## 3 Volatile Organic Compounds Emitted from WBPs
### Terpenes and Terpenoids  
Terpenes are hydrocarbon compounds and the largest class of plant secondary metabolites, usually serving as defensive molecules against biogenic enemies or as signal compounds to e.g., attract pollinators [55, 56, 57]. Terpenes can undergo several reactions such as hydrogenation, dehydrogenation, and oxygenation to form terpenoids [58, 59]; however, the term \"terpene\" can also be used to include terpenoids. Terpenes are composed of isoprene units (2-methyl-1,3-butadiene) and can be classified according to the number of isoprene units or carbon atoms. The number of isoprene units is related to a terpene's volatility, as fewer isoprene units are associated with higher volatility. Volatile terpenes are part of wood's primary VOC emissions and include hemiterpenes (1 unit, C5), monoterpenes (2 units, C10), and sesquiterpenes (3 units, C15) [60]. The presence of terpenes in wood is primarily linked to natural resin, as the sapwood cells through which resin flows contain volatile and non-volatile terpenes in their composition [54], and are largely responsible for the characteristic fragrant smell of wood [45, 48]. Due to being rich in natural resin, softwoods not only have characteristically high emissions of monoterpenes and monoterpenoids, but also emit several sesquiterpenes and sesquiterpenoids [59]. Of the many softwood species studied over the years, the highest concentrations of VOCs have been found in the pine family species, where terpene emissions predominate significantly in their VOC profile. Compounds such as \(\alpha\)-pinene, \(\Delta 3\)-carene, \(\beta\)-pinene, and limonene are characteristic volatile terpenes of coniferous wood species; moreover,camphene, myrcene, and \(\beta\)-phellandrene are also frequently found in softwoods, although at lower amounts. A more extensive list of the volatile terpen emitted from pine, spruce, larch, and fir can be seen in Table 4.  
Prolonged storage or exposure to high temperatures--such as those used for drying and thermal treatment or hot-pressing--can significantly reduce terpene emissions in wood and WBPs. High temperatures are known to cause most volatiles to escape the wood material or to degrade into other compounds during heat treatment, which is evidenced by the high terpene emission rates obtained from studies conducted in a chip dryer [71]. In a comparative study of untreated and heat-treated pine (_Pinus sylvestris_ and _Pinus_ spp. ), emissions of the most abundant terpen, \(\alpha\)-pinene and limonene, decreased by 85-89% and 95%, respectively. Additionally, compounds like camphene, \(\beta\)-phellandrene, and \(\beta\)-myrcene were no longer emitted by the heat-treated pine samples [65; 66]. Similar findings were observed in spruce (_Pica abies_), where emissions of \(\alpha\)-pinene and \(\Delta\)3-carene decreased by 99%, and limonene by 79%. Heat-treated spruce also showed no emissions of \(\beta\)-phellandrene or \(\beta\)-myrcene [66]. This same behavior is reflected in WBPs produced with softwoods. In a study performed by [PERSON] et al. (2013) [72], an assessment of VOC emissions from several WBPs showed that VOC emissions from fresh PB (2 weeks old) were four times higher than those from a 10-year-old PB, in which \(\alpha\)-pinene and \(\Delta\)3-carene were the most noticeable terpen. Emissions from non-resinated wood sawdust and shavings, obtained from the same source as the 2-weeks-old PB, had a similar terpene composition. In 2013, [PERSON] et al. [73] determined the emission factors of natural VOCs in different softwood lumber and WBPs such as PB, MDF and plywood. A significant decrease in emissions was seen after 10 days of analysis and when comparing the emission factors resulted from the several lumber samples to those from WBPs. These PB and MDFs, which were produced with waste wood and UF resin, had \(\alpha\)-pinene, \(\beta\)-pinene and \(\Delta\)3-carene as their highest emitted compounds, as well as lower emissions of limonene, \(\beta\)-myrcene, camphene, \(\alpha\)-terpinene, and \(\gamma\)-terpinene.