Volatile Organic Compounds (VOCs) in Heritage Environments and Their Analysis: A Review

Abstract

In the recent years, there has been an increased interest in indoor air quality in heritage environments, specifically in relation to volatile organic compounds (VOCs). These could originate from objects, furnishings, visitors and staff, as well as from olfactory exhibitions. This interest led to a number of studies investigating the “typical” emissions for diverse materials and their impact on the surrounding environment. The analysis of volatile compounds emitted by objects helps in the characterization of the material composition, its conservation history or its degradation processes. This contribution reviews how volatiles are emitted from objects and the commonly used sampling techniques for heritage science applications. A variety of methods are available, from bulk air sample collection to preconcentration using samplers. The commonly studied object types contributing to indoor VOCs are discussed. These include emissions from heritage objects, conservation products, furnishing materials and display cases. Furthermore, olfactory exhibitions are discussed in terms of indoor air quality. Finally, the findings are compared with the current guidelines on indoor volatile concentrations.

1. Introduction

Liquid and solid materials emit volatile organic compounds (VOCs), gaseous substances containing one or more carbon atoms, and are characterized by a high vapour pressure at room temperature [1]. The total concentration of VOCs indoors is as an indication of indoor air quality. In indoor environments, VOCs originate from external sources, such as traffic, and internal sources, such as emissions from objects, building materials, solvents, cleaning agents, visitors and human activities [2,3].

Although volatile compounds are generated in both indoor and outdoor environments and have been of great interest in different fields, this review focusses specifically on indoor heritage environments, such as galleries, libraries, archives and museums (GLAMs) [4,5]. In a museum context, many objects, particularly their organic components, emit volatile compounds.

VOCs are categorized according to their boiling point at a standard atmospheric pressure of 1 atm [6]:

• VVOCs: very volatile organic compounds, boiling point < 100 °C.
• VOCs: volatile organic compounds, boiling point ≤ 250 °C.
• SVOCs: semi-volatile organic compounds, boiling point > 250 °C, up to 380–400 °C.

To evaluate the emissions from items, the chemical composition of the emitted volatiles, the emitted mass and the emission rates are of interest. The emission rate is defined as the unit of mass emitted per unit of time, typically expressed in µg/(m²h) [7,8], and is affected by several factors: physical processes (evaporation, boiling temperature, diffusion, sorption), chemical composition and environmental variables (temperature, relative humidity, ventilation, air velocity) [7].

The identification and quantification of VOC species is crucial, as volatiles can be highly reactive [7]. The study of emissions from historic objects provides information about the materials and their degradation processes [9,10,11], which could include biologically formed VOCs as a consequence of metabolic processes. This could mean that the detection of VOCs could be used as a chemical marker to detect the presence of biodeteriogenic pathogens at early stages, even before the consequences of biodeterioration become visible [12,13]. Furthermore, emitted VOCs are the cause of the perceived smell of a material [14].

In the past, many researchers studied the volatiles present in heritage environments from different perspectives. Grzywacz’s handbook on indoor air pollution considers the effects on collections, sampling techniques and suggests threshold values for the main pollutants [1]. Material damage and tools to assess the risk in museum collections are thoroughly addressed in the literature review by Tetreault [15]. The literature review by Kumar describes the most common methods for sampling volatiles, presenting sampling devices, different types of sorbents and the main analytical techniques [16]. Current research is now investigating the materials used in the construction of display cases to determine which volatiles are emitted and to explore the effects these may pose on heritage objects [2,6].

This review paper looks at the emissions of volatiles from materials as a source of risk, identifying the main volatiles emitted by cultural heritage materials, and provides a state-of-the-art overview of the research carried out in the field and of the standards that have been developed. This work focusses only on organic compounds and does not include inorganic volatiles, such as ammonia, hydrogen sulfide and carbon dioxide. Olfactory exhibitions, in which smells are used as a core element of visitor engagement, are considered with several examples. Despite their increasing popularity, the impact of intentionally introduced VOCs during olfactory exhibitions is largely unexplored. Particular attention is given to the sampling methods used, and the analytical techniques used to characterize VOCs. An overview of how volatiles can provide reliable information about object condition is presented, with the aim of helping conservation managers in their decision making. The current guidelines for preventive conservation in relation to VOCs are reviewed as well.

2. Volatiles in Heritage Environments

Numerous volatiles are present in heritage environments as a consequence of emissions from cultural heritage objects, furnishings and conservation materials and treatments. In addition, visitors and people working in the environment can be emitters as well, both through human activities (respiration, skin emissions) and by the use of personal care products (soaps, deodorants, detergents, perfumes) [3,17,18,19].

It is fundamental to determine the typical concentrations at which organic volatiles are found, to establish a practical background concentration value. The total amount of volatile organic compounds detected is defined as the tVOCs value. This refers to the sum of all the individual concentrations of the detected compounds converted into an equivalent concentration of isobutylene, or another gas chosen for the calibration of a VOC detection device. This value informs conservators on the concentration within the museum. According to the study by Fenech et al., it appears to be fairly constant over time [20]. In her work, Fenech detected a typical concentration of 30–60 ppb in an archival repository environment with peaks of 200–250 ppb corresponding to cleaning activities (Figure 1).

