Extreme heat, health inequalities, and the net zero transition: future directions

Older adults

Older adults are consistently identified as one of the population groups most vulnerable to extreme heat. Ageing is associated with physiological changes that reduce thermoregulatory capacity, including diminished sweating responses, impaired thirst perception, and reduced cardiovascular efficiency (Núñez-Rodríguez et al., 2025). These changes can limit the body’s ability to dissipate heat and maintain thermal equilibrium during periods of high temperature.

Research has shown that the cardiovascular response to heat in older individuals involves a reduced ability to pump and redistribute blood to the skin, increasing strain on the heart (Hale, 1997). This can exacerbate existing cardiovascular or respiratory conditions and increase the risk of heat-related mortality (Kenney et al., 2014). In the United Kingdom, the growing proportion of older adults, particularly those living in care or nursing homes, represents a significant public health concern during heatwaves (Hajat et al., 2007). Evidence from previous heat events indicates that hospital admissions during heatwaves are disproportionately concentrated among individuals aged over 75 (Paterson and Godsmark, 2020). Furthermore, gender differences have been observed, with heat-related mortality risks found to be higher among women than men in older age groups (Hajat et al., 2007).

Individuals with chronic health conditions

Individuals living with chronic health conditions are also at increased risk during extreme heat events. Conditions such as hypertension, diabetes, cardiovascular disease, and respiratory illness can heighten physiological stress during heat exposure (Meade et al., 2023). Epidemiological studies have shown that mortality risks during heatwaves increase among individuals with cardiovascular and pulmonary disease, with reported increases in mortality of 2.48% and 1.61% respectively (Paterson and Godsmark, 2020). In addition, many medications used to treat chronic illnesses can interfere with thermoregulation, fluid balance and/or impair sweating, increasing the risk of dehydration (ibid). These physiological and pharmacological factors together increase susceptibility to heat stress.

Pregnant women and infants

Pregnant women and infants represent additional population groups with elevated vulnerability to extreme heat. Heat exposure during pregnancy has been associated with increased risks of preterm birth (Chersich et al., 2020), low birth weight, and a range of maternal health complications (Wyrwoll, 2023). These risks are likely to increase as extreme heat events become more frequent.

Infants are also particularly susceptible to heat-related illness due to immature thermoregulatory systems and limited capacity to regulate their own environmental conditions. Unlike adults, infants depend entirely on caregivers to maintain adequate hydration and manage their surrounding thermal environment (van de Kamp and Daanen, 2025). However, evidence on the specific environmental contexts in which infant heat vulnerability is greatest remains limited.

Socioeconomically disadvantaged households

Socioeconomic inequalities further shape vulnerability to extreme heat. Households experiencing energy poverty often lack the resources required to maintain safe indoor temperatures during periods of extreme heat. While fuel poverty has traditionally been associated with winter heating needs, there is increasing recognition that cooling requirements must also be considered within definitions of energy poverty in warming climates (Sanchez-Guevara et al., 2019).

Empirical research examining overheating risk in English dwellings indicates that certain housing types and tenure groups are particularly exposed. Analysis of more than 11,000 households suggests that privately or socially rented flats built between 1965 and the present, particularly those classified as non-decent homes and occupied by households in the lowest income quintile, are most likely to experience overheating (Zhang et al., 2025). These findings highlight the intersection between housing quality, socioeconomic disadvantage, and heat vulnerability.

Institutional and confined environments

Individuals living or working in institutional settings face additional risks due to limited control over their thermal environments. In such contexts, occupants often have restricted ability to modify indoor conditions, adapt behaviours, or access cooling strategies.

Prisons represent a particularly challenging environment. Security restrictions may limit access to natural ventilation, fans, thermostats, or other cooling measures. Prisoners typically spend extended periods in their cells—on average approximately 19 hours per day—which can result in prolonged exposure to elevated indoor temperatures (Dogbeh et al., 2015). Research suggests that prisoners may therefore have a lower upper thermal comfort limit than populations for whom conventional comfort standards were developed.

