Many studies have shown the adverse effects of temperature on human mortality due to cardiovascular, respiratory diseases, or all causes      . There is also evidence that shows the effect of atmospheric conditions on human mortality. This evidence is mainly related to the effects of temperature, especially heat waves   . External causes, which account for a large proportion of the causes of death in some countries, include accidents, injuries or situations  . The importance of these factors on mortality and burden of diseases has led to the conduct of many studies in this field. Hajat et al.  examined the relation between temperature and mortality due to external causes in the UK. Basagana et al.  also examined this relation in Catalonia, Spain. Both studies have shown that high temperatures are associated with increased death due to external causes.
Death due to external causes includes the codes V0 - Y98 and E800 - E998 in the International Classification of Diseases, version 10 and version 9 respectively. These categories involve many intentional and unintentional injuries. Intentional injuries include intentional murder or suicide by self-immolation, poisoning, hanging, drowning, and etc. Unintentional injuries include traffic accidents, falls, electrical shock, and etc. The above causes are classified in Chapter 20 of the ICD10 book. External causes also cause damage to various areas of the body which are known as the consequences of external causes. These consequences are classified in Chapter 19 of the ICD10 book   .
Several studies have examined the effect of temperature on death due to external causes, separately    . There are also systematic reviews that have assessed certain causes, separately. For example, in a review conducted by Kampe et al., the relation between temperature and only unintentional injuries has been investigated  .
In general, studies that examine the effect of climate on death or disability due to external causes can be divided into two groups. One group investigates death due to all external causes together instead of investigating deaths due to external causes separately based on each specific cause. The other group, studies deaths due to external causes based on each specific cause separately. It is possible that no significant relation appears between temperature and death due to specific external causes analyzed separately, but significant relations appear between temperature and death due to all the external causes together  , because of the increased sample size. Also there are studies that although have studied human mortality and temperature, but have excluded deaths due to external causes    . Excluding mortality due to external causes may lead to problems in climate studies. For example, by excluding deaths due to external causes from all causes of death, the effect of temperature on death due to all cause will be underestimated in countries where mortality is high due to external causes.
Evidence about a relation between deaths due to external causes and temperature will help both researchers who study climate change and death and policy makers to development plans for preventing death. The purpose of this review article is to summarize the available information about the relation between temperature and death due to all external causes.
2.1. Search Strategy and Selection Criteria
A systematic search was conducted in PubMed, Scopus, Science Direct, Embase, and Web of Science; until January 2017 with no restriction on language, time and location. Climate, temperature, heat, cold, weather, hot, and ambient temperature were the key words used for exposure. The key word “external causes” was used as the outcome of this study. The search strategy has been shown in Table 1 and the search strategy in each database has been shown in Supplementary (Table S1). In addition to searching the mentioned databases, the reference list of included articles was also reviewed.
Articles that reported the relation of temperature with deaths due to specific external causes, were not included in this study. Many studies have been conducted about certain types of deaths due to external causes such as suicide and temperature studies   . Review articles have also been done about this topic   . However, the purpose of this article was to review the studies about death due to all external causes together, not death due to specific causes among external causes.
Articles with different designs were included in this review, if they had 1) an indicator as exposure to outside temperature, 2) mortality due to external causes as the outcome, 3) and the outcome had been investigated in humans. Articles were excluded if they 1) had assessed only one type of death due to external causes out of all external causes, 2) had assessed non external causes, 3) or had defined a medical visit or morbidity as the outcome.
2.2. Study Selection and Their Data
Table 1. The search strategy in the review.
We couldn’t find a standard checklist for appraising ecological studies. Nonetheless we evaluated the quality of the selected studies using the methods used in the literature   . The articles’ quality were assessed by considering: study design, study populations, information about exposure and outcome, statistical methods, measures of air temperature, and adjustment for important confounders such as meteorological factors, long-term trends, and day of the week.
Information was extracted from articles that met the inclusion criteria. A table was created and used to extract information from the studies. Where ever necessary, further information was inquired from the corresponding author of the studies by personal communication. The extracted information included the author, study period, population, study design, exposure and outcome variables, confounders that had been adjusted for, statistical methods and the main result.
Figure 1. Diagram of study selection.
external causes according to ICD10 and ICD9, and consequence of external causes (S - T) are separated from death due to external causes (V - Y) in ICD. Three studies had examined only death due to external causes as the outcome, but only the V00 - Y89 codes were included in the study    . From the 9 included studies, only one of them had defined external causes correctly  . One article did not mention the codes included in the study  . Also two studies had excluded death due to high natural heat (X30), and had analyzed it separately   .
Except one study  , all studies had controlled for potential confounders. One study had controlled for only age and sex  and two studies had analyzed the relation between temperature and death due to external causes in different age categories. The rest of the studies had adjusted for the variables that had changed over time.
The characteristics of the studies included in this review and their results have been summarized in Table 2. From 6 papers that examined the effect of extreme hot days or heat waves on death due to external cause, 5 showed a significant relation. One study  did not find a relation between hot days and mortality due to external causes. As the authors of the study pointed out, their results did not match the results from developed countries. This may be because of the low statistical power of their study, as they mentioned, themselves. Also Hajat  and Ishigami  , used mean ambient temperature, and reported that high temperature was related to mortality due to external causes. Ouro  also concluded
Table 2. Characteristics of included studies.
that hot days, which were defined using maximum daily temperature, significantly increased mortality due to external causes. Rey  and Joey  , despite the fact that they excluded death due to excessive natural heat (X30), saw a significant relation between deaths due to external causes and heat.
