Journal Archive

Indoor temperature and humidity conditions are critical to public health because people in industrialized countries generally spend more than 90% of their time in indoor environments (Hppe and Martinac, 1998). Heat and cold stress in home environments are particularly important to examine. In New York, 80% of heat stroke morbidity during the summer of 2007 was attributed to heat exposure at home (NYC, 2013). A similar situation was observed in France; half of all heat stroke fatalities occurred inside homes during the 2003 heat wave (Fouillet et al., 2006). Similarly, there are an estimated 30,000-60,000 fatalities in the United Kingdom and 1,500-2,000 fatalities in Ireland due to low residential temperatures each winter (Clinch and Healy, 2000). Low relative humidity (less than 30%) has been associated with dryness of skin, throat, and mucous membranes, and sensory irritation of the eyes (Reinikainen and Jaakkola, 2003; Sato et al., 2003). High relative humidity in indoor environments may also affect air quality (Fang et al., 1998) and induce the growth of mold, causing respiratory allergies and discomfort (Bornehag et al., 2004). Therefore, the maintenance of adequate indoor temperature and humidity levels in homes is necessary to achieve comfortable thermal conditions, protect human health, and improve quality of life.

Low-income households may not have adequate indoor environments. Chartered Institution of Building Service Engineers (CIBSE) recommends 25°C and 24°C as acceptable indoor temperatures for living areas and bedrooms, respectively, during warm weather (CIBSE, 2006). Therefore, use of an air conditioning (AC) unit is the strongest protective factor against heat-related deaths. However, individuals with low incomes may not have AC units in their homes and may thus be vulnerable to high temperatures during the summer months (Naughton et al., 2002). In low-income households in Greece, indoor temperatures consistently exceed 30°C during heat waves. One study reported that 216 continuous hourly indoor temperatures were over 30°C; in very-low-income households, indoor temperatures were above 33°C over 6 continuous days (Sakka et al., 2012). To maintain adequate health during the winter months, several studies have suggested an indoor temperature of between 18°C and 24°C (WHO, 2007; DEFRA, 2004; Healy and Clinch, 2002a). However, approximately 15-25% of the low-income population of southern Europe and Ireland are unable to afford the expenses associated with this suggested temperature level (Eurofound, 2003). For example, 29% of indoor temperatures in low-income households in southern Europe are lower than 18°C and 10.8% of indoor temperatures in low-income households in Ireland are below 18°C (Healy and Clinch, 2002b).

Indoor temperature and humidity have been investigated for the general population (Bae and Chun, 2009) including in public buildings in Korea (Yeom et al., 2014; Yang et al., 2009). In general residences of Korea, indoor temperatures during summer and winter were 27.5±2.0°C and 23.9±1.6°C, respectively and indoor relative humidity during summer and winter were 59.5±9.7% and 31.6±7.7%, respectively (Bae and Chun, 2009). These factors have also been investigated in low-income housing during short periods of the summer or winter in other countries (Santamouris et al., 2014; Sakka et al., 2012; Oreszczyn et al., 2006) but not in Korea. In addition, to appropriately assess the effects of indoor temperature and humidity conditions on one’s daily life, it is essential to monitor these factors over the long term, ideally over the course of 1 year. Therefore, we characterized the indoor temperature and humidity in various types of low-income residences over 1 year in Seoul, Korea.

The percentage of residence with AC units may be associated with residential income. In a previous Korean survey, 67.6% of residences in Korea had AC units (Statistics Korea, 2013). However, only 23.9% had AC units in this study population. The difference by residential income was similar to data reported from Greece, where AC units were installed in 48% of low-income households and 69% of high-income households (Santamouris et al., 2007).

Monthly average temperatures were significantly higher indoors than outdoors, while indoor relative humidity was significantly lower than outdoors. However, monthly average absolute humidity was not significantly different indoors and outdoors. These findings are similar to those of a previous study where indoor temperature, relative humidity, and absolute humidity were measured over a 1-year period in residences in Boston, United States. The daily average indoor temperature and relative humidity were 21.4°C and 46.4% respectively, while the corresponding values outdoors were 13.1°C and 64.8%. However, daily average indoor and outdoor absolute humidity were 8.9 and 8.2 g/m³ (Nguyen et al., 2014). In two types of middle-income residence (apartment and detached house) in Korea, indoor temperature and relative humidity were measured over a year (Yeom et al., 2014). The ranges of monthly average indoor temperature were from 22.3 to 29.3°C for apartments and from 20.3 to 30.1°C for detached houses. The lowest monthly averages of indoor temperature were higher than the low income residences in this study. It might indicate that people in middle-income residences used heating system more frequently than low-income residences.

The values of all parameters differed among the three types of residence, particularly during summer and winter. Among residences, the indoor temperatures in rooftop residences were the highest during the summer and the lowest during the winter. Because such homes are located on the tops of buildings, they are more affected by outdoor conditions. Moreover, poor insulation or construction material of building envelopes may also influence the indoor temperature in such houses (Santamouris et al., 2014, 2007). The indoor relative humidity and absolute humidity in basement residences were highest during the summer. Because basement residences are located underground, they have limited sunlight and ventilation. The indoor temperatures in public rental apartments were higher than in the other types of residence during months when heating was required. The relative humidity in such apartments was lowest during these same heating months.

