At the same time, solitary death among elderly individuals has appeared as a new social problem related to the ageing society. The phrase ‘solitary death’ came into use in the mass media's reporting of a case that occurred in Japan during the 1980s in which the skeletal remains of an elderly person living alone was found as a result of the prolonged length of time from death until the finding of the dead body.² The Japanese people were shocked by the individual's remarkable postmortem bodily changes. Cleaning solitary deaths up has been established as a business for stable incidents of elderly solitary deaths. That unusual job was noticed by foreigners and elderly solitary death was introduced as just a Japanese problem.³ However, this Japanese issue has been spread over the sea. In Korea, several solitary deaths were reported in 2011.⁴ Korean rapid ageing of society with similarity to Japan has also influenced the social function around family.

However, what is ‘solitary death’ has not been defined clearly yet. Most of articles, which had tried to reveal the ‘solitary death,’ mentioned that this lack of definition is the biggest problem. One letter article from Japan showed the 35 cases treated as solitary deaths in 8 years from 2000. They showed the association between incident of solitary death and person who found resident's status in the public housing complex.⁵ In this report, they defined whether a case is solitary death or not depending on subjective opinion of person who found the case. One study operationally defined solitary death as unexpected death among the people living alone,⁶ in the other study, they added the death occurred at their own house on that definition.⁷ As a broader definition, there were some articles said solitary death is occurred not only among people living alone.^{8 9} One study researched the municipal bodies’ practices for solitary death.¹⁰ They revealed that the definition of solitary death was varied.

Under these confused background, many studies tried to explain solitary death with various view points, such as household,^{6–8 10–12} age of subject,^{6 7 10 11} cause of death,^{7 8 10 11} the place where the subject was found,^{8 10} or the person who found the subject.^{7 8 12} All of these articles dealt with the time spent after the death^{6–8 10–12} as main characteristic of solitary death. The death left for long time and making remarkable postmortem bodily change must be avoided and the time spent after the death can be an important view point on solitary death. Therefore, in this study, we focused solely on this time spent after the death and analysed the factors affected on it.

About the time spent after the death, the ‘postmortem interval’ (PMI) is the essential parameter in forensic medicine. PMI is defined as the time from death until examination. There is one study on this PMI and they showed that the average of PMI was longer among the elderly living alone.⁶ However, because our study focused on the finding of a dead body, we defined the ‘PMI until finding’ (PMI-f) as the period from the estimated time of death until the finding of the dead body.

To obtain information about PMI-f, we focused on forensic medical data. Forensic medical data, especially the postmortem examination (inspection) records, are informative in understanding the circumstances of deaths, including those involving elderly cases with long PMI-f.

Crude and adjusted HRs and 95% CIs for increasing of PMI-f were shown in table 2. In the crude models, the four variables with significant HRs were age, household situation, the place at which the dead body was found, and the person who found the dead body. The adjusted risk of living alone was the highest of all independent variables (HR 3.73, 95% CI 3.37 to 4.13). In the adjusted model, younger age (0.99, 0.98 to 0.99), unnatural death due to an accident (1.50, 1.28 to 1.75) and found at one's own home (1.37, 1.22 to 1.55) were also significant risk factors that delayed the finding of bodies.

The results of the model including interactions are also shown in table 2. Living alone showed the strongest influence (2.34, 1.64 to 3.33) also in the fully adjusted model including interactions. Other variables related to long PMI-f: age (0.99, 0.98 to 0.99), male (0.89, 0.82 to 0.96), unnatural death (1.45, 1.24 to 1.72) and finding at own home (1.32, 1.14 to 1.52). Significant interactions of the household situation (=living alone) were observed among male (1.31, 1.10 to 1.56) and finding at one's own home (1.47, 1.06 to 2.05).

