Digital Technologies for Sustainable Management of Visitor Carrying Capacity in Heritage Enclosed/Confined Spaces

Abstract

Cultural tourism has become an increasingly significant phenomenon in urban areas, especially in cities rich in heritage sites. However, when the number of visitors exceeds sustainable capacity thresholds, both the physical and psychological comfort and safety of individuals may be compromised. A higher number of visitors inside historic buildings leads to elevated concentrations of carbon dioxide (CO₂), particularly in poorly ventilated enclosed or confined spaces, primarily as a result of human respiration. Such conditions not only accelerate the deterioration processes affecting heritage materials but also introduce potential health risks for visitors. Parameters such as CO₂ concentration, indoor air temperature, and relative humidity represent key measurable parameters for assessing environmental Indoor Air Quality (IAQ) within heritage buildings. Digital real-time monitoring of these parameters plays a crucial role in preventive heritage conservation, sustainable site management, and in ensuring visitors’ comfort and well-being. This paper presents a procedure and methodology that use digital technological tools to efficiently estimate and monitor the Visitor Carrying Capacity (VCC) of enclosed/confined heritage spaces, especially Heritage Building Information Modelling (HBIM) and Sensor Technology. These kinds of spaces require particular attention due to their spatial characteristics. In order to do so, it is necessary to know the geometry of the site, and to consider IAQ conditions. This study also considers the number of People at One Time (PAOT) and Visitor Occupancy (VO). The results focus on the procedural development of the analysis and emphasise the role of digital tools not only due to their efficiency and accuracy in spatial analysis for estimating VCC, but especially for the real-time monitoring of visitors and surveying specific environmental parameters. The experimental phase of this study uses the Chapel of the Holy Chalice of the Valencia Cathedral (Spain) as a pilot case. Monitoring this space reveals how quickly high CO₂ levels are reached with continuous visitor presence, and how long it takes for them to decay in absence of people and under passive ventilation conditions. The outcome of this research is a detailed methodological framework designed to assess and monitor Visitor Carrying Capacity (VCC) in enclosed/confined heritage sites by integrating digital technologies, thereby enhancing sustainable management, planning and decision-making processes.

1. Introduction

Visitor Carrying Capacity (VCC) is one of the most widely recognised tools for the sustainable tourism management of heritage assets. This concept refers to the maximum number of visitors (People At One Time -PAOT-) that a heritage site can accommodate without causing negative impacts on the site’s physical integrity, environment, cultural significance, or the quality of the visitor experience, comfort and safety, in order to ensure sustainable and pleasurable visitation.

The practice of estimating VCC as a methodological tool dates back to the 1970s. It was originally linked to protected natural areas [1,2,3], particularly by the US National Park Service. Later, this tool was transferred to the scope of cultural heritage, with the calculation models refined and adapted accordingly. There have always been two key concepts that repeatedly appear in the analyses: PAOT, which Manning [4] established as a quality indicator for a visitation area, and Usable Surface for Visitors (USV), which is defined as the available floor area that can comfortably accommodate PAOT once any areas that are inaccessible or unusable for reasons of conservation, safety, fragility, incompatibility of uses, legality, or due to the physical distribution of their internal components have been subtracted [5,6,7,8]. Both remain core components of current VCC studies. However, their relative importance varies depending on the type of the heritage space. USV is a fundamental parameter for open spaces, but it is not as decisive for enclosed/confined spaces as the IAQ is.

In recent decades, the need to monitor VCC in real time has become apparent, since beyond setting an occupancy threshold, it is also necessary to determine how these capacity limitations will be managed, as visitor flows are dynamic elements that are constantly evolving. Another recurring trend identified in published works is that the focus of analysis remains on people’s comfort; few studies have addressed the cumulative impact that a continuous excess of visitors has on heritage sites [9].

One of the factors to consider in the analysis is the type of heritage site, as no two heritage sites are alike. Therefore, preliminary diagnostic studies must be carried out to determine the spatial characteristics, environmental conditions and vulnerabilities, and to identify the key element that will determine the limitations on the use of the heritage site.

It can therefore be seen that VCC studies have evolved and are moving towards the analysis of very specific spatial and environmental processes so that they can truly be an efficient tool and enable real-time management decisions to be made.

Currently, one of the challenges facing VCC studies is the need to use digital technologies, such as those relating to Reality Capture (RC): Terrestrial Laser Scanning (TLS), Photogrammetry, Simultaneous Localisation and Mapping (SLAM), Unmanned Aircraft System (UAS) or Drone Technology, Light Detection and Ranging (LiDAR). Also those that involve 3D Modelling, such as the Heritage Building Information Modelling (HBIM); and the ones that are Integrated Tools such as Geographic Information Systems (GIS), Techniques of Geospatial Data Processing, Digital Twins (DT), Artificial Intelligence (AI), and Sensor Technology. All of them have proven extremely useful in the analysis of heritage sites and can be used individually or collectively and be integrated into data platforms. Numerous studies [10,11,12,13,14] show the benefits of using digital technologies to address issues such as zoning, metric and volumetric spatial estimates, the determination of spatial attributes and vulnerabilities of monuments and urban heritage spaces, the determination of the USV, the analysis of visual viewsheds, touring patterns, and the identification of assembly and meeting points for groups, among others. However, they are particularly useful in monitoring VCC, for real-time people counting or to control IAQ [15,16,17,18]. Creating DT of heritage spaces offers a unique opportunity to find solutions to various challenges without exposing the heritage element to any kind of stress [19,20,21].

