According to recent studies, most people spend approximately 80%–90% of their time indoors (Al horr et al., 2016), so the quality of the interior environment is a topic that has drawn extensive scholarly attention. Recent research aimed at enhancing Indoor Environmental Quality (IEQ) is closely associated with the increasing demand for building rehabilitation across Europe and the imperative to decrease energy consumption while promoting sustainability and efficiency (Anderson et al., 2015). The confluence of these factors highlights the critical need for ongoing investigation into the optimization of indoor environments to meet contemporary ecological and health standards (Soares et al., 2017; Ortiz et al., 2017). However, most studies and research focus on commercial typologies and workspaces, leaving the IEQ in residential buildings understudied (Al horr et al., 2016; Carton et al., 2022). The Association for Media Research (AIMC) estimates that the time spent in houses is around 65% in Spain. This percentage increases to 85% for individuals over 65 and up to 95% for those over 85 (Hughes et al., 2019).

Previous research indicates that the quality of interior environments—including thermal, acoustic, visual, humidity, air quality, odors, and vibrations—significantly impacts the comfort and health of building occupants (Apte, 2000; Wolkoff, 2018; Wu et al., 2023a). Furthermore, the relationship between the occupants’ wellbeing and satisfaction and the interior environmental quality is based on complex parameters. This complexity presents challenges in measurement and can lead to discrepancies affected by contextual factors such as local climate, building typology, and the interior layout of the structures. Additionally, the interplay between occupant wellbeing and satisfaction and the interior environment’s quality is influenced by many parameters, which can be both diverse and complex. While these contextual elements may not be explicitly categorized as determinants of interior environmental quality, they directly influence user comfort, perceptions, and how individuals inhabit and utilize the spaces within buildings (Al horr et al., 2016; Carton et al., 2022).

The quality of the interior environment affects diverse population groups in different ways. Age-related variations are particularly notable, as seen in infants and elderly individuals (Putri et al., 2023). The World Health Organization also identifies other vulnerable groups including people with reduced mobility or disabilities, as well as pregnant women. These considerations highlight how interior environments interact uniquely with the specific needs and conditions of each group.

Allab et al. (2017), Dias Pereira et al. (2017), and Zuhaib et al. (2018) addressed IEQ issues related to building energy efficiency or performance, user needs, and design strategies (Šujanová et al., 2019). Usually, only one of the aspects or parameters defining the quality of the interior environment is analyzed (Ortiz et al., 2017; Bluyssen, 2010). Understanding these parameters is essential to designing comfortable, healthy, and inclusive buildings for all user profiles. In addition, according to the WHO, healthy buildings must adapt to the future and, therefore, be protected under the now more latent criteria of sustainability, efficiency, and low consumption and cost without damaging the surrounding environment (Šujanová et al., 2019; Bluyssen, 2010).

However, the behavior of the occupants inside these buildings sometimes conflicts with the strategies designed for low energy consumption. To effectively adapt to future requirements, people’s needs must be balanced with IEQ parameters (general comfort, energy efficiency, building design, and climate control), also called quality of life indicators (Bluyssen, 2010). Thus, a three-level connection is related, as Figure 1 shows (Šujanová et al., 2019).

Besides IEQ domains, psychological and physiological aspects related to the wellbeing of occupants influence the overall perception of comfort. All of these domains must be considered to achieve the right quality of the interior environment (Bluyssen, 2010). As relevant studies show, physiological and psychological domains, such as the intrinsic characteristics of each occupant, the age range (Zalejska-Jonsson and Wilhelmsson, 2013; Frontczak et al., 2012; Chiang et al., 2001), sociocultural conditions, and the type of space (Frontczak et al., 2012; Heinzerling et al., 2013) (19) are important when evaluating IEQ and its impact. However, defining these domains can be difficult, as specific criteria for different user profiles may not always be known or considered (Bluyssen, 2010). Recent studies highlight the importance of adopting multi-domains approaches, as comfort cannot be fully understood by isolating individual parameters and factors. Instead, the interactions between IEQ domains (thermal, visual, acoustic and air quality), combined with psychological and physiological dimensions, provide a more comprehensive framework for assessing occupant wellbeing (Schweiker et al., 2020).

