the drowning rate decreased from 18.70 per million people to 12.87. Also, the overall trend of death due to drowning was downward for both genders.

• The highest yearly rate of drowning was 15.58 per 1 000 000, and men had the highest rate of death (25.91) compared with women (4.98) in 2019.

• In spatial-temporal analysis, four significant high-risk clusters were detected in which the most likely cluster was located in the north of Iran (Mazandaran).

• The relative risk of drowning in the most likely cluster was 4.8, and the risk of events inside the cluster was 4.8 times more than outside the cluster.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• The findings of this study provide useful information about the epidemiology of drowning in the last 17 years. This information provides the opportunity to plan for the implementation of effective interventions in order to prevent and reduce mortality according to the geographical location of each region.

Introduction

Drowning is the third main cause of mortality due to unintentional injuries based on the WHO report and about 300 000 people around the world die due to drowning every year.¹ Unintentional drowning can lead to severe injury and disability in people rescued from it. Among unintentional injuries in 2019, drowning had the second rank in the DALY rate (disability-adjusted life years) per 100 000 populations.² The high-risk groups for drowning are men and children under 5 years of age, and it is 1 of the 10 causes of death in people aged 1–20.³ Nearly 80% of drowning cases are men.⁴ More than 90% of drowning cases occur in low- and middle-income countries and Asia (south, east and southeast) had the highest number of years of life lost.⁵ Iran is an upper middle-income country and results of studies show a decrease in the drowning mortality rate until 2018, but this change is not impressive. Still, every year, on average, a thousand people die due to drowning in Iran annually.^{6 7}

The distribution of mortality rate due to drowning is varied by province and geographical location. The highest crude mortality rates of drowning were 33.19 in the southwest (Khuzestan) and 29.45 in the north (Mazandaran), while southern Khorasan in the east of the country had the lowest rate (1.26) in 2018.⁷

The Caspian Sea is located in the north of Iran and the country has a long coastline in the south (Persian Gulf and Oman Sea) with many rivers and dams, and this geographical diversity may be one of the main causes of the different mortality patterns.⁷ The highest drowning mortality rate occurs in men with an age group of 15–29 years, which has outcomes such as the high number of lost years of life and the direct and indirect cost to the country.^{8 9} In the north of Iran (Gilan province), the direct cost of drowning and non-fatal drowning incidents was reported as US$119 192, and the average cost was US$494 per drowning death in 2011.¹⁰ The majority of injured victims in this area were in the age group of 20–29, which accounts for 34% of direct medical costs.¹⁰

Although drowning is a serious problem for public health, fortunately, it is preventable. Many Asian countries such as the Philippines, Thailand, Vietnam and India have succeeded in implementing drowning prevention programmes at the national level.¹¹ Also, in high-income countries, significant progress has been made in the prevention of child drowning because they use national and international drowning prevention programmes.¹²

Sporadic plans are implemented by local authorities at the subnational level, such as the provision of a safe zone for swimming in the Caspian Sea with lifeguards present in the summer. Other activities include holding a conference (drowning day) or educational activities.^13–16 However, there is no integrated and targeted programme at the national level to reduce deaths and these activities are limited to certain areas or times.¹⁷ Understanding information about the epidemiology of drowning such as the trend of mortality and identifying high-risk areas provide the possibility of effective interventions to prevent and reduce mortality according to the conditions of each region. So, the first aim of this study is to investigate the trend of drowning mortality incidence rate by province, year and gender. The second aim is to examine the space-time distribution of mortality to identify high-risk clusters.

Materials and methods

Data sources

There are some mortality registration systems in Iran, which record different types of all causes of deaths.¹⁸ But Legal Medicine Organisation (LMO) is the main source of recording fatal injuries in the whole of Iran. According to the law, all fatal injuries and all suspected cases of death must be reported to the LMO for evaluation and determination of the cause of death. Therefore, this organisation is responsible for issuing death certificates for such deaths, and drowning is also one of the cases mentioned in the definition.¹⁹

Therefore, the LMO database is the most reliable source for these registries (the golden standard source) and the most complete database in this field. LMO records a wide range of variables including full name, age, gender, date of injury, date of death, place of death, cause of death and other details based on the death certificate. Death registration in this organisation is based on the International Classification of Diseases and data collection is done by doctors in the dissection room in each province.^{7 20} According to the results of studies, the registration of fatal injury cases by LMO has an under-reporting of about 13% and the completeness of registration coverage was reported as 86–87%.^{20 21} This repeated cross-sectional study was conducted using registry data extracted from the Iranian Legal Medicine Organisation during the years 2005–2022. Until 2011, only information related to the number of deaths was recorded without considering gender and location of drowning, so trend analysis of these variables will be focused on 2011 and after. The crude mortality rate of drowning per 1 million people was calculated by province, gender and year of incidence.