Regarding the context of this research, it should be noted that it is closely related to the problems faced by many heritage tourist destinations around the world that have always suffered from overtourism. This is reflected in highly unsustainable situations such as visitor overcrowding, degradation of cultural heritage, etc., that cause dissatisfaction among visitors and local residents. Moreover, the data provided by the main international institutions that deal with these records suggests that there has never been such an alarming growth in visitor numbers in such a short period as there has been recently [22,23,24,25]. Therefore, the current oversizing of tourism due to the large number of visitors at heritage sites means that the managing of these spaces, particularly monitoring VCC, is impossible without using digital technologies.

Enclosed and confined spaces are the type of heritage assets that this work will focus on. An enclosed space is a restricted area that is surrounded by walls, floors and ceilings. A confined space is a specific type of enclosed space with additional hazardous characteristics, such as limited entry/exit points, and a potential for dangerous atmospheres. This makes it unsafe for continuous human occupancy and requires special safety precautions, as defined by organisations involved in drafting labour regulations, such as the Occupational Safety and Health Administration (OSHA) [26], the UK Confined Spaces Regulations 1997 No. 1713 [27], or the European Union [28,29] with the EU Directive 89/391/EEC Framework Directive on Safety and Health at Work and EU Directive 89/654/EEC Framework regulating minimum safety and health requirements in the workplace.

While there is a wide range of enclosed/confined spaces in the workplace, there are also heritage sites, both above and below ground, that can be classified as such (e.g., crypts, tombs, air-raid shelters, chapels, show caves, tourist mines). These are delicate spaces where elements such as humidity, temperature and CO₂ concentrations can be significantly affected by human presence. High CO₂ concentrations will be the main limiting factor in determining the Visitor Occupancy (VO) of these heritage spaces. It should be noted that most enclosed and confined heritage sites were not intended for continuous VO, and they lack features such as permanent ventilation and easy access, which must be considered when planning tourist visits.

Of all the types of immovable heritage elements, enclosed/confined spaces are possibly the most sensitive, yet the least scientific attention has been devoted to them regarding VCC studies. Arguably, the most analysed type of enclosed/confined heritage site is the show caves. These heritage sites often face significant challenges due to their fragile ecosystems and numerous physical constraints. While they are highly attractive to the public, organising visits to these environments can be complex. Some interesting studies have investigated relationship between the number of visitors and IAQ in caves. Examples include research on the El Castillo and Covalanas caves in Cantabria [15], or that on the Mogao Grottoes in China [30]. Some studies have also analysed the IAQ of built enclosed/confined spaces, such as the case of the Tomb of Tutankhamen in Egypt [31].

The objective of this paper is to propose a methodological procedure for estimating and monitoring VCC in enclosed/confined heritage sites in real-time using digital technologies. The Chapel of the Holy Chalice in the Valencia Cathedral (Spain) was chosen as the testing ground for validating the procedure. Consequently, this pilot study aims to assess and analyse IAQ in specific scenarios, with the objective of elucidating the relationship between VO levels within enclosed/confined spaces and the resulting CO₂ concentration, during religious and visiting activities, and to study their dynamic behaviour. To set sustainable visitation limits, we consider factors such as visitor flow, available space per person, operating hours and specific site conditions.

This research was conducted in the framework of the Universitat Politècnica de València research project “Analysis and development of the integration of HBIM in GIS for the creation of a protocol for cultural heritage tourism planning”, funded by the Ministry of Science and Innovation, Spain (2021–2024).

2. Methodology

Given the study’s solution-focused nature, the Design Science Research (DSR) methodology was chosen as a guideline for the research process. The DSR methodology is a problem-solving approach used across various disciplines and frameworks to solve real-world problems through innovative solutions [32,33].

Inspired by the DSR Model [34,35], this work takes a five-step approach to achieving research outcomes (Figure 1): (a) problem identification (Section 3); (b) definition of a solution (Section 4); (c) solution development (Section 5); (d) solution implementation and demonstration (Section 6); and (e) solution evaluation (Section 7). The authors of this paper frame the development of their methodological steps as an integral outcome or result of their research.

Figure 1. Phases of the DSR methodology applied to this research work. Source: own elaboration.

3. Problem Identification: Constraints in Estimating and Monitoring the Visitor Carrying Capacity (VCC) in Enclosed/Confined Heritage Spaces

Traditionally, VCC studies have been conceived as static assessments limited to specific moments and locations. However, visitor behaviour and environmental conditions within enclosed/confined heritage spaces can vary considerably, even over short time periods. Therefore, estimating VCC should be conceived as a dynamic analysis.

3.1. Physical Constraints of the Enclosed/Confined Spaces

The distinctive physical characteristics of enclosed/confined heritage spaces make it more challenging to determine and monitor their VCC than in other types of heritage sites, and therefore more difficult to analyse and implement. This analysis uses various qualitative and quantitative methods.

Firstly, the USV must be estimated, as should the geometry of the site, because it defines the boundaries. Then, the volume of the space must be measured in cubic metres (m³). It should be noted that the geometry of these spaces is rarely regular. In many cases, this poses a significant spatial constraint, as they tend to have complicated layouts and irregularities, as well as limited accessibility.

3.2. Consideration of the Indoor Volume and Air Quality

It is essential to know the air volume and its quality in enclosed/confined spaces when estimating the VCC, as these are limiting factors for human occupation of these sites.