It is essential to consider not only all factors, but also the capacity for adaptation to change, and above all, the time frame in which it occurs, as this will be decisive. Even if actions with less impact and lower energy consumption are implemented, occupants’ behaviour and their adaptation will be uncertain, influenced by actions such as (Šujanová et al., 2019; Coccolo et al., 2016; Gunay et al., 2013): -

behavioural adjustments (change in habits or tolerance limits).

These actions may occur consciously, in the first case; subconsciously, in the second case, or automatically by the peripheral nervous system, in the last case.

The number of occupants in a space is another aspect to consider when measuring the IEQ. A higher occupancy can lead to increased indoor air pollution. Additionally, the interior air quality can be affected by the construction materials used and the types of equipment present within the building (Al horr et al., 2016; Bakó-Biró et al., 2004). It is, therefore, essential to accurately assess both space occupation and interior pollutants. Numerous researchers studied various types of public buildings, such as offices (Kong et al., 2022), schools (Wu et al., 2023b; Tao et al., 2022), shopping centers (Deng et al., 2022), hotels (Xu et al., 2022a; Xu et al., 2022b) and nursing homes (Wang et al., 2022). These studies have used a variety of methods, including field measurements, simulations, behavioral observations, questionnaires, and interviews, as well as tools such as virtual reality (VR) and electroencephalography (EEG) (Wu et al., 2023a). However, the relationship between the objective aspects and the perception of the environment remains unclear and undefined. There is often a discrepancy between actual conditions and how user profiles respond to or evaluate their environment (Bluyssen, 2010).

The WHO confirms that approximately a quarter of the diseases around the world are due to modifiable environmental factors (Šujanová et al., 2019). Many studies have confirmed that the quality of the interior environment is correlated with diseases, such as cardiovascular, respiratory, and reproductive conditions, which are visible both in the short and long term (Al horr et al., 2016; Wu et al., 2023a; Bluyssen, 2010; Houtman et al., 2008; Fisk et al., 2007). Assessing diseases and discomfort among different populations, particularly vulnerable individuals, is crucial for understanding their relationship with IEQ domains (Bluyssen, 2010). Air pollution occurs due to factors such as the presence of mold, dust, mites, allergens, indoor aldehydes, volatile organic compounds (VOC), airborne fungi, pesticides, tobacco smoke, lighting, air exchange or circulation rates, carbon monoxide, carbon dioxide. These elements can lead to respiratory health issues (Al horr et al., 2016; Takigawa et al., 2009). Mechanical ventilation, as opposed to natural ventilation in buildings, may increase the incidence of diseases by 30%–200%, resulting in more hospital visits, particularly among women (Al horr et al., 2016; Preziosi et al., 2004).

Sick Building Syndrome (SBS) comprises a group of health issues caused by the indoor environment of a building (Al horr et al., 2016; Takigawa et al., 2009; De Dear and Brager, 2002). Various factors contribute to SBS, including temperature, humidity, chemical and biological contamination, and physical condition and psychosocial status of the occupants (Simonson et al., 2002; Wang et al., 2007; Wolkoff and Kjærgaard, 2007; Stolwijk, 1991). People experiencing SBS can suffer eye, nose, and throat irritation, headache, cough, wheezing, cognitive disorders, depression, sensitivity to light, gastrointestinal upset, fatigue, and similar to those of the flu (Šujanová et al., 2019; WHO, 1983; Mendell and Smith, 1990).

