Determinants of carbon dioxide exposure for residents and staff of nursing homes: a field monitoring study in Spain

Research Article Determinants of Carbon Dioxide Exposure for Residents and Staff of Nursing Homes: A Field Monitoring Study in Spain Alba Fuertes , 1 Rory V. Jones , 1 María Teresa Baquero , 2 Roger Vergés , 2 and Nuria Forcada 2 1 School of the Built Environment, University of Reading, Reading, UK 2 Group of Construction Research and Innovation (GRIC), Universitat Politècnica de Catalunya (UPC), Terrassa, Spain Correspondence should be addressed to Alba Fuertes; [email protected] Received 23 September 2024; Accepted 24 March 2025 Academic Editor: Faming Wang Copyright © 2025 Alba Fuertes et al. Indoor Air published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Providing high air quality is crucial to the health and well-being of older people living in nursing homes. A measurement study was undertaken in 22 communal rooms of five nursing homes in Spain to investigate the effects of heating, ventilation, and air-conditioning (HVAC) systems, room characteristics, and occupant activities on the indoor air quality. The study included 196 periods of data collection (equivalent to 5282 measurements). To the authors’knowledge, this is the first study of indoor air quality in Spanish nursing homes. The study found that mean CO 2 concentrations were consistently below established standards, although notable peaks were evident due to specific activities. Natural and cross ventilation had a clear role in maintaining CO 2 concentrations below recommended levels. The findings indicate that lower occupancy density may be required in rooms where high-CO 2 -generating activities take place, such as gym–physiotherapy rooms. The results showed that the older residents and staffwere both more thermally comfortable at higher CO 2 concentrations. This suggests that striking a balance between air quality and thermal comfort is necessary. The study provides useful insights for the design of ventilation systems and spatial layouts of nursing homes, which can achieve higher levels of indoor air quality and occupant well-being. Keywords: carbon dioxide; heating; indoor air quality; nursing homes; older people; ventilation and air-conditioning 1. Introduction Built spaces, both indoors and outdoors, must be adapted to the needs of the older population. Research in this field is rising as the number of people over 65 years worldwide is projected to triple by 2037, and in Spain specifically, they will constitute 27% of the population [1]. Consequently, demand for assisted living and nursing home facilities will grow rapidly, and understanding how such spaces should be designed and operated for the benefit of their primary users is crucial. Older people spend almost 90% of their time indoors [2–4], and therefore, indoor environmental conditions have greater effects on their health and well-being [5–8]. Asthma, respiratory infections, cardiovascular sickness, chronic obstructive pulmonary disease, cancer, nausea, upper respiratory difficulties, and cognitive impairment are just some of the health conditions that have been linked to poor indoor air quality (IAQ) [9–11]. Low ventilation rates and crowded indoor environments have also been shown to increase levels of bacteria, gases, particulate matter, and viruses which can lead to negative health outcomes for the older people [12]. Furthermore, indoor environmental factors, including carbon dioxide (CO 2 ) levels, have been associated with behavioural issues like wandering, aggression, and agitation in older people with dementia which negatively impacts their quality of life [13, 14]. Previous research, such as by Reddy et al. [3], argued that in addition to introducing enforceable IAQ standards, nursing homes should be regularly monitored to proactively identify and address weaknesses and that resources should be made available for remedial measures to improve IAQ, Wiley Indoor Air Volume 2025, Article ID 4153784, 17 pages https://doi.org/10.1155/ina/4153784 such as source control, improved ventilation and filtration, and the use of air cleaners. The authors propose that this would result in improved health, well-being, and quality of life for the older people and as a result reduce medical costs. Many previous studies have undertaken measurements of CO 2 concentration and assessed its effects on humans [15–18]; however, field studies specifically addressing nursing homes are lacking in the literature compared to other building types. Related to the current paper, Guerra Santin et al. [19] studied a range of spaces in a Passivhaus care home for people with dementia in England and found that the percentage of time that CO 2 concentrations exceeded 