The volume of a space is equivalent to the total amount of air it can hold, assuming the space is entirely filled with air. This volume of air will largely determine how many people can be accommodated inside the enclosed/confined space. However, the IAQ may not be suitable for visitors due to a buildup of gases such as CO₂. This gas, being heavier than air, can be accumulated in these spaces and displace oxygen, leading to oxygen deficiency. High concentrations of CO₂ negatively affect human health, leading to increased heart rate, reduced blood oxygen levels, fatigue, inflammation, headaches, dizziness, and impaired cognitive performance [36,37]. In addition, it should be noted that CO₂ also affects the materials that make up confined spaces. In the case of calcareous materials, CO₂ can dissolve in infiltration or condensation water that comes into contact with the supporting materials, leading to gradual chemical interactions that may alter their physical and mechanical properties. This phenomenon has been studied especially in karstic caves [38,39,40,41,42].

In the enclosed/confined types of spaces, a higher indoor concentration of CO₂ is usually caused by people breathing, inadequate ventilation, and a complex spatial geometry. The continuous presence of people in poorly ventilated spaces can lead to an increase in temperature and relative humidity, as well as a cumulative increase in CO₂ levels over time. Many studies related to this topic have been conducted, primarily in residential settings, classrooms, offices and other workspaces [43,44,45]. In VCC studies, CO₂ is an important constraint which results in a progressive reduction in PAOT on the site and, as the visiting hours progress, the site reduces its capacity to safely accommodate people over time.

4. Definition of the Solutions

Following the identification of the problems, this phase of the investigation focuses on the conception of solutions. To this end, a literature review has been conducted, dealing particularly on human comfort and safety thresholds for air quality, as well as building on the research team’s experience in developing and implementing solutions to VCC issues on heritage sites. It should be noted that the VCC estimation is unique to each site, although some of the key procedures are common to all sites.

4.1. Determination of the Usable Surface for Visitors (USV), Site Geometry, and Air Volume

Estimating the USV in an enclosed/confined space requires identifying the available floor area (in m²) where visitors can move around (optimum space for movement) or remain at rest (standing or sitting) comfortably and safely. It is also necessary to consider the geometry of the site, as well as the volume of air within the space itself.

Measured building survey is a challenging and time-consuming method that relies on manual measurements, sketches, and calculations. These are prone to error and can be difficult to execute in complex or geometrically constrained areas. For these reasons, it is best to use digital technologies such as TLS, Photogrammetry and 3D modelling techniques, as well as HBIM, since these tools offer more advanced and efficient solutions for capturing complex and non-standard geometries than traditional methods. They provide 2D or 3D topographical plans, measurements and 3D models along with other valuable information, such as the physical characteristics of the space (e.g., morphological features, ventilation openings, location of entrances and windows, etc.), connectivity between spatial units (e.g., corridors, hallways, etc.), walkability, grades, obstacles, physical vulnerabilities (level of exposure of heritage elements to various risks, etc.), and zoning into smaller spatial units if necessary. HBIM was essential for these sites as the pilot case analysed in this study. The point cloud integrated into a parametric HBIM model allowed a precise and verifiable calculation of the interior volume, something that would not have been reliably achievable with traditional survey methods.

4.2. Assessment of the Indoor Air Quality (IAQ)

Another necessary step is to define the IAQ conditions that are important for visitors, as these affect their health and performance at the heritage site. It is also necessary to define the conditions that will prevent damage to heritage elements [21].

The desired IAQ conditions are those that do not include known pollutants at harmful concentrations, as determined by the competent authorities, and that do not cause dissatisfaction among a substantial majority of exposed people (80% or more) [46]. CO₂ has been debated for decades as an indicator of IAQ, yet this gas is still commonly used in ventilation guidelines for non-industrial indoor spaces across many countries [47,48,49,50].

The reference values or threshold concentrations of CO₂ for indoor enclosed/confined spaces have been taken from relevant regulations and building codes. In this work, Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings [25] has been adopted as a guiding framework. The reference value generally used for these kinds of spaces must not exceed 500 ppm (parts per million) above the outdoor CO₂ concentration. The nominal value of the latter in the Northern Hemisphere, as recorded by the Mauna Loa Observatory, is estimated at 426 ppm [51]. Therefore, a value of around 1000 ppm is considered the maximum tolerable threshold for the Category II of the above-mentioned EU Directive that includes activities in offices, classrooms, museums, and libraries. While this concentration indicates acceptable air quality, ventilation is recommended. In contrast, 1500 ppm indicates poor air quality and ventilation is required.

Ideally, the process of measuring specific indoor environmental air conditions would involve collecting data on sites over extended periods without human disturbance. However, this situation is often not possible. Nevertheless, field experiments involving visitors in various scenarios and numbers can be used to monitor environmental parameters to adjust VCC.

Another factor to consider is the CO₂ dispersion capacity of the space (how quickly outdoor air replaces indoor air), which in this way provides an indication of ventilation effectiveness. This information is obtained by calculating the Air Change Rate per Hour (ACH), which is defined as the number of times the air (in m³) is completely replaced per hour by supply and/or recirculated air. It should be remembered that air renewal is not easy in confined spaces. If visitors are constantly occupying the space and producing metabolic CO₂ through exhalation, it becomes very difficult to guarantee the availability of quality air.

