Original research

Causes of death and characteristics ofnon-­survivors rescued during
recreational mountain activities in Japanbetween 2011 and 2015: a
retrospective analysis
Kazue Oshiro ‍ ‍,1,2,3 Tomikazu Murakami3

To cite: Oshiro K, Murakami T.
Causes of death andcharacteristics of non-­survivors rescued during
recreational mountain activities
in Japan between 2011
and 2015: a retrospective
analysis. BMJ Open
2022;12:e053935. doi:10.1136/
bmjopen-2021-053935► Prepublication history forthis paper is available online.
To view these files, please visit
the journal online (http://dx.doi.​org/10.1136/bmjopen-2021-​
053935).

Received 03 June 2021
Accepted 20 January 2022

© Author(s) (or theiremployer(s)) 2022. Re-­use
permitted under CC BY-­NC. No
commercial re-­use. See rights
and permissions. Published by
BMJ.
1

Department of Cardiovascular
Medicine and Director of
Mountain Medicine, Research,and Survey Division, Hokkaido
Ono Memorial Hospital, Sapporo,
Hokkaido, Japan2
Department of Internal
Medicine, Nihon University
School of Medicine, Tokyo,
Japan
3
Director, Association for
Mountain Medical Rescue
Japan, Hokkaido, Japan
Correspondence to
Dr Kazue Oshiro;​Kazooshiro@​gmail.​com

ABSTRACT
Objectives This study aimed to describe the causeof death and characteristics at the prehospital setting
associated with care and rescue processes of non-­survivors rescued in the mountain of Japan.
Design Retrospective analysis.
Setting Prehospital setting of mountain searches
and rescues in Japan. A total of 10 prefectural police
headquarters with >10 cases of mountain death from2011 to 2015.
Participants Data were generated from the existing
records. Of the total 6159 rescued subjects, 548 mountain
deaths were caused by recreational activities.
Results Among the 548 mountain deaths, 83% weremen, and major causes of death were trauma (49.1%),
hypothermia (14.8%), cardiac death (13.1%) andavalanche-­related death (6.6%). The alive rate at rescue
team arrival in all non-­survivors was 3.5%, with 1, 4 and14 cases of cardiac, hypothermia and trauma, respectively.
Cardiac deaths occurred in 93.1% (67/72) of men andindividuals aged >41 years, and 88.7% (63/71) were foundon mountain trails. In hypothermia, callouts were madebetween 17:00 and 6:00 at 49% (40/81) and by persons
not on-­site in 59.7% (46/77). People with >6 hours intrauma or >1 hour in cardiac death already died on rescue
team arrival, but some with hypothermia after 6 hours
were alive.
Conclusion This study is one of the first large-­scale
retrospective analyses of prehospital non-­survivors inmountain emergencies. The alive rate at rescue arrival in
all mountain deaths was only 3.5%. These data showedthat the circumstances related to onset and the processuntil the rescue team arrives have different characteristics,
depending on the cause of death. Survival may beenhanced by targeting better use of the time before
rescue team arrival and by providing further education,
particularly mountain rescue-­related medical problems to
rescuers including bystanders.

INTRODUCTION
Numerous people visit mountainous areas
for recreational activities; in the 2016 official statistics of Japan, 11.37 million people
aged >10 years joined mountaineering inStrengths and limitations of this study► This study is one of the first large-­scale analyses ofprehospital non-­survivors who were rescued during
recreational mountain activities.
► This study provides the alive rate of deaths on rescue team arrival, and the care and rescue processes of the prehospital setting time are evaluated forenhancing survival for each major cause of death.
► This research targeted the rescued people in Japan,
and from a universal perspective of injury and illness, the results of this study can be generalised;however, the generalisation of the results may belimited since the topography, weather and rescue
systems vary, depending on the country and mountain area.