900 ppm was highest in winter, and the researchers also raised concerns about the positioning of the mechanical ventilation (MV) in bedrooms to maintain good IAQ. Baudet et al. [20] reported a mean CO 2 concentration of 572 1± 106 0 ppm measured over 4.5 days in four elderly care facilities in France, although peaks in excess of 1000 ppm were also observed. Mendes et al. [7] studied six urban care centres for older people in Portugal and found that CO 2 concentrations exceeded an international recommended level of 1080 mg/m 3 (600 ppm CO 2 ) in winter but fell below this level in summer. The authors concluded that low ventilation rates and crowded indoor conditions were associated with the higher concentrations observed. Gupta et al. [21] measured CO 2 levels in two lounges in two different Londonbased care homes and found mean concentrations of 1027 ppm in one and below 500ppm in the other. The variation was attributed to a larger floor area, lower occupancy density, and greater ventilation. Although not specifically studies of nursing homes, Chen et al. [11] investigated the indoor environment of rural older people’s houses in several climatic zones of China and discovered high CO 2 levels at night, which were associated with decreased ventilation rates due to the closure of doors (Ds) and windows (Ws). Also, Serrano-Jiménez et al. [22] investigated naturally ventilated social housing occupied by older adults during different seasons in Spain. High levels of CO 2 were particularly evident in winter, and this was attributed to lower ventilation rates (i.e., fresh air infiltration and occupant ventilation behaviour, such as opening Ws) during the winter season. These previous studies have shown that CO 2 concentrations in buildings occupied by older people sometimes exceed internationally recommended levels, and this may result in negative health and well-being consequences for this population. The current study adds to this limited but growing body of literature by reporting on, to the authors’ best knowledge, the first study of CO 2 concentrations in nursing homes undertaken in Spain. The aims of the current paper were (i) to measure the concentrations of CO 2 in a range of different space types (living, dining, occupational therapy, and gym–physiotherapy rooms) in five Spanish nursing homes and compare these with recommended values from national and international standards; (ii) to assess the effects of heating, ventilation, and air-conditioning (HVAC) regimes, room characteristics, and occupant activities on the levels of CO 2 observed; and (iii) to discuss the implications of the findings for improving air quality in nursing homes. 2. Methodology The study was carried out in five nursing homes located in Madrid, Spain, between April and August 2021 (warm months) and between October 2021 and February 2022 (cold months). The CO 2 measurements were taken in 22 communal rooms used in the five nursing homes by the building residents (older people) and the support staff(caregivers and therapists) during the day while doing their typical activities. These include the living room, where the nursing home residents spend time watching TV and doing other quiet activities such as playing cards or reading; dining room, where residents have their meals; gym–physiotherapy room, where residents performed directed physical exercises, such as light movements with the help of the physiotherapist, in groups while seated; and occupational therapy room, where residents typically participate in group activities, such as word games and singing. The nursing home residents were all older than 65 years old with a range of different cognitive and physical abilities. The support staffwere nonresidents and were generally younger adults (25–35 years old). In relation to the study, they were those with the capacity to open or close Ws and Ds to ventilate the rooms. The research was approved by the ethics committee of the Universitat Politècnica de Catalunya (UPC, Spain), and informed consent was obtained from participants before data collection commenced. The reference number of the ethics application is 2020.01. 