2.1. Volatiles Emitted from Heritage Objects

Heritage objects may emit organic or inorganic volatiles that may be harmful to themselves and surrounding objects [18]. The type of emitted volatile substances depends to a large extent on the materials from which the object is made and their state of degradation, and the impact on the surrounding environment depends on the sensitivity of the material. This section focusses on the most common and studied materials, in terms of volatile emissions, in heritage environments: wood, paper, plastic, wax, medicinal and perfumed objects.

Wood is one of the most common materials found in museums. The most studied compounds emitted by wood are formic and acetic acid, which are more intensively emitted at higher temperatures and during ageing [21]. Acetic acid is formed by the hydrolysis of acetyl esters in hemicellulose. Formic acid, on the other hand, is formed during the microbial fermentation of the sugars present in the wood. The content of hemicellulose is higher in hardwood than in softwood, so that more acetic acid is emitted, while formic acid emissions are generally lower [22]. High values of relative humidity and temperatures increase emissions. However, in museums, where the environment is controlled, such emissions are also under control as a consequence [22]. Other compounds emitted by wood are formaldehyde, which is formed from cellulose, hemicellulose and lignin, to varying degrees, depending on humidity, temperature and treatments [6,23]. Other volatiles include saturated and unsaturated aldehydes that contribute to the often strong odour of wood, such as terpenes: α-pinene, β-pinene, 3-carene and limonene [3,23].

Paper has been widely studied in the field of cultural heritage due to its ubiquity in museums, its susceptibility to degradation and to better define conservation strategies. In the studies by Lattuati-Derieux et al. [11,24], the main volatiles identified were carboxylic acids, aldehydes, alcohols, benzene derivatives, esters, aliphatic hydrocarbons and polycyclic aromatic hydrocarbons. The main compounds were acetic acid, formic acid, hexanal, benzaldehyde, furfural, vanillin and guaiacol [3]. A comparison of VOC emissions from books of different periods [25] also revealed that both contemporary and old books emit acetic acid, furfural and straight-chain aldehydes.

Under ambient conditions, cellulose is degraded to low-molecular-weight organic acids by hydrolytic and oxidative reactions. Acid hydrolysis of cellulose, hemicellulose and lignin leads to the formation of reducing alcohol groups [24,25], while oxidation leads to the formation of carboxylic acids [25]. Other frequently investigated compounds are acetaldehyde and acetone [3,25].

Furfural is formed either during oxidative degradation of glucose, arabinose or xylose or during acid-catalysed hydrolysis followed by a dehydration reaction [3,11,24]. In both cases, furfural is considered a marker for degradation. Gibson et al. [26] have shown that the emission of furfural is more intense at the edges of book pages than in the centre, illustrating how different extents of degradation in the same object can lead to different emissions. Higher concentrations of furfural are also associated with storage rooms rather than reading rooms, due to book density and lower air exchange rates [3,26]. In contrast to furfural, vanillin is a degradation product of lignin and can be considered as its main degradation marker. Other compounds such as benzyl alcohol, benzaldehyde and guaiacol (2-methoxyphenol) are also emitted from lignin. Other minor volatile components can be found in lignin-based paper, including unsaturated fatty acids such as oleic acid, linoleic acid and linolenic acid, which are caused by resin residues in wood fibres [25]. Other paper components such as binders, glues, printing inks, resin residues and waxy compounds degrade as well, producing compounds such as alcohols, aldehydes, ketones, carboxylic acids and hydrocarbons.

The investigation of VOC emissions contributes to the determination of material properties. The production of volatile degradation products can be quantitatively correlated with paper properties, such as rosin, lignin, degree of polymerization of cellulose, carbonyl group content and acidity of the paper [14].

In addition, some of the emitted volatiles have a distinct odour, making it possible to identify old books by smell alone. The most commonly detected by smell are benzaldehyde, ethylbenzene, furfural, toluene, 2-ethylhexanol with a sweetish, almond-like and floral odour [11,25], vanillin with a vanilla-like odour [14] and guaiacol, which is produced during lignin degradation, with a typical smoky odour [25].

Plastic objects are found as works of art in many modern art collections, and the most commonly investigated objects are made of cellulose nitrate, cellulose acetate and poly(vinyl chloride) [27,28].

Objects made of cellulose nitrate produce NO₂ as a degradation product, and volatile emissions include those from plasticizers used in the production of plastics. In cellulose nitrate artifacts, we can detect camphor, which was traditionally used as a plasticizer, and in some objects also diethyl phthalate [27,28,29]. In addition, the emission of furfural, which is formed during the acid-catalysed hydrolysis of cellulose, and camphor derivatives—camphene and campholenal—has also been detected [27,28]. Cellulose nitrate releases NO₂, and although it is not a VOC, it is important as it can corrode metals upon reaction with atmospheric moisture, and the conversion to nitrous acid and nitric acid catalyses the hydrolysis of glycosidic bonds in cellulose, degrading paper and wood [29].

Cellulose acetate objects mainly release acetic acid. This common degradation phenomenon is called “vinegar syndrome” due to the strong odour of acetic acid [27,28]. Cellulose acetate is mainly degraded by hydrolysis, in which the acetate groups on the glucose ring are replaced by hydroxyl groups and acetic acid is released. Therefore, acetic acid can be regarded as a specific marker for the degradation of cellulose acetate [27]. Two common plasticizers are dimethyl and diethyl phthalate, as well as the antioxidant butylated hydroxytoluene in smaller amounts. Accelerated degradation studies of cellulose acetate have detected toluene and phenol. In this context, phenol is an indicator for the degradation of the plasticizer triphenyl phosphate (TPP) [10,27].

