The Interplay between Air Quality and Energy Efﬁciency in Museums, a Review

: Energy efﬁciency in museums and buildings that house works of art or cultural heritage appears to be a difﬁcult achievement if indoor air quality has to be kept at appropriate levels for artefacts’ long-term sustainability. There is a gap in our scientiﬁc literature on the relationship between indoor air quality and energy efﬁciency, meaning that there are no numerical data that examine both of them simultaneously, although this is a theme that is broadly discussed by museum managers, curators, and scientists. It is certain that the two parameters, indoor air quality (IAQ) and energy efﬁciency (EEF) are conﬂicting and difﬁcult to reconcile. Furthermore, IAQ is not only the determination of temperature, relative humidity, and CO 2 , as is usually presented. Using green or renewable energy does not make a building “energy efﬁcient”. Hence, in the manuscript we review the literature on IAQ of museums and exhibition buildings, in conjunction with the consideration of their EEF. Hopefully, reviewing the literature for this problem may lead to carefully designed monitoring experiments. The selection, application, and testing of appropriate technological measures can lead to a new balance between the two conﬂicting parameters. Not only must solutions be found, but these solutions are necessary in the mitigation battle against climate change.


Introduction
Climate change is a reality in today's world [1]. It is strongly dependent on the energy production by fossil fuels and the energy consumption by diverse human activities, both of which contribute to CO 2 and other greenhouse gases emissions. Climate change and occasionally erratic power supply conditions sped up the political decisions to reduce energy demand and to produce renewable energy, i.e., energy derived from natural sources that are replenished at a faster rate than they are consumed. Furthermore, energy efficiency (EEF) is strongly pursued in an effort to use less energy to achieve the same task or product.
The built environment is critical to achieving energy efficiency and other environmental objectives. Homes and buildings that are energy efficient use less energy to achieve comfortable living and working conditions. The Directive 2012/27/EU and the 2018 amending Directive on Energy Efficiency in the EU seek to save energy while also promoting good indoor air quality and comfort. However, they do not specify how to achieve both the goals of energy savings and satisfactory indoor environmental quality. The legislation governing indoor air quality (IAQ), in particular, is fragmented.
The amount of particulate matter (PM), airborne inorganic, organic, and biological particles, such as SARS-CoV-2, fungi, bacteria, and gaseous pollutants, present in the indoor Indoor air quality is a crucial parameter for preserving the high value and cultural significance of objects housed in museums or historical buildings, over the centuries. Gaseous pollutants, particulate matter, and microorganisms, if present in a museum, can create sometimes irreversible damage to works of art. For example, soiling, corrosion, color fading, cracks, and mold. In times of climate change, the world of museums encounters a new challenge: energy saving.

Materials and Methods
The publications that are included in the present work were written in English and were retrieved from several databases, such as Google, Scopus, Google Scholar and Web of Science. Examples of the key words used were: climate change; indoor air quality; electricity consumption; energy efficiency; greening; indoor environmental quality; retrofit; refurbish; thermal conditions; decarbonization; sustainability; controlling IAQ; energy saving. These key words were used in combination with the word "museum" or "cultural heritage" or "works of art" or "historical building".
This review is organized as follows: (a) a presentation of a basic IAQ model that describes the parameters that make up the IAQ and how the IAQ is related to the building's energy consumption; (b) a description of the threat to the conservation of the housed cultural heritage, a summary of proposed indoor air quality guidelines for some key atmospheric pollutants, and a summary of their measured atmospheric concentrations in museums; (c) a summary of the studies that reported indoor atmospheric pollutant monitoring with energy consumption/saving in museums; (d) interventions to improve IAQ in museums, in conjunction with EEF measures.
The current review highlights gaps in our understanding of the relationship between IAQ and EEF, i.e., what knowledge is well established and what requires further scientific investigation.

Indoor Air Quality and Energy Consumption Fundamentals
IAQ is affected by the concentrations of air pollutants, gases, or particles in the atmosphere. Indoor air pollutants can be transported from outside or emitted from inside. The indoor air pollutant concentrations and their diurnal variation will be shaped by building characteristics such as air exchange rate, material of the indoor surfaces, and occupant activity. Depending on the indoor activities, atmospheric pollutants emitted indoors may be the most serious threat to poor indoor air quality. To demonstrate this, a simple indoor model, shown in Figure 1, is discussed below.
