High, low, and non-optimum temperatures exposure on road injuries in a changing climate: a secondary analysis based on the Global Burden of Disease Study 2019

Global warming in the twenty-first century has gradually made high temperatures a major threat to the global population. Health problems associated with extreme temperatures have become a growing public health concern worldwide. This study aimed to estimate road injuries stratified by sex, age, geographic location, and sociodemographic status attribute to high, low, and non-optimal temperatures in 21 regional and global. We used the Global Burden of Disease (GBD) Study Results Tool to examine the age-standardized death rates (ASDR) and disability-adjusted life years (DALYs) due to road injuries in 2019 by Joinpoint regression. In addition, we reported high, low, and non-optimal temperature exposures for road injuries across different groups by gender, age, region, and disease. Moreover, we examined temporal trends in the burden of road disease caused by high, low, and non-optimum temperatures from 1990 to 2019. Trend analyzes were conducted for five sociodemographic index (SDI) regions. Globally, both ASDR and DALY declined from 1990 to 2019, with average annual percent change (AAPC) values of − 1.3% and − 1.2%, respectively. In 2019, the indicators (death and DALYs) steadily declined, while SDI quintile increased in most regions. Road injuries related to death and DALYs rate attributed to high temperatures were 0.17 and 8.50, respectively, in 2019. From 1990 to 2019, DALYs for road injuries caused by low temperatures showed the most significant upward trend in most regions, especially in low-latitude countries. This study provides a comprehensive understanding of the road injury burden caused by high, low, and non-optimum temperatures, which remains high in regions with low SDI. Therefore, special attention should be paid to road injuries in poor countries or in areas with extreme temperatures.


Introduction
The World Health Organization (WHO) released the Global Status Report on Road Safety 2018 and established performance indicators focused on road safety targets in the WHO's overall work plan for 2019-2023 (Organization). According to GBD data, China is the world's most populous country with nearly 262,000 road traffic deaths in 2017, accounting for about 21% of global road traffic fatalities (Anonymous 2018a). Moreover, road injuries encompass injuries involving motor vehicles, pedestrians, motorcyclists, and cyclists in GBD. Currently, there are many studies on the risk factors that contribute to road injuries. But few have studied the impact of risk factors such as temperature or air pollutants on road injuries (Liang et al. 2021). Despite being preventable, road injuries are still the eighth leading cause of death worldwide, especially in developing countries s Responsible Editor: Lotfi Aleya (Zhan et al.2020). Globally, high temperature caused 0.54% of death and 0.46% of disability-adjusted life years (DALYs) in 2019, highlighting it as a global health threat (Song et al. 2021). Global Burden of Disease (GBD) 2019 study is the first to include high temperature exposure as a risk factor for many diseases and injuries, and it used the standardized data processing methods to systematically estimate the burden of high and low temperature diseases in 204 countries and territories between 1990 and 2019 (2020a). Furthermore, climate change will be the greatest health threat of the twenty-first century. Health problems associated with increased ambient temperatures have become a growing public health concern (Zhao et al. 2021). Over the past 20 years, high temperature mortality among people aged 65 + has increased by 53.7%, with the total number of deaths reaching 296,000 in 2018 (Watts et al. 2021). It is well known that ambient temperatures can seriously affect traffic safety and lead to road injuries. In some previous published studies, ambient temperature has been used as one of the main risk factors in assessing road traffic accidents (Jung et al. 2010;Zeng et al. 2020). There are also studies that prove that temperature changes increase the risk of road injuries (Liu et al. 2017). High temperatures can also lead to personal neglect, fatigue, and reduced cognitive and motor function and may also affect work capacity (Otte im Kampe et al. 2016). Exposure to temperature extremes is also an important factor significantly affecting traffic injuries, possibly due to the increased fatigue perceived by drivers (Daanen et al. 2003). Temperature factors are associated with physical and psychological effects. Some researchers have found that the effects of low temperatures on mortality can last for a long time (Anderson and Bell 2009). Extreme temperatures may produce a range of reactions in frail individuals (Bhaskaran et al. 2013). Therefore, adverse weather conditions may influence human behavior (Theofilatos and Yannis 2014).
As a result of previous research, alcohol consumption is one of the main risk factors for road injuries. WHO has also identified alcohol consumption as a behavioral risk factor for road injuries (World Health Organization 2013). Therefore, reducing alcohol consumption is considered to be an effective way to reduce drink driving rates. However, many studies show that exposure to high temperatures is an important risk factor for many adverse outcomes (Arbuthnott et al. 2020;Qi et al. 2021). The risk of collision increases by 2.9% (95% CI: 0.7-5.1%) in high temperature (Basagaña et al. 2015). Even so, the impact of low temperatures and the estimate of the cause-specific relative risks of non-optimal temperature on specific diseases cannot be ignored (Burkart et al. 2021). The risk of road injury increases by 2.1% for each 1 °C decrease in average daily temperature in Korea (Lee et al. 2014). In recent years, extreme heat and cold waves have been experienced in many regions around the globe due to extreme climate change (Burkart et al. 2021;He et al. 2022).
To date, previous studies on high or low temperatures have focused on one region, or one country, and no studies have assessed the burden of road injuries due to temperature extremes by age, gender, or socio-economic status on a global scale. This could help road injury prevention and control efforts in the face of climate change. In current research, the risk factors we explore in relation to exposure to road injuries are climate change, such as high, low, and non-optimum temperatures. And we also assessed the burden of road injuries due to non-optimal temperatures in 21 regions and globally from 1990 to 2019.