High exposure to poorly ventilated interior spaces or areas with airborne pollutants, such as s soil fungal concentrations due to high humidity (Redd, 2002) or dust in chairs, is often associated with allergies, irritation of lung functions, asthma, and pneumonitis (Al horr et al., 2016; Redd, 2002; Fisk et al., 2007). These health issues, along with poisoning, are closely linked to indoor air quality, which is one of the leading contributors to health-related deaths (Šujanová et al., 2019; Franchi et al., 2006; WHO, 2017). The severity of these diseases depends on the intensity, duration, and source of harmful exposure (Šujanová et al., 2019). Prolonged exposure to poor Indoor Air Quality (IAQ) can lead to serious illnesses like legionellosis and CO poisoning. Although symptoms like asthma, cough, and pulmonary infiltration may appear after brief exposure, they can become chronic and acute respiratory diseases. Many of these conditions can be avoided with proper indoor air quality management (Franchi et al., 2006).

Beyond air quality, other environmental domains also have measurable health impacts. High noise levels can cause cardiovascular risks and hypertension (Šujanová et al., 2019). In addition, heart rate variability has been related to indoor air temperature and may be a predictor of mortality (Bluyssen, 2010). Thermal comfort, air quality, and noise collectively influence not only health but also concentration, and motivation (Šujanová et al., 2019; Kallio et al., 2020). For example, increased temperatures can elevate the concentration of harmful substances in the air from the devices, relating temperature dependence to indoor air quality or ventilation (Šujanová et al., 2019). On the other hand, poor ventilation or fungal concentration can also cause fatigue (Šujanová et al., 2019; Redd, 2002). High humidity worsens the quality of sleep (Wolkoff, 2018).

Lighting conditions are equally critical. Inadequate lighting disrupts sleep quality and circadian rhythms, causing discomfort throughout the day (Šujanová et al., 2019; Chang and Chen, 2005). The effects of poor lighting can vary based on gender, age, and the time of year (Serghides et al., 2015). Similarly, poor acoustic environments are associated with insomnia and alter sleep patterns (Šujanová et al., 2019), particularly among the elderly (Putri et al., 2023). High noise levels, their frequency, and sound pressure differences cause discomfort in people (Landström et al., 1995). A deficit in the optimization of indoor acoustics not only affects cognitive decline in hearing but also causes distractions, irritability, stress, discomfort, and fatigue (Al horr et al., 2016); Šujanová et al., 2019; Passero and Zannin, 2012). It can even affect the development of learning disorders, such as dyslexia or voice problems (Bottalico and Astolfi, 2012).

This systematic review aims to identify key factors affecting building environmental quality in residential buildings and their impact on different vulnerability groups. The study also examines how sensory perception, cognitive function, and building design and operation interact with building environmental quality in vulnerable people.

This review addresses vulnerable groups from an age-related perspective, considering the elderly and children as classifications recognised by the World Health Organisation. However, other vulnerable groups, such as pregnant women and people with disabilities, are also mentioned. This classification is not addressed from a socio-economic perspective or based on housing characteristics, which could be another way of classifying vulnerable groups.

This literature review aimed to document the key research and theories linking IEQ to design, focusing on the most vulnerable occupants in residential building typologies.

Thermal comfort directly impacts buildings’ energy consumption. Over 60% of a building’s energy consumption is used for heating and cooling a space (Šujanová et al., 2019). Even a slight feeling of discomfort can lead users to adjust the controls to suboptimal levels (Al horr et al., 2016; Catalina and Iordache, 2012; Corgnati et al., 2009).

Indoor air quality (IAQ) impacts the health of the building occupants in the short and long term (Simonson et al., 2002; Wargocki et al., 2002). Addressing IAQ comprises three critical factors: external weather conditions, air renewal rates, which directly affect the consumption of HVAC systems (Mancini et al., 2020) and a comprehensive assessment of various air quality parameters. The complexity of pollutant selection goes beyond simple measurements of air temperature and relative humidity, which are typically reported. The wide range of pollutants found in indoor environments complicates the identification of any single pollutant as a sole indicator of IAQ (Ortiz et al., 2017; Wong et al., 2016).