A significant proportion of modern art objects in museums is made of PVC, whose main degradation product is HCl, which can accelerate degradation, causing discoloration and decomposition of other objects [27,30]. In PVC samples, the loss of plasticizers is reflected in the formation of bis(2-ethylhexyl)phthalate and its degradation product 2-ethylhexanol [28]. Various volatiles emitted by PVC have been identified, including hydrocarbons from pentane to octadecane, butoxyethanol, cyclohexanone, 2-ethoxyhexanol, 2-ethylhexanol, 2-ethylhexanoic acid, tri-methylbenzene, phenol, diethylbenzene and toluene. These are mainly “non-specific” VOCs, and it is not possible to define a specific marker. Museum objects made of PVC mainly decompose with the formation of HCl as a degradation product, which can accelerate degradation of exhibited objects, such as ceramics and metals [30,31]. The study by Curran et al. [29] shows the cross-infection effect between cellulose and diverse plastics. In the case of cellulose acetate, large amounts of acetic acid are emitted, which catalyses the hydrolysis of the glycosidic bonds of cellulose and accelerates the degradation of cellulose. When this emission occurs at room temperature, it has been shown to decompose other items made of organic materials, metals or other plastics.

Wax is not an extensively studied material in terms of identifying the main volatile emissions in the cultural heritage field, but a strong resinous and waxy odour has been detected in association with wax sculptures stored in display cases [5]. SPME analyses identified 40 compounds such as alkylbenzene derivatives, naphthalene derivatives, n-alkanes, alcohols, n-aldehydes, n-carboxylic acids, benzene derivatives, cinnamic derivatives and monoterpenes. Among these, the most important categories identified as markers for beeswax are n-carboxylic acids with 6–12 carbon atoms, n-alkanes, benzene and cinnamic derivatives. Some of the compounds detected are similar to those found in paper and wood analysis, such as furfural, vanillin and various benzene and cinnamic derivatives [5]. The predominant ones are benzenemethanol, cinnamaldehyde and benzoic acid, compounds defining the typical strong odour of beeswax [5].

Museum collections may also include medicines, drugs, pharmaceutical products and perfumed products (perfumes). These are characterized by both inorganic and organic volatile emissions. In a systematic study at the Smithsonian Institution’s National Museum of American History, the most important volatile organic emissions from pharmaceutical products and cosmetics were analysed [2]. When analysing fragrances, cosmetics and disinfectants, most aroma compounds of plant or animal origin are terpene derivatives: camphene, α-pinene, thujene, phellandrene, terpinene, limonene, terpineol, menthone derivatives, 3-carene and 4-carene. Most of them are strong-smelling compounds such as limonene—which is also often used in cleaning products—with a citrus odour, the isomers 3-carene and 4-carene, which are known for a sweet and pungent smell, and others that smell of resin, rosemary, herbs and wood [2]. Similar is the analysis of medicinal compounds: the most important compounds identified are thymol, isopropyl alcohol, eucalyptol, methyl salicylate, menthol, paraldehyde, 2-propanol, 1,1,1-trichloro-2-methyl-, tribromomethane, acetophenone, trans-Z-α-bisabolene, phenol and camphor [2]. These compounds are characterized by a distinct odour of herbs, wood, citrus, freshness and mint.

In museums, galleries and archives, objects are usually stored in containers such as display cases, storage boxes and cabinets. This affects the indoor air quality, as the material of the enclosures can emit additional volatile substances. In addition, low ventilation or a lack of air exchange in the microenvironment leads to the accumulation of substances generated in the box [6,32]. When different materials are present in the same environment, the volatile organic compounds emitted by an object can affect the behaviour of others. The damage can be caused by direct contact, i.e., when an object consists of several materials, or by indirect contact, i.e., when volatile organic compounds are transferred to the object through the air [23,29].

Emissions from display case materials, such as VOCs from plastic, adhesive, rubber and wood components, and their effects have been studied [23,33]. Wood is often chosen for display cases because of its availability and relatively low cost, but it is a precarious material that affects very different types of objects [34]. Oak is well known as a material emitting high content of acetic acid; therefore, its use is usually avoided in museums [35].

Carpeting materials nowadays include synthetic fibres such as nylon, olefin and polyester and natural ones such as wool fibres [36]. Wool and other proteinaceous materials may emit hydrogen sulfide [6] and thus represent a risk. Carpeting contributes significantly to the total VOC concentration indoors due to the large surface area [37]; on the other hand, they can also absorb and re-emit volatiles [37].

2.2. Volatiles Emitted by Conservation Treatments

The analysis of VOC emissions provides not only information about the original materials but also about subsequently added products, such as restoration products (including pesticides). In this respect, the study of volatile emissions has mostly focused on pesticides, rather than other conservation products, which is clearly due to their toxicity. It was a common practice and part of general care to use substances to protect objects from pests [38]. The most common chemicals used to brush or immerse objects in were dichlorodiphenyltrichloroethane (DDT), paradichlorobenzene, lindane and pentachlorophenol, and among inorganic compounds, arsenic compounds, boric acid and mercuric chloride. Not all of these are volatile compounds, and their investigation is therefore beyond the scope of this review. A description of how these chemicals were used and applied can be found in the study by the National Museum of Natural History at the Smithsonian Institute [39].

