## 2 Volatiles in Heritage Environments
### 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 terpen: \(\alpha\)-pinene, \(\beta\)-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 [PERSON] 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]. Figure 1: tVOC concentrations measured in the photosore in repository 7A of the National Archives, Kew (UK), between 18 th and 24 th February 2009. Reproduced from [20] with permission. Furfural is formed either during oxidative degradation of glucose, arabin 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. [PERSON] 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-methoxyphenyl) 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 linoleic acid, which are caused by resin residues in wood fibres [25].