The scientific literature shows that the most prevalent indoor air pollutants include volatile organic compounds (VOC), detected in 84% of cases; carbon dioxide (CO₂), present in 65%; asbestos in 45%; particulate matter (PM10 or PM2.5) in 16% (Karami et al., 2018). Other studies consider the former factors and add environmental parameters such as temperature (T) and relative humidity (RH) (Vilčeková et al., 2017), highlighting the close relationship between environmental conditions and thermal comfort (Šujanová et al., 2019). In addition to the pollutants already mentioned, several other airborne particles adversely affect occupant health. Tobacco smoke, mites, pet allergens, cockroaches, mold, pollen, nitrogen oxide, formaldehyde, radon, and mineral fibers must be mitigated by adequate ventilation and air quality management strategies (Franchi et al., 2006).

These parameters are classified into intervals by classes, depending on the different contaminants obtained from the EN 13779: 2007 standard and their relationship to temperature and relative humidity. Table 2 illustrates the relationship of these parameters.

An acceptable concentration of CO₂ indoors is usually up to 1,000 ppm (Lai et al., 2009). However, the concentration levels of pollutants usually differ in summer and winter and in different rooms (Zhang et al., 2022). Under external weather conditions, the interior air relative humidity usually goes unnoticed. Not only does relative humidity affect thermal comfort, but it also affects IAQ and the occupants’ health (Simonson et al., 2002). Previous research states that high relative humidity implies a higher formaldehyde concentration. Some material finishes of walls and floors can also produce volatile organic compounds (VOCs) emissions, contributing to unpleasant odors and the perception of stale air (Wolkoff and Kjærgaard, 2007). It is also important to consider air filtration systems, heat gains, airflow patterns, and pressure ratios (Šujanová et al., 2019). The tightness of the building and the use of synthetic construction or furniture materials also impact air quality (Lai et al., 2009). Occupants’ habits and ventilation methods can also be relevant in maintaining good IAQ. Poor ventilation is a significant issue that typically leads to unhealthy indoor air conditions (Zhang et al., 2022).

Recommended values for IAQ can be found in standard EN 16798-3 for non-residential buildings, ASHRAE 62.1 and 62.2, and CR 1752. These standards outline ventilation requirements and offer guidance for designing buildings with adaptable 1AQ. IAQ control can be achieved through natural ventilation or mechanical systems for air renewal (Šujanová et al., 2019).

Good visual comfort is beneficial for users’ wellbeing, who also achieve greater comfort (Serghides et al., 2015; Leech et al., 2002; Veitch, 2001). To ensure this wellbeing through visual comfort, it is crucial to consider the lighting conditions and views of the interior space. These aspects are likely to cause a significant therapeutic impact (Al horr et al., 2016; Aries, 2005; Aries et al., 2010).

To maintain good quality of the indoor environment, it is essential to protect users from noise and provide a comfortable acoustic environment. Indoor spaces must be protected from outdoor noise and noise generated by neighboring equipment or facilities (Al horr et al., 2016). Sound perception depends not only on the intensity of the sound and its temporal and spectral characteristics but also on the individual’s activities, psychological state, and a variety of other contextual factors (Šujanová et al., 2019).

Although there is still a limited understanding of human perception’s neurological and biological mechanisms, it is necessary and beneficial to understand how humans perceive and what effects it brings to the environment. Researchers must address psychological and sociological methodologies (Wu et al., 2023a). Indoor natural light landscapes can influence physiological indicators (Kong et al., 2022), while colors can affect emotional indicators (Tao et al., 2022). These findings provide supporting data for future developments in the therapeutic effects of indoor environments on health (Wu et al., 2023a). In addition, natural or biophilic elements, vegetation, and even artificial greenery are incorporated into buildings to improve mental health, help stress, and achieve cognitive recovery and mental fatigue. Being surrounded by plants causes positive stimulation that favors the recovery of diseases (Chang and Chen, 2005).