In each room or in an entire building, where air can be assumed to be well mixed, the rate of change of an air pollutant concentration can be described by a mass balance equation, as follows [37,38]: where C in and C out are the respective indoor and outdoor atmospheric pollutant concentrations (µg m −3 ); f is the air flow rate (m 3 h −1 ); the room air exchange rate (AER) is equal to f /V (h −1 ); and S represents the emission rate of the air pollutant from the indoor sources (µg h −1 ). The air pollutant loss rate R can be considered to be equal to u d AC in , where is the average deposition rate of each pollutant on indoor surfaces. V is the volume of the room (m 3 ), A is the total interior surface area (m 2 ), and u d is the average deposition velocity of each air pollutant (m h −1 ). The parameter P only refers to PM and is defined as the particle penetration coefficient (dimensionless) [39]. In the case of gaseous pollutants, P = 1. In each room or in an entire building, where air can be assumed to be rate of change of an air pollutant concentration can be described by a ma tion, as follows [37,38]: where Cin and Cout are the respective indoor and outdoor atmospheric p trations (μg m −3 ); f is the air flow rate (m 3 h −1 ); the room air exchange rat to f/V (h −1 ); and S represents the emission rate of the air pollutant from the (μg h −1 ). The air pollutant loss rate R can be considered to be equal to = (h −1 ) is the average deposition rate of each pollutant on indoor s volume of the room (m 3 ), A is the total interior surface area (m 2 ), and u deposition velocity of each air pollutant (m h −1 ). The parameter P only ref defined as the particle penetration coefficient (dimensionless) [39]. In the pollutants, P = 1.
Certainly, the simple Equation (1) does not account for all of the var ence indoor air pollutant concentrations. Indoor chemical reactions, as we or phase change processes in the case of PM, interfere with the terms S an velocities are not stable and can be different for the diverse orientation other phenomena also influence the indoor air pollutant concentrations [ Based on the simple model above, we can refute the widely held beli the AER of a room would always improve its air quality. Indoor air pol are not always solved by simply changing the AER. The solution is depe bination of outdoor and indoor air pollutant concentrations, indoor air po and loss rates, etc., and each combination is unique and may not be appli Certainly, the simple Equation (1) does not account for all of the variables that influence indoor air pollutant concentrations. Indoor chemical reactions, as well as coagulation or phase change processes in the case of PM, interfere with the terms S and R; deposition velocities are not stable and can be different for the diverse orientation of the surfaces; other phenomena also influence the indoor air pollutant concentrations [38,[40][41][42].
Based on the simple model above, we can refute the widely held belief that changing the AER of a room would always improve its air quality. Indoor air pollution problems are not always solved by simply changing the AER. The solution is dependent on a combination of outdoor and indoor air pollutant concentrations, indoor air pollutant emission and loss rates, etc., and each combination is unique and may not be applicable for all pollutants under consideration [43,44].
Many studies use CO 2 concentrations as an indicator of IAQ. Indoor CO 2 concentrations are affected by the number of occupants, their metabolic rate, and in buildings without combustion sources, such as museums, they are only indicative of whether the AER is adequate for bio-effluent dilution [45]. Numerous other indoor air pollutants contribute to poor indoor air quality [46]. VOCs, for example, can come from both outdoor and indoor sources, such as human breath, personal care products, and cleaning products [47][48][49][50]. The materials that are used in the construction of the display cases can be also a source of dangerous VOCs, such as formic and acetic acid [51]. Particulate matter of various chemical compositions and sizes causes concern for human health, but it also endangers works of art [52,53].
As a result, there is no universal solution for every difficult indoor air pollution situation. An extensive IAQ monitoring program is required to propose IAQ control measures in each case.