Similar constraints have been documented in care settings, where residents frequently have limited autonomy over their environment and depend on staff to manage thermal conditions (Brown and Walker, 2008). These structural limitations highlight the need for targeted interventions and building design strategies that prioritise heat resilience in institutional environments.

Homes

Private homes represent the primary environment in which heat exposure occurs. During the 2022 heatwave, deaths in private homes were 11.6% above average, compared with 9.2% in care homes and 5.8% in hospitals, highlighting the central role of residential environments in heat-related mortality (Office for National Statistics and UK Health Security Agency, 2022).

HEARTH will evaluate the interaction between extreme heat, mitigation measures, and adaptation strategies across different housing tenures and typologies, including private rented housing, owner-occupied dwellings, and social housing. Building types will range from detached homes to high-rise flats, while neighbourhood-scale analysis will consider urban heat island effects.

Overheating risks in housing are influenced by several factors, including poor ventilation, excessive solar gains, inadequate shading, and limited opportunities for behavioural adaptation. Behavioural adaptation also plays a role in shaping thermal exposure. Qualitative research by Tweed et al. (2015) demonstrated that older residents actively seek microclimates within their homes by adjusting blinds, moving between rooms, or altering daily routines. However, some vulnerable residents may also be unable to modify their indoor environments through ventilation or shading or relocate to cooler spaces during heat events (Gupta and Gregg, 2013). These findings highlight that thermal comfort depends not only on building performance but also on occupant agency and environmental control, suggesting that adaptation strategies should address both physical and behavioural dimensions of overheating.

Care settings

Care settings represent a particularly critical environment because they house some of the most physiologically vulnerable individuals in society. Residents of residential care homes, nursing homes, and extra-care housing are typically older adults with multimorbidity, reduced mobility, and increased sensitivity to heat stress.

HEARTH will assess how building design, operational practices, and adaptation measures influence overheating risk and health outcomes across these settings. The research will evaluate the impacts of extreme heat and the net zero transition on resident wellbeing, staff working conditions, and building performance.

Early evaluations of extra-care housing in England identified several environmental shortcomings. Barnes et al. (2011) found that overheating was particularly problematic for residents with limited mobility who were unable to move to cooler areas. Further operational challenges in managing indoor temperatures include conflicting thermal comfort preferences between staff and residents, limited access to environmental controls, and lack of awareness among staff regarding overheating risks or heatwave response procedures (Gupta et al., 2016; Gupta and Howard, 2020). Monitoring in these studies have also reported persistent overheating, with indoor temperatures frequently exceeding 30°C and night-time bedroom temperatures remaining above daytime levels.

Building design features can further exacerbate these risks. Limited cross-ventilation, sealed windows, and safety practices such as keeping bedroom doors closed overnight restrict nocturnal cooling. Thermal modelling suggests that by the 2080s, indoor temperatures during heatwaves in some care homes could reach 32–36°C, posing serious health risks for frail residents (Gupta et al., 2021).

Hospitals

Hospitals represent complex environments in which climate resilience must be balanced with strict clinical and operational requirements. HEARTH will investigate overheating risks across acute hospitals, community hospitals, and mental health facilities, examining how extreme heat interacts with patient vulnerability and building performance.

Hospitals face dual challenges during heat events. Patients may be highly susceptible to heat stress due to illness, age, or medical treatment, while healthcare facilities must maintain controlled indoor conditions for infection prevention and clinical safety. Retrofitting ageing hospital estates without compromising these requirements presents a significant challenge.

Evidence indicates that overheating risks are already widespread across the National Health Service estate. Approximately 90% of UK hospitals are considered at risk of overheating (Betts and Brown, 2021; Climate Change Committee, 2022). In 2021–2022, NHS England recorded 5,554 overheating incidents, representing a substantial increase over previous years (HM Government, 2022).

Extreme heat also affects healthcare delivery. In England, in-patient mortality rates have been shown to increase by 6–26% during heatwave periods (Thompson et al., 2022). In addition, operational disruptions have been reported, with nearly one in five UK doctors reporting cancellation of elective procedures during the 2022 heatwave due to overheating or staff shortages (GreenSurg Collaborative, 2023).