Population data

The census of the population in Iran is conducted in 5-year intervals, so the population in the denominator of the incidence rate fraction was estimated by using the census data (2005, 2010 and 2015) and calculating the average growth rate.

Statistical analysis

The trend of fatal unintentional drowning was assessed by computing the annual per cent change (APC) and the average annual per cent change (AAPC) with joinpoint regression programme V.5.0.2. Joinpoint regression, which is known as piecewise regression, is a method that fits a series of joined straight lines on a log scale to the trends and these linear segments are joined at points called joinpoint.²² Also, we used spatial scan statistics to detect and evaluate the spatial and temporal geographical distribution of drowning deaths to identify high-risk clusters.²³ The space-time scan statistic imposes cylindrical windows that move in the specified locations in space and time with a circular base corresponding to geography and the height shows the timespan of clusters.²⁴ The likelihood ratio statistic is estimated for each cylindrical window based on observed and expected cases, and the expected cases are computed by observed cases within and outside the window.²⁴ Finally, the likelihood function is maximised for all cylindrical windows, and the cluster with the maximum likelihood ratio is the most likely cluster and other clusters are secondary clusters.²⁴

This method indicates which areas have a greater risk of drownings in the study region. For each location of the scanning window, the alternative hypothesis is that the risk of death due to drowning within the window is higher as compared with outside.²⁴ In this method for detecting significant clusters, Monte Carlo hypothesis testing is used to obtain the p values. The SaTScan software V.10.1 was used to identify significant clusters.²⁴

Patient and public involvement

Patients and/or the public were not involved in the design, or conduct, or reporting of this repeated cross-sectional study, and analysis was performed based on registry data.

Result

Between 2005 and 2022, a total of 19 541 people died from drowning in Iran (6900 deaths during 2005–2010 and 12 641 deaths during 2011–2022). About 84% (10 681) of victims were men, and 16% (1960) were women from 2011 to 2022. The highest and lowest drowning death numbers were 1275 and 879 deaths in 2019 and 2015, respectively. The total mortality rate has changed from 18.69 in 2005 to 12.84 in 2022. From 2011 to 2022, men had a remarkable proportion of deaths, where the highest rate of mortality for men and women were 25.91 and 4.98 in 2019, respectively (table 1).

Table 1. The frequency of drowning deaths and the crude mortality rate per 1 million people, (N=19 541), 2005–2022, Iran.

Years	Number of deaths			Mortality rate
	Total	Men	Women	Total	Men	Women
2005	1280	NA	NA	18.70	NA	NA
2006	1160	NA	NA	16.45	NA	NA
2007	1220	NA	NA	17.09	NA	NA
2008	1162	NA	NA	16.07	NA	NA
2009	1007	NA	NA	13.75	NA	NA
2010	1071	NA	NA	14.43	NA	NA
2011	1072	902	170	14.26	23.80	4.56
2012	1074	892	182	14.12	23.24	4.83
2013	1063	894	169	13.80	23.01	4.43
2014	898	783	115	11.52	19.91	2.98
2015	879	748	131	11.13	18.78	3.35
2016	913	798	115	11.42	19.70	2.92
2017	1026	858	168	12.74	21.02	4.23
2018	935	800	135	11.52	19.45	3.37
2019	1275	1074	201	15.59	25.92	4.98
2020	1188	984	204	13.49	22.05	4.69
2021	1166	1009	157	13.14	22.43	3.59
2022	1152	939	213	12.88	20.72	4.83

The mortality rate reduction was observed in 16 (52%) provinces in 2022 compared with 2011, while in one province the rate was stable and in the rest of the area, mortality rates increased (see online supplemental appendix 1).