Data sources
GBD 2019 is the most systematic database of epidemiological study in the world. It provides a very comprehensive assessment of 359 injuries and diseases, 84 risk factors, Fig. 1 Temporal trends in age-standardized death rate (A) and age-standardized DALYs rate (B) for road injuries in different SDI regions Environmental Science and Pollution Research (2023) 30:11012-11024 11013 and 282 reported causes of death among 195 countries, 21 regions, and 7 super-regions (Murray et al. 2012). Road injuries are from vital registration, surveillance systems, verbal autopsies, police reports or censuses (GBD 2017 Causes of Death Collaborators) (Anonymous 2018b). Global Health Data Exchange query tool (GHDx, http:// ghdx. healt hdata. org/ gbd-resul ts-tool) was used to collect data on the burden of road injuries attributable to high, low, and non-optimum temperatures database by gender and age from 1990 to 2019. It uses DisMod-MR 2.1, the Bayesian meta-regression tool as the primary assessment method to ensure consistency between incidence, prevalence, causes of death, and risk factors for each disease (Anonymous 2020). Road injuries mortality data from vital registration systems and mortality projections were used as input data into CODEm (Death Ensemble Model) (Foreman et al. 2012).
In this study, we also collected the burden caused by 3 risk factors. In short, a non-optimal temperature was defined as a temperature above (i.e., high) or below (i.e., low) the ambient temperature of the day associated with the lowest risk of death. Taking into account different annual mean temperature zones and different exposure-response  curves, as well as spatially and temporally different etiologic composition, the theoretical minimum risk exposure level (TMREL) was considered as the daily temperature associated with the lowest mortality rate for all diseases with an incorporated etiology for a given location and year. Where high temperature is the temperature above the TMREL and low temperature is considered as the temperature below the TMREL (Hersbach et al. 2020). In addition, the sociodemographic index (SDI) is based on a lagged distribution of per capita income, average educational attainment over age 15 years, and total fertility under age 25 years (Roth et al. 2017). It is used to position countries along the development continuum and to classify countries into five SDI quintiles (high SDI, high-middle SDI, middle SDI, low-middle SDI, and low SDI) based on GBD results. The index ranges from 0 to 1, i.e., least developed to most developed. Moreover, we extracted estimates of road injury deaths, DALYs, and corresponding age-standardized rates from GBD 2019 with their 95% uncertainty intervals (UI). We also chose DALYs to model the attributable burden of cause and road injuries. All data are publicly available from the IHME website and can be accessed and reviewed online.

Analysis presented in this paper
Road injuries burden variations were assessed geographically using map construction, stratified by country and region, and based on different temperature changes. We also explored the road injury burden in all 21 regions from 1990 to 2017, using restricted cubic splines with flexibility to establish associations between death and DALY rates and SDI. Statistical descriptions and analyses above were performed using the R program (version 3.6.0, R Core Team). We estimated the annual percent change (APC) and annual average percentage change (AAPC) in death rates, DALYs through Joinpoint regression analysis. In this model, death and DALY rates for different SDI areas in different years are determined by breakpoints, which more visually describe temporal trends in road injuries burden. Among them, APC and AAPC are used to characterize the trend of age-standardized death and

Road injuries burden and sociodemographic transition
This study presented significant trend changes and estimated annual percentage changes in death and DALY for trend period in Fig. 1 and Table 1. The Joinpoint results showed a significant downward trend for global and all SDI quantiles regions. Among the high SDI quintile, the death rate decreased from 1990 to 2019, with an overall AAPC of − 2.1%. However, the low-middle SDI quintile death rate showed upward first from 1990 to 2000 and 2004 to 2010. Similarly, the DALY rate for high SDI quintile showed significant downward trend from 1990 to 2019, with an overall AAPC of − 2.2%. The DALY of low-middle SDI quintile showed a slight downward trend with an overall AAPC of − 0.6%. The higher the level of the regional SDI, the more significant the downward trend for the death rate and DALY rate. We used death and DALY rates for each year from 1990 to 2019 to plot against the sociodemographic status index for the region in the same year, and we can observe a different pattern of epidemiological change in each region as shown in Fig. 2. Both a decreasing trend in SDI and a decline in road injuries on death and DALYs occurred in many regions. Despite the significant decline in SDI, the road injury DALY rate and death rate in Eastern Europe showed an upward and then a downward trend. The relationship between SDI and death rates and DALY rates are shown by the black line. There was a first decline, then rise, and finally decline for death rates as SDI increases, with the more rapid decline at the SDI quintiles of 0.5-0.6.