Prisons

Prisons represent another high-risk institutional environment due to a combination of structural constraints, high occupancy densities, and limited occupant autonomy. HEARTH will examine overheating risks across multiple prison typologies, including closed prisons, ageing Victorian-era facilities, and overcrowded wings. Thermal conditions in prisons are often shaped by restricted ventilation, security requirements, and limited opportunities for behavioural adaptation. Prisoners typically have little control over their thermal environment, including access to windows, fans, or temperature controls.

Prisoners are also more likely than the general population to experience chronic physical and mental health conditions that increase sensitivity to temperature extremes (Holt, 2015). Approximately 70% of prisoners meet diagnostic criteria for two or more mental health conditions (House of Commons Justice Committee, 2021). Many prisoners experience multiple overlapping vulnerabilities. A high prevalence of complex physical and mental health comorbidities and the medications used to treat these conditions put prisoners at heightened risk.

The ageing prison population further compounds this risk. In the United Kingdom, the number of prisoners aged over 50 has increased by more than 150% over the past two decades (Sturge, 2021). Older prisoners are more likely to experience chronic illness, reduced thermoregulatory capacity, mobility limitations and increased sensitivity to heat exposure, which may heighten vulnerability during extreme heat events. Women in prison may face additional vulnerabilities, including higher rates of chronic illness, trauma histories, and mental health conditions (Swavola et al., 2016). Extreme environmental conditions may therefore exacerbate both physical and psychological health risks in incarcerated populations.

Cross-setting synthesis

Across the four environments examined by HEARTH several common mechanisms influence heat exposure and vulnerability as shown in Figure 5. First, many occupants experience limited control over their thermal environment, whether due to physical frailty, institutional rules, or building design constraints. Second, these environments frequently combine high occupancy densities with limited ventilation, increasing internal heat gains. Third, many occupants exhibit elevated physiological vulnerability, including older age, chronic illness, or medication use that affects thermoregulation.

Figure 5

At the same time, each setting presents distinct adaptation challenges. Housing requires scalable retrofit strategies across diverse building types and tenures. Care homes must balance thermal comfort with resident safety and staffing constraints. Hospitals must maintain strict environmental control while ensuring patient safety. Prisons represent highly constrained environments where security and infrastructure limitations restrict adaptation options.

HEARTH addresses these challenges through an integrated research approach combining multi-scale climate modelling, empirical indoor environmental monitoring, behavioural studies, and health outcome analysis. This approach enables comparative analysis of overheating risk across building types and supports the development of evidence-based adaptation strategies aligned with the net zero transition.

Compound heat risk and climate futures

The programme first establishes the hazard context within which exposure and vulnerability occur. High-resolution weather and climate scenarios are developed to characterise extreme heat events and co-occurring hazards under different climate mitigation pathways, including net-zero futures. The analysis integrates downscaled regional climate projections, urban heat island modelling, socio-spatial risk mapping, and future energy demand modelling. Multi-scalar climate modelling across regional, urban, and neighbourhood scales enables the assessment of compound heat risks under both 2°C and 4°C warming scenarios, thereby defining the macro-climatic and systemic conditions shaping subsequent exposure and health impacts. Metrics for temperature extremes will be evaluated through stakeholder workshops.

Key outputs include new climate scenarios, datasets describing co-occurring climate extremes, and methodological advances for heat-health monitoring and early warning systems. The work also generates stakeholder-informed metrics for extreme heat and cold events and humidity indicators tailored to vulnerable indoor settings. Together, these outputs contribute to the development of a holistic indicator framework linking climate, health, and economic data to assess the trade-offs and unintended consequences of net-zero policies.

Outdoor environmental exposure

The programme then quantifies spatial variation in outdoor heat exposure from city to neighbourhood scales to understand interactions between climate processes and the built environment. Urban heat exposure is influenced by urban morphology, surface materials and albedo, building density, and the distribution of green and blue infrastructure. Microclimate modelling, remote sensing, and empirical environmental measurements are used to quantify variation in heat intensity, duration, and cumulative exposure across diverse urban typologies.