When analysing the four domains of environmental comfort (Tables 1, 3, 4, 5)—thermal, visual, acoustic and indoor air quality—together, it is evident that, although there is a solid base of studies addressing metrics, standards and effects on health and wellbeing, research remains fragmented and with significant gaps. Each domain focuses on a limited set of technical variables, leaving integration with the psychological, social and behavioural aspects that are essential to understanding the actual experience of occupants in the background. Furthermore, the diversity of contexts and populations is underrepresented: studies favour offices, schools or institutional environments, while vulnerable groups—such as children, older adults, or people with disabilities—receive little attention.

Even in Table 6 (Critical synthesis across studies in IEQ), research lacks of consensus and standardisation in metrics and indices across studies. This limits comparisons between building types, cultural contexts and population groups:

Lack of standardized metrics and methodologies, along with limited international comparability, restricts the application of results of practical design and regulatory criteria.

The World Health Organization (WHO) defined healthy buildings in 1990. In many investigations, wellbeing is related to the design of buildings and quality standards. However, to achieve and seek objectives to improve the interior environment’s quality, it is important to formulate pertinent inquiries that facilitate effective design strategies to promote wellbeing and accommodate forthcoming factors (Bluyssen, 2010; Altomonte et al., 2020). Attributes of the building, such as the construction materials, the climatic conditions, or those related to users (gender and age, needs and comfort) (95) must be considered at early design stages since they affect the quality of the interior environment (Frontczak et al., 2012). Additionally, economic factors—about financing and the final valuation—coupled with environmental considerations, such as resource utilization, sustainable management practices, and life cycle assessments, must also be incorporated (Bluyssen, 2010). These behavioral aspects remain absent in building certifications such as LEED, BREEAM, or CASBEE. The challenges in defining a healthy building may be due to the conflicting interests between sustainability and the quality of life of the occupants, among others (Bluyssen, 2010; Hua et al., 2014; Schiavon and Altomonte, 2014). This review observes a scarcity of literature that connects IEQ with energy efficiency (Manfren et al., 2019; Lee and Guerin, 2010).

Renovation projects provide clear opportunities to improve building efficiency, including upgrades such as window replacement, roof and wall insulation, renewable energy integration, and the installation of adapted heating or indoor humidity control systems for enhanced thermal comfort (De Santoli et al., 2018; Mikučioniene et al., 2014). In contrast, in new buildings, there must also be a physical adaptation of the environment at the early stages of the design, avoiding the inefficiency and cost associated with renovations (Jazizadeh et al., 2014; Indraganti et al., 2014; Lozinsky et al., 2025). In this adaptation, it is important to consider the climate and the influence of culture (Lovins, 1992). For instance, mechanical cooling is necessary in the Middle East to maintain an optimal level of comfort for occupants (Nicol et al., 2008), whereas, in tropical climates, natural ventilation consumes significantly less energy and provides users with a closer connection to nature (Fisher, 2000).

Design strategies to achieve thermal comfort have evolved to adapt to environmental variability (Table 8). These strategies include operable windows, blinds devices, or automated controls that change their settings in response to changing weather conditions. Among sustainable design strategies, natural ventilation and other passive climate approaches have proven to be the most effective (Deuble and de Dear, 2012). Other factors determining the occupant’s final perception are behavior, physiological adaptation, and psychological expectation (Bluyssen, 2010; Nikolopoulou and Steemers, 2003). As a reference for the design, the ASHRAE 55 and the ISO 7730 standards (1994) define thermal comfort worldwide.

IAQ is a more complex problem because it depends on external weather conditions and air renewal rates, which directly affect the consumption of HVAC systems (Mancini et al., 2020). Energy-saving strategies must include building design, the implementation of demand-controlled ventilation (DCV) (Merema et al., 2018), or building automation and control systems (BACs). These systems optimize HVAC operation by regulating airflow rates and employing diverse ventilation methodologies (Chenari et al., 2016; Ben-David and Waring, 2016; Mancini et al., 2019; Aste et al., 2017). At the time of day, intelligent or Smart control systems control airflow, temperature, and humidity, among others. Another way to control humidity is by selecting suitable building materials capable of moisture retention, such as wood, which is an alternative approach to humidity control (Simonson et al., 2002).