2.1. Room Characteristics. The communal rooms investigated were mostly located on the ground floor (L0) of the nursing homes. The dimensions of the rooms varied between nursing homes and the activity they were designed for, with the living rooms and dining rooms being the largest. Apart from one room (occupational therapy room in NH4), the rest all had at least one W and one D, which could be open or closed during the measurements to provide natural and cross ventilation. All rooms were also ventilated by MV systems, which were solely controlled by a preprogrammed schedule of operation. The MV systems in the nursing homes were centralised and operated according to specific preprogrammed schedules. In NH1, the HVAC system includes two boilers, one chiller, and two-pipe fan coil units. Ventilation is provided by four supply and exhaust fans strategically positioned at different ends of the building, each serving specificfloors within their respective zones. These fans do not include heat recovery systems. Ventilation operates twice daily, following peak occupancy periods, but it is not linked to any monitoring system to optimise airflow or adjust based on real-time needs. In NH2, the HVAC system is a VRV with 16 outdoor units: three per floor dedicated to the rooms and one serving the common areas. The ventilation system features heat recovery units, with two units per floor located in the false ceilings of the common areas. NH3 and NH4 both employ an HVAC system with two boilers, two chillers, and twopipe fan coils for heating and cooling. In NH3, forced 2 Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License ventilation is delivered via five zoned fans that operate for 2 h around midday. In contrast, NH4 relies entirely on natural ventilation, achieved by opening Ws early in the morning and after lunch to promote airflow. In NH5, the HVAC system consists of a two-pipe setup with one chiller and one boiler connected to fan coil units and air handling units that serve the common areas. The airflow rates in these systems were not manually adjustable by staffand were set to function according to the aforementioned schedules, though specific airflow rates were not measured in this study. Maintenance of the MV systems was carried out periodically to ensure their proper functioning, though the frequency and extent of these procedures varied across the nursing homes. Fan coils were used to cool or heat the spaces. Table 1 presents the main characteristics of the investigated rooms in each nursing home. All rooms had a similar floor to ceiling height of 2.8–3.1 m. Figure 1 provides some additional information of the investigated rooms. All five nursing homes were in areas not classified as highly polluted. Each facility was located within 2 km of a State Meteorological Agency (Agencia Estatal de Meteorología, AEMET) weather station, ensuring reliable environmental monitoring. Outdoor CO₂levels remained between 400 and 420 ppm, consistent with typical background concentrations and indicating no significant elevation due to pollution. Note that the exact locations of the nursing homes are not disclosed to maintain anonymity in accordance with the study’s ethical procedures. 2.2. Data Acquisition. The data collection was undertaken between April and August 2021 and October 2021 and February 2022 in five different nursing homes. During those months, each nursing home was visited 7–11 times by the researchers. In each visit, CO 2 concentrations were measured in the four types of rooms discussed in the paper. The measurements took place between 10:00 am and 6:00 pm, in intervals up to 60 min (excluding the stabilisation period) per room, depending on the room and the activity being carried out by the residents. A total of 22 communal rooms were monitored during the study, resulting in 196 periods of data collection (equivalent to 5282 measurements). This sample is comparable to that of other studies such as [23], which focused on four academic buildings and 15 rooms, and [24], which analysed six buildings. The number of occupants present in each room during each measurement period was counted and recorded directly by the researcher. During the measurement period, the room occupancy and the ventilation conditions (by means of the MV or the opening of Ds/Ws) did not change. 2.2.1. CO 2 Measurements. A portable air quality datalogger (Delta OHM HD 21ABE17, accuracy: ±50 ppm + 3%of the reading, range of 0–5000 ppm) was used to record CO 2 measurements every minute. Before data collection commenced, the sensor was calibrated according to the manufacturer’s instructions. The equipment was allowed a 10-min stabilization period before measurements began. Regular checks were performed during the study to ensure the sensor remained within its specified accuracy range. The sensor was placed at a height between 1.2 and 1.5 m from floor level (according to the ANSI/ASHRAE Standard 62.1 [25], the height of the breathing zone is described as 0.75–1.8 m above the floor) and more than 1 m away from walls, Ws, and ventilation outlets. 