Persily and Emmerich [54] explain that in a building with a heating, ventilation, and air conditioning (HVAC) system, the main link between IAQ and EEF is the parameter f in Equation (1), which is the air flow rate that infiltrates or exfiltrates (or exhausts) from the room per unit of time. The energy consumed to heat or cool the air, and to purify and circulate it within the building's duct system is analogous to the f ; the higher the f, the greater the power requirement [54]. The air flow rate (f ) in a naturally ventilated building is uncontrolled and varies over time. Buildings must consume energy in this case to achieve appropriate microclimatic conditions (heating or cooling), to purify the atmosphere, or to humidify or dehumidify the air [55]. Energy for lighting is also a significant consumer in museums [56]. To summarize, museums' primary energy (E) consumers are: E = heating (cooling) + lighting + air cleaning + air humidifying (or dehumidifying) (2) The air flow rate (f ) and indoor and outdoor air pollutant sources and sinks are the main factors that define the relationship between IAQ and EEF.

IAQ in Museums
It is well established that air pollutants can harm works of art, and in some cases the damage is irreversible [33,57]. For example, O 3 and NO 2 concentrations must be close to zero in order to preserve works of art for an extended period of time [8,58]. Particulate matter in the air comes from both indoor and outdoor sources. The impact on works of art is determined by their size, concentration, and chemical composition [52,59]. The concentration and chemical composition of airborne particulate matter inside and outside of five museums in California (USA) were studied as part of an extensive monitoring program [60]. The Sepulveda House is an historical building with natural airflow and this museum had the highest indoor/outdoor (I/O) fine PM ratio = 0.94, i.e., the same levels as outdoors. On the contrary the Norton Simon Museum (with HVAC and PM filtering system) had the lowest I/O fine PM ratio, which was 0.18. The concentrations of black elemental carbon particles and fine soil-dust particles in all of the museums ranged from 20 to 100% of the outdoor levels, depending on the presence or absence of an HVAC and PM filtering system. At the sites with a HVAC and a PM filtering system, the indoor airborne particles were mostly fine, consisted largely of organic matter and at two sites, indoor concentrations of organic matter exceeded the respective outdoor concentrations. According to the authors, PM deposition can affect the surfaces of works of art by soiling them or by attacking them chemically, depending on PM chemical composition [60]. Table 1 compares guidelines developed by the World Health Organization (WHO) for the most common air pollutant concentrations for human health protection, with guidelines proposed for the preservation of housed works of art for the same pollutant. The preservation target is the amount of time (in years) that the objects can be exposed to the indicated level of pollutants without deterioration.  Table 2 summarizes the findings of some monitoring campaigns that measured indoor and outdoor pollutant concentrations (and the respective I/O ratio) in museums or historical buildings. Only the results for the pollutants presented in Table 1 are included in  Table 2. It should be noted that the average time for the reported atmospheric pollutant concentrations and the applied monitoring method varies by museum.  Several studies, both old and new, have reported particle number concentrations and size distribution, PM chemical composition, VOCs, organic and inorganic acids, SO 2 , H 2 S, CO 2 , and microorganisms [16,17,28,32,[72][73][74][75][76][77][78][79][80]. Table 2 shows a small sample of such studies to demonstrate that the issue of elevated indoor air pollutants is always current.