In building design, the most effective strategies for improving IAQ involve either increasing the ventilation rate, reducing air pollutants (Daisey et al., 2003), or minimizing pollution sources inside and outside the building (Wargocki, 1999). Another passive strategy is the use of natural ventilation (Al horr et al., 2016; Chenari et al., 2016), which can significantly lower cooling energy costs and reduce instances of Sick Building Syndrome (SBS) among occupants (Borgeson and Brager, 2011; Seppänen and Fisk, 2002).

Nevertheless, natural ventilation is not always adequate if there is a significant temperature difference between the indoors and outdoors, which limits its effectiveness in removing indoor pollutants (Zhang et al., 2022). Other passive ventilation systems include solar chimneys on the facade or roof, Trombe walls, and pipes with geothermal air, which are based on solar energy and the residual heat of buildings, increasing the ventilation rate and air temperature. However, those strategies are ineffective in eliminating indoor pollutants and depend on an unstable value of variable solar radiation (Zhang et al., 2022). A more reliable solution may be mechanical ventilation. This solution controls the airflow and starts and stops ventilation to control the pollutants. Additionally, underground ducts can help preheat incoming cold air using the ground’s temperature, improving air quality and enhancing the overall benefits of the ventilation system (Bluyssen, 2010; Wang et al., 2016).

It is essential to ensure visual comfort inside buildings, as it affects the wellbeing of users throughout the day. Some researchers estimate that 30% of energy consumption corresponds to building lighting (Budhiyanto and Chiou, 2022). Decisions such as the geometry and location of windows, surface photometry, the amount of glass, the dense distribution of space, the use of colors, and shading devices are critical for achieving high-quality interior lighting (Al horr et al., 2016; Serghides et al., 2015). By focusing on these elements, building designers can avoid some of the negative impacts associated with poor lighting, such as compromised sleep quality, vision loss, and glare (Chang and Chen, 2005; Serghides et al., 2015; Veitch, 2001). Natural light is considered an essential factor to consider when designing buildings (Optimizing et al., 2022). It not only enhances aesthetic appeal but also reduces the energy demand for electric lighting (Wu et al., 2022b). In renovations, especially in protected facades, the entry of natural light through ceiling openings must be incorporated whenever possible (Wu et al., 2022a). Light control sensors are a system that is implemented in building design, which achieves optimal results and reduces consumption by up to 60% (Jia et al., 2023).

An effective interior design must balance visual comfort with acoustic design without neglecting key acoustic control methods in the form of physical barriers, which often occur in open spaces (98). The geometry of a room plays a significant role. A square area will provide greater acoustic comfort than a long, narrow one, creating a “bowling alley” effect and causing the sound to bounce between the walls (Acoustics in Educational Settings, 2005). ASHRAE Standard 50 recommends minimizing hard surfaces, as these can reduce sound absorption and increase interior noise levels (Anderson, 2008). Therefore, traditional methods include using sound-absorbing materials such as textiles on ceilings, walls, and floors to enhance the acoustic environment.

The selection of appropriate building materials is essential for ensuring good IEQ (Takigawa et al., 2009). The selected materials can significantly impact a space’s thermal, visual, and acoustic comfort (Al horr et al., 2016).