2.2.2. Thermal Comfort Survey. During the CO 2 measurement periods, the nursing home residents and support staff were also surveyed to determine their thermal sensation vote (TSV). A customised right-here-right-now survey was used to collect subjective comfort evaluations. The questionnaire consisted in one simple question about the TSV, which was assessed using the ASHRAE 7-point thermal sensation scale (−3cold,−2cool,−1slightly cool,0neutral,+1slightly warm,+2warm,+3hot) [26]. The TSV was assessed by asking participants: “How do you feel at this precise moment?” Afigure-based survey was given to residents to facilitate their understanding of the thermal scales. On average, 80% of the occupants of the room being measured completed the TSV survey (see occupancy numbers in Table 1). Throughout the study and its multiple measurement periods, a total of 3359 surveys were completed: 2873 were filled by residents (2189 women, 684 men), and 486 were completed by support staff(435 women, 51 men). 2.3. Data Analysis 2.3.1. IAQ Standards. The Spanish technical standard for the design of HVAC systems [27] establishes different IAQ levels depending on building type. For nursing homes, it recommends “Good air quality”(IDA2), as a maximum CO 2 concentration level of 500 ppm above the outdoor air CO 2 concentration (considered to be 300–400 ppm by this Spanish standard) [28]. Therefore, the Spanish standard recommends maximum CO 2 levels of 800–900 ppm in nursing homes. Similarly, the European standard EN 13779 [29], which sets up the performance requirements for ventilation and room-conditioning systems for nonresidential buildings, recommends a “Medium IAQ”(IDA2) of 400–600 ppm CO 2 concentration levels above the level of outdoor air (considered to be 400 ppm by this EU standard). The CO 2 concentrations measured during the experimentation were classified according to the IAQ categories defined by EN 13779 (Table 2), and an 800–900 ppm CO 2 concentration range was used as a recommended IAQ for nursing homes in line with the Spanish technical standard. 2.3.2. Methods of Analysis. Descriptive statistical analysis (minimum, maximum, mean, median, and standard deviation) was used first to describe CO 2 levels and data trends in relation to (i) room type, (ii) ventilation regime (MV ON/OFF and Ws and Ds open/closed), (iii) heating or cooling regime (heating/cooling ON/OFF), and TSV (unidimensional). For each of these scenarios, the cumulative frequency of CO 2 levels falling in each of the IAQ categories was analysed. 3Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Predictive statistical analysis (regression) was used to understand how CO 2 levels (dependent variable) can be predicted given different independent variables. In this study, regressions were used to show how mean CO 2 levels are related to average occupancy density (groups of 1 m 2 /p) and (i) room type, (ii) ventilation regime, and (iii) heating/ cooling regime. In evaluating regression fits, consideration was given to the expected CO 2 levels in a virtually infinite space (around 400 ppm, as a representation of an open space), indicating a nonlinear decrease with occupancy density. An assessment of multiple regression lines (linear, logarithmic, power, and exponential) indicated that a power TABLE 1: Main characteristics of the investigated communal spaces. Nursing home Space Floor area (m 2 ) Floor level Window area (m 2 ) Average occupancy (min–max) NH1 Occupational therapy room 35.0 L0 1.6 9 (7–16) Living room 123.3 L0 14.4 17 (10–34) Gym–physiotherapy room 47.0 L0 12.6 14 (10–16) Dining room 54.3 L0 3.4 19 (11–27) NH2 Occupational therapy room 101.9 L1 29.9 16 (8–20) Living room a 56.7 L2 11.4 19 (11–27) Living room b 145.3 L1 29.9 17 Living room c 156.4 L0 11.4 14 (9–19) Gym–physiotherapy room 172.5 L1 14.7 19 (14–25) Dining room 167.6 L0 26.8 37 (28–49) NH3 Occupational therapy room 65.4 L0 17.3 23 (17–30) Living room 126.4 L0 17.3 36 Gym–physiotherapy room 61.0 L0 10.3 10 (8–12) Dining room 81.1 L1 8.1 23 (17–31) NH4 Occupational therapy room 89.2 L0 —14 (12–17) Living room 125.5 L0 21.7 25 (18–35) Gym–physiotherapy room 89.2 L0 3.0 18 (12–19) Dining room 81.5 L0 29.1 41 (28–48) NH5 Occupational therapy room 97.5 L0 15.5 20 (7–32) Living room 103.7 L0 28.9 28 (15–36) Gym–physiotherapy room 47.1 L0 14.5 11 (7–15) Dining room 81.5 L1 17.5 37 (31–43) (a) (b) (c) (d) Figure 1: Examples of the investigated rooms in the nursing homes: (a) occupational therapy room, (b) living room, (c) dining room, and (d) gym–physiotherapy room. 4 Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License regression line provided the best coefficients of determination. A power regression line is represented as y=ax b, R2=n, where yis dependent and xis the independent variable. The R2was used to measure the proportion of the variation in the dependent variable which is explained by the model [30]. Data were analysed using Microsoft Excel and Statistical Package for the Social Sciences (SPSS). 