In some cases, pesticide residues are trapped in the object, e.g., in fabric fibres. Various studies in museums have detected pesticides in objects and even quantified the main ones such as camphor, chloronaphthalene, dichlorvos, naphthalene and thymol. Dichlorvos, a mixture of α-, β-, γ- and δ-hexachlorocyclohexane, dieldrin and aldrin were detected only qualitatively [38,40,41,42,43,44]. Other studies were compound-specific, such as the determination of pentachlorophenol (PCP), one of the most commonly used wood preservatives in the 20th century [45], and the detection of naphthalene, a widely used pesticide used against moths and moth larvae [2,40,46]. Due to its slow rate of degradation, it is still present in display cases after years and can remain absorbed into the material so that it is slowly released over years. In closed display cases, the pesticides give off vapours that volatilize and recrystallize on museum objects, thus leading to their deterioration [47].

2.3. Volatiles Emitted by People

Organic volatiles are emitted not only from objects or furnishings but also from activities and the personal care products people use. Human biogenic volatile organic compounds (HVOCs) can have various sources ranging from respiration to emissions through the skin [19,48].

Personal care products, including cosmetics, perfumes, deodorants and hair care products, often contain diverse VOCs. Several studies have demonstrated that the main volatiles measured in public spaces such as offices, schools, cinemas and museums are strongly influenced by the previous consumption of food or alcohol and by the use of personal care products [49,50,51,52].

The main volatiles emitted by people are acetone, isoprene and methanol [53]. Other components are aldehydes, alkanes and alkenes. In particular, the oxidation of skin oil by reaction with ozone produces 4-oxopentanal (4-OPA) and 6-methyl-5-hepten-2-one (6-MHO) [52]. Slight changes in temperature and relative humidity do not contribute to major changes in the types of volatiles emitted. However, elevated temperature and RH lead to greater dermal emission, sweating and thus the production of more carboxylic acids [19]. The main types produced by human skin are acetic acid, propanoic acid and butyric acid.

Studies looking at the contribution of personal care products have identified monoterpenes as the main contributors [49,54] and studied the efficiency of their removal through ventilation. Despite the extensive research on volatiles generated by human activities (respiration and skin emissions), there is a lack of research on the impact of personal care products on the environment at cultural heritage sites.

3. Olfactory Exhibitions

An olfactory (or smell) exhibition is a type of exhibition in which smells are intentionally part of an exhibition as “olfactory objects” in their own right or related to material works of art. Here, volatile compounds are used to enhance a display or an exhibition and add an olfactory dimension to better engage visitors or an interaction tool for visually impaired people [18,55]. Although it used to be possible to touch and interact directly with artworks in museums in the past, with the rise of tourism in the 19^th century, museums mostly turned to exclusively visual interaction to avoid tactile interaction [56].

This has led to the development of several olfactory exhibitions in recent years. One of the first was the exhibition at the Jorvik Viking Centre in York, England, which olfactorily recreates how the world smelled during the Viking occupation in the 10^th century [57]. Later, the historian Caro Verbeek worked on several exhibitions about olfactory reconstructions [58,59,60]. More recently, the Prado Museum in Spain presented the exhibition “The Essence of a Painting. An olfactory exhibition” [61]. All these examples show that interest in odours has increased significantly in recent years, which has led to the development of various olfactory interpretation, conservation and engagement projects [55].

Currently, when fragrances are exhibited in a museum, laboratory tests are normally carried out on the safety, cost and stability of the products. Safety is a fundamental parameter primarily taking into account the safety of visitors in terms of negative effects on health and sometimes the safety of the surrounding collection materials [62,63]. Recent studies have shown that the presence of volatiles at low concentrations (acetic acid < 100 ppb) is not harmful to humans and does not affect the collection [14,18]. Nevertheless, these studies are still few and there seems to be a significant lack of research on the subject, particularly to determine how potentially new VOCs interact with the volatiles already present in the environment.

It is crucial to compare the tVOCs background value with the one measured during olfactory exhibitions to be able to assess the risks associated with the introduced VOCs. As previous research highlighted, high peak concentrations (150–240 ppb) are detected during routine cleaning activities [20], and similar measurements in heritage environments can define the impact of human activities.

Recently, our VOC monitoring results obtained at the National Museum of Slovenia showed an average value of 200 ppb in the exhibition area and 300 ppb in the storage room. Further research is exploring the tVOCs concentrations during olfactory exhibitions. This contributes to the evaluation of the possible risk associated with olfactory exhibitions and the VOCs they contribute to the overall VOC background in an indoor environment, allowing heritage institutions to take informed decisions about the potential impacts of olfactory objects. It is important to highlight that olfactory exhibitions need to be studied on a case-by-case basis, taking into account the specific environmental conditions, as there are currently no guidelines on how and in what concentration VOCs can be introduced into heritage environments.

4. Characterization of VOC Emissions

The analysis of volatile organic compounds in heritage environments is fundamental to indoor air quality [2,25,28,64], and it involves several methods that differ in the type of sampling, the extraction method or the type of material under investigation [16]. When dealing with volatile compounds, sampling involves their collection into a suitable container, or their preconcentration on sorbents, and either active or passive systems are used to collect pollutants on an absorbent material and then analyse them [1]. In addition, sensors can be used in any of these systems to monitor volatile compounds and provide real-time measurements [65].

An active system consists of a calibrated pump that pulls air into or through the sampling device, allowing for short sampling periods while the sampling rate and volume are controlled [66]. It requires instruments to analyse the extracted sample.