Road injuries burden affected by different age groups
Among children under 5 years old, Sub-Saharan, South Asia, and Southeast Asia were the leading regions for road injury DALYs (Fig. 3). In early neonatal (0-7 days), DALYs for road injuries were the highest among the population in Western Sub-Saharan Africa and North Africa and the Middle East. In Western Sub-Saharan Africa, DALY rates increases rapidly around the age of 50 and is the leading cause of road injuries burden between the ages of 50 and 94. As a result, the burden of road injuries for young and middle-aged people is lower than children and older people. In the age of 30 to 50, DALYs for road injuries were the highest among the population in Tropical Latin America, South Asia, and North Africa and the Middle East.

Overall status of road injuries burden attributed to high, low, and non-optimum temperatures
Age-standardized death and DALY rates caused by high, low, and non-optimal temperatures showed similar trends in the global and SDI quintiles (Fig. 4). In 2019, road injuries related to death and DALYs rate attributed to high temperatures were 0.17 and 8.50, respectively, with a slight upward trend in the high SDI quintile. Correspondingly, the age-standardized DALY rate attributed to low temperatures in Global was − 24.10, with the greatest increasing in the high SDI quintile (57.33%) and in the high-middle SDI quintile (54.55%). Similarly, the age standardized DALY rate attributed to non-optimum temperatures showed significantly upward trend in the high SDI quintile (70.92%) and in the high-middle SDI quintile (57.69%).

Age standardized DALY rate of road injuries attributable to high, low, and non-optimum temperatures of different countries in 2019
The regions with the highest rates of road injuries due to high temperatures in 2019 were Western Sub-Saharan Africa ( showed the most significant upward trend in most regions, especially in low-latitude countries. For the trend in death and DALYs for road injuries due to high temperatures, an increasing trend was observed in a small part of regions ( Fig. 5 and Table 2).

Risk factors attributable to road injuries burden
Globally, we plotted DALY attributable to high, low, and non-optimum temperatures of road injuries by sex (Fig. 6). In 2019, the DALY rates of road injuries attributable to high temperatures for male were more than 3.0 times higher than female in Global. The DALY rate for road injuries attributed to low temperatures is slightly higher in female than in male. High temperature and non-optimum temperatures are the main risk for DALYs in Western Sub-Saharan Africa by male and female. Low temperature and non-optimal temperature are the lowest DALYs in Eastern Europe by the male.

Discussion
A study of GBD 2019 showed trends and patterns in global road injury death and DALY, as well as risk factors for high, low, and non-optimum temperatures. This study showed that the burden of road injuries is high and varies considerably between countries and regions, with low SDI regions having higher DALY rates than other SDI regions and a rapid downward trend in all 21 regions except Eastern Europe. The ASDR is on a downward trend in all SDI regions. In 2019, the burden of road injuries varies by age group, with early neonatal being the highest value. The DALY rate for road injuries due to high temperature in males was 85.5 in the Western Sub-Saharan region (Anonymous 2018b).