3. Results 3.1. Overview of CO 2 Concentration Measurements. During the study, 5282 CO 2 concentration measurements were collected in the dining rooms (1817), living rooms (1192), gym–physiotherapy rooms (908), and occupational therapy rooms (1365) of five nursing homes. Measured CO 2 levels ranged from 287 to 1558 ppm, with the highest frequency of measurements falling within the 500–800-ppm range. Table 3 contains the descriptive statistical analyses (minimum, maximum, mean, median, and standard deviation) of the CO 2 levels recorded during the study. It can be observed that all mean values are below the 800-ppm threshold, indicating that all rooms achieved at least an IAQ category of IDA3, as shown in Table 2. The numerical data on the number and percentage of CO 2 concentration measurements falling in the four IAQ categories are presented in Table 4. In this study, CO 2 levels exceeded 800 ppm in 15.8% of the cases, while 4.2% of the measurements were above 1000 ppm, and 0.3% exceeded 1400 ppm. 3.2. CO 2 Concentration by Room Type. This section provides an overview of the CO 2 concentrations in each room type studied in the five nursing homes: occupational therapy room, gym–physiotherapy room, living room, and dining room. TABLE 3: Descriptive statistical analysis of the CO 2 concentration measurements. No. of measurements Min (ppm) Max (ppm) Mean (ppm) Median (ppm) SD CO 2 levels by room type (5282 measurements) Dining room 1817 373 1334 645 619 158 Living room 1192 379 1090 541 521 119 Gym–physiotherapy room 908 384 898 596 574 118 Occupational therapy room 1365 287 1558 713 669 119 CO 2 levels by ventilation regime (5173 measurements) (MV = mechanical ventilation) MV OFF 1103 379 1558 638 596 179 MV ON 4070 287 1558 632 592 175 MV OFF: W+D closed 333 395 1558 660 596 235 MV OFF: W+D opened 193 386 1090 613 582 144 MV OFF: Either W or D opened 577 379 1202 633 602 147 MV ON: W+D closed 526 395 1558 739 703 250 MV ON: W+D opened 830 384 997 555 547 117 MV ON: Either W or D opened 2714 287 1334 635 603 160 CO 2 levels by heating/cooling regime (5282 measurements) Heating 1817 387 1557 665 628 170 Cooling 1564 287 1314 561 540 124 No cooling/heating 1901 386 1558 655 594 197 CO 2 levels by thermal sensation vote (TSV) (residents and support staff) (3359 measurements) TSV <0 447 336 1058 619 615 123 TSV = 0 2528 336 1058 625 599 136 TSV >0 384 336 1058 622 599 140 TABLE 2: Recommended CO 2 values according to EN 13779 [29] standard. IAQ category Description CO 2 level above level of outdoor air (ppm) (typical range) Total CO 2 level (ppm), based on outdoor CO 2 level of 400 ppm IDA1 High IAQ ≤400 ≤800 IDA2 Medium IAQ 400–600 800–1000 IDA3 Moderate IAQ 600–1000 1000–1400 IDA4 Low IAQ >1000 >1400 5Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License The min, max, and mean (with SD) of CO 2 concentrations measured in each room type are presented in Table 3. In this study, mean CO 2 levels were below the 800–900-ppm recommended levels for nursing homes [27] in all room types, with the highest mean CO 2 concentration of 713 ppm measured in the occupational therapy room. However, the maximum values suggest that all room types, apart from the gym–physiotherapy room (with a CO 2 concentration of 898 ppm), reached CO 2 levels above the recommended values. Again, the occupational therapy room recorded the highest CO 2 concentration value of 1558 ppm. Figure 2 shows the cumulative frequency (%) of CO 2 measurements falling in the four IAQ categories for each room type. For example, 72% of the CO 2 measurements taken in the occupational therapy rooms were within category IDA1 (CO2< 800 ppm), while 28% of the measurements exceeded this recommended level. From those, 17.7% fell in category IDA2 (800 < CO2< 1000 ppm), 9.1% in category IDA3 (1000 < CO2< 1400 ppm), and 1.2% in category IDA5 (CO2> 1400 ppm). Overall, the room type with the highest frequency of measurements in category IDA1 and therefore with the highest IAQ during the study was the living rooms (95.6%). Categories IDA3 (moderate IAQ) and IDA4 (low IAQ) had the highest frequencies in the occupational therapy rooms (combined frequency value of 10.3%) and dining rooms (combined frequency value of 3.6%). 