Acoustics is an essential domain to avoid problems in the health and comfort of users (Andersen et al., 2009; Anderson, 2008). A comprehensive assessment of acoustic performance must encompass internal and external noise factors and consider anticipated occupancy patterns within the rooms (Bluyssen et al., 2011a; Bluyssen et al., 2011b). Different strategies can be adopted depending on the source of the noise. If the noise comes from outside, using absorbent materials and acoustic insulation in the ceiling, façade, and windows is advisable. Incorporating absorbent materials and acoustic insulation in the ceiling, façades, and window assemblies is recommended for external noise sources (Qu et al., 2022). If the noise comes from the interior space, physical barriers such as panels in the distribution or electronic sound masking techniques will be used (Loewen and Suedfeld, 1992). The volume, spatial configuration, and the selection of materials or barriers are decisive in achieving acoustic comfort (Šujanová et al., 2019). Despite an increasing emphasis on acoustic comfort factors, the criteria for efficient certifications are often not mandatory in this area, and their indications and guidelines are often incomplete (Schiavon and Altomonte, 2014).

A building design process that integrates social, environmental, and economic factors into the building can lead to enhanced energy performance and improve IEQ (Steemers and Manchanda, 2010; Kua and Lee, 2002; Iwaro and Mwasha, 2013). However, a conflict between these buildings’ performance and users’ wellbeing remains. A lower ventilation rate means high energy efficiency but also increases the concentration of suspended particles indoors (Lai et al., 2009; Koponen et al., 2001). Moreover, optimizing ventilation rates to prioritize occupants’ comfort might increase acoustic issues due to airflow background noise (Deuble and de Dear, 2012). Decisions regarding ventilation and choices about materials, lighting levels, and other factors ultimately affect occupant wellbeing. Unfortunately, it is a general practice for building designers to promote energy efficiency over the wellbeing of the user (Lai et al., 2009; Koponen et al., 2001; Liang et al., 2014). This practice often leads to pursuing certifications for sustainable design, such as LEED, BREEAM, and GSAS, at the expense of the user’s experience (Elsarrag and Alhorr, 2012).

The use of these certifications generates a contemporary metric depending on the energy efficiency of the building, thanks to simulation tools of the proposed design (WBDG Home), which prioritizes low energy consumption (Šujanová et al., 2019). Usually, one-third of the buildings with these certifications consume more energy than those that do not have it because they typically fail to account for the behavioral patterns of the building’s occupants (Liang et al., 2014; Liang et al., 2019). The introduction of smart controls could lead to a reduction in energy consumption by as much as 10% (Costa et al., 2013; Dean et al., 2016; Anda and Temmen, 2014), compared with the percentage of consumption in the sector in developed countries (Šujanová et al., 2019; Liang et al., 2014). The implementation of renewable energy could favor these buildings since results yielded that 15% of the energy demand is consumed in residential buildings in the European Union (Šujanová et al., 2019).

A building can only be deemed genuinely energy efficient when it neither induces nor exacerbates health issues among its occupants, concurrently ensuring their comfort while minimizing energy expenditures to achieve optimal conditions, as outlined by the Health Optimization Protocol for Energy Efficient Buildings (HOPE) (Cox, 2025).

According to HOPE (Health Optimization Protocol for Energy Efficient Buildings), a building can only be deemed energy efficient when it does not cause or aggravate health issues among its occupants, guarantees comfort, and minimizes the use of energy to achieve the desired conditions (Cox, 2025). The building must be used consciously, without exceeding the occupation, and using efficient technology for its maintenance (Costa et al., 2013; Liang et al., 2019). The IEQ must be evaluated throughout the building’s life cycle, with sustainable strategies at the early design stages and appropriate maintenance of this building until the disposal of materials at the end of its cycle (Al horr et al., 2016). This evaluation must focus on the occupants, considering various user profiles influencing needs based on gender, age, and vulnerabilities. In addition, the quality of the interior environment has been related not only to the wellbeing of these users but also to its impact on health and diseases in the short and long term, being relevant to the development of adequate actions to reduce or eliminate harmful effects on health (Srinivasan et al., 2003).