Road injuries burden in different regions by different age and sex
China is the most populous country in East Asia, with nearly 262,000 road traffic deaths in 2017, accounting for 21% of road traffic deaths worldwide (Wang et al. 2019). Some studies have all confirmed an increased risk of road injuries for extreme temperatures in the USA, China, and Saudi Arabia (Cheng et al. 2017;Islam et al. 2019;Nick et al. 2018). The Swedish study also finds that extreme high temperature causes more road injuries (WHlberg 2008).
Another study has shown that there are significant differences between regions and countries in terms of temperature and disease, as temperatures vary largely between regions in terms of longitude and latitude (Burkart et al. 2021). A study concluded that most deaths in countries spanning different climatic zones and sociodemographic conditions were caused by exposure to high and low temperatures (Gasparrini et al. 2015). Another study has shown that higher temperatures increase the risk of traffic accidents in different regions (Liang et al. 2022). Moreover, in our study, the road burden associated with high temperatures was much higher in low and low-middle SDI regions than in other SDI regions, probably due to the fact that most of the world's tropical regions are concentrated in developing and economically disadvantaged countries (Li et al. 2015). Most of the countries with low SDI are located at low latitudes which have low exposure of low temperatures. However, the increase of road injury burden in high SDI regions at low temperatures may be the frequency of extreme weather due to global climate change (Stott 2016).
In the Netherlands study, the burden of road deaths, injuries, and health was higher among children aged 12-17 years; young people may be spared from road injuries, and older people may be fatally injured (Twisk et al. 2017). Over the past 20 years, high temperature mortality among people over 65 years of age has increased by 53.7% (Watts et al. 2021). This suggests that future high temperature-related diseases and injuries may be amplified by an ageing population (Lee and Kim 2016). And in our study, in addition to early neonatal, the 1-4 and 70-74 age groups presented two peaks, so older people are also at high risk for road injuries. Therefore, future health outcomes associated with extreme temperatures may be influenced by population ageing. A recent study suggested that high temperature deaths may be expand by 2.3 to 5.8 times due to an ageing population in China (Yang et al. 2021).
Previous studies have shown that men and the elderly have a higher risk of road traffic mortality than women (Eva et al. 2016). The fact is that men are more likely than women to be exposed to high temperature work in their occupations and that most drivers are male. This may increase their exposure to heat, while heavy alcohol consumption and the stress of a hot working life may also increase the risk of road injuries in men . Consistent with our research, DALY rates are higher for men on roads injuries than for women globally and in 21 regions.

High, low, and non-optimum temperatures risk factors
Frequent exposure to temperature extremes will directly affect human health, and the occurrence of some extreme weather can indirectly affect human health (Melillo et al. 2014). At the same time, extreme temperatures also have an important impact on road injuries. Among them, one study found that people driving efficiency decreased by 16% in low temperatures and 13% in high temperatures (Hein et al. 2003). Some studies also have shown that the extreme ambient temperatures chosen have an effect on human skin temperature and that people's body temperature only increases slightly in cold and warm conditions. The increase in temperature during cold temperatures is mainly due to an increase in metabolism caused by shivering (Daanen and Van de Linde 1992). Temperature control may be more important to the driver than driving control. High or low temperatures may divert the driver's attention and thus reduce performance (Wyon et al. 1996). At the same time, high temperature environments may also lead to reducing driver decision-making ability and impulsively may result in traffic injuries (Brewerton et al. 2018). High temperatures can affect vehicle speed (Jgerbrand and Sjbergh 2016), vehicle performance (Kraemer et al. 2014), and road conditions . Thus, high temperatures are associated with red light running and wrong lane driving, and it can increase the risk of road injuries (Fu and Liu 2020). As a result, driving ability is worse in extreme ambient temperatures than when the temperature is suitable (Daanen et al. 2003). Cold weather crashes may also be influenced by road conditions, such as a snowy and icy roads, which can increase the risk of crashes (Lee et al. 2014). Extreme cold temperatures can cause ice to form on the road, which not only reduces driving performance, but also adversely affects vehicle condition, leading to an increased risk of road injuries in East Asia (Chowdhury 2015). Therefore, ambient temperature is an important risk factor (Ye et al. 2012).
Many previous global studies have shown that temperature has a long-term and short-term impact on mortality (Huang et al. 2015). Similarly, a number of global studies on temporal trends have confirmed the global upward trend in the burden of non-communicable diseases due to high temperatures (Chen et al. 2019). In addition, many scholars have found that the effects of low temperature mortality tend to be more persistent. Extreme temperatures can increase the risk of death (Huang et al. 2015). For example, among cardiovascular diseases, the burden of heat-related diseases is the highest for stroke (Dereje 2020). High temperatures will also significantly increase hospitalization rates for people with asthma and heart disease (Khalaj et al. 2010). All the above studies show that the health of traffic drivers can be affected even after a certain temperature has been reached.
There are a number of limitations to this study. (1) The road injury burden indicator values attributable to high, low, and non-optimal temperatures in the GBD 2019 data are only an estimate, and temperature refers to short-term effects that occur on the day of exposure, so our results may underestimate the burden of disease associated with high, low, and non-optimal temperatures. (2) As the global climate warms and extreme temperatures become more important, heat waves are in fact playing a substantial role. But the burden of disease caused by heat waves is not analyzed in GBD 2019. (3) Our study only addresses regional climate differences across SDIs, so further assessment of countryspecific disease burden and injury due to climate differences is needed. (4) Data estimates are based on data from a number of countries, many of which lack raw data, and estimates are influenced by parameters and models.

Conclusion
In summary, the present analysis of the global burden of road injuries shows a decreasing trend in both global death and DALY rates, with low SDI regions remaining at high levels and contributing to a considerable disease burden. The main risk factors for road injuries studied in this paper are high, low, and non-optimum temperatures, and we should take measures to reduce the burden of road injuries. The burden of road injuries varies differently across different SDI regions. Therefore, disparities between countries at different levels of development should be shortened.