The joinpoint model result showed four-line segments joined in the years of 2013, 2016, 2019 and 2022. Nationally, the crude mortality rate from drowning decreased from 18.69 to 13.80 per 1 000 000 from 2005 to 2013 (APC: −3.86; 95% CI: −5.57 to −2.12). Between 2013 and 2016, the crude mortality rate decreased from 13.80 to 11.42 per 1 000 000 people (APC: −5.82; 95% CI: −21.90 to 13.58). Unlike the previous two segments, the drowning crude mortality increased from 11.42 per 1 000 000 in 2016 to 15.59 per 1 000 000 in 2019 (APC: +10.05; 95% CI: −7.95 to 31.55). Finally, in the last segment (2019–2022), the average crude mortality rate decreased by −4.44% per year (95% CI: −11.91 to 3.67) (figure 1). On average, during the 17-year study period, the drowning mortality rate decreased by 2% per year (AAPC: −2; 95% CI: −5.87 to −2.02). This value for men was −1.36 (95% CI: −5.09 to 2.50), and for women it was −0.97 (95% CI: −14.64 to 14.88). More details about the trend of mortality due to drowning by gender split are depicted in figure 2.

Figure 1. The time trend of crude mortality rate due to drowning (N=19 541), 2005–2022, Iran. APC, Annual Percent Change.

Figure 2. The time trend of crude mortality rate due to drowning by gender (N=12 641), 2011–2022, Iran. APC, Annual percent Change.

Hamadan had the considerable rise (APC: 6.21; 95% CI: −22.08 to 44.81) and Golestan had the most decline (AAPC:−13.10, 95% CI: −49.63 to 49.89) in average annual mortality percent and these changes were not statistically significant. The largest decrease and increase in the death trend were assigned to East Azerbaijan (APC: −19.47 to 56.34) and Southern Khorasan (APC: 21.71 to −38.65). Statistically significant percent change was observed for Bushehr (APC: −29.98, p-value=0.02) in 2011–2014 and East Azerbaijan (APC: −19.27, p-value=0.02) during 2017–2022 (subset table 2). (See the complete table in the online supplemental appendix 2).

Table 2. Result of joinpoint regression model analysis of mortality due to drowning at subnational level (N=12 641), 2011–2022, Iran.

Province	Time point	APC	Lower and upper CI	P-value	AAPC	Lower and upper CI	P-value
East Azerbaijan	2011–2014	−19.47	47.85 to 24.35	0.23	−3.39	−20.82 to 17.87	0.73
	2014–2017	56.34	−35.78 to 280.64	0.23
	2017–2022	−19.27	−32.42 to −3.56	0.02*
Bushehr	2011–2014	−29.98	−48.10 to −5.54	0.02*	−2.27	−16.87 to 14.89	0.78
	2014–2017	36.05	−36.35 to 190.88	0.323
	2017–2022	−2.13	−13.16 to 10.30	0.64
Southern Khorasan	2011–2013	0.60	−93.94 to 1572.39	0.99	3.80	−33.18 to 61.25	0.86
	2013–2020	21.71	−7.61 to 60.34	0.11
	2020–2022	−38.65	−88.98 to 241.70	0.47
Golestan	2011–2014	−33.27	−83.50 to 169.86	0.46	−13.10	−49.63 to 49.89	0.61
	2014–2019	5.67	−66.50 to 233.38	0.90
	2019–2022	−18.34	−82.67 to 284.71	0.73
Hamadan	2011–2013	−4.68	−86.13 to 555.17	0.94	6.21	−22.08 to 44.81	0.70
	2013–2019	19.76	−14.07 to 66.94	0.20
	2019–2022	−10.19	−55.68 to 81.97	0.69

^*p<0.05

AAPC, Average Annual Percent Change; APC, Annual Percent Change.

In the spatial analysis, four geographically significant space-time clusters were detected for mortality due to drowning from 2011 to 2022. The most likely cluster is located in Sari (centre of Mazandaran) with 1707 observed cases and 463.11 expected cases. The relative risk (RR) of mortality in the cluster was 4.8 (p-value=10⁻²⁶); that is, the risk of mortality inside the cluster is 4.8 times higher than on the outside of the cluster (table 3). The first significant secondary cluster was located in Ahvaz (centre of Khuzestan) with 912 observed cases and 321.03 expected cases and the relative risk of mortality within the cluster was 2.98 (p-value=10^–26). The third cluster was located in Rasht (centre of Gilan) and the relative risk of death within the cluster was 5.6 during 2011–2016. The big cluster containing 10 cities (Esfahan, Bushehr, Charmahale Bakhtiari, Semnan, Fars, Qom, Kerman, Kohgiluyeh, Yazd, Southern Khorasan) was identified with RR value 1.43 (p-value=10⁻¹²) during the years 2019–2020 (figure 3).