Despite the advances summarised in this review, it is important to recognise the limitations that make it difficult to draw definitive conclusions about the relationship between IEQ and the health of vulnerable populations. Firstly, the observed associations and relationships may be influenced by uncontrolled confounding factors with a significant impact on indoor environments, which are rarely considered in studies. These factors include external variables, such as outdoor air quality, the presence of urban vegetation or proximity to sources of pollution; internal variables related to the use of everyday items, such as air fresheners or recent renovations; and other factors, such as exposure to electromagnetic fields generated by devices both indoors and in the immediate outdoor vicinity. Secondly, the heterogeneity in the classification and definition of vulnerable groups, as well as in measurement methods, makes it difficult to compare results. Future research must consider these factors in order to reduce bias, improve the validity of findings and establish a more accurate framework.

It is important to bear in mind the heterogeneity that exists between the different types of groups considered vulnerable, as well as within each of them. Table 8 contrasts how different vulnerable groups (children, elderly people, pregnant women, and people with disabilities) are affected by the main IEQ domains (thermal, air quality, lighting, acoustics). While there is a significant impact at different stages of life, children are affected in their cognitive development, while older people suffer from chronic diseases. All groups are highly sensitive to poor air quality.

In the case of older people, for example, age is not the only parameter to consider; other parameters must also be taken into account, such as lifestyle, home characteristics, gender, and physiological characteristics such as mobility and chronic diseases (Beard et al., 2016). This highlights the need for an IEQ index model adapted to this heterogeneity, incorporating weights for both variability and environmental domains. While some studies (Laskari et al., 2016; Tiele et al., 2018) have begun to propose such indices and interpret the related weights, further research is needed.

The elderly population is significantly vulnerable due to limited mobility and the high prevalence of chronic diseases. However, there is insufficient scientific literature connecting IEQ with its effects on the physical health of older adults. Building designers need to tackle variables affecting indoor and outdoor environments, as these domains significantly impact the health of vulnerable adults. For instance, studies have shown the importance of bedrooms, indoor gardens, green roofs, and patios, but evidence in this area is limited. Besides involving vulnerable adults in creating inclusive buildings, additional studies need to establish assessment criteria and monitoring standards that can lead to well-rounded design recommendations for housing where vulnerable adults reside. Empirical research on social and economic impacts will help facilitate the sustainable development of aging societies and create more inclusive and healthy designs.

Beyond the energy credentials granted to green and sustainable buildings, specific considerations are required, often insufficient to guarantee user comfort and wellbeing. This document presents a review of current knowledge on user wellbeing and comfort related to IEQ. To address existing research gaps and challenges, a holistic framework for building environment design guidelines should be developed, in addition to future lines of research. These guidelines should provide recommendations for building designers and urban planners that are applicable across various contexts rather than confined to specific locations.

Building designs for vulnerable people should prioritize spatial distribution, focusing on places such as bedrooms, dining areas, and other rooms frequented by occupants, ensuring thermal comfort, adequate ventilation, and natural lighting. Future research should prioritize establishing the quantitative relationship between IEQ, comfort conditions, and occupant health, as this remains unclear.

Quantifying the spatial distribution of green spaces in relation to air pollution and extreme heat is imperative, as they act as comfort modulators. In addition, it is a priority to optimise ventilation and air quality through hybrid systems that combine natural and mechanical ventilation to ensure adequate levels of air renewal in spaces that are used for long periods of time. Similarly, the thermal design of buildings should be based on passive strategies, such as insulation, cross ventilation and solar protection, complemented by low-energy technologies. Specific comfort ranges must be defined that take into account the greater sensitivity of certain groups. Lighting is another fundamental aspect: natural light should be encouraged in frequently used rooms and supplemented with adjustable artificial lighting that respects circadian rhythms and reduces glare, thereby contributing to the visual health and rest of the occupants. At the same time, noise exposure must be reduced through more stringent acoustic standards, the use of insulating materials, and landscaping solutions that act as sound barriers.

Environmental monitoring and modelling methods can be used to verify and improve the building environment. This can be achieved by installing low-cost, accessible sensors that allow residents to monitor the environmental quality of their homes in real time.