Air renewal can be estimated using the Decay Step-down method [52,53,54,55], which calculates the ACH based on the natural decay in indoor CO₂ concentrations compared with the outdoor reference concentration level of 426 ppm recorded at Mauna Loa, and considering the time elapsed between the space becoming vacated by visitors and being reoccupied. Air renewal can be influenced by the site layout and its geometry, the visitor traffic, and seasonal environmental variations, particularly through the gradient between interior and exterior air temperature and ventilation (natural and mechanical). Natural ventilation occurs when a space is opened and natural air currents cause air to flow into, through and out of the space. These currents develop due to thermal conduction within the space, the presence of heavier or lighter gases within the space, or a variance in atmospheric pressure between the inside and outside of the space. The American Conference of Governmental Industrial Hygienists [56] recommends 20 air changes per hour for ventilating confined spaces, which is significantly higher than the general ventilation recommendations for other types of spaces. The Directive 2010/31/EU mentions about 12–15 air changes for museums [25].

Analysing ventilation airflow rates is crucial when implementing a mechanical ventilation system to improve IAQ. Such is the case of the Scrovegni Chapel in Padua [57], the Sistine Chapel in the Vatican [58] and the Tomb of Tutankhamen [32], among others.

Environmental sensors are used to measure CO₂ concentrations. The data are then compared with that provided by people counters. In this way, it is possible to determine how CO₂ concentrations evolve in relation to the presence of people. Digital Twins of the heritage sites, together with AI, allow for research into virtual occupation scenarios with different characteristics to help prevent critical situations.

4.3. Estimation of People at One Time (PAOT) and Visitor Occupancy (VO)

Once the USV is known, it is necessary to calculate the number of PAOT, considering the personal spatial proxemic requirements of visitors in enclosed/confined areas (m² per person), according to health, comfort and safety regulations, as well as considering space for visitor movement. The type of activity also entails different considerations of personal space, even when carried out in the same space (e.g., static worship activities in a church vs. dynamic visiting activities) [59].

There are technical publications and building codes that suggest personal space standards depending on the type of space and the activity to be carried out. However, these guidelines do not provide much information on enclosed/confined spaces. The International Building Code [60] uses a series of occupancy factors to determine the maximum number of people that a space can safely accommodate rather than using a fixed minimum area per person.

However, it should be noted that, in addition to counting the number of people entering and leaving an enclosed/confined heritage space, it is useful to know the average or total number of people in the space at any given time, i.e., the VO. In this context, it is more important to determine VO levels hourly or at any given time than calculate the average visit time of each individual, since doing the former helps to continuously maintain appropriate IAQ parameters throughout the entire daily visiting hours. This requires a comprehensive approach that considers the building’s design, the activities that take place within, and the applicable health and environmental regulations. Monitoring VO is crucial precisely for determining adequate ventilation airflow rates and establishing early warnings of overcrowding.

The VO rate can be obtained by tracking visitor entries and exits to the site, either by performing manual counts or with image sensors, and then calculating a weighted average of the number of people present at any given time. However, the ideal solution is to have a Visitor Occupancy Detection System [61]. These systems use various technologies such as video cameras, infrared sensors or thermal imaging to monitor movement and detect when people enter or exit a defined area. These Wi-Fi connected image sensors use AI for detecting and counting people, which send data in real time.

4.4. Defining of the Metabolic CO₂ Produced by a Person

CO₂ generation by occupants is essential for determining VCC in an enclosed/confined heritage space, as visitors are the main sources of CO₂ emissions in these sites [48,62,63].

The volume of CO₂ generated per person can vary between 12.90 and 16.10 litres per hour (L/h) depending on factors such as age, gender, local temperatures, and other variables [64]. The ASTM D6241-18 standard [65] suggests using a formula that includes constant coefficients to estimate the metabolic rate of CO₂ generation. This formula considers the respiratory coefficient (with a value of 0.85) and the Dubois body surface area in square metres (with a reference value of 1.8 m² for an adult of average height and weight). The type of activity is also taken into account, with the proposed value of 1.4 used for ‘filing and standing’ activities according to ASHRAE [66]. Therefore, the value of the CO₂ emission rate per person considered in this study is 22.03 L/h.

4.5. Monitoring Environmental Parameters

The monitoring of fundamental environmental parameters, such as CO₂ concentration, temperature, and relative humidity, under varying visitor flow rates and during occasional public closures is essential for establishing the baseline data required for the sustainable management of enclosed/confined heritage spaces. These data provide information on the fundamental characteristics of the state and environmental dynamics in the context of human impact. There are interesting studies on this topic, especially in caves, as they are confined spaces very popular with visitors [67,68,69].

At this stage of the process, it is apparent that digital technologies are essential for the real-time monitoring of environmental parameters. There is a wide range of sensors with varying degrees of performance. Ideally, an advanced measurement, data logging, and storage system should be available, capable of continuously recording data according to a predefined schedule, while also enabling real-time viewing for subsequent processing and analysis. In this way, efficient data transmission systems play a critical role.

5. Solution Development for the Pilot Site

This phase of the research describes the solution developed for a designated test area. In our case, this is the Chapel of the Holy Chalice in the Valencia Cathedral, Spain (Figure 2).

Figure 2. Location map of the Valencia Cathedral, Spain (left image)); situation of the Chapel of the Holy Chalice (central image), and photo of the interior of the Chapel (right image). Source: own elaboration, and photo from J.M Gandia (July 2025).

5.1. Identify and Understand the Characteristics of the Pilot Case

This Cathedral is the most important religious building in the city of Valencia and receives a significant number of visitors and parishioners on a daily basis (±1500 people on average). The Cathedral houses the Holy Chalice used by Jesus Christ during the Last Supper, considered to be one of the most significant relics of Christianity [70]. It is located in an indoor Chapel that is frequently visited, which justifies its selection as the site of the pilot experimental study for testing the IAQ.