Differences in indoor temperature and relative humidity in the three types of low-income residences may be due to the type of residence, ventilation, and heating and cooling systems in place. When indoor temperatures were measured over a 1-year period in four types of residence in the United Kingdom, the indoor temperature of terraced houses was lower than that of the other residences investigated (Yohanis and Mondol, 2010). Ventilation may have a greater effect on indoor temperature and humidity than a house’s building materials (Kalamees et al., 2009). For example, the ventilation rate was significantly higher in multifamily houses than in detached houses in both Norwegian and Swedish studies (Bornehag et al., 2005; Øie et al., 1998). Studies to determine the ventilation rate in various types of residence in Korea are needed.

The heating systems in rooftop residences, basement residences, and public rental apartments might be used when the outdoor temperature was below 3.9°C, 9.9°C, and 17.1°C, respectively. In a previous study on the general population in Korea, the heating point was determined to be 15°C (Yeom et al., 2014), which is higher than in rooftop and basement residences in this study. The findings of this and previous studies demonstrated that low-income residences might be vulnerable to cold conditions. The reason why the heating point in public rental apartments was highest of the three types of residence was that 10 of the 18 public rental apartments used a central heating system. Similar results were observed in China. When measurements were made in nine cities in China during a short period, high indoor temperatures were observed in households using a central heating system (Zhang and Yoshino, 2010). However, cooling threshold was not found in this study. Since people living in low-income-residences did not use AC units during summer, the correlation between indoor and outdoor temperatures were not significantly changed. It demonstrated people living in low-income residences could not control indoor temperature during summer.

In a previous Korean study, it was reported that people turned on their AC units when the indoor temperature was 30°C and turned the units off at 26.9°C (Bae and Chun, 2009). In 2005, the Japanese government recommended an AC unit start up temperature of 28°C until the beginning of September (Roaf et al., 2010). However, in this study, the indoor temperature in rooftop residences continuously exceeded 30°C during tropical nights. People living in rooftop residences did not have or operate AC units during the study period. Moreover, living in a poor-quality house with a low residential income can increase the likelihood of experiencing adverse indoor environmental conditions that cause health issues during hot summers (Conti et al., 2007; Haines et al., 2006).

In this study, hourly average indoor temperatures in rooftop residences were significantly different between winter and cold-wave advisory days. Furthermore, the pattern of hourly average indoor temperatures in rooftop residences was similar to the hourly outdoor temperature pattern during the winter, including cold-wave advisory days. This indicates that people living in rooftop residences did not operate heating systems over the course of the whole winter. A study conducted in low-income residences in Greece also found large differences between indoor temperatures during the winter; on very cold days there were high levels of variation in indoor temperatures (Santamouris et al., 2014). People living in low-income residences have an increased likelihood of experiencing adverse indoor environmental conditions that cause health issues during cold winters (Santamouris et al., 2014; Eurofound, 2003).

This study has a few limitations. This study was conducted in small number of each type of residences. When this study was designed, more than 60 residences (20 rooftop residences, 20 basement residences, and 20 public rental apartments) were expected to participate. However, many occupants in low-income residences were reluctant to show their poor living conditions to researchers. And some participants moved during the study period. Therefore, we were unable to recruit the expected number of residences. Two slightly different data logger models (UX-100 and U10, Onset, Cape Cod, MA, USA) were used. Since they have slightly different accuracies, use of different models may cause some potential bias. However, use of the two models was evenly distributed to three types of residences. Thirty nine UX-100-003 data logger models (9 rooftop residences, 15 basement residences, and 15 public rental apartments) and Seven U10 data logger models (3 rooftop residences, a basement residence, and 3 public rental apartments) were used in this study. Although the manufacturer did not provide accuracy in RH below 25%, actual measurements in winter were often below 25% (KMA, 2015). The data in such dry condition may not be accurate. It should be noted that many low income residences can be suffered from very dry conditions in winter.

This study could not evaluate effects of heating system, cooling system and age of residence to indoor temperature and humidity. In order to determine the effect of heating and cooling systems, energy use information is needed. Although presence of heating and cooling systems and age of residence were collected, people living in low-income residents did not want to provide information. Since they usually rented their residences, some of them did not know the exact age of residence. In future study, data of heating and cooling systems usage should be collected continuously.

Indoor temperature and relative humidity differ by type of residence in low-income residences in Seoul, Korea. In rooftop residences, the indoor temperature is highest during summer and lowest during winter, with a high level of variation. In the basement residences, the indoor humidity is the highest during summer. In the public rental apartment, the indoor relative humidity is the lowest during winter. The residents of low-income residences in this study were unable to control their indoor temperature and humidity during the summer and winter. Therefore, people living in low-income residences could be at risk from extreme hot and cold conditions. The results highlight the urgent need for improved management of indoor temperature and humidity in low-income residences throughout the year.