A passive system is based on the free flow of analyte molecules into a collection medium following their natural diffusion, where the flow continues until equilibrium is reached or sampling is stopped [16,67]. In a passive system, sampling, analyte isolation and preconcentration are performed in a single step, and often no solvent is required, unless extraction is required [67].

The advantage of passive sampling is its simplicity, which allows for straightforward deployment in a heritage environment as no power supply is required, and its low cost. The main drawbacks of some devices, such as SPME fibres, tVOCs sensors and diffusion tubes, are the lower quality of information compared to active sampling, as no information on the flow rate and volume is acquired, leading to less informative results [1]. In addition, passive sampling requires longer exposure times as natural diffusion is typically three orders of magnitude slower than the sampling rates used in active sampling.

Several studies show how exposure time, air velocity and temperature can affect the final uptake, observing that environments with good ventilation and high air exchange result in lower concentrations of volatile compounds [67,68,69]. In contrast, display cases with no air exchange lead to an accumulation of compounds [1,7]. It has been observed that microenvironments tend to have higher volatile concentrations than macroenvironments due to their lower air exchange rates, resulting in the trapping of emitted compounds [1].

4.1. Sampling Methods

4.1.1. Sampling Setups

The challenge of sampling VOCs emitted from heritage objects is that volatiles are emitted in a dynamic process, i.e., from the surface of objects. Therefore, the emission rate and the surface area from which volatiles are emitted are difficult to determine as it is often not possible to sample heritage objects and often surfaces have complex shapes. In addition, in contrast to environmental analysis, where a large volume of well-mixed air is sampled, these often involve smaller environments, and volatiles are present in lower concentrations. To better define the analytical parameters (sampled area or volume), it is therefore customary to enclose the object, or a part of it, before sampling to isolate the object from the general environment and enable the volatiles to accumulate. It is also essential to ensure that the temperature is constant or at least monitored and, if possible, increased, to increase the emission rates of volatile compounds. The more volatile compounds accumulate in the headspace (gas phase) surrounding the object, as Henry’s constant increases with increasing temperature [11,70].

However, in heritage science, heating an object is only allowed in rare cases. In most cases, the entire object is enclosed in a sampling enclosure, following which the headspace is sampled at room temperature. The migration of the compounds into the headspace depends on their volatility, and after a certain time, a dynamic equilibrium is reached [70,71]. The most important factors to consider are the object size and whether or not the sampling involves preconcentration of the analytes. When sampling with preconcentration, the absorption of VOCs on a substrate is involved either in active or passive mode.

In bulk air sampling (i.e., without preconcentration), no sorbent material is involved, and multiple analyses can be performed on the same air sample as a large volume of air is often collected.

When the object under investigation is placed in a sampling enclosure, the latter must be non-adsorptive and non-reactive. Glass enclosures/vials can accommodate small- or medium-sized objects, while very large objects can be sampled in their entirely by enclosing them in customized emission chambers (Figure 2a) or enclosure bags, or a part of them can be isolated and sampled [25,62]. A sealed display case also accumulates volatiles and can be used as a sampling enclosure. Attention has to be paid when the display case materials are emissive themselves, such as wood, which would influence the outcome of the analysis. Typically, emission chambers are made of stainless steel and/or glass to reduce the “sink effect”, i.e., the adsorption and desorption (re-emission) of volatiles from the inner surfaces of the chamber [7,72,73]. They are available in different sizes, from ca. 50 cm³ microchamber devices to 1 m³ chambers (Figure 2), and they offer the possibility to adjust parameters such as temperature, relative humidity, air velocity and sampling flow. The released VOCs are then collected onto a sorbent material (sorbent tubes or fibres) selected according to the compounds of interest.

The Flat and Laboratory Emission Cell (FLEC cell) enables standardized quantification of VOC emission rates from flat surfaces [74,75,76]. The geometry of the emitting surface plays a crucial role for the emission characteristics, as rough surfaces often exhibit higher emission rates due to the larger effective surface area. While FLEC cells allow for standardized emission rate determinations, they require the objects to have large flat surfaces, which is often a limitation in heritage contexts.

Figure 3 describes different setups for sampling heritage objects with sample preconcentration in active or passive mode with partial or complete enclosure. Enclosures allow for passive sampling to be performed with a passive sampler (Figure 3a) in a static air space by placing the sampler within the enclosure or attached through a cap, e.g., SPME fibres and sorbent tubes used in passive mode. This setup is the simplest, as no air movement is required; however, long sampling times (e.g., days) may be required.

If it is not possible to enclose an entire object due to size or fragility, it may be possible to isolate a part of it with a cover (Figure 3b). In this setup, both passive and active modes can be used for sampling. In the latter case, a continuous flow of air through the cover is required (as in, e.g., FLECs). The air flow is regulated using a flow regulator while air is pulled through a sampling or detection device by an air pump. If the surface is not perfectly flat or if the geometry of the object does not follow the shape of the cover, losses and contamination are possible.

In perfectly sealed enclosures, volatiles can be collected in active mode using an air circulation pump (Figure 3c). The advantages of this system are the circulation of air, which allows for the collection of continuously emitted volatiles. The amount of tubing and collection devices is also minimized, reducing the opportunities for absorption on sampling device surfaces and thus loss. In the system suggested in Figure 3d, an air inlet and an air outlet are present where scrubbed air enters the enclosure while it is drawn out through a sorbent tube.

