The surface air temperature has increased significantly by about 1.5℃ during the past 100 years on the Korean peninsula which is higher than the global average increase of 0.74℃ [1]. Given the increase in the frequency and intensity of extreme weather events such as heat waves predicted in the future due to climate change [2], a number of researchers have drawn attention to the effects of external temperatures on human health and have characterized vulnerable groups [3-13].

Low-income groups are highly vulnerable to extreme weather events such as high temperatures and natural disasters [3,4]. Naughton et al. [5] reported that air-conditioners are the strongest protective factor against heat-related death, showing that low-income groups can be very vulnerable to the effects of high temperature. Poverty can be a risk factor for heat-related illnesses and deaths because the poor are more likely to live in heat-island urban areas [6] and are less likely to be able to afford air-conditioning [7-9].

In addition, it is well known that elderly people do not adjust compared to young people to sudden changes in temperature because they are more likely to have a chronic medical condition that changes normal body responses to heat [9-13]. The most serious short-term effects of excessive environmental heat are caused by the impact on the body's thermo-regulating mechanism, water and electrolyte balance, and the increased demands imposed on the cardiovascular system [13], which means that elderly people may be much more susceptible to heat stress than younger people because they are more likely to have cardiovascular diseases. Previous studies [14,15] reported that elderly persons may be particularly susceptible to temperature-related variations in blood pressure. The baroreflex, which is one of the mechanisms of blood pressure regulation, is modified in elderly subjects [14]; it has been hypothesized that disorders of baroreflex control and enhanced vasoreactivity could contribute to the aging-associated increase in cardiovascular morbidity [15].

Although both poverty and old age are critical risk factors for heat-related illness and death, there are few studies on the health effect of heat waves on the poor and elderly people. Kim and Joh [3] studied the effect of high temperatures on low-income elderly people in terms of excess mortality due to heat stress. However, their study did not include changes in physical condition and the disease burden.

In order to reduce the adverse health impact of extreme hot weather, potential health effects should be predicted based on an understanding of vulnerable groups who are susceptible to climate change. One of the best ways to predict potential health effects of heat stress on the elderly living in poor conditions is to measure the changes in primary outcomes, including the blood pressure and body temperature, caused by the hot weather. This would lead to a better understanding of the health effects of climate change and would allow the formulation of an adaptation strategy considering the socioeconomic status of people.

The purpose of this study is to investigate the indoor environmental climate of dosshouses and to assess the acute effects of heat stress on body temperature and blood pressure in elderly individuals living in poor housing conditions.

We investigated the indoor environmental climate of dosshouses located in the center of metropolitan Seoul during hot summer days to assess their health impact on those whose health was most likely to be affected by weather-related changes, i.e., elderly people who lived alone in poor housing conditions.

According to the definition of a previous study [18], heat waves were categorized by intensity and duration. Hajat et al. [18] considered 6 heat wave types: periods of 2 or more or 4 or more days of continuous temperatures that are more than the 98.5th, 99th, or 99.5th percentile of the community's temperature distribution. Considering that the average temperature of Seoul is 12.9℃ (SD, 10.2℃) for the last ten years, 2001-2010, and that the 99th and 98.5th percentiles of the average temperature in Jongno-gu, Seoul are 28.6℃ and 28.3℃, respectively, a heat wave was occurred in the study period. The ambient average temperatures of August 5 and 6 in 2010 were 29.2℃ and 28.3℃, respectively, thus matching or exceeding 28.3℃ and having occurred continuously. Furthermore, considering the indoor temperature, the residents in the dosshouses were under a heat wave during all the study period because the daily indoor temperatures were always over 28.6℃, the 99th percentile of the last ten years, showing a range of 29.1-34.8℃ (Figure 1). During the hot days, the indoor DI was very high, showing an extremely uncomfortable condition. That is, the average discomfort index was 84.22 (SD, 1.77), which was higher than the level of 80 at which most people complain of discomfort (Table 1). The average morning and afternoon temperatures were 31.1℃ (SD, 0.8℃) and 31.9℃ (SD, 1.0℃), respectively, and there were not significant differences between them. In Korea, when comparing the air temperature in the summer under the same conditions, the appropriate indoor cooling temperature is suggested to be between 26 to 28℃. Therefore, based upon this information, people residing in dosshouses live in temperatures 4 to 5℃ higher than the advised standard indoor temperature in the summer. According to the records of the hourly room temperature of two participants, the room temperature was comparatively constant regardless of the daily time, having remained at more than 30℃ (Appendix 1). That is, the high temperature lasted all night, which means that the residents in the dosshouses were directly exposed to the high temperature and tropical night phenomenon during the hot summer days.

Dosshouses, with an average room size of 5.1 m² (SD, 3.5 m²) according to the measurements of 19 dosshouses and without window in half of them, are characteristically clustered together in a narrow space comparable to a labyrinth, which inhibits ventilation and air circulation. Furthermore, residents in dosshouses typically cook in their room using a potable burner and an electric cooker because they have no separate kitchen. This can cause the temperature to rise in the dosshouses during the day. The small building fronts could also lead to less heat escaping, even at night. In addition, due to poor security at the dosshouses, the residents could neither open their doors nor their windows at night, which exacerbated the high-temperature condition.

The rise in body temperature was significantly associated with indoor and outdoor temperature rises (Table 3). The rise in body temperature was more affected by the indoor temperature (0.21℃ of increase in the body temperature per 1℃ increase in the ambient temperature increase) than the outdoor temperature (0.07℃), which reflects the fact that the residents spend most of their time in the dosshouses due to the general difficulty they experience when moving. In addition, the effect of the temperature on the body temperature was higher in hypertensive subjects than non-hypertensive subjects (Table 4).