The HRs of the multivariate model with interactions, 0.89 for sex, 2.34 for household situation and 1.34 for their interaction, mean that the males of living alone prolong the median PM-f 0.92 day, the males with family shrink it 0.01 day, and the females of living alone prolong it 0.29 day, with the reference of the females with family, which are obtained by transforming these HRs to median PM-f days in the model (not shown in table). The stratified analysis in table 3 showed the significant relations with longer PMI-f, among subjects living alone, finding at own home (1.99, 1.48to 2.67), unnatural death (1.72, 1.02 to 2.91), male (1.23, 1.04 to 1.44), younger age (0.97, 0.96 to 0.98) in the adjusted model shown in table 3. Among subjects living with family, sex of male related shorter PMI-f (0.92, 0.85 to 0.99) as opposed to the case in living alone.

The analyses demonstrated that living alone was the strongest predictor of delays in finding elderly dead bodies; the risk was about four times higher in this group than in those living with family. This finding suggests that elderly subjects living alone were isolated and contactless. One study surveyed survival after falling among people living alone in San Francisco.¹³ Of 387 people, 90 were left un-rescued for an average of 18 h after becoming helpless in their own homes and died. Citizens living alone were difficult to find if they fell down. In Japan, the proportion of community-dwelling elderly individuals living alone is increasing with ageing. The proportion of elderly individuals living alone among whole elderly people was 13.8% in 2000 and 16.4% in 2010.¹⁴ This high risk to left the dead body for long time in living alone became an urgent public health issue.

We found the contrary results regarding sex between household situations (living alone or with family). ‘Male subjects living alone’ and ‘female subjects living with family’ tended to prolong their PMI-f. Male elderly people, in general, are said to have poor social relation. Thus, their dead bodies should be difficult to be found by their neighbourhoods or friends. However, among the subjects living with family, male subjects tend to be found their dead bodies than female subjects. In cases of male people, their family member will make more effort to seek. This might be due to the presence of souse. In Japan, the life expectancy is longer in female people than in male¹⁵ and the age of a husband used to be older than his wife.^{15 16} It is suggested that the number of female widows were greater than male.

Interaction between living alone and dying at one's own home showed effect on PMI-f. This interaction showed the situation that elderly individual's home could be a risk factor to make them more isolated. This risk indicates the need for automatic alarm-and-notification systems. Some municipalities provide such systems connected to local rescue services.¹⁷ If accidents occur, the systems can notify to rescue services. In addition to these systems, an important first step in preventing solitary death is to raise public awareness of the likelihood that elderly people living alone may become helpless and isolated in their homes within a community.

Regarding the causes of death included in our model, unnatural causes (eg, drowning, suffocation and falling) were associated with the fact that they are found later. It is natural that the people who were under medical care with some internal diseases can be found earlier. In contrast, external accidents happen suddenly, and therefore it is difficult for relatives to find the dead bodies. In really, the biggest single cause of incapacitation that was left alone was reported to be a fall.¹³

In our adjusted model, younger individuals’ deaths were found later. This suggests that accidents may have had a bigger impact on younger elderly than older individuals because the former may have been less concerned about death. The situation in which an individual dies and her/his dead body is found can be affected by her/his life and relationships while alive, especially in cases of unfortunate accidents. With a little of individual's concern and correspondence about her/his possibility of death, the people around her/him also can less take care of her/him death and find the dead body later.

We found differences between the crude and adjusted models with regard to significant variables. The identity of the person who found the dead body did not affect the PMI-f in the adjusted model. This indicates that who found the dead body depended on other factors and so that there was no directive this variable's association to PMI-f.

Our present study had several limitations. Variable settings were based on the official record written about the living and dying among the subjects by police officers. Variables including basic characteristics and other components were confirmed to be recorded. Those records were based on the inquires by the police office; however, the accuracy of the information was not secured and it is difficult to assess the validation retrospectively. All of our study subjects were the cases that were reported as unexpected deaths and that were examined by police officers. This reporting system abides by law with compulsion for medical doctors. However, there should be unexpected deaths that are not reported and not performed autopsies. For example, if the doctors have known the family and attended the elderly before, they may not report the unexpected death. If such underreported cases were present, our PMI-f should be longer than actual one because PMI-f in such cases may not be so long. Other situations also may present. Our results should be captured under this limitation.