The Chapel has a square floor plan measuring 13.42 m by 13.72 m, resulting in a total area of 185 m². The walls reach a maximum height of 16.20 m, while the apex of the star-shaped dome at the keystone attains a height of 18.79 m (Figure 3). The interior volume of the Chapel was determined using HBIM, based on the 3D point cloud obtained from the digital model of the Cathedral and was found to be 3100 m³, without considering the volume occupied by furniture and objects. This religious space features a single entrance and an interior door connecting it to the Cathedral Museum. As there is no natural ventilation via openable external windows, air renewal is difficult. The Chapel is equipped with air-conditioning for cooling or heating, and the temperature is adjusted as required, but there is no additional mechanical ventilation.

Figure 3. (a) Floor plan of the Holy Chalice, where the environmental sensors (black dots) and image sensor (black square) are located (heights: sensor 01: 2.00 m, sensor 02: 1.80 m; sensor 03: 1.80 m; sensor 04: 4.70 m); (b) Chapel Section with dimensions; (c) Interior view of the Chapel and altarpiece displaying the Holy Chalice. Source: Own elaboration, and photo from K. Smaha (July 2025).

The Cathedral’s external environmental conditions are determined by the climate of Valencia, which is classified as a hot-summer Mediterranean climate (Csa) according to the Koppen-Geiger classification. The default value for CO₂ is considered to be 426 ppm.

It is a space that combines religious use (e.g., masses, baptisms and wedding events) with public visits, but at different times for each activity. The Cathedral is frequented twelve hours a day, except on Sundays when it closes early, and also on certain days for exceptional celebrations.

Recent studies on the Carrying Capacity of parishioners and visitors [59], based on its USV, provide an estimate of 42 visitors, considering the available floor space for transit of people, standing and viewing areas, and seating facilities. For religious activities with seated parishioners, the maximum capacity was 93.

5.2. Defining Chapel Environmental Conditions

Various punctual experiments have been carried out to determine the Chapel VO based on IAQ. This was the only feasible option, given that the Chapel has been open to the public for many decades, and no studies have ever been conducted before. Thus, several CO₂ surveys were conducted under different occupancy conditions using various counting equipment and sensors to determine the most effective approach. The analysed events were: (a) a 12 h period without occupancy, recording CO₂ by sensors (8–9th October 2024); (b) a 12 h period of intense occupancy, with manual people counting (1st of November 2024); (c) a 9 h period of variable average occupancy, with manual people counting (1st of June 2025); (d) a 5 h 30 min period of occupancy, with manual people counting compared to automatic counting and collated with the evolution of CO₂ registered by sensors (5th of July 2025).

5.3. Design and Application of a Monitoring Environmental System

The following section presents the equipment implemented to environmental monitor parameters. It should be noted that the digital devices and tools used in this pilot project are commercially available and affordable, and that software used is open source. Furthermore, the information produced by the system is easy for end users to understand.

CO₂, temperature and humidity have been recorded using three commercial sensors, employing capacitive elements for humidity and NTC thermistors for temperature, ensuring reliable characterisation of the indoor environment. The Testo 160 IAQ, sourced from Testo SE & Co. KGaA (Titisee-Neustadt, Germany), the Witeklab CO₂ sensors supplied by Witeklab SL (Barcelona, Spain) and the Milesight AMD319 sensors sourced from Milesight Technology Co., Ltd. (Xiamen, China), are all based on the non-dispersive infrared (NDIR) principle. They were placed at average human height, in areas with restricted access to prevent damage, while always respecting the heritage, and also to avoid possible anomalous readings caused by one or more visitors standing too close to the sensor. They were located away from entry points to minimise disturbances from localised airflows. The sensors were fixed with clamps, without drilling. An on-site validation was carried out over a two-hour period, confirming consistency among commercial CO₂ systems, with deviations of approximately 5–10 ppm.

Collected data are transmitted via Wi-Fi, 4G-LTE or LoRa (Long Range Wide Area Network) to their respective cloud platforms. They ensure secure transmission and storage as well as enable real-time monitoring, analysis and alarming on computers and mobile devices.

The CO₂ trends also aligned with observed occupancy, as verified through manual counting. In parallel, the people-counting device was installed at the entrance, although its data dashboard revealed potential inaccuracies, requiring additional on-site calibration to evaluate the effects of camera orientation and mounting height.

5.4. Design and Implementation of a People Counting System for the Chapel

Manual and automatic methods were used to count the number of people in the Chapel of the Holy Chalice. Manual counting was performed using the Count Counter app (iOS 2024.12.1), which records precise timestamps. This feature allows users to aggregate the records into customisable time intervals. This enables more precise identification of occupancy peaks and analysis of occupancy patterns at different times of the day.

For automatic counting people, the Milesight VS121 was used. It applies AI algorithms for zone detection, density estimation and directional flows recognition. When installed at the entrance of the Chapel at a height around 4 metres, it covers an area of up to 78 m² and transmits only aggregated counts, thus ensuring user privacy. Following its installation, data dashboard revealed potential inaccuracies, requiring additional on-site calibration to evaluate the effects of camera orientation and mounting height.

5.5. Procedure of Data Acquisition

Special attention was paid to the data acquisition procedure to ensure the integrity, availability, and applicability of environmental and occupancy information within the heritage space. In this study, secure transmission systems were combined with various data management platforms.

The Testo 160 IAQ data logger operates as a recorder with an internal memory for 32,000 samples, that are recorded at configurable intervals and stored locally for up to three months. Data are synchronised with the Testo Cloud platform via Wi-Fi or 4G-LTE, allowing for real-time access and review of historical environmental data, as well as alarm management, and generation of automatic or manual reports. The local memory buffer ensures continuity during network outages by overwriting the oldest entries when full.