In order for the human body to maintain a consistent temperature, a continuous exchange of heat with the environment is necessary according to the law of thermodynamics. Body heat is primarily produced through a metabolic process in the interior of the body, and heat exchange between the body and the exterior environment is accomplished through mechanisms such as conduction, convection, radiation, and evaporation. However, when a body is exposed to high temperatures for an extended period, body temperature regulation is inhibited as heat exchange cannot occur properly. Furthermore, the failure of thermoregulation caused by extremely high temperatures may be impaired by dehydration, salt depletion, and increased surface blood circulation [19].

As a consequence of impaired body temperature regulation, our subjects experienced a significant increase in body temperature during the hot days, and at worst some had a slight fever. In addition, a high level of subjective symptoms such as muscle pain, headaches, movement disorders, and difficulty in breathing were also reported; two participants experienced dyspnea, one participant experienced nausea, five participants experienced headache, and six participants experienced muscle aches and stiffness. In addition, during periods of sultry weather, over 50% of the research subjects complained of sleeping disorders; subjects were able to sleep only one third of their normal sleep hours as the average number of sleep hours during the research period was 2.5 compared to the normal sleep hours of 7.4 according to their self-reports.

Some previous studies [20-22] reported that both hyperthermia and hypothermia are generally linked to cardio-respiratory morbidity or mortality. After examining the functions of the cardiovascular, cerebrovascular, and respiratory systems at high temperatures, Michelozzi et al. [23] reported that elderly women had a 4.5% decrease in function per 1℃ increase in body temperature. Therefore, an increase in the body temperature in the poor and elderly can be a potential risk factor to aggravate cardiovascular, cerebrovascular, and respiratory diseases. 21% of the residents living in dosshouses in Korea have one ailment among hypertension, respiratory, and cardiovascular disease [24].

The blood pressure showed a tendency to decrease in relation to indoor and outdoor temperature increases except for the effect of outdoor temperature on the DBP. These results are in good agreement with previous research results related to blood pressure changes according to the season, with one finding reporting that the blood pressure in summer is lower than that in winter [25,26]. In summer, the blood pressure is lower because the veins of the skin expand in order to diffuse body heat. The change in the blood pressure is more obvious among the elderly, and people who are thin react more sensitively to this change. Upon comparing our research results to those of a study that investigated the effect of a change in the temperature on the blood pressure among young adults in Korea [27], which reported that the SBP and DBP decreased by 0.659 mmHg and 0.368 mmHg, respectively, due to a 1℃ increase in the ambient temperature, the effect of the outdoor temperature on the SBP of the elderly people living in the dosshouses was in accordance with findings of the previous research. However, that on the DBP of the elderly people living in the dosshouses was different from the previous study (Table 3); the DBP of the elderly people in the dosshouses increased by 0.20 mmHg due to a 1℃ increase in the ambient temperature although our results were not statistically significant due to the smaller sample size.

The impact of the indoor temperature on the blood pressure was higher than that of the outdoor temperature; the effect of the indoor temperature on the SBP was about 5.1 times of that of the outdoor temperature. In addition, the effect of the indoor temperature on the DBP was statistically significant while that of outdoor temperature was not (Table 3). It can be inferred that the change of the blood pressure is related more to the indoor temperature than to the outdoor because the residents in the dosshouses spent most of their time indoors. Correlation coefficients between the blood pressure and temperature also support this assumption (Table 2).

Alpérovitch et al. [28] pointed out that because decreased blood pressure can be a risk factor for increased mortality in persons who have environmental hyperthermia, the decrease of blood pressure in the elderly people could be an important consideration during a heat wave. they reported that in August of 2003, there was an unprecedented heat wave in France that resulted in approximately 15,000 deaths, mainly of the elderly. After their second follow-up examination of the blood pressure of the affected participants, they also reported that the seasonal fall in the blood pressure values was sharper in August of 2003 (mean SBP, 132 mm Hg) than in August of 2004 (138 mm Hg) [28]. In addition, it is estimated that a decrease in the blood pressure among the elderly due to high temperatures increased the risk of fainting, heat fatigue, and death related to intense heat. This is also likely to be related to deaths caused by hypertension that occur frequently in the summer time [29]. Therefore, attention should be paid on the acute changes in the blood pressure during hot summer days.

This research has limitations, including the fact that we used a relatively small sample size. Considering the problem with public security, the tropical night phenomenon and an assessment of health during the affected times was not included. In addition, the data used in the current study contain some bias, including measurement error such as that caused by differences between the nurses measuring the blood pressure. Despite these limitations, this study is meaningful in that it is the first to assess the effect of a heat wave on the health indicators of elderly people living in extremely poor housing condition as well as to investigate the indoor environmental climate of dosshouses. It is expected that these research results can be used as basic data to construct heat wave adaptive plans that are differentiated and actualized in terms of age and region. In addition, this research can contribute to climate change vulnerability assessments that can be used to minimize the detrimental health effects of heat waves.

In conclusion, the poor and elderly are directly exposed to heat wave both night and day during hot summer days. In a poor housing condition, their vital signs respond sensitively to indoor temperature increase. As weather changes accelerate, it is predicted that the occurrence of heat waves in the summer will increase; thus, more substantial and systematic investigations of the health effects on elderly populations during the heat wave season are needed. In addition, careful adaptation strategies as regards to climate change are required considering the socioeconomic status of at risk groups.