Table 3. Result of spatio-temporal analysis of mortality due to drowning (N=12 641), 2011–2022, Iran.

Cluster	Time frame	Number of cases	Expected cases	Observed/expected	Relative risk	Log likelihood ratio	P-value
Most likely cluster (Sari centre of Mazandaran)	2016	994	220.78	4.5	4.8	746.88	10−26
Secondary clusters 1 (Ahvaz centre of Khuzestan)	2011–2016	912	321.03	2.84	2.98	375.66	10−26
Secondary clusters 2 (Rasht centre of Gilan)	2011–2016	589	174.08	3.38	3.5	309.99	10−26
Secondary clusters 3 (Esfahan, Bushehr, Charmahale Bakhtiari, Semnan, Fars, Qom, Kerman, Kohgiluyeh, Yazd, Southern Khorasan)	2020	686	489.04	1.4	1.43	36.80	10−12

Figure 3. High-risk clusters of mortality due to drowning (N=12 641), 2011–2022, Iran.

Discussion

This study aimed to demonstrate the trend of drowning mortality and examine the spatial distribution of death to identify high-risk clusters in the years 2011–2022. We should mention that due to a lack of demographic data (such as gender, drowning place) in some years, the overall mortality trend was computed from 2005 and the trend based on gender split was estimated from 2011. According to the result of the study, the majority of deaths were in men (84%) for 11 years. In many countries, the risk of drowning was higher for men than women and in low- and middle-income countries, men account for 75% of deaths.^{25 26} One of the essential causes of this phenomenon is that men are more active in water-based jobs, and they also have high-risk behaviours.^{5 27 28} In many cultures, men have more freedom outside the home to swim in rivers and ponds.²⁹ In Iran, due to specific cultural conditions, men have more access to swim in public areas such as the sea and the river.³⁰

The results showed the largest mortality rate for men (25.91) compared with women (4.98) per 1 000 000 in 2019. We did not find a study that assessed the incidence of drowning in Iran in recent years. The trend of deaths was decreasing (APC: −3.86) during 2005–2013, and this decline had a steeper slope (APC: −5.82) until 2016. Consistent with our findings, Derakhshan et al reported that the overall trend of death decreased with APC: −5.3 and −10.7 for men and women in the years 1990–2015.⁶ It should be mentioned a set of factors and actions, including the legal obligation of swimming pools to comply with safety standards, the drying up of rivers and water bodies due to the construction of a dam in the Zayandeh Rood river (in the centre of Iran), the drying up of the country’s largest lake in the north western (Urmia lake) and the creation of protected swimming spots on the shores of the Caspian Sea in summer have been effective in reducing mortality.⁶

Urmia ake is the second-largest Salt Lake in the world³¹ and the largest lake in Iran (area 6000 km²), which is located in the northwest of the country.³² Due to the economic functions, cultural, ecotourism and recreational, scientific, educational,³³ its condition is of great importance. Lack of management of resources and severe climate changes caused a severe decrease (70%) in water, and the highest decrease in the water level was from 2010 to 2015.³⁴ This event has had destructive environmental, social and economic effects, including the stagnation of tourism and migration from the areas around the lake.³⁴

In our study, the percentage of annual changes increased (APC=10.5) and an upward trend was observed from 2016 to 2019, which was not statistically significant. Several factors are effective in the occurrence of drowning deaths, including individual factors, geographical conditions and preventive intervention. Shahbazi et al. reported that the mortality rate from drowning was raised from 11.13 to 12.68 during 2016–2018, and unsuccessful implementation of preventive plans was reported as one of the reasons.⁷

Fortunately, we observed, the death trend decreased again until 2022, and in line with this result, Omidi et al predicted a decline in the number of deaths during 2019–2022 in Iran.³¹ During this time, the COVID-19 pandemic affected the world and intervention measures were taken to prevent further spread of the disease. Similar to our findings, Kiakalayeh et al. reported mortality rates declined after COVID-19 in the Caspian Sea from 2020 to 2021.³² Restrictions such as the ban on providing tourism services, lockdown policy and closing of swimming pools had effects on reducing events such as drowning.^{32 33}

A downward trend in mortality was detected for both genders (AAPC: −1.36 and −0.97) from 2011 to 2022. Rare studies examined trends of drowning death and none of them reported AAPC. In a study, researchers observed a decline in mortality trend (APC: −5.3, -10.7) for men and women in the years 1990–2015,⁶ which part of this timespan was similar to our study.