3.3. CO 2 Concentration by Ventilation Regime. CO 2 concentrations were analysed for different room ventilation scenarios, including whether the MV was active or not (ON/OFF) and whether the Ws or Ds were opened or closed at the time of the measurements. Ws and Ds were considered to be opened or closed based on visual inspection by the researchers. The min, max, and mean (with SD) of CO 2 concentrations measured in the different ventilation scenarios are presented in Table 3. When studying the CO 2 levels based on only the MV being ON/OFF, no particular differences were observed in the average concentration or the range of measurements (min and max values). While the recommended value of 800–900 ppm was exceeded in both scenarios, the mean levels [638 ppm (MV OFF), 632 ppm (MV ON)] stayed below these. The effect of the Ws and Ds being opened or closed was also investigated. Mean CO 2 levels were below the 800–900-ppm recommended levels in all scenarios, with the highest mean CO 2 concentration of 739 ppm occurring when the MV was ON and both Ws and Ds were closed. In contrast, the lowest mean CO 2 level (555 ppm) was associated with the scenario when the MV was ON and both Ws and Ds were opened. Maximum values suggest that all ventilation scenarios reached CO 2 levels above the recommended values. The scenarios where the Ws and Ds were closed (regardless of the MV being ON or OFF) recorded the highest concentration value of 1558 ppm. TABLE 4:CO 2 concentration measurements and frequency (%) by IAQ category. All data No. of measurements IDA1 (<800 ppm) IDA2 (800–1000 ppm) IDA3 (1000–1400 ppm) IDA4 (>1400 ppm) 5282 4447 84.2% 612 11.6% 207 3.9% 16 0.3% CO 2 levels by room type (5282 measurements) Dining room 1817 1506 82.9% 246 13.5% 65 3.6% 0.0 0.0% Living room 1192 1139 95.6% 36 3.0% 17.0 1.4% 0.0 0.0% Gym–physiotherapy room 908 819 90.2% 89 9.8% 0.0 0.0% 0.0 0.0% Occupational therapy room 1365 983 72.0% 241 17.7% 125 9.1% 16 1.2% CO 2 levels by ventilation regime (5173 measurements) MV OFF 1103 951 86.2% 100 9.1% 44 4.0% 8 0.7% MV ON 4070 3387 83.2% 512 12.6% 163 4.0% 8 0.2% MV OFF: W+D closed 333 287 86.2% 14 4.2% 24 7.2% 8 2.4% MV OFF: W+D opened 193 176 91.2% 12 6.2% 5 2.6% 0 0.0% MV OFF: Either W or D opened 577 488 84.6% 74 12.8% 15 2.6% 0 0.0% MV ON: W+D closed 526 346 65.8% 100 19.0% 72 13.7% 8 1.5% MV ON: W+D opened 830 799 96.3% 31 3.7% 0 0.0% 0 0.0% MV ON: Either W or D opened 2714 2242 82.6% 381 14.0% 91 3.4% 0 0.0% CO 2 levels by heating/cooling regime (5282 measurements) Heating 1817 1444 79.5% 312 17.2% 53 2.9% 8 0.4% Cooling 1564 1489 95.2% 65 4.2% 10 0.6% 0 0.0% No heating/cooling 1901 1514 79.6% 235 12.4% 144 7.6% 8 0.4% CO 2 levels by thermal sensation vote (TSV) (residents and support staff) (3359 measurements) TSV <0 447 416 93% 24 5% 7 2% 0 0% TSV = 0 2528 2224 88% 250 10% 54 2% 0 0% TSV >0 384 333 87% 45 12% 6 2% 0 0% 6 Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Figure 3 shows the cumulative frequency (%) of CO 2 measurements falling in the four IAQ categories for each ventilation scenario. Overall, the ventilation scenario with the highest frequency of measurements in category IDA1 (<800 ppm) and therefore with the highest IAQ during the study involved the MV being ON and the Ws and Ds opened (96.3%). Categories IDA3 (moderate IAQ) and IDA4 (low IAQ) had the highest frequency in the scenarios when the Ws and Ds were closed, regardless of whether the MV was ON or OFF. Figure 3 also provides an indication of the effect that the state (opened/closed) of Ws and Ds had on the CO 2 concentrations. For example, with the MV ON, the percentage of CO 2 measurements under 800 ppm was reduced from 96.3% (with Ws and Ds opened) to 65.8% (with Ws and Ds closed). This effect was less significant when the MV was OFF, where the difference between having the Ws and Ds opened (91.2%) or closed (86.2%) was only 5% for measurements under 800 ppm. 3.4. CO 2 Concentration by Heating/Cooling Regime. CO 2 concentrations were also analysed for different room heating and cooling regimes, including whether the room was being cooled or heated or no heating/cooling (free-running) was taking place at the time of the measurements. The min, max, and mean (with SD) of CO 2 concentrations measured in the different heating/cooling regimes are presented in Table 3. Mean CO 2 levels were below the 800–900-ppm recommended levels in all scenarios, with the highest mean CO 2 concentration of 665 ppm occurring when the heating system was on. In contrast, the lowest mean CO 2 level (561 ppm) was associated with the scenario when the cooling system was on. Maximum values suggest that all heating/cooling scenarios reached CO 2 levels above the recommended values. The cumulative frequency (%) of CO 2 measurements falling in the four IAQ categories for each heating/cooling scenarios is shown in Figure 4. The highest IAQ (category IDA1, <800 ppm) was measured in situations when the cooling system was on (93.0% of the measurements). In contrast, categories IDA3 and IDA4 had the highest frequency when there was no heating or cooling in operation (cumulative frequency of 8.2% of measurements over 1000 ppm). 