The Witeklab monitoring system transmits data to a centralised server in real time, with no local storage on the sensor. This information is stored in a secure MySQL database hosted on a redundant server infrastructure and accessed via a web platform or mobile app. This enables remote monitoring and configuration, although a constant internet connection via a Wi-Fi network or cellular data gateway (3G/4G/5G) is required. Data can be downloaded in .pdf or .csv formats from both systems.

The counter people is configured locally via Wi-Fi (2.4 GHz), but it transmits measurements using LoRaWAN^®, which is a low-power IoT protocol that can be adapted to large areas or environments with limited infrastructure. This device sends compact data packets to a gateway that connects to the cloud server, reducing the risk of data loss and simplifying the implementation. Figure 4 illustrates the key components of the two sensor systems (environmental parameters and people counting) and the data collection/transfer processes.

Figure 4. Sensor systems implemented in the Chapel of the Holy Chalice. Source: Own elaboration.

6. Solution Implementation and Demonstration in the Chapel of the Holy Chalice

Once the sensors had been installed and their correct functioning verified, data collection and subsequent analysis were carried out.

6.1. Air Change per Hour (ACH)

The air renewal in the Chapel was estimated using the Decay Step-down method, particularly focusing on the estimation of decay (A_Decay-A_D-) of the ACH [55], considering the time from when the Chapel was vacated after occupancy (t₁) until it was reoccupied (t₂). During this interval, CO₂ concentration was known at both time points, allowing the natural reduction in indoor CO₂ to be measured directly under passive ventilation conditions. This allowed the ventilation rate to be derived directly from observed CO₂ decay during real vacancy periods, providing a value representative of the Chapel’s actual behaviour without relying on theoretical ventilation models. The value obtained using this procedure was:

A_Decay = 0.1575 ^{h − 1}

Since this value is less than 1, it indicates that in one hour the air in the Chapel (3100 m³) has not been renewed even once. Therefore, to achieve an ACH rate of 1, that is, a full renewal of the Chapel, at least 6.36 h are required. This is a particularly low rate, because in one hour, not even a sixteenth of what needs to be renewed is replaced.

6.2. CO₂ Decay (V_DCO₂) by Passive Ventilation

The CO₂ decay was calculated to estimate the reduction in CO₂ in the Chapel through passive ventilation (V_DCO₂), i.e., natural air exchange through leaks or small openings. This data is essential for determining the amount of CO₂ that visitors can emit without exceeding the established maximum concentration of 1000 ppm, and thus for estimating occupancy based on IAQ. The same period without visitors used for A_Decay estimation was applied, during which the Chapel remained vacated so that the only variation in CO₂ corresponded to natural ventilation.

A linear regression analysis was then applied to determine a CO₂ loss rate, which was found to be 35.73 ppm per hour, with the concentration decreasing to 505 ppm. Considering the volume of the Chapel (3100 m³), this corresponds to 110.82 litres of CO₂ being removed per hour. This served as the basis for calculating the amount of CO₂ that can be reintroduced when visitors are present.

V_DCO₂ = 110.82 L/h

6.3. Evolution of CO₂ Concentrations According to the Visitor Presence

The results of monitoring the various experimental tests (Figure 5) show a clear relationship between visitors’ presence and CO₂ concentration in the Chapel.

Figure 5. Monitoring of the number of visitors (vertical line histogram) inside the Chapel of the Holy Chalice in relation to CO₂ concentration (red curve) throughout 1st of June 2025, where high concentration levels of CO₂ are associated with the evolution of visitor numbers throughout the day. Source: own elaboration.

For example, on Sunday 1st of June 2025, during the early hours of the day (0:45 a.m.), in the absence of visitors, the CO₂ concentration was 828 ppm. This gradually decreased to 569 ppm by 7:30 a.m., reflecting the natural ventilation of the space.

From 9:00 a.m. onwards, a sustained increase in CO₂ concentration was observed in parallel with the arrival of visitors, exceeding 1000 ppm at around 11:45 a.m., and reaching a maximum of 1040 ppm at 12:15 p.m. Between this time and 1:30 p.m., there was a plateau with a slight decrease in concentrations, coinciding with an interval without visitors. Subsequently, during the mass with the Chapel door closed, and with a constant attendance of about 18 people for approximately 45 min, a new increase was recorded, reaching the maximum peak of the day at 1119 ppm. It should be noted that between 9:00 a.m. and 2:15 p.m., the Chapel reached peaks of up to 76 PAOT, which confirms the direct correlation between high attendance and deterioration in IAQ. During the afternoon, between 2:15 p.m. and 6:00 p.m., there was a significant decrease in both visitor numbers and CO₂ levels. CO₂ concentrations remained between 830 and 958 ppm, with a maximum PAOT of 44 people being reached only in brief intervals.

The increase in CO₂ in the Chapel, as shown in the previous Figure 5, is directly linked to the presence of visitors, who generated it through their metabolic activity (22.03 L/h).

6.4. Carrying Capacity of Visitors and Parishioners and Visitor Occupancy (VO)

As previously mentioned, in confined spaces, such as the Chapel of the Holy Chalice, it is not the USV (floor available area) that determines the carrying capacity, but rather the IAQ and CO₂ concentrations. This analysis is closely linked to the presence of people, as mentioned above.