2015.1 Considering the topographic features,
weather conditions, activity duration, andheavy and/or inadequate equipment, mountain activities can be physically demanding
and can lead to serious injuries or death. The
mountain rescue team in Japan comprises
police organisations. According to the
National Police Agency report, the total callouts in 2015 were 3043, of which 11% died(335/3043). Considering this information,>300 people die in Japan’s mountainous areas
annually.2 Mountain fatalities were retrospectively studied, and trauma, acute altitude
illness (AAI), hypothermia, avalanche burialand sudden cardiac death (SCD) were themost common causes.3–6 However, few studies
have examined the cause of death and thecharacteristics at the prehospital setting inrecreational mountain activities internationally3 7 or in Japan. We conducted a large-­scale
analysis of non-­survivors rescued in Japanbetween 2011 and 2015. We aimed to clarify
the causes and frequency of mountain deaths
and identify the onset and rescue response

Oshiro K, Murakami T. BMJ Open 2022;12:e053935. doi:10.1136/bmjopen-2021-053935

1

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Open accesscircumstances and the prehospital setting time course
to develop possible measures to strengthen the response
and enhance survival chances during emergency situations in the mountain.

METHODS
Data source
After each mountain rescue mission, police officers
record the event on the rescue report form, which is kept
for the annual rescue report for 5 years. This report covers
all rescued people in Japan. We collected such reports
from police headquarters within all prefectures that hadover 10 mountain death cases between January 2011 andDecember 2015 (the last 5 years in which records were
kept as of 2016 when this study began). The head of police
mountain rescue personnel in the included prefectures
retrospectively obtained data from the documented information survey items, such as age; sex; date and time of six
points (start of activity, onset, callout, rescue team arrival,
hospital arrival and death confirmation); the mechanism;
number of groups; person making the callout; presenceor absence of any signs of life based on four points (at theonset, callout, rescue team arrival and hospital arrival);
implementation and type of medical intervention on-­site;
trigger of accident; cause of death; coexistence findings;
information of onset and the found site such as weather,
altitude, terrain features; and mode of travel such as
ascent, descent, ridge, rest, at the hut, sleeping, which did
not contain individual-­level identifying data. Any signs oflife were considered ‘alive’. The National Police Agency
and the competent authority of each prefectural police
headquarters approved the use of the aforementioned
information for this study and mailed it to us. All postmortem examinations were done by physicians or coroners. Autopsies were not conducted because no crime
and/or trouble was suspected.
Patient and public involvement
No patients or public were involved in designing,
conducting, reporting or disseminating this research.
Statistical analysis
Continuous data are expressed as median and IQR,
whereas categorical variables are presented as counts andpercentages. Proportions were compared using χ2 or Fisher’s exact test, and the continuous data were comparedusing one-­way analysis of variance or Kruskal-­Wallis test.
Furthermore, a p value of <0.05 was considered statistically significant. Data were stored using Excel V.2019
and analysed using EZR V.1.54 (Jichi Medical University,
Saitama, Japan) modified R commander.

RESULTS
Figure 1 illustrates the process of inclusion and exclusionof cases. After exclusion, 548 cases were analysed, with
2

Figure 1 Flow diagram illustrating the inclusion andexclusion processes.