It seems that the effect of cold days on mortality due to external causes is less significant. One study  had shown that the proportion of death due to external causes is small in comparison to death due to all causes during cold waves (about 3%). Ingole et al.  did not see any relation between cold waves and death due to external causes. But one study  that had defined exposure correctly, found a significant relation and death due to external causes increased on cold days.
As far as we know this is the first literature review done about the relation between temperature and death due to all external causes together. This review suggests that high temperatures are probably more related to deaths due to external causes than low temperatures. Rey et al.  despite excluding deaths due to excessive natural heat (X30), observed the greatest excess mortality for other deaths(ICD: O - E) due to external causes. Similarly, Joe et al. excluded deaths due to excessive natural heat and still observed a direct relation between temperature and deaths due to external causes.
The results of studies that have addressed death due to one or more external causes separately, are consistent with these results. For example, several studies have examined the effect of temperature on death due to suicide and have shown that high temperatures are associated with an increase in suicides    . Also studies that have examined the effects of temperature on road accidents have also concluded that high temperatures increase crashes   . In a review conducted by Campe et al., from 13 studies that were included, 11 of them showed an increase in unintentional injuries related to high temperatures  .
In the present review, due to the small number of studies about the effect of cold or low temperature on deaths due to external causes, it is difficult to conclude how cold is associated with death due to external causes. Most studies that have investigated death due to specific external causes have also focused more on high than low temperatures  . Therefore, further investigation on the effects of cold is needed. A study conducted by Ingole et al.  did not show any significant effect of cold in three different lags (0, 0 - 4, 0 - 14) on death due to external causes. However, this study had used oral autopsy for data collection and included only deaths in which an oral autopsy with the deceased person’s family member had been done.
Shaposhnikov et al.’s study showed that although death due to external causes had a significant relation with cold waves, but this relation was weak. They showed that the age group over 65 was more vulnerable to cold than the younger group (under 65). A study conducted by Ishigami et al.  showed that deaths due to external causes are associated with both age and high temperature, and as age increases, the risk of death increases more steeply. But Hajat et al. reported that the highest risk of death due to high temperature happened in the 0 - 64 age group  . Considering this point is necessary that if deaths due to specific external causes (for example deaths from road crashes, get studied separately; the results may be different. For example in a study conducted by Ishigami et al., the younger age group was at a higher risk of deaths due to road traffic injuries, falls and drowning related to high temperature, than the elderly. This is probably due to the fact that younger people are more likely to seek adventure and encounter external causes than the elderly.
Increased mortality due to external causes at high temperatures is justifiable through mechanisms and intermediate variables and also depends on the specific cause. For example, road accidents, which account for a large proportion of deaths due to external causes in some countries, can occur due to changes in road or car conditions that happen in high temperatures   . Also it can occur due to changes in human behavior in hot temperatures  . For example high temperatures reduce people’s performance and skill in doing their routine work and the risk of unintentional injuries, such as work accidents or occupational injuries, increases. One of the other reasons is that recreational activities such as swimming increase in hot temperatures, and this increases the risk of drowning. Also outdoor fires occur more frequently in high temperatures   .
Some studies have shown that injuries occur more on certain times of the year and locations. For example a study conducted in British Columbia  showed that the majority of injuries occurred in the afternoon and on weekends. They also showed fatal incidents most often occurred during summer (particularly in the month of August) and the majority of fatalities occurred during daylight hours, and on dry roads. They recommended prevention programs such as strengthening individual knowledge and skills, promoting community education, and changing organization practices. Also special safety laws and regulations are needed for injury prevention in specific seasons, and weather conditions. For example safety tips are recommended for safe driving in the cold season. Although recommendations have been made for certain causes such as drowning in the hot season as well, this review shows that the heat season needs more attention to prevent accidents and injuries.
Some population subgroups are more vulnerable to injuries than others. A study that assessed the association between high temperature and work-related injuries in Australia  , mentioned that people working outdoors are at high risk of weather-related heat exposure and male workers, young workers aged ≤ 24 years, those working in industries such as “agriculture, forestry and fishing”, “construction” and “electricity, gas and water”, laborers, production and transport workers, and tradespersons in small and medium-sized businesses were more vulnerable than others. Therefore, industry-specific and hot-weather alert and response mechanisms need to be developed through multi-sector cooperation to improve risk perceptions and knowledge of vulnerable groups about minimizing harm strategies during extremely hot weather.
However, the results of this review are not enough for making concrete conclusions, and further studies are needed to determine the relation between temperature and death due to external causes. Most studies about the effects of ambient temperature have excluded death due to external causes. Moreover, due to inaccuracy in recording the causes of death in some countries, it is possible that death due to external causes may have been underestimated   . One reason maybe because the consequence of these external causes (Chapter 19 of ICD10 book) was recorded as the main cause of death, instead of the external cause itself (Chapter 20 of ICD10). This may lead to inaccuracy of the estimates in investigating the relation between mortality due to external causes and its risk factors. Therefore, the results of these studies should be interpreted cautiously.
A limitation of this review is that due to the ecological nature of most of the articles included in this study, their quality was not evaluated using an existing standard checklist. It is suggested that more robust studies be done in this field.
The results of this systematic review show that high temperatures (heat) are more likely to cause mortality due to external causes than low temperatures. Considering the increase in average global temperature and the increasing number of deaths due to high temperature in many parts of the world, effective intervention programs are need to raise awareness among people, especially vulnerable groups such as specific job groups, children, people with disabilities, and the elderly to prevent and reduce the adverse effects of high temperatures.
However, there were a small number of studies about cold and death due to external causes, and further studies about this topic should be done.
This study was financially supported by Kerman University of Medical Sciences, Kerman, Iran.
Authors state no conflict of interest.
All authors contributed equally and participated in the study design, data collection, and writing the final article.
Table S1. Search strategy and search terms used in the databases.
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