3.5. CO 2 Concentration by Occupancy Density. Occupancy density was calculated as the amount of floor area of the room per occupant (m 2 /p). In this study, the occupancy density ranged from 1.0 to 17.0 m 2 /p. The effect of occupancy density on the CO 2 concentration levels was studied for the different room types, ventilation regimes, and heating/cooling regimes. The mean CO 2 level for each period of data collection was plotted against the corresponding averaged occupancy density (in groups of 1 m 2 /p). 0 100 200 300 400 500 600 700 800 900 1000 1200 1300 1400 1500 1600 1700 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Percent (%) CO2 concentration level (ppm) IDA1 IDA2 IDA3 IDA4 1100 Occupational therapy room Gym-physiotherapy room Living room Dining room Total Figure 2: Cumulative frequency (%) of CO 2 measurements by room type. 7Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 3.5.1. CO 2 Concentration by Occupancy Density and Room Type. The mean occupancy density varied from room to room: dining room, with a mean of 3.3 m 2 /p (ranging from 1.0–11.0 m 2 /p); living room, with a mean of 5.6 m 2 /p (ranging from 2.0–17.0 m 2 /p); gym–physiotherapy room, with a mean of 5.8 m 2 /p (ranging from 2.0–12.0 m 2 /p); and occupational therapy room, with a mean of 4.8 m 2 /p (ranging from 2.0–13.0 m 2 /p). Figure 5 shows the mean occupancy density plotted against the corresponding mean CO 2 concentration levels for each monitoring period and by room type. The results show that the mean CO 2 levels decreased as the floor area per person (m 2 /p) increased for all room types. However, different correlation levels were found for each room type. For instance, the gym–physiotherapy room power line showed an R2of 0.89, whereas the occupational therapy room provided an R2of just 0.26. This would suggest that the specific activities carried out in each room type influence differently the CO 2 concentrations relative to the mean occupancy density. Thus, activities with high metabolic demands, such as those in a gym–physiotherapy setting, presented a stronger correlation with the occupancy density compared to less physically demanding rooms, such as a living room. In addition, results also showed that to maintain a mean CO 2 level of 800 ppm, occupant density should be at least 3.8 m 2 /p in the dining room, 3.5 m 2 /p in the gym–physiotherapy, 1 m 2 /p in the living room, and 3.8 m 2 /p in the occupational therapy room. 3.5.2. CO 2 Concentration by Occupancy Density and Ventilation Regime. The occupancy density in each measurement was plotted against the corresponding mean CO 2 concentration levels in two different ventilation scenarios: with the MV ON or OFF. Figure 6 shows that, in both ventilation scenarios, mean CO 2 levels declined as the mean occupancy density increased. Results suggest that mean CO 2 concentrations is strongly correlated (R2=081) with the mean occupancy density when the MV was ON. Conversely, when the MV is inactive, mean CO 2 concentrations can not be confidently explained by the mean occupancy density (R2=046). That would suggest that there are other factors beyond just occupancy density, such as room type, that would influence the CO 2 levels. Further research in other nursing homes is required to establish these relationships. 3.5.3. CO 2 Concentration by Occupancy Density and Heating/Cooling Regime. Mean CO 2 levels decreased as the floor area/person ratio increased in all three scenarios of heating, cooling or no heating/cooling (free-running) (Figure 7). Results show correlations equal or above 0.6 across different heating/ cooling scenarios: cooling (R2=073), heating (R2=060), and free-running (R2=076). This would suggest that the heating/ 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 CO2 concentration level (ppm) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Percent (%) MV OFF: W + D opened MV OFF: Either W or D opened MV OFF: W + D closed MV ON: W + D opened MV ON: Either W or D opened MV ON: W + D closed IDA 4IDA 3IDA 1 IDA 2 Figure 3: Cumulative frequency (%) of CO 2 measurements by ventilation regime. 8 Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License cooling regime is an important indicator to consider when evaluating the mean CO 2 concentrations based on occupancy density. While the R-squared values indicate moderate to strong correlations, it is essential to acknowledge that other factors, such as room type as aforementioned, may contribute significantly to understanding the nature of this concentration decrease. 3.6. CO 2 Concentration Over Time. Temporal dynamics of CO 2 are plotted in Figure 8 based on room type and ventilation regime. The results indicate that cross ventilation was the most effective method for maintaining low CO 2 levels. Temporal evolution of CO 2 levels appears to vary by room type, regardless of the ventilation regime. Specifically, dining rooms—and to a lesser extent, occupational therapy rooms—tend to accumulate CO 2 over time. Interestingly, a longer total monitored time was associated with higher CO₂concentrations, indicating a tendency for accumulation, particularly in cases with insufficient ventilation. 