Works of art have been and continue to be threatened as a result of a variety of indoor and outdoor pollutant sources [81]. Museums are typically located in congested, central areas with significant outdoor air pollution, which can be transported indoors via windows, ventilation systems, or by visitors (for example road dust from their shoes) [82]. Indoor sources, on the other hand, are unambiguous in museums. Visitors are an obvious and significant source of indoor air pollution. People emit particulate matter, including bioaerosols, through their skin and clothing, and by speaking and breathing, and they also cause PM resuspension [83][84][85]. Other chemical compounds, such as VOCs, are also emitted by people through their breath and personal care products, as well as from building and construction materials [50,86,87]. Vacuuming and cleaning the rooms can release a variety of gaseous and particulate pollutants into the atmosphere [49,88]. A sophisticated HVAC system with appropriate air filtration is the most commonly recommended intervention to reduce indoor air pollutants [89]. Without air filters, the HVAC system cannot adequately protect the artworks housed [90]. Hu et al. [69] measured indoor air pollutants above the recommended levels in five museums (three of which had HVAC systems but no filtration) ( Table 2). Hisham and Grosjean [64,91] monitored the IAQ in museums in California, USA; these authors measured indoor and outdoor concentrations of O 3 , NO 2 , SO 2 , H 2 S, total reduced sulfur (TRS), peroxyacetyl nitrate (PAN), methyl chloroform, and tetrachloroethylene in three of the 12 examined museums. In the remaining nine museums, they monitored NO 2 , PAN, nitrate, methylchloroform, tetrachloroethylene, and nitric acid. They concluded that the chlorinated hydrocarbon I/O concentration ratios were greater than unity in all locations, indicating indoor sources. Many consumer and industrial products, such as cleaning products, contain chlorinated hydrocarbons [64,92]. Hisham and Grosjean [64,91] observed that the influx of outdoor air diluted the chlorinated hydrocarbons and reduced their I/O ratio. On the contrary, because pollutants like PAN originate outside, the influx of outdoor air increased their I/O ratio. Except for two museums, the authors reported that PAN indoor concentrations were close to outdoor levels in all of them. They observed that, in museums without an HVAC system, indoor air pollutant concentrations were very close to outdoor levels. They found a modest removal of NO 2 , PAN, and chlorinated hydrocarbons in museums with HVAC systems. Only one of the three museums equipped with HVAC and chemical filtration achieved low I/O air pollutant concentration ratios [64].
Nazaroff et al. [60] monitored airborne particles indoors and outdoors of museums in California, USA: a historical museum, an archaeological museum, and three art museums. In the historical museum, the Sepulveda House, the indoor fine-particle concentrations were nearly identical to those outdoors. However, the coarse particle concentrations were much lower than outdoors. The indoor particle concentrations at the three museums with sophisticated HVAC systems and particle filtration were significantly lower than those measured outdoors. In particular, the lowest indoor particle concentrations were found at the Norton Simon Museum. In this museum, with a high air recirculation rate, the infiltrated air from outdoors and the recirculated air passed through a series of air filters. Thus, among all the museums studied, its indoor PM levels were the lowest [60]. Godoi et al. [90] found that the Oscar Niemeyer Museum (MON) in Curitiba, Brazil has satisfactory air quality, which may be favoring the conservation of its art collection. The quantity of bulk particles in the MON was lower than outdoors. However, air filters with a higher efficiency were suggested for use in the HVAC system of the MON, particularly to capture fine particles.
HVAC systems with an appropriate air filtration system (and heat recovery) make it possible to prevent outdoor atmospheric pollutants from entering the museum. Thus, the inflow of fresh outdoor air can be increased (f in Equation (1)) to supply clean outdoor air indoors to dilute the indoor-generated atmospheric pollutants. However, applying such a system to historical buildings, such as churches, is difficult, and it is unclear whether they will function properly to provide stable conditions for the conservation of works of art [93]. In any case, it is an expensive addition to any museum, old or new. To keep these systems running properly, not only for the safety of visitors but also to maintain stable conditions for the housed works of art, they must be constantly monitored, resulting in an increased cost for their operation and maintenance. Failure of these systems can have serious consequences for the conservation of works of art [94][95][96]. Humidifiers or desiccant systems are also used in museums to control humidity. These systems must be carefully installed and maintained in order to function properly [18,97].

IAQ and EEF in Museums
Saving energy and producing energy from renewable sources has become a global necessity, and it is a critical component of the "green building" concept. The IEQ is a fundamental credit category in many green building rating systems (GBRSs), including LEED, Green Globes, WELL, and BREEAM [98]. However, in a study published by Licina and Langer (2021), measurements did not demonstrate that these GBRSs reduced indoor atmospheric pollutants [99]. In the case of museums, particularly historical buildings, it appears to be a difficult endeavor to reduce indoor air pollution while also saving energy [100].
Several studies have emerged that propose solutions for retrofitting museums to save energy. For example, Katsaprakakis et al. [101] present the findings of a proposed project aimed at improving the energy efficiency of the Natural History Museum of Crete, Greece, from class D to class A+ using a combination of passive and active measures. The IAQ was not assessed. The new Audain Art Museum (Whistler, BC, Canada), a Class AA building, was opened in 2020. In this museum, overall heating energy savings account for 70% of total energy savings. As long as the sophisticated air filters work properly, a combination of them ensures good IAQ. However, measurements were not used to assess the IAQ in this study [94].