455 (83.0%) men and 95 (17.0%) women and a medianage of 60 (46–68) years and 62 (53–68) years, respectively.
Causes of death and alive rate at rescue team arrival
The overall fatality rate (FR) (number of death/numberof callouts) was 12.0% (737/6159).
Figure 2A shows the cause of death and the number ofalive victims during rescue team arrival. The causes of the
majority of the deaths were the following: trauma (49.1%,269), hypothermia (14.8%, 81), cardiac death (13.1%,72) and avalanche-­related death (6.6%, 36). Duringrescue team arrival, only 3.5% (19) were alive. Of these19 alive cases, 14 (5.2% of 269), 4 (4.9% of 81) and 1(1.4% of 71) exhibited trauma, hypothermia and cardiac
death, respectively.
Regional differences in mortality outcomes
Figure 3 shows the differences between the north island(Hokkaido) and Japanese Alps range (Nagano, Toyama,Gifu) in terms of the cause of death, its frequency and FR.
The major cause of death was hypothermia in Hokkaido(FR: 4.1%, 32/784) and trauma in the Alps range (FR:15.5%, 358/2308).
Table 1 summarises the characteristics stratified by the
top four causes of death, namely, trauma, hypothermia,
cardiac death and avalanche, in 458 subjects. In cardiac
death, the percentage of male victims was high (93.1%,
67/72), and all victims were >41 years of age (p<0.001).
Meanwhile, those who had avalanche-­related deaths were
the youngest.
Type of trauma and avalanche-related death
Figure 4 shows the trauma types, avalanche-­related death
and alive victim proportion during rescue arrival. The
types of trauma included traumatic brain injury (TBI)(36.4%, 98/269), multiple injuries (35.7%, 96/269),
cervical spine injury (7.4%, 20/269), chest injury (3.7%,
10/269), pelvic fracture (2.6%, 7/269) and others/
undocumented (14.1%, 38/269), but only 7.1% (7/98),
2.1% (2/96), 5.0% (1/20), 0% (0/10) and 28.6% (2/7)
were alive at rescue arrival, respectively.
Avalanche-­related death was mainly caused by asphyxia(84.6%, 22/26) followed by trauma (15.4%, 4/22).
Oshiro K, Murakami T. BMJ Open 2022;12:e053935. doi:10.1136/bmjopen-2021-053935

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Figure 2 Cause of death, time distribution until rescue arrival, and the number of alive subjects between 2011 and 2015
in Japan. (A) The overall number of deaths and the number of alive subjects during rescue team arrival according to cause(B) differences in time distribution and the number of alive subjects from the onset to rescue arrival between the top four causesof death.

Unfortunately, none were alive when the rescue teamarrived.
Characteristics at the time of occurrence
In trauma death, the mechanisms of injury (excluding
the undocumented) were fall (94%, 222/236) and falling
rocks (3.0%, 7/236). The predominant injury was blunt

Figure 3 Regional differences in the proportion of alive
subjects on rescue arrival and in the cause of death.
Oshiro K, Murakami T. BMJ Open 2022;12:e053935. doi:10.1136/bmjopen-2021-053935

trauma. Meanwhile, figure 5 shows the characteristics
of hypothermia. Hypothermia deaths were recorded
every month and most frequently in May (26%, 20/81),
followed by April (10.4%, 8/81), both spring months,
and then in December (10.4%, 8/81), which is the wintermonth. The callout time peaked at 17:00 and was between
17:00 and 6:00 at 49% (40/81); those made by themselves
and persons not on-­site accounted for 5.2% (4/77) and
59.7% (46/77), respectively. Regarding cardiac death,the callout time peaked at 11:00 and was 7:00–13:00 in
70.8%. In addition, 88.7% (63/71) of cardiac death cases
were found on mountain trails (p<0.001), while 59.2%(126/223) of trauma cases and 53.5% (38/71) of hypothermia cases were found at the hazardous areas such as
snow, rock and ice (p<0.001) (table 1).
Figure 2B shows the differences of time distribution
from onset to rescue arrival between the top four causes.
The proportion of within 1 hour from the onset to rescue
arrival was 17.1% (24/140) for trauma, 3.1% (2/67) for
hypothermia, 50.0% (22/44) for cardiac death and 0%
in avalanche.
On-site treatment
On-­site treatment was done in 31.1% (59/190) of documented cases. The most applied treatment was cardiopulmonary resuscitation (CPR) followed by automatedexternal defibrillators (AEDs) (applied with or without
record of shock), rewarming, bleeding control, immobilisation and oxygen therapy (44, 7, 6, 4, 1 and 1,
3

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Open accessTable 1 Characteristics of four major causes of death in Japan from 2011 to 2015
TraumaHypothermia