3.7. CO 2 Concentration and TSV. CO 2 concentrations were also studied in relation to the TSVs reported by the nursing home residents and the supporting staffsurveyed during the monitoring periods. The frequency (%) of TSVs reported in each scale (−3 cold,−2cool,−1slightly cool,0neutral,+1slightly warm, +2 warm,+3hot) [26] falling in the four IAQ categories is shown in Figure 9. In this study, the results suggest that the percentage of the nursing home residents (older people) reporting feeling neutral (TSV = 0) and therefore thermally comfortable increases with the CO 2 concentration levels. The highest percentage (83.9%) of residents feeling neutral (TSV = 0) was recorded in situations when the CO 2 levels were between 1000 and 1400 ppm (IDA3). In contrast, the lowest number (78.4%) of residents feeling neutral (TSV = 0) was recorded when CO 2 concentrations were below 800ppm (IDA1). In those circumstances, 14.9% of residents reported feeling from slightly cool to cold (TSV < 0), compared to 6.8% who reported feeling slightly warm to hot (TSV > 0). Similarly, the support staffreported feeling neutral (TSV = 0) in greater frequency (63.9%) at CO 2 levels between 1000 and 1400ppm (IDA3) and at lower frequency (48.8%) at CO 2 concentrations between 800 and 1000 ppm (IDA2). When comparing the TSV of the nursing home residents and the support staff, it can be observed that residents were generally feeling colder than the staff. This was particularly evident at CO 2 levels below 800 ppm (IDA1) (14.9% of residents with TSV < 0, compared to 9% of staff, and 6.8% of residents with TSV > 0, compared to 37.1% of support staff). The mean thermal sensation vote (MTSV) of all votes recorded for each monitoring period was calculated and subsequently related to the CO 2 measurements of the period. Figure 10 shows box plots of MTSV of nursing home residents and support stafffor the four IAQ categories. Average 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Percent (%) IDA1 IDA2 IDA3 IDA4 CO 2 concentration level (ppm) Cooling Heating No cooling/heating Figure 4: Cumulative frequency (%) of CO 2 measurements by heating/cooling regime. 9Indoor Air ina, 2025, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1155/ina/4153784 by Readcube (Labtiva Inc.), Wiley Online Library on [16/04/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License sensation gap widening at spaces with lower CO 2 levels. Therefore, achieving a balance between air quality and thermal comfort is essential. While this study focused specifically on CO₂as an indicator of ventilation effectiveness, further research should explore the full spectrum of IAQ factors. In particular, studies investigating the presence of particulate matter, VOCs, and other pollutants in nursing homes are needed. Such research will contribute to a more comprehensive understanding of how various indoor air pollutants influence the health and comfort of vulnerable populations, including older residents and nursing home staff. Data Availability Statement Data will be made available upon request. Conflicts of Interest The authors declare no conflicts of interest. Author Contributions Alba Fuertes: writing –original draft, investigation, conceptualization, data curation, formal analysis. Rory V. Jones: writing –original draft, conceptualization, investigation. María Teresa Baquero: writing –original draft, investigation, data curation, methodology. Roger Vergés: writing – original draft, investigation, data curation, visualization. Nuria Forcada: funding acquisition, project administration, methodology, investigation, writing –review and editing. Funding This research was supported by the Spanish Ministry of Economy, Industry and Competitiveness under the R&D project Thecoelen, reference no. PID2019-106777RB-C21. Support for this research was also provided by the Catalan agency AGAUR under its research group support program (2021 SGR 00341). Acknowledgments The authors thank Sanitas Mayores and especially Marc Vallet and Albert Ayala for providing their nursing homes and helping to collect the data. The authors would like to extend our appreciation to all the older people and support and maintenance staffwho participated in this project. References [1] National Institute of Statistics, Spanish Population Projections 2022-2072 (Press note, 2022), https://www.ine.es/prensa/pp_ 2022_2072.pdf. [2] C. Hughes, S. Natarajan, C. Liu, W. 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