This is the first published study, to our knowledge, to investigate regional difference in climate on drowning mortality in China. Drowning is the first major cause of young people (< 14 year) in China, according to China Health Statistics Yearbook in recent ten years (2008–2017), about one quarter of 1–14 years child died of drowning.
Precipitation, duration and temperature in summer, river and lake density, regional development level are crucial factors of drowning, as one of the biggest country in the world, land of China covers a range of 4000 kilometers from the north to the south and climate varies a lot, annual precipitation varies from less than 200 millimeter in northwestern China to more than 2000 millimeter in southeastern China, with a fluctuation of as much as 1800 millimeter. Southern China has abundant water bodies, while northern China has serious problem of water shortage. This study found precipitation is the primary leading factor of north-south difference in drowning mortality (OR: 1.589, 95%CI: 1.350 ~ 1.871), precipitation and water system are direct reason for drowning, precipitation in China are concentrated in summer, especially in the Yangtze River basin in June and July, it is called “rainy season”. In addition, southern China usually suffered by typhoon (a very violent tropical storm) in summer and autumn that is generate in tropical Pacific Ocean, rainy season and typhoon brings short term heavy rainfall that always lead to flood. The soared water level and violent water flow increase proportion of drowning of local residents.
Temperature in summer (OR: 1.252, 95%CI: 1.075 ~ 1.459) is easy to be ignored, average temperature in July varies from 19 centigrade in northeastern China to 31 centigrade in the south. Swimming is one of main causes of drowning, people prefer to swim to against hot temperature in summer, especially for children, they always play in the water body together, water temperature in southern China is suitable for swimming and playing, while low atmospheric temperature resulting lower water temperature and less drowning mortality rate. Water temperature in northern China is cold for swimming and playing. Temperature in summer can be used for explaining why Xinjiang Uygur Autonomous Region is the single northern province that has high ranking in drowning mortality, different from other northern provinces, average temperature in July in southeastern Xinjiang can be 33 centigrade because of the unique terrain, the temperature is even higher than hottest province in southern China (Fig. 5), higher temperature increases the probability of water loving.
Per capita income is a crucial factor to decrease drowning, the OR is 0.471 (95CI%:0.391 ~ 0.567) in this study. The result is in accordance with formal studies about urbanization and drowning, drowning mortality rate in rural place and less developed region is much higher than urban and developed region. Urbanization could affect the risk of drowning in many ways (Wang, et al., 2018). Urbanization would reduce people’s access to natural waters because there are many swimming pools in city. Urbanization increases income and drowning rates vary inversely with per capita income (carter, et al., 2011; baca, et al,. 1988). In addition, safety and first aid education in urban is more adequate than rural place, 83.1% of rural residents had no knowledge of first aid skills for drowning (Wang, et al., 2020). Western China with lower per capita income has a higher drowning mortality although some western provinces are not so hot and rainy. There is no statistical difference of per capita income and urbanization rate between northern and southern China (Table 1), therefore, they are not main reason for the north-south difference in drowning.
Distance between residence and water system is also an important factor, the young people are more likely to swim if they live near the water system. But the result of this study doesn’t find statistically significant evidence (OR: 1.111, 95%CI: 0.956 ~ 1.291). This may attributed to the reason that the resolution of our data is too large to find meaningful results, the drowning incidents usually occurred within 100 meters of a body of water (Wang, et al., 2020), but the resolution of our data (i.e. population density) is 1kilometer, the distance is too long for children to find a natural water body.
Children and teenagers are high-risk age groups of drowning because they are curious and motivated with high-frequency wide-range activities and a lack of self-safety awareness (Wang, et al., 2018). Abundant water resources and higher temperature increases drowning mortality of young people in southern China. With society and economy development of China, mortality rate attributed to drowning will continuously decrease in future, but drowning mortality will still higher in southern China than in northern China in future because of plenty rainfall and higher temperature.
Therefore, we can conclude that the spatial heterogeneity of regional drowning mortality could be attributed to comprehensive effects from multiple climate-economy factors. Consequently, it is indispensable for local government to prevent drowning according to local temperature performance. For example, southern China should take longer defense measurements because the daily mean temperature is above 25 centigrade as long as six month (May to October); the Yangzi river basin and south Xinjiang should pay attention to hottest temperature (usually above 30 centigrade) in July and August. The central government should take more effective measures to mitigate economic inequality in China, which will be helpful to ensure the appropriate measures would be taken in low income regions to better respond to drowning, especially in western China.
There are some limitations of this study. For example, the periods of drowning and risk factors are not entirely consistent with each other, which affects the accuracy of the results.