Cardiac death

Avalanche-­related

269

81

72

36

 Male213 (79.2)

70 (86.4)

67 (93.1)

32 (88.9)

0.022 Female56 (20.8)

11 (13.6)

5 (6.9)4 (11.1)

0.022

60 (47–67)

59 (43–68)

64.5 (58.8–71)

48 (39–58.3)<0.001 ≤20

5 (1.9)3 (3.7)0 (0.0)0 (0.0)0.29 21–4044 (16.4)

15 (18.5)

0 (0.0)11 (30.6)<0.001 41–60

98 (36.4)

26 (32.1)

29 (40.3)

18 (50.0)

0.286 61–80

119 (44.2)

37 (45.7)

43 (59.7)

7 (19.4)

0.001

 ≥81

3 (1.1)0 (0.0)0 (0.0)0 (0.0)0.548 By oneself

3 (1.1)4 (5.2)0 (0.0)0 (0.0)0.177 Accompanied

91 (34.7)

18 (23.4)

26 (37.1)

17 (48.6)

0.194 Passerby

77 (29.4)

9 (11.7)

27 (38.6)

9 (25.7)

0.015 Not on-­site

91 (34.7)

46 (59.7)

17 (24.3)

9 (25.7)

0.001 Solo130 (48.3)

44 (54.3)

21 (29.2)

6 (16.7)<0.001 Group

139 (51.7)

36 (44.4)

51 (70.8)

30 (83.3)<0.001 On trail (multiple choices)

87 (41.0)

35 (49.3)

63 (88.7)

1 (2.9)<0.001 Hazardous

126 (59.2)

38 (53.5)

6 (8.5)35 (100.0)<0.00118 (6.7)

7 (8.6)24 (33.3)

0 (0.0)<0.001n

P valueSex

Age (years) Median (IQR)
Age group (years)

CallerGroup size

Contact location

Treatment Donerespectively). Of the 44 CPRs, 22, 11, 2 and 1 were casesof cardiac death, trauma, stroke and drowning, respectively. In cardiac death, 75.9% (22/29) underwent CPR,
and 6.8% (5/7), including a bystander, underwent AED.
However, the CPR initiation and duration time, presenceor absence of shock and return of spontaneous circulation achievement were not specified in the record. Meanwhile, rewarming was provided in six hypothermia cases(three were initiated by bystanders) and zero trauma
cases.

Figure 4 (A) Types of trauma and the proportion of alive
subjects. (B) Causes of avalanche-­related death. TBI,
traumatic brain injury.

Figure 5 Characteristics of non-­survivors in hypothermia.
(A) Month of occurrence. (B) Proportion of people who madethe callout. (C) Distribution of callout time.