In dynamic sampling systems, the air flow must be regulated to be able to determine the volume of sampled air. Compared to passive systems, such methods allow for reduced sampling times, as larger air volumes can be sampled in less time. The disadvantages of using pumps are the often-limited energy autonomy, which reduces the sampling times, as well as the noise, which may be disturbing in gallery settings.

4.1.2. Sampling Devices

The most commonly used devices used for bulk sampling are stainless steel canisters and plastic bags. Canisters can be used passively under pressure or actively with pumps. The main parameters to consider are the moisture content of the air, polarity of the analytes and reactivity of the analytes to water [16]. An adequate level of relative humidity within the canister is fundamental to obtain good recoveries of volatiles preventing absorption and chemical interaction with the inner surface of the canister. Requirement of water vapour for volatiles stability depends on the type of analytes investigated. Polar volatiles require great relative humidity to overcome their affinity with the canister surface and vice versa when non-polar analytes are studied. The study by Ochiai et al. [77] showed good recovery on day 0 when polar compounds were stored with RH > 27% (with the exception of alcohols with RH > 99%) and RH > 8% for non-polar VOCs. The disadvantages of sampling with canisters are large dimensions, difficult cleaning between sampling events, the risk of water condensation in the container and the tendency for reduced flow rate at the end of sampling due to a decreased difference between container and ambient pressure [16].

Sampling bags are made of polyvinyl fluoride (Tedlar), aluminized Tedlar, polytetrafluoroethylene (Teflon), Kynar^® polyvinylidene Fluoride (Kynar^® PVDF) or polyethylene terephthalate (Nalophan^®) [78,79,80]. Their advantages are low cost, ease of use and availability in a variety of sizes. Similarly to canisters, they can be cleaned and reused, and unlike canisters, the sampled air needs to be pumped into the bag, which is why active sampling is only possible [16].

Extraction sampling devices are differentiated depending on whether sampling is active or passive. SPME fibres are widely used passive samplers in heritage applications and consist of a syringe with a fused silica fibre coated with a retentive coating. The coated fibre absorbs VOCs during the set sampling time, and afterwards the fibre is retracted into the needle, followed by analysis [70,81,82,83,84]. Two thermodynamic aspects are relevant: the vapour phase that interacts with the sample and the vapour phase that interacts with the fibre [2]. The advantages of SPME are that it is solvent-free and provides reproducibility, sensitivity and selectivity, with no sample preparation (exception is when an SPME fibre is derivatized, as in the case of aldehyde sampling). SPME sampling is widely used in heritage science [5,14,25,56,85,86,87]. SPME fibres are available with different types of retentive coatings (including mixed coatings) to improve the selectivity, such as polarity. The different criteria for selecting an SPME fibre are analyte size, polarity, analyte concentration and sample matrix complexity. Carboxen fibres are used for low-molecular-weight samples, while they are often used for general-purpose extractions due to their broad analyte compatibility. Fibres such as polyacrylate (PA)-coated fibres and polydimethylsiloxane/divinylbenzene (PDMS/DVB) are suitable for applications such as sampling of highly polar volatile analytes, e.g., amines and alcohols. Multibed fibres such as divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) are used for a wide range of analytes. After sampling, the fibre is desorbed and analysed by gas chromatography or by liquid chromatography after extraction with a solvent.

DVB/CAR/PDMS fibres are most commonly used in heritage science due to their wide range of applications [11]. The duration of sampling depends on the application. For laboratory sampling in closed vials, 1 h sampling is commonly used [88,89], while for bulk air sampling of rooms, studies have shown examples of 1 day sampling up to 15 days [86,88,90], with 1 day recommended to obtain the best repeatability [10]. SPME Arrow is a fibre with a larger surface area that allows for higher capacity and analyte extraction compared to conventional SPME fibres [91,92,93]. Further developments include thin-film solid-phase microextraction (TF-SPME). This method uses a sorbent on a carbon mesh strip, which has a larger surface area compared to traditional SPME fibre, increasing the collection volume and extraction rate. In addition, it is suitable for a wider range of volatiles, making it suitable for analysing VVOCs and trace-level compounds [94,95]. Similarly to SPME fibre, the Magic Chemisorber ^® (Frontier Lab, Fukushima, Japan) PDMS is made by a thin film of 500 µm of PDMS on a titanium tube used mainly for the trace analysis of volatile compounds in air or solutions. The sampling device is then introduced into a pyrolyzer to thermally desorb the extract components followed by GC-MS analysis [96].

For sampling SVOCs, polyurethane foam (PUF) is most commonly used [97,98], in both active and passive sampling modes, although it has recently been used primarily for passive sampling [99]. It has a large surface area characterized by a porous structure capable of collecting a wide range of volatiles over long periods of time. Moreover, they are relatively inexpensive, easy to use and compatible with a variety of sampling setups [100].