To calculate this, an outdoor CO₂ concentration of 426 ppm was taken as the starting point. The indoor CO₂ limit adopted for this site was 1000 ppm. The difference between these two values indicates the additional amount of CO₂ that can be safely introduced into the Chapel before reaching the established limit. Applying this difference to the volume of the Chapel gives an approximate result of 1777 litres. This volume is then combined with the CO₂ naturally removed by passive ventilation over the 12 h opening period, which was previously calculated to be 110.82 litres per hour. This results in a total of approximately 3109.30 litres of CO₂ that the space can contain without exceeding the 1000 ppm threshold.

The final result is expressed as the maximum PAOT who can occupy the Chapel simultaneously without exceeding the CO₂ threshold during continuous occupation (VO) and taking into account the outdoor CO₂ concentration and natural ventilation. In this case, this equates to 12 people (visitors or parishioners) at the same time during the Chapel’s 12 h opening period:

VO = 3109.3/(22.03 L × 12 h) = 12 persons simultaneously for 12 h constantly

It is important to note that the VO is not constant; visitor numbers fluctuate throughout the day. Nevertheless, this calculation provides a safe upper limit to maintain CO₂ concentrations within recommended levels, ensuring the safety and comfort of all users.

7. Solution Evaluation

Once the VCC and VO have been estimated and implemented, the results should be validated and areas for improvement in the procedure identified.

The suitability of the different devices used in the project has been considered, and it is believed that the potential of each instrument should be fully understood in order to select the most suitable for each situation and location, and to ensure that they can be used in a coordinated manner.

Another relevant aspect is the significance of the data transmission system. Special attention has paid to ensure that it operates under optimal conditions and to identify any potential issues that could cause malfunctions. Based on the insights gained from the pilot case, the convenience of data loggers in storing data for a reasonable period of time is evident, which also allows for the development of these studies with challenging data connectivity.

On the other hand, manual visitor counting is also considered relevant, because it has served to validate the accuracy of the sensor data, and has also produced more detailed raw information instantly, which allows for the identification of specific peaks that go unnoticed by the sensors.

At this stage, the procedure was validated by comparing the data obtained from CO₂ concentration measurements and recorded visitor occupancy throughout the operating hours, cross-checked against manual counts. An important consideration is viewing the work procedure as a flexible tool that must adapt to the multiple occurrences and scenarios that may occur at heritage sites.

Finally, following the implementation of the procedure in the pilot case, it is worth noting the advisability of developing a DT based on the HBIM of the site and with the support of AI, all possible visitor scenarios can be tested to give the system maximum predictability. This allows to anticipate possible non-intrusive solutions and preventive strategies for alerts, visitor carrying dispersion strategies and conservation of the heritage asset.

8. Discussion

Following the development and validation of the successive phases of the methodological procedure, the results are presented and discussed.

The results of this study clearly demonstrate that conventional area-based calculation tools, such as USV and PAOT, are insufficient to estimate VCC, since elevated CO₂ concentrations considerably constrain VO. In these enclosed/confined spaces, CO₂ serves as a reliable tracer gas for VCC assessment, owing to the linear relationship between CO₂ concentrations and number of visitors. This relationship provides a solid empirical basis for estimating real-time occupancy and defining maximum capacity thresholds. Furthermore, monitoring CO₂ concentrations and their temporal variation in indoor settings provides a valid and practical means of estimating the actual number of occupants as demonstrated in the Chapel of the Holy Chalice pilot case. While the estimation of PAOT remains fundamental metric in VCC studies, particularly for open spaces, enclosed/confined sites require more precise information on VO. This is because PAOT represents a static parameter, given that the USV remains constant. Conversely, IAQ is a dynamic variable fluctuating continuously during visiting hours as a result of the ongoing presence and activity of visitors, which directly influence CO₂ concentrations and then, it determines the real capacity of these enclosed/confined spaces. Therefore, establishing a representative average VO throughout the day becomes essential to ensure stable occupancy conditions that do not exceed the CO₂ concentration thresholds established for spaces without mechanical ventilation.

Digital technologies have significantly enhanced the efficiency, sustainability and productivity of Visitor Carrying Capacity (VCC) assessments, but they are especially required in the case of VCC estimation for enclosed/confined spaces. It should be noted that the estimation of the volume of enclosed/confined spaces with complex geometries requires the use of HBIM, following a laser scan of the site to obtain accurate and detailed 3D models. This HBIM-based volume spatial analysis has proven essential for the integration of environmental and visitor flow data. Within this context, the role of digital monitoring systems through environmental and image sensors becomes crucial, as they provide real-time information on dynamic phenomena, such as visitor flows and environmental parameters, that fluctuate according to VO levels. Definitively, the integration of digital technologies provides access to extensive datasets, offering substantial potential for predictive analysis and sustainable management strategies driven by real-time information. Such integration enables early warning systems for overcrowding or excessive CO₂ concentrations, and therefore, potential risks to the heritage assets and visitor well-being can be mitigated. Consequently, it enables cultural and tourism managers to make evidence-based, timely decisions that respond effectively to changing conditions, while balancing conservation priorities with visitor comfort and safety. It is important to note that the digital equipment employed in the pilot project is based on open-source software and commercially available devices, making the system economically accessible for heritage institutions. Furthermore, the data generated are presented in a user-friendly format, facilitating comprehension and management by non-specialist end users.

Although the proposed methodology and findings provide valuable insights into VCC assessment in enclosed/confined heritage sites, several limitations must be acknowledged.

First, this methodology could be further enhanced by incorporating additional pilot cases to include other scenarios and variables. Different architectural configurations, materials, ventilation systems, and visitor behaviour patterns may influence IAQ and CO₂ dynamics in ways not captured within this study. Consequently, broader application across diverse heritage contexts is therefore required to refine the proposed procedure.