4

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DISCUSSION
In Japan, the major cause of mountain deaths during thedesignated period was trauma followed by hypothermia,
cardiac death and then avalanche-­related death. In ourstudy, the alive rate of all non-­survivors on rescue teamarrival was only 3.5%. In a mountainous prehospitalsetting, on-­site physicians and technical support such as
cardiac monitoring, which confirms death, are generally
unavailable in Japan. Confirming death by the absenceof vital signs is unreliable; hence, correctly recognising
death immediately after death is extremely difficult.8 The
classification of being alive can still be uncertain, thereby
possibly affecting the number of alive subjects on rescue
arrival. In addition, although all deaths were confirmedby physicians or coroners, autopsy was not performed for
all, which may influence the cause of death of an unwitnessed cardiac arrest. Highly frequented regions such as
Nagano, and victims who were found skeletonised morethan a few months later might also have influenced theresults. However, our 5-­year data showed no hitting peaks
or troughs of fluctuating trends by year, and many factors
affected the length of interval from callout to recovery.
Factors such as terrain, weather and search technique cangreatly affect the outcome of mountain rescue. Thus, ourresults can be considered valid for the overall picture ofmountain death in Japan for 5 years. The cause of deathreported by international research varies by region. In
Denali, between 1903 and 2006, the main cause of deathamong the 96 included cases was trauma followed by hypothermia, AAI and cardiac death (43, 44.8%; 16, 16.7%; 7,7.3%; and 1, 1%, respectively).5 In Aconcagua Provincial
Park, between 2001 and 2012, death in 33 included cases
was also mainly caused by trauma but followed by AAI,
hypothermia and cardiac death (9, 27.2%; 7, 21.2%; 5,
15.2%; and 4, 12.1%, respectively).6 In our study, trauma
and hypothermia showed similar trends, but no AAI
deaths were noted. The incidence of high-­altitude pulmonary oedema, which is fatal, is reportedly 0.3% at 3500 m
altitude.9 However, our study showed that Japan’s mountains are not high enough to cause altitude-­related deaths
often. Even in Japan, the causes and frequency of deathand the FR differed by region. In the present study, theFR was not the mortality rate for the number of mountaineers because such number remains uncertain. This study
intended to consider enhancing the survival by focusingon people who need rescue. The FR was clearly lower
in the north island than in the Alps range (figure 3). In
contrast to trauma and cardiac death, hypothermia gradually developed and progressed, which allowed survival
for a longer time until rescue. This notion could be oneof the reasons causing the regional differences in FR in
Japan. Previously, the alive rate of all non-­survivors on
rescue team arrival was 7% (4/57) in Scottish mountain,10
and 85% of rural trauma victims died at the prehospitalsetting in Norway,11 with 2 years of observation period.
Our 5-­year results support that in remote environments,
most deaths occur on the site before rescue team arrival.
Oshiro K, Murakami T. BMJ Open 2022;12:e053935. doi:10.1136/bmjopen-2021-053935

According to previous studies, the main mechanism of
injury associated with trauma in the mountains is a fall
(59.6%).5 10 12 In our study, 95% of trauma deaths werecaused due to falls. Therefore, falls are the major cause ofdeath and the major mechanism of trauma. The larger thefall distance, the higher the risk of fatality3; however, we
could not obtain data regarding the fall height for most of
the victims; these data could enhance our study’s contributions to the field. Recently, the incidences of major
trauma have increased in the elderly, and the main mechanism is a fall from a height of >2 m.13 The pathophysiological response to trauma, anatomical and physiologicalfragility, and potential higher serious comorbidities inolder individuals can increase the probability of major
trauma from low-­velocity mechanisms in this age group,
which leads to a higher FR and early death than those inyounger people.14 15 These findings potentially impact the
low alive rate at rescue arrival in the current study. The
occurrence of cardiac death was higher in men aged >41
years; this was similar to previous reports focusing on theEuropean Alps, in which 90%–95% were men, all were
>34 years of age.4 In our study, the time of collapse in
an unwitnessed arrest was impossible to estimate, but atleast 48.8% of the cases resulted in death within 1 hourof onset, which corresponded to SCD.16 Ageing is astrong risk predictor of sudden cardiac arrest (SCA),
which increases after the age of 35 years. Moreover, SCD
is more common among men rather than women in all
age groups.17 The prevalence of SCD in mountain fatalities can be attributed to such finding. Additionally, theincidence of sudden death is several times higher during
exercise than at other times.18 Unfortunately, in Japan,
where many mountaineers are already in middle age, SCD
will continue to occur, according to previous findings.
Regarding the time factors, the major causes of death
were significantly different from the onset to rescue arrival(figure 2B). Concerning the time from the onset to callout,
if signal is available, callout can be made in a short time intrauma-­related, cardiac death-­related and avalanche-­related
accidents with apparent onset. In hypothermia, approximately half of the callout was made after around twilight.
Previously, hypothermia deaths tended to occur as a resultof injury.5 In our study, secondary to trauma by 17.5%(14/81); thus, most of the hypothermia cases were primary
because of prolonged exposure to heat loss, bad weather,and fasting. When hypothermia progresses to stage ≥2, thevictims lose their judgement because of the decreased levelof consciousness. Moreover, lone subjects were mostly men,and the onset of hypothermia was unclear. Consequently,
making callouts on the site becomes impossible once thestage progresses. These views are the reasons why 59.7% of
the callout was from a person who was not at the site, suchas a family member, after recognising that the casualty had
not returned home. Therefore, the dispatch of the rescue
team was delayed, and the hypothermia stage had further
progressed. Additionally, most cardiac deaths occurred on a
mountain trail, which shortened the required time to searchand locate casualties. Conversely, searching for fall (trauma)
5

