## 3 Volatile Organic Compounds Emitted from WBPs
### Aldehydes, Carboxylic Acids, and Ketones  
Another important group of VOC emitted from wood and WBP are the carbonyl compounds, which include aldehydes, carboxylic acids, and ketones. Most aldehydes derive from the oxidative degradation [2; 48; 50; 59; 80] and/or thermal degradation [2; 59] of components abundant in wood. These processes may occur naturally or be a consequence of WBP manufacturing. Biogenic formaldehyde is present in low concentrations and may occur through the oxidation of unsaturated terpen such as \(\alpha\)-pinene, \(\beta\)-pinene, and limonene, as well as due to the decomposition of lignin [2; 50; 59]. In contrast, anthropogenic formaldehyde results from the synthetic resin added in WBP production [50; 81]. Authors [PERSON] et al. [48] and [PERSON] [80] suggest that oxidation of unsaturated fatty acids in wood leads to the formation of saturated aliphatic aldehydes, of which hexanal is usually the most abundant. Furthermore, pyrolysis of wood leads to the breakdown of polysaccharides and results in compounds such as formaldehyde, acetaldehyde, propenal, and butanone. Thermal degradation of lignin produces benzaldehyde and other aromatic compounds [2; 59]. Formic, acetic, and hexanoic acids are organic acids widely emitted in relatively low amounts from wood, although other acids may appear in specific wood types [59]. Acetic acid may occur due to the hydrolysis and cleavage of acetyl groups in lignin and hemicellulose [48]. Formic and hexanoic acids may originate from the decay of fatty acids (and further degradation of formaldehyde and hexanal, respectively) [82]. The formation of organic acids and/or exposure to high temperatures leads to the degradation of polysaccharides, resulting in the formation of furan aldehydes and derivatives such as furfural (furan-2-carbaldehyde), 5-hydroxymethylfurfural and 5-methylfurfural [45; 83; 84; 85; 86]. Their formation may also occur through the hydrolysis of carbohydrates [51]. Acetone is the most emitted ketone from wood and WBP. However, emissions of other ketones such as acetophenone [64], 6-methyl-5-hepten-2-one, and hydroxypropan-2-one (hydroxyacetone/acetol) [87] have also been reported. While a study showed a possible correlation between acetone emissions from pine trees and photosynthesis [88] and another reported the formation of acetone by the photolytic degradation of cellulose [89], little is known about the mechanism behind the formation and emission of acetone in wood. Although some mechanisms responsible for the formation of certain aldehydes and ketones through thermal, enzymatic, and microbial degradation have been studied, they fail to explain the diverse range of compounds detected. Furthermore, these mechanisms may occur in conditions significantly different from those encountered in the WBP manufacturing [2]. Although hardwoods have substantially lower emissions than softwoods due to their general lack of volatile terpen, they have considerably high carbonyl emissions (Table 6) [45; 48]. Hardwood hemicelluloses have a higher number of acetyl groups. Therefore, the concentration of acetic acid emitted from hardwood is usually higher than that from softwoods [45]. A study from [PERSON] et al. (1998) [48] focused on the emissions of acetic acid and other VOCs from several wood species. Results showed significantly higher acetic acid emissions for oak (_Quercus robur_) and cherry (_Pruus serotina_), followed by rubberwood (_Hevera brasiliensis_), ash (_Franxinus excisior_), maple (_Acer saccharum_), and beech (_Fagus sylvatica_). Birch wood (_Betula pubescens_) was the only hardwood sample to show lower acetic acid emissions than the two softwoods tested (pine and spruce). Regarding other carbonyl compounds, hexanal was among the most emitted compounds from all Wood drying and pressing procedures in the WBP manufacturing process also affect the emissions of carbonyl compounds, although this effect differs from observed with terpenes. [PERSON] et al.