Indoor overheating: A review of vulnerabilities, causes, and strategies to prevent adverse human health outcomes during extreme heat events

Abstract

The likelihood of exposure to overheated indoor environments is increasing as climate change is exacerbating the frequency and severity of hot weather and extreme heat events (EHE). Consequently, vulnerable populations will face serious health risks from indoor overheating. While the relationship between EHE and human health has been assessed in relation to outdoor temperature, indoor temperature patterns can vary markedly from those measured outside. This is because the built environment and building characteristics can act as an important modifier of indoor temperatures. In this narrative review, we examine the physiological and behavioral determinants that influence a person's susceptibility to indoor overheating. Further, we explore how the built environment, neighborhood-level factors, and building characteristics can impact exposure to excess heat and we overview how strategies to mitigate building overheating can help reduce heat-related mortality in heat-vulnerable occupants. Finally, we discuss the effectiveness of commonly recommended personal cooling strategies that aim to mitigate dangerous increases in physiological strain during exposure to high indoor temperatures during hot weather or an EHE. As global temperatures continue to rise, the need for a research agenda specifically directed at reducing the likelihood and impact of indoor overheating on human health is paramount. This includes conducting EHE simulation studies to support the development of consensus-based heat mitigation solutions and public health messaging that provides equitable protection to heat-vulnerable people exposed to high indoor temperatures.

Keywords: Extreme heat events; built environment; climate change; dwellings; heat balance; heat stress; thermoregulation.

Figure 1.

Summary overview of the complex interplay of factors discussed in this review that mediate indoor overheating risk, the adverse health impacts caused by exposure to hot indoor environments and the need for heat protection strategies to protect heat-vulnerable occupants when indoor temperature reach dangerous levels during hot weather or extreme heat events.

Figure 2.

Overview of intra- and inter-individual factors that can modulate the body’s physiological capacity to dissipate during a heat stress such as during a daylong exposure to an overheated home in an extreme heat event.

Figure 3.

Schematic depicting the change in physiological strain and mood state in an older adult exposed to indoor overheating during hot weather or an extreme heat event. Data represents physiological responses measured in older adults (n = 39; 61–78 years; 11 females) during a daylong (9-hour) exposure to indoor overheating (40°C) [77,201]. Tco, core temperature; bpm, beats per minute.

Figure 4.

Determinants of the urban thermal environment. Factors such as high building density (panel A), lack of green space (panel B), paved surfaces (panel C) and others can worsen (upward pointing arrow, bottom right of each panel) the urban heat island effect and indoor overheating. In contrast, the addition of green infrastructure (panel E), parks and bodies of water (panel F), use of high-reflective materials (panel G), increased vegetation and green space around dwellings (panel H) and others can lessen the impact (downward pointing arrow), lowering outdoor temperature and reducing the risk of indoor overheating.

Figure 5.

Dwellings without an air conditioner system or a heat pump system can overheat during hot weather or an extreme heat event placing the occupant(s) at risk of a heat-related injury or death. A recent report showed that maintaining indoor temperature at or below 26°C will ensure that occupants are protected (see ref [297]).

Figure 6.

Schematic depicting structural and design characteristics of the home that can reduce heat gain and heat retention. Modifying the home with operable energy efficient windows, employing different forms of shading (e.g. window blinds, window awnings, planting trees around the home), utilizing appropriate insulation and attic ventilation techniques can promote greater cooling of the home helping to achieve safe indoor temperature conditions (i.e. 26°C, see ref [297]).

Figure 7.

While the use of home-based strategies for staying cool such as the use of limb immersion has been widely advocated by public health agencies, recent investigations demonstrate they are not effective for limiting increases in core temperature during indoor overheating. Data represents physiological responses measured in older adults (n = 17; 69–74 years; 8 females) during 6-hour exposure to indoor overheating (38°C) with (left panel) and without (right panel) of the feet to mid-calf in 20°C water for the last 40 minutes of each hour [407]. Tco, core temperature; HR, heart rate.

Figure 8.

Accessing cooling centers for a brief period (1–3 hours) can provide protection for people who are vulnerable to heat-related illnesses. While it has positive effects on physiological and mood-state, these responses are transient. Continued caution and use of appropriate countermeasures to mitigate indoor overheating should be employed upon reentry to the heat. Data represents physiological responses (see ref. [438]) and mood-state (see ref. [439]) measured in older adults (64–79 years) underwent a 9-hour simulated heat wave (40°C) with (cooling group, n = 20, 9 females) or without (control group, n = 20, 8 females) a cooling intervention consisting of 2-hour rest in an air-conditioned room (˜23°C, hours 5–6). Tco, core temperature; HR, heart rate; bpm, beats per min.