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Open accessand hypothermic casualties in places that require a technical rescue process was more time consuming. Regarding
the time from the onset to rescue team arrival, our results
revealed that at least 83% of trauma deaths took >60 min to
be rescued, and no alive cases were rescued in >6 hours. In
previous studies, an out-­of-­hospital time (OHT) exceeding
60 min was observed in 84% and 67% of mountain trauma
and helicopter rescue cases, respectively,12 19 which led to
an increased mortality rate in seriously injured patients.20
Meanwhile, mountain rescue operations have a longer
OHT (mean: 117.4±142.9 min) associated with a prolonged
treatment-­free interval (TFI) than rescue operations in non-­alpine areas (68.7±28.6 min), but there was no difference inthe hospital mortality.12 Moreover, the mortality within thefirst hour of injury was high, between 70% and 80% regardless of rurality.11 In our study, though some casualties possibly
had life-­threatening injuries, we found alive cases even witha prolonged OHT. Factors that can contribute to survival
other than a short OHT should be specified.
In the cardiac death group (non-­survivors), only one
victim was alive when the rescue team arrived, despite
an increased rate of rescue team arrival within 1 hour.
Survival from SCA is inversely correlated with the intervalfrom collapse to definitive care,21 22 and in SCD, the best
outcome reported was from the occurrence site witha publicly recognisable advanced emergency response
system.23 In the present study, 52.1% (38/73) of callouts were made by a companion or a passerby and at
least 24.7% (18/73) of cardiac arrest incidences were
witnessed. However, the current emergency response
system in the mountains has to overcome various hurdles
before better outcomes can be achieved.
Considering on-­site treatment, the number of on-­site
interventions was greater than that of the alive subjects.
Given that some cases had undetectable vital signs, death
could hardly be confirmed.
With correct implementation of first aid in blunttrauma, mortality potentially decreased by 4.5%.24 In thepresent study, 28.6% (2/7) had pelvic fracture, and 7.1%
(7/98) had TBI death after rescue arrival. Early diagnosis
and treatment may prevent death caused by pelvic exsanguination.25 Pelvic binders were 70% effective in stabilising the pelvis.26 In severe TBI, basic airway managementwith oxygen administration obtained a better outcome
than endotracheal intubation with oxygen administration for hypoxia prevention (SaO2 ≤94%).27 28 Despite
the fact that our results do not demonstrate statisticalsignificance, they encourage the implementation of andtraining on these procedures for life-­saving in remoteenvironments, considering the background of reported
evidence. For hypothermia, on-­site rewarming is safe andeffective,29 but the implementation rate on rescue arrival
is low. Appropriate rewarming with mountain equipment
showed good recovery at the site by non-­medical persons
when casualty transportation was difficult during bad
weather or at night.30 In this study, even after 6 hours from
callout, alive cases were found. Early callout, first-­aid skill
education and dispatcher-­assisted rewarming contribute6