Sorbent tubes [16] can be used passively by exposing an open end directly to bulk air, or actively when combined with an air pump. The advantages are their versatility, as they can be tailored to increase specificity, as well as their high sensitivity and detection limit. However, their limitation lies in their limited capacity for high-volume sampling [16]. Several types of sorbents are available depending on the compounds of interest, and more than one sorbent can be used in a series or as multiple beds in the same tube [62,71,101]. Typical sorbent materials are charcoal, graphitized carbon, silica gel, porous organic polymers (Porapak, Tenax^®, Chromosorb), quartz wool and coconut charcoal. Several of these sorbents can be used for a wide range of applications, such as silica gel tubes used for sampling polar hydrocarbons, low MW thiols, amines and inorganic acids. Charcoal tubes are applied for the collection of non-polar compounds. Sorbent tubes filled with Tenax^® are used to collect high boiling compounds: Tenax^® TA is typically used for high boiling T compounds at low concentration, while Tenax^® GR (Tenax/graphite) has a wider collection of analytes reaching low boiling compounds. Other sorbent materials are compound-specific, such as sulfur tubes to sample volatile sulfur compounds and Porapak^® tubes specific for the analysis of volatile nitriles. In heritage science, Tenax^® TA sorbent tubes are most commonly used due to their broad range of high volatile compounds collected [26,27,40,43,56].

Similar to sorbent tubes, gas detection tubes such as Dräger tubes^® can be used in active and passive mode and directly provide quantitative data [102]. These consist of glass tubes filled with a specific chemical reagent reacting with the compound or family of compounds of interest, such as aldehydes, acetic acid and formic acid [102]. The main advantages of Dräger sampling tubes are their low cost and speed of analysis, with a wide range available for accurate measurements.

The Radiello^® sampler is a passive sampling device for VOCs consisting of a cartridge of graphitized charcoal inserted into a radial diffusive sampler. After sampling, the cartridge is thermally desorbed and analysed by gas chromatography. The advantage of a radial diffusive sampler is the large diffusion area parallel to the cartridge and the tortuous diffusion path, which means that the uptake rate is not affected by wind and air currents [68,103,104].

Another type of diffusion tubes is Palmes^TM tubes, which were originally developed for nitrogen dioxide sampling. These are now commonly used for passive sampling in heritage environments to measure the concentration of formic and acetic acid, normally followed by ion chromatography [22,99].

4.2. Analysis of Volatiles

The technique of analysis is selected depending on the compounds of interest and the method by which they are sampled. Direct-reading devices allow for real-time readings without the need for sampling and subsequent laboratory analysis. Examples include devices based on photoionization detectors (PIDs), flame ionization detectors (FIDs), electrochemical sensors (ECs, amperometric or potentiometric) and metal oxide semiconductor sensors (MOSs) [105,106].

PIDs are based on ionization of the target molecule by a UV lamp, which generates a current flow that is then converted into a voltage signal. The signal is proportional to the ionized compound, and only compounds with an ionization potential lower than the energy of the UV lamp photons are detected [105]. The greater the difference between these two, the better the response of the detector is. In addition, these detectors are only sensitive to volatiles within a certain range of photoionization energy or measuring a potential or current difference and therefore may exclude some compounds that are present in high concentrations in the environment. PIDs are typically used to measure the tVOCs present, rather than being compound-specific, and their sensitivity is in the range of 1 ppb–1000 ppm [105].

Electrochemical sensors detect organic volatiles based on an electric current generated through the interaction of the analytes with an electrode. The compounds detected are organic volatiles such as formaldehyde and ethanol as well as inorganic ones such as NH₃, NO₂, SO₂ and CO, with a detection range of 100 ppb–50 ppm [105]. MOS sensors are based on a change in resistance when the target volatile interacts with a metal oxide receptor surface. Sensors are available in two types: n-type, in which the change in resistance is given by a reducing gas; p-type, in which the change in resistance is given by an oxidizing gas [106]. They can be used to measure the concentration of a wide range of volatiles, such as alcohols, ethers, ketones, esters, carboxylic acids, nitroalkanes, amines or aromatic compounds. Nevertheless, in contrast to other sensors, MOS sensors have a lower sensitivity and can only measure volatiles in the ppm range.

Sensor-based devices are portable, easy to use and require minimal training to operate, making them suitable for applications in heritage environments. However, there are limitations in terms of lower accuracy and often higher limits of detection compared to laboratory-based analytical methods. In particular, if the sensor only measures the sum of all tVOCs such as a PID sensor, it needs to be coupled with some analytical techniques to obtain more information about which species are present in the environment.

An established method for the separation and analysis of VOCs is chromatography, usually gas chromatography (GC) in conjunction with mass spectrometric detection [78,105,107]. The sample, e.g., collected on SPME, is introduced directly, or else extracted first, e.g., from sorbent tubes, where extraction proceeds thermally with solvents. Liquid extracts can also be analysed using high-performance liquid chromatography (HPLC), commonly used for the analysis of aldehydes [108,109], or with ion chromatography (IC) for the analysis of low-molecular-weight acids, such as formic and acetic acid [110,111].

To complement the analytical information with sensory information, it is necessary to use olfactometric detection, in which the human nose is used to detect and describe odour signals [78]. In GC-MS-O, the two detection techniques are used in parallel and are complementary, as not all MS-detectable compounds are odour-active and vice versa [62,72,81,82]. The output is the chromatogram, which is overlaid with the aromagram (or olfactogram) representing the eluted odorous VOCs [72,78]. GC olfactometry has found few applications in the field of cultural heritage so far [56], while it is widely used in other sectors, such as perfumery [62,78] and the beverage and food industry [112,113].

5. Museum Guidelines

Several research studies have investigated the most common indoor volatiles and the associated risks to collections [4,111,114]. In addition, studies have shown that some toxic or otherwise hazardous compounds found in museum air can affect human health (depending on the exposure time, exposure level, concentration and the person’s medical condition and age), which emphasizes the need to define limits for indoor pollutant concentrations in heritage environments [2,115]. However, no standard threshold values have yet been developed.

