# Essential Oils, Phytoncides, Aromachology, and Aromatherapy--A Review
## 5 Phytoncides and Their Functions  
Phytoncides are a naturally available, complex blend of EOs or VOCs produced from plants and trees, explained as \"extreminated by the plant\" [66; 67]. Approximately 400,000 known aromatic and medicinal plants have essential oils [68]. Phytoncides were first devised by a Russian biochemist, [PERSON], in 1928. They are volatile chemicals emitted by plants to defend against viruses, bacteria, saccharomyces, molds, and protozoans [69] or to prevent decay and herbivore attacks. Phytoncides also exhibit other beneficial effects, such as anti-microbial, antibacterial [70], anti-fungal, anti-inflammatory, anti-stress, analgesic, and anti-spoilage activities, and they can be used as food preservatives [36; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82]. They also exhibit anti-mycoplasmal activity [83], anti-larvicidal activity against malaria [84; 85] and dengue [86], anti-septic activity, and anthelmthic activity; in addition, they facilitate wound healing [87], can act as cholesterol inhibitors [88], can enhance sleep [89; 90; 91], and even enhance bacterial susceptibility to antibiotics [92; 93]. Phy  
Figure 2: Schematic representation of the perception of aroma. Aroma enters via either the orthonasal or retronasal paths, reaching the olfactory centers of the brain through a signal cascade. Aroma transfers from olfactory epithelial neurons into the olfactory bulb. The olfactory efferent enters the piriform cortex (PC), entorhinal cortex, periamygdaloid cortex, and hippocampus. Olfactory neurons from the PC enter the orbitofrontal cortex (OFC) and thalamus. Thalamus and OFC neurons are connected to the insular cortex [52]. toncides exert alleopathy effects, where these secondary metabolites from plants affect microorganisms, acting as a defence mechanism for plants. Hence, phytoncides are recognized as allelochemicals [94]. Phytoncides can prevent mucosal damage in the digestive tract and have been proven to have anti-inflammatory effects in the stomach, colon, and other digestive parts such as the esophagus, small intestine, and duodenum [70] (Figure 3). The human brain can process olfactory stimuli even during sleep [95; 96]. The sleeping brain and waking brain receive the aroma and react differently. In our study, the temporal activity of the brain was found to increase upon the inhalation of lavender aroma during sleep. Sleep quality was also found to increase depending upon the aroma stimuli [97]. Many plants, such as pine, garlic, cedar, and onion, can emit phytoncides [98]. [PERSON] and his team studied the sleep-enhancing effects of phytoncides derived from pine oils. Pine essential oil's most important volatile terpenoid phytoncides are myrcene, \(\alpha\)- and \(\beta\)- pinene, \(\beta\)-thujene, bornyl acetate, and 3-carene [99]. Amongst these terpenoids, \(\alpha\)-pinene is the most abundant and a significant phytoncide with a characteristic odor, hence its use in perfumes [100]. In our study, inhalation or oral administration of \(\alpha\)-pinene showed enormous biological activities, such as anxiolytic, sleep-enhancing, anti-microbi, anti-nociceptive, and anti-inflammatory activity [89; 90; 101; 102; 103; 104]. The aroma of sweet orange (_Citrus sinensis_; _C.sinensis_) produces an anxiolytic effect in individuals exposed to an anxiogenic situation [105]. The aroma of bitter orange (_C. aurantium_) has shown anxiolytic activity in chronic myeloid leukemia patients [106], crack cocaine users [107], and rodents [108]. 3-carene has a sweet and pungent odor and has been used as raw material in perfumes and cosmetics [109; 110]. Like \(\alpha\)-pinene, the oral administration of 3-carene is reported to have a sleep-enhancing effect [90; 91]. Previous studies show that the Non-Random Eye Movement Sleep (NREMS) augmented by \(\alpha\)-pinene and 3-carene prolongs GABAergic anti-nibitory postsynaptic signaling. Thus, phytoncides enhance sleep with fewer side effects than conventional hypnotics [98].