to a shortened TFI and increase the chances of survival.
In our study, CPR interventions were highest in cardiac
death. Most SCAs were found on busy mountain trails
where a bystander is likely to be present. However, thequality of chest compressions apparently decreases in
5–10 min at an altitude of 3454 m.31 CPR implementationin the mountains is restricted by various technical issues,
including chest compression quality, AED availability,
ventilation requirement and human resources, as well as
the safety of rescuers, such as the bystander’s advanced
age, the terrain, the longer interval from onset to rescue
arrival, and the weather. Moreover, dispatcher-­assisted
CPR is effective in treating out-­of-­hospital cardiac arrest.32
When heart attack is suspected, prompt callouts should
be made to use dispatch-­assisted CPR instructions. The
decisions to implement, hold and terminate CPR should
be supported.
The limitation of this study lies in its retrospective
nature; analyses were performed based on data extracted
from the information recorded in each regional format.
Police record formats are unified by region but not nationwide. Although the facts of field activities by rescue teams
are recorded, the emergency setting in the mountain thathas restricted circumstances can hinder the recording of
accurate prehospital time-­stamped patient data. For thisreason, some data were missing. Future research needs toconstruct a standardised data collection system to reduce
missing data in collaboration with the dispatcher centre
and hospitals and to collate with other topographicalareas, considering the limited number of deaths.
In conclusion, the leading causes of death in themountains in Japan are trauma, hypothermia, SCD andavalanche-­related deaths. The alive rate on rescue arrival
in non-­survivors was only 3.5%. No survival was observedin >6 hours in trauma and >1 hour in cardiac death cases,
but a longer survival time was observed in cases of hypothermia. Early intervention into pelvic fracture and TBI,
which have higher alive rate on rescue arrival, and major
improvement in early recognition leading to early callout
and initiation of effective rewarming in hypothermia
cases can possibly enhance survival by shortening TFI.
Heart attack cases mostly correspond to SCD; however,
due to the obvious onset, occurrence on mountain trail,
probable early callout and easy locating, the time until
rescue arrival can be better used for survival.
Twitter Kazue Oshiro @KazueOshiro
Acknowledgements We acknowledge the support of the following organisations
in collecting the data: National Police Agency, Hokkaido, Aomori, Yamagata,
Fukushima, Niigata, Gunma, Nagano, Toyama, Gifu and Shizuoka Prefectural
Polices. The authors also thank Enago (​www.​enago.​jp) for the English language
review.
Contributors KO contributed to the acquisition and analysis and drafted themanuscript. KO and TM contributed to the conception and design, criticallyrevised the manuscript, contributed to interpretation, gave the final approval,
agreed to be accountable for all aspects of the work to ensure integrity andaccuracy, and read and agreed to the publication of the final version of themanuscript. All authors have read and agreed to the published version of themanuscript. KO is the guarantor of the work and accepts full responsibility for thepresented content.
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Open accessFunding The authors have not declared a specific grant for this research from any
funding agency in the public, commercial or not-­for-­profit sectors.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in
the design, conduct, reporting or dissemination plans of this research.
Patient consent for publication Not applicable.
Ethics approval This study does not involve human participants.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data may be obtained from a third party and are notpublicly available. All data provided were from the National Police Agency and thecompetent authority of each prefectural police headquarters, which approved theuse for this study by the authors. National Police Agency +813-­3581-­0141Hokkaido
Prefectural Police +8111-­251-­0110Aomori Prefectural Police +8117-­723-­4211
Yamagata Prefectural Police +8123-­626-­0110Fukushima Prefectural Police +8124-­
522-­2151 Niigata Prefectural Police +8125-­285-­0110 Gunma Prefectural Police+817-­243-­0110 Nagano Prefectural Police +8126-­233-­0110Toyama Prefectural
Police +8176-­441-2­ 211Gifu Prefectural Police +8158-­271-­2424Shizuoka
Prefectural Police +8154-­271-­0110
Open access This is an open access article distributed in accordance with theCreative Commons Attribution Non Commercial (CC BY-­NC 4.0) license, which
permits others to distribute, remix, adapt, build upon this work non-­commercially,and license their derivative works on different terms, provided the original work is
properly cited, appropriate credit is given, any changes made indicated, and the use
is non-­commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.
ORCID iD
Kazue Oshiro http://orcid.org/0000-0003-1248-0737

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