## 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 \({}^{\circ}\)C.
* VOCs: volatile organic compounds, boiling point \(\leq\) 250 \({}^{\circ}\)C.
* SVOCs: semi-volatile organic compounds, boiling point \(>\) 250 \({}^{\circ}\)C, up to 380-400 \({}^{\circ}\)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 \(\mu\)g/(m\({}^{2}\)h) [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 biodeterigenic 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. [PERSON]'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 [PERSON] [15]. The literature review by [PERSON] 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.