The results of the study showed that mortality rates decreased in the coastal provinces (Mazandaran, Golestan, Bushehr and hormozgan) in 2022 compared with 2011. We should mention these provinces have a long coastline with the sea in the north and south of Iran and a high rate of drowning.^{35 36} In these provinces, most cases of drowning occur in the sea (70–85%), rivers (27–30%) and other places (13–18%).^{10 30} Preventive actions such as sea sanitisation plans (safe areas for swimming), the presence of lifeguards and rescue forces and interventional studies have been effective in decreasing mortality.^{37 38}

In a semi-experimental study on the residents and tourists of the coastal areas of the Caspian Sea, the effects of an intervention package, including preventive measures, were evaluated. These interventions include increasing community awareness (education through public media and health workers) and increasing monitoring of beach safety by lifeguard service stations for residents and tourists. The results of the study showed that increased surveillance and increased community awareness can reduce the risk of drowning.³⁷

In other provinces, we observed different changes in the rates compared with 11 years ago. In Isfahan, Chaharmahal and Bakhtiari and Khuzestan, the highest number of drownings are often related to rivers (40–80%), canals (11–34%) and 7% related to dams.^{33 34 39} Hamadan has many rivers and dams,³² and swimming in these places is common in Iran. In semidesert provinces such as Qom and Yazd, there is very limited access to water resources and about 75% of drowning mortality in Yazd are in pools.³⁴

Various factors including changes in awareness and attitude, safety knowledge, water safety behaviours and skills, environmental changes, policies and regulations are effective in increasing or decreasing drowning.³⁹ For example, preventive measures such as installing fences around dams have been effective in reducing mortality due to drowning in Ardabil and Zanjan.^{6 7} We have very little information about preventive measures and their effectiveness in different provinces.

In this study, we used space-time analysis because this technique provides valuable information from an epidemiological point of view. This method enables the identification of high-risk clusters, the establishment of a geographical surveillance system and reveals changes in risk over time.³⁵

Mazandaran province was detected as the most likely cluster of drowning mortality (RR=4.8, p value=10⁻²⁶) in 2011–2016. Also, three secondary clusters were identified, including two small clusters that were located in Khuzestan and Gilan provinces.

Mazandaran and Gilan have a long border with the Caspian Sea and the results of studies have shown that the majority of the drowning in these provinces occurred in the Sea.^{8 36} Access to water is one of the important risk factors of drowning,³⁷ and this feature is one of the main causes of remarkable mortality rates in these provinces.³⁸ Approximately 67% of all deaths happened in unprotected areas of the sea, where emergency services were not easily accessible.²¹

Mazandaran is a popular tourist destination, and it is noteworthy that a significant part of the victims (78–94%) were tourists in this area.^{36 40 41} Residency status is important because in most cases of drowning they lived outside the event place, so they probably were less familiar with the high-risk areas and had little knowledge of swimming skills.⁴⁰ The reasons for the cluster being statistically significant only in 2011–2016 may be related to the changes (prevention of drowning) that happened after this period. For example, population-based studies for drowning prevention (increasing awareness and education) have been conducted in Mazandaran province during the years 2010–2012.^42–44 In addition, the ‘sanitization of sea’ plan has been implemented with the participation of local officials and the Red Crescent Organisation of this province. In this plan, measures such as creating safe spots for swimming and entertainment for tourists and the presence of rescue forces at various coastal points are also carried out in the summer seasons.³⁶