Guidelines for museum environments have been developed with suggestions on the concentrations of volatile substances in cultural heritage environments in order to preserve collections [1,15,31]. These guidelines aim to understand potential risks given by volatiles in cultural heritage environments, propose thresholds for the main compounds and develop new approaches to risk assessment. In these, different materials are evaluated because of their different sensitivities to volatiles and degradation at different rates [15].

The main VOCs identified in the guidelines are the most reactive ones and those normally found in higher concentrations, such as acetic acid, formic acid, formaldehyde and acetaldehyde.

Table 1. List of the main volatile organic compounds present in museum environments, their indoor sources and the effect they may have on heritage materials when present in high concentrations [15]. On the right, suggested threshold values in ppb are reported according to Grzywacz guideline and PAS 198 guideline [1,113,116].

Compound	Indoor Sources	Effects on the Material	Suggested Threshold (ppb)
			Sensitive Material (Grzywacz 2006)	General Collection (Grzywacz 2006)	Long-Term Storage (PAS 198:2012)
Acetaldehyde	Wood products, polyvinyl acetate adhesive	Oxidation to carboxylic acid in high RH and/or in the presence of oxidants	1–20	–	–
Formaldehyde	Furnishing, coating for wood products, carpet finishing products	Oxidation to carboxylic acid in high RH and/or in the presence of oxidants, metal corrosion	0.1–5	10–20	300
Acetic acid	Wood products, furnishing, cellulose acetate objects (vinegar syndrome), acid-type silicon sealants, degradation of organic materials, emulsion paints, adhesives	Metal corrosion, efflorescence on calcareous materials and soda-rich glass objects, reduction in cellulose degree of polymerization	5	40–280	100–1000
Formic acid	Wood products, degradation of organic materials, oil-based paints	Metal corrosion, efflorescence on calcareous materials and soda-rich glass objects, reduction in cellulose degree of polymerization	5	5–21	500

shows the existing recommended threshold concentrations for organic volatiles in heritage environments and their sources.

The threshold value is defined as the concentration at which no or insignificant reaction with collection materials takes place. When assessing the risk in an environment with objects consisting of several materials, the most sensitive material is considered to make preventive conservation decisions.

It is essential to control the concentration of these volatile substances in heritage environments, as a high concentration could cause health issues as well as lead to object degradation. The World Health Organization (WHO) has developed guidelines to assess the health risks and suggested thresholds for the most common toxic volatile substances in indoor environments [117]. In the list of volatile substances found in heritage environments (Table 1), formaldehyde is included in the list of toxic pollutants that may be found in concentrations relevant to human health indoors. It specifies short-term exposure (30 min) limits to below 0.08 ppm in order to avoid sensory irritation [117]. In addition, a comparison of the values for tVOCs proposed in the latest guidelines shows that a limit value of 85 ppb is recommended for long-term exposure [118,119].

Although no quantitative studies have yet been conducted to evaluate the impact of olfactory exhibitions, the above guidelines might also be useful in the assessments of the risks related to olfactory exhibitions in museums.

The guidelines propose control strategies to reduce the concentration of volatiles indoors and limit the risk of damage to the collections. Monitoring methods should then be used to control the concentrations of volatiles. These include minimizing outdoor and indoor sources through blocking systems, filters or sorbents, selecting an appropriate ventilation system and low-emission materials for enclosures.

6. Conclusions

VOCs represent a significant challenge in museum conservation due to their potential to contribute to the degradation of cultural heritage materials. Understanding the sources, concentrations and behaviour of organic volatiles is essential for implementing effective preservation strategies and ensuring the long-term sustainability of museum collections.

The main challenges in investigating volatiles emitted in heritage environments are related to the different materiality and non-homogeneous surfaces often present in the same object. Therefore, it is essential to characterize the volatile contributions of each material present and assess the possible interactions, especially when different types of objects are stored in the vicinity. In addition, objects usually emit low concentrations of volatiles, requiring longer periods of accumulation time compared to traditional traffic-generated pollutants. Furthermore, since it is not possible to take physical samples of heritage objects, it is crucial to develop suitable sampling enclosures and to select the most appropriate sampling devices (in active or passive mode) according to the analytes of interest and the availability of the object, i.e., the possibility of handling as guided by its fragility and relevant museum policies. The current research on volatiles contributes to the understanding of VOC emissions and their impact and provides guidance on thresholds and control strategies that should be followed in heritage facilities. On the other hand, this review has identified the following knowledge gaps:

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The emission rates of volatiles from objects are still poorly understood, especially of potentially harmful VOCs, e.g., organic acids. Quantitatively determined emission rates are essential to enable environmental managers to establish appropriate air exchange requirements in high-density storage areas or in enclosures.

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The availability of sorbents for sampling of the diverse VOCs emitted from heritage objects is still limited, especially for organic acids. Their low emission rates require either long sampling times or elevated temperatures (e.g., for model materials), which is impractical. More research and development are needed to implement more effective sorbents for sampling.

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There is a significant lack of data on the thresholds for VOC concentrations in indoor heritage environments from the viewpoint of preventive conservation. More research is needed into the assessments of the relative impact of VOCs on heritage materials, as compared to temperature and relative humidity.

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The recent growth in popularity of olfactory exhibitions requires the development of a practical methodology for the assessment of the effect of smells intentionally introduced into exhibition spaces on the displayed objects, as well as the development of guidelines to ensure the safety of both collections and visitors.