Outputs include meteorological datasets derived from urban environmental models for the five case-study cities; analysis of the cooling impacts of urban greening and building design interventions; nationally aggregated data from citizen weather stations addressing urban monitoring gaps; spatial maps of green infrastructure characteristics and associated health benefits; and integrated maps combining heat exposure with social and demographic vulnerability. These outputs provide the evidence base required for HEARTH decision-support tools and evaluation frameworks designed to assist national and local governments in prioritising equitable and effective climate adaptation strategies.

Indoor environmental exposure

Because individuals spend most of their time indoors, indoor environments represent the primary locus of heat-health risk. The programme therefore characterises indoor exposure pathways by empirically measuring physical and mental health impacts associated with indoor heat exposure among high-risk populations living or working in residential and institutional environments.

This analysis combines building energy simulation, dynamic thermal modelling of retrofit and new-build archetypes under future climate scenarios, continuous monitoring of temperature, humidity and carbon dioxide, and wearable exposure sensors. In addition, questionnaires and focus groups with occupants, and semi-structured interviews and workshops will be performed to capture behavioural analysis of occupant practices such as window opening, shading use and occupancy patterns, staff and operational practices and constraints. A mixed-methods approach, including quantitative and qualitative fieldwork questionnaires and interviews, to understand occupants’ thermal comfort experience and habits in their living environment has precedent (Attia, 2020; Gupta et al., 2016).

By linking regional climate projections with building-level modelling and empirical monitoring, the programme demonstrates how macro-scale warming translates into lived indoor heat exposure across homes, care homes, hospitals and prisons. Outputs include longitudinal datasets of indoor environmental conditions, physiological data capturing core body temperature, skin temperature and cardiovascular strain via non-invasive wearables, and qualitative insights from semi-structured interviews with residents, patients, and operational staff. These datasets establish a robust empirical evidence base on overheating risks and inform heat-stress prevention guidance for vulnerable groups.

Vulnerability and health burden

Environmental exposure data are subsequently integrated with health indicators to quantify differential vulnerability and heat-related health burden. Linked health datasets—including hospital admissions, primary care records, mental health indicators, and wellbeing surveys—are analysed to examine relationships between heat exposure and cardiovascular, respiratory, renal, metabolic, and mental health outcomes. Workshops will be held to identify theories of systemic and behaviour change used in policy/practice and evidence of effectiveness along multiple dimensions including cost and equity, identify enablers of and barriers to implementation, including evidence on implementation processes, public acceptability and cost-effectiveness.

Equity-sensitive analyses assess how heat risks are distributed across socioeconomic groups and vulnerable settings, enabling identification of populations and environments where intervention is most urgent. Outputs include health impact assessment frameworks for evaluating climate interventions, economic analyses incorporating Disability-Adjusted Life Years (DALYs) and NHS cost estimates, and integrated health-risk metrics capturing multi-sectoral policy impacts on specific population groups. These outputs support evidence-based recommendations for reducing health inequalities within climate adaptation policies.

Solutions and intervention evaluation

The programme evaluates both existing and future net-zero interventions to determine whether they reduce overheating risk while delivering health and carbon co-benefits. Interventions assessed include building fabric retrofit strategies, passive cooling and solar shading measures, ventilation optimisation, nature-based cooling solutions, and climate-resilient new-build standards. To inform the development of evidence-based recommendations for policymakers, estate managers and practitioners, stakeholder engagement and practice-based workshops will be undertaken throughout the programme. Each intervention is evaluated using a multidimensional framework incorporating reductions in overheating hours, implications for winter energy demand, carbon emissions performance, health co-benefits, distributional equity, and cost-effectiveness. Outputs include evidence on the effectiveness and limitations of current heat mitigation measures, simulation results for retrofit strategies under future climate scenarios, and a comprehensive database of intervention impacts on temperature exposure, thermal comfort, energy consumption, and greenhouse gas emissions. These findings inform guidelines for climate-resilient, low-carbon buildings and support evidence-based decision making by policymakers and practitioners.