Khuzestan has a coastal border with the Persian Gulf, and this province is one of the most water-filled provinces in the country with large rivers (Karun and Dez rivers).⁴⁵ From 1990 to 2015, the age-standardised mortality rate due to drowning per 100 000 in this province was more than 1.9 per year.⁶ According to the results of a study, the highest number of deaths in Khuzestan occurred in rivers (75%).⁴⁶ The Dez river was one of the dangerous rivers in Khuzestan and the lack of necessary facilities and equipment to prevent drowning was presented as one of the reasons for the high incidence of death.⁴⁷ A large secondary cluster including 10 provinces was detected in central and southern Iran in 2019–2020. In a study about the pattern of drowning in Chaharmahal and Bakhtiari provinces, it was reported that the death trend in 2020 has increased by 115% compared with 2018.⁴⁸ Detection of this high-risk cluster can be related to heavy rainfall and flooding of canals and rivers in this time period.⁴⁹

One of the causes of drowning is natural floods.⁴⁹ The flooding from March 2019 to April 2019 affected the west and southwest of Iran, and continued in January 2020 in the south and southeast of Iran.^{49 50} Heavy rainfall that occurred in 2019 caused flooding of 140 rivers in three quarters of Iran’s provinces, (mostly west and southern) including Khuzestan, Fars, Lorestan, Chaharmahal and Bakhtiari, Yazd and Kerman provinces.⁴⁹ Most of the deaths were related to Khuzestan and Lorestan provinces in the southwest of Iran.⁵¹

In general, a wide range of factors (hazard characteristics, individual, socioeconomic, environmental and management factors) have had an effect on flood mortality in Iran.⁵² Floods with a flow rate of more than 500 m³/s, night floods, flash floods and floods with floating debris have the highest risk of death due to floods, respectively.⁵³ Among the individual factors, high-risk behaviors, crossing flooded roads and being under 18 years of age increase the odds of death to a thought-provoking level.⁵⁴ According to the results of a qualitative study titled investigating the underlying factors affecting death due to flooding, the most item mentioned by the participants was the perception of danger by people (trust in the warning system, awareness and attention to safety).⁵²

It seems that factors such as the weakness of the flood warning system, the lack of coordination of the media in sending warning and preventive messages, the structural weakness of the crisis management organisation in terms of interdepartmental coordination and the lack of appropriate training programmes are issues that should be considered.⁵⁵

It should be mentioned that areas in the fourth cluster were located far from the sea and results of studies have shown that in provinces such as Isfahan,⁵⁶ Chaharmahal Bakhtiari⁴⁸ and Khuzestan (areas far from the sea),⁴⁷ most deaths have occurred in rivers. While in the other three high-risk clusters, most drowning cases were related to unprotected sea areas and rivers were reported to be the second most dangerous location for drowning.³⁸ Therefore, according to the characteristics of each area, different preventive measures should be planned to prevent drowning (table 4).

Table 4. Summary of drivers behind the high-risk clusters and preventative measures.

High-risk area	Drivers behind the high rates	Preventive measures
(Sari centre of Mazandaran) (Rasht centre of Gilan)	Swimming on unprotected beaches of the Caspian Sea, lack of protective equipment (life jacket), existence of splitting current, lack of knowledge of dangerous areas for swimming, seasonal and non-continuous monitoring of lifeguards.	Using an approved personal flotation device, improvement and increase of protected beach areas for swimming (with the presence of lifeguards), full-time supervision by lifeguards, wide and cheap access to rescue equipment, determining the places of splitting waves and unsafe areas, training swimming and resuscitation skills.
Secondary clusters (Esfahan, Bushehr, Charmahale Bakhtiari, Semnan, Fars, Qom, Kerman, Kohgiluyeh, Yazd, southern Khorasan)	Swimming in unsafe places and lack of lifeguards, insufficient supervision of the rivers and dams in terms of safety, lack of warning signs around water bodies, absence of flood warning systems, construction operations in flood-prone areas.	Installation of drowning warning signs in dangerous places, compliance with protective measures (creating fences and barriers around dams, pools and canals), restricting construction around the rivers and flood paths, training and promoting the culture of prevention.

Limitations

This repeated cross-sectional study used registry data that lacked epidemiological information such as age, economic and social status, site of drowning (river, pool, dam, etc) and residence status (native or tourist). Second, it was impossible to identify smaller high-risk clusters (city or village). Studying on a smaller spatial scale allows a more accurate assessment of the high-risk area, which will lead to more effective preventive actions.

Conclusion

Although the overall mortality rate in 2022 decreased compared with 17 years ago, there is heterogeneity at the subnational level. This study also provides valuable information about the geographical distribution of high-risk clusters to design and develop more preventative strategies effectively.

Data availability statement

Data are available upon reasonable request.