Second, the temporal scope of the monitoring campaign was limited. Short- to medium-term data collection restricts the ability to analyse long-term seasonal variations in IAQ, occupancy trends, and environmental responses. Extended monitoring periods would help establish more robust patterns and enhance the accuracy of predictive modelling.

Third, while CO₂ concentration was identified as the principal indicator of VO and IAQ, other environmental parameters, such as temperature, humidity, particulate matter, or volatile organic compounds, were not examined in depth. These may also affect visitor comfort and heritage material conservation, and their integration into future analyses could offer a more holistic understanding of environmental performance.

In addition, although the use of digital tools such as HBIM and real-time sensors proved effective, limitations related to data integration, synchronisation, and system calibration were observed. Variations in sensor accuracy, connectivity issues, and delays in data transmission may introduce small measurement uncertainties. Developing standardised protocols for data validation and system interoperability will be essential in future studies.

Finally, while the proposed methodology emphasises affordability and accessibility through open-source tools, successful implementation still depends on the technical capacity and organisational structure of heritage administrations. Institutional readiness, staff expertise, and resource allocation may influence the feasibility and long-term sustainability of deploying such systems at scale.

9. Conclusions

As final conclusions on this work, several reflections emerge that should be considered in VCC studies to ensure the sustainable management of eclosed/confined heritage sites, and to guarantee their long-term conservation and a comfortable, safe, and high-quality experience for visitors.

First and foremost, it should be noted that this study on visitor carrying capacity in enclosed/confined heritage buildings is pioneering in its field because, with the exception of natural caves, no other studies have addressed this visitor management challenge.

The findings of this study demonstrate that conventional area-based methods developed for open spaces, such as USV and PAOT, are insufficient for determining VCC in enclosed/confined spaces. In these environments, IAQ, particularly CO₂ levels, emerges as a dynamic parameter, reflecting human presence and fluctuations. Among IAQ parameters, CO₂ concentration is identified as the most critical, since elevated CO₂ levels directly limit human presence and affect visitor comfort.

Another key conclusion is that both the estimation and the real-time monitoring of VCC in enclosed/confined heritage sites are unfeasible without the integration of digital technologies, particularly TLS, HBIM, and Sensor Technology. Real-time, data-driven monitoring is no longer merely a methodological advancement; it is a fundamental requirement for safeguarding the long-term conservation of such heritage sites and maintaining the quality of visitor experience in destinations with high tourism pressure.

The results obtained from the pilot study conducted in the Chapel of the Holy Chalice at the Valencia Cathedral by using digital technologies validate this approach. Continuous monitoring of CO₂ concentration and its temporal variations proves to be an accurate and practical method for estimating real-time VO. Incorporating VO into VCC analysis enables the establishment of more precise and adaptive capacity thresholds, ensuring that IAQ remains within acceptable limits and that visitor numbers can be dynamically regulated throughout the day.

Building upon the findings and limitations identified in this study, several directions for future research can be outlined. First, expanding the analysis to multiple case studies would enhance the robustness and transferability of the results. Future investigations should therefore apply and validate the proposed VCC methodology across a wider spectrum of enclosed/confined heritage sites that differ in spatial configuration, construction materials, and visitor dynamics. Comparative analyses across such diverse contexts will facilitate the refinement and generalisation of the model, as well as the identification of typological patterns that influence IAQ behaviour. Furthermore, long-term monitoring campaigns are required to capture seasonal variations, long-term occupancy trends, and the cumulative effects of environmental conditions on both heritage materials and visitor comfort. Continuous, year-round data collection would provide the foundation for developing more accurate, reliable, and resilient predictive models.

Future research should also address the integration of additional environmental parameters beyond CO₂, including temperature, humidity, particulate matter, and volatile organic compounds. Incorporating these variables will enable a more comprehensive assessment of IAQ and its implications for both heritage conservation and visitor well-being.

Moreover, future studies are encouraged to advance digital modelling and simulation approaches by exploring the integration of Computational Fluid Dynamics (CFD), Digital Twins (DT), and Artificial Intelligence (AI). The combined use of these tools can simulate complex IAQ dynamics and facilitate automated, real-time monitoring. Such developments would significantly enhance the predictive capacity, responsiveness, and adaptability of VCC systems under diverse environmental and operational conditions.

An important avenue for future research concerns the development of comprehensive data integration and management platforms. Further studies should focus on designing robust and interoperable systems capable of aggregating sensor data, HBIM information, environmental models, and visitor flow datasets. Particular emphasis should be placed on enhancing data synchronisation, minimising latency, and establishing standardised protocols for data exchange across different systems and institutions.

Finally, institutional implementation and policy frameworks must also be addressed. Future work should examine the organisational and governance dimensions of digital VCC management including the development of training programmes, decision-support tools, and policy guidelines. These efforts will be crucial to facilitate adoption by heritage institutions and to ensure the long-term operational sustainability of such digital systems.

Overall, this research provides a foundation for advancing the sustainable management of indoor environments in heritage sites, while supporting evidence-based decision-making for conservation and visitor comfort. By proposing a replicable methodology and identifying key directions for future research, it contributes to bridging the gap between environmental monitoring, digital technologies, and heritage management practices. In this regard, the study aligns with the objectives of Sustainable Development Goal (SDG) 11, which seeks to make cities and human settlements inclusive, safe, resilient, and sustainable. Specifically, it supports Target 11.4 by promoting the protection, safeguarding, and sustainable use of cultural and natural heritage as integral components of sustainable urban development.