The balance between acceptable microclimatic conditions (for works of art and visitor thermal comfort) and the EEF was examined in museums by monitoring indoor air temperature and relative humidity, and the EEF was typically evaluated using simulation tools [97,102,103]. CO 2 concentrations were also monitored and reported as an IAQ indicator in some studies. The latter is far from scientific reality. Table 3 compiles a list of relevant publications.  Table 3 shows that proper IAQ monitoring (aerosol, VOCs, etc.) simultaneous with EEF modeling/monitoring is largely absent in museums. Despite this, there are few studies for buildings other than museums. Asere and Blumberga [24] investigated the effect of reducing energy consumption on IAQ in several public buildings in Latvia using a simulation tool. They conclude that energy efficiency measures in public buildings reduce national energy consumption while worsening IAQ. It was dubbed the "energy efficiency/indoor air quality dilemma". Chatzidiakou et al. [108] compared two schools, an old Victorian building and a new, low-carbon designed building. Their findings revealed that school building maintenance and operation, as well as occupant behavior, had a significant impact on IAQ and energy consumption.
Hence, it is more than obvious that a nearly acceptable IAQ requires an appreciable amount of supplied energy (energy consumption), proving the contrasting effects of the two parameters. High IAQ leads to low EEF and vice versa. It is important here to emphasize that dedicated, specific, and systematic studies of these two interplaying parameters are very few or absent from the literature.

Promoting IAQ and EEF in Museums
To ensure the longevity of the aesthetic value of the exhibits, specific and consistent environmental conditions must be maintained in all buildings housing works of art, whether modern or historical. These conditions must be maintained at all times, whether there are visitors or not. Furthermore, the building must provide visual and thermal comfort, as well as good IAQ for visitors and staff. These objectives necessitate a significant amount of energy, primarily for lighting, heating or cooling, and systems that humidify or dehumidify the atmosphere in the exhibition halls, as well as in the storage rooms and offices. Nonetheless, energy conservation efforts in museums are inevitable [109,110].
Ventilation is one of the most important aspects of a museum's IAQ and EEF. The majority of published studies agree that a sophisticated HVAC system with appropriate air filtration can control indoor atmospheric pollutants in the museum, at least to some extent [60,90]. Other control strategies are also available, as is guidance for museum authorities and curators to prevent the deterioration of works of art due to air pollutants. Every museum can set its own air pollutant concentration targets based on the materials that make up its collections (marble, ceramic, silver, copper, and so on). The relevant literature discusses showcases, standalone air purifiers, and non-invasive treatments for exhibits. However, measures that can be taken to achieve adequate IAQ must take other factors into account, such as resource availability and priority setting.
Concerning EEF, several studies proposed museum-specific interventions [105,111,112]. These include using renewable energy sources, natural daylight lighting and replacing old lighting with non-energy-consuming lamps, mixed mode ventilation, phase change materials, display cases, and other building modifications where possible, such as adding an intelligent facade system with external shading, natural ventilation, and night cooling systems [34,56,104,[113][114][115][116].
In addition, new indices [117], such as life cycle analysis, used to calculate energy consumption and greenhouse gas emissions [118], and "choosing by advantages", are being developed to aid decisions for innervations [119].
Controlling energy consumption as well as IAQ in museums and historical buildings that preserve our world's history remains a challenge. Extensive IAQ and EEF monitoring campaigns must be carried out in each museum, tailored to each special housed collection. Every proposed strategy for improved IAQ, comfort, and climate resilience, as well as the goal of energy savings, should be based on the "world" of hard numerical data.
Finally, in the context of climate change, the Coalition of Museums for Climate Justice has encouraged the museum world to consider "their tremendous potential to influence climate action through their programs, research, and advocacy" [120,121]. Museums can contribute to climate change actions, and, because they attract a large number of visitors, these structures can serve as a model for balancing IAQ and EEF [34][35][36].

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.