Essential Oils, Phytoncides, Aromachology, and Aromatherapy—A Review

: Chemical compounds from plants have been used as a medicinal source for various dis-eases. Aromachology is a unique ﬁeld that studies the olfactory effects after inhaling aromatic compounds. Aromatherapy is a complementary treatment methodology involving the use of essential oils containing phytoncides and other volatile organic compounds for various physical and mental illnesses. Phytoncides possess an inherent medicinal property. Their health beneﬁts range from treating stress, immunosuppression, blood pressure, respiratory diseases, anxiety, and pain to anti-microbial, anti-larvicidal, anti-septic, anti-cancer effects, etc. Recent advancements in aromatherapy include forest bathing or forest therapy. The inhalation of phytoncide-rich forest air has been proven to reduce stress-induced immunosuppression, normalize immune function and neuroendocrine hormone levels, and, thus, restore physiological and psychological health. The intricate mechanisms related to how aroma converts into olfactory signals and how the olfactory signals relieve physical and mental illness still pose enormous questions and are the subject of ongoing research. Aromatherapy using the aroma of essential oils/phytoncides could be more innovative and attractive to patients. Moreover, with fewer side effects, this ﬁeld might be recognized as a new ﬁeld of complementary medicine in alleviating some forms of physical and mental distress. Essential oils are important assets in aromatherapy, cosmetics, and food preservatives. The use of essential oils as an aromatherapeutic agent is widespread. Detailed reports on the effects of EOs in aromatherapy and their pharmacological effects are required to uncover its complete biological mechanism. This review is about the evolution of research related to phytoncides containing EOs in treating various ailments and provides comprehensive details from complementary medicine.


Introduction Aromachology and Aromatherapy
The emerging study of aromachology first began in the late 20th century, initiated by a scientist from Japan, Shizuo Torii. Torii studied the association between aroma and emotion and found that the fragrance of lavender and chamomile enhances relaxation. The term aromachology was first coined by the Sense of Smell Institute in 1982. Aroma has the extraordinary power to amend our physical, mental, and emotional state of well-being. More than 3500 years ago, the fragrance was used in human life rituals by ancient Egyptians. Our modern frenetic, chaotic lifestyle has brought back the use of aromas to obtain better physical and mental health [1]. Aromachology is the study of the interrelationship between components in the EOs can change their aroma. Oxygenated EOs are more fragrant than the EOs containing monoterpene hydrocarbons [26]. EOs with C5 units are termed monoterpenes. Monoterpenes are the major component of EOs, responsible for aroma and flavor, and they act as important ingredients in the agricultural, pharmaceutical, cosmetic, and food industries [27]. They include different types, such as nerol, linalool, citronellol, citronellal, and citral [28]. Sesquiterpenes usually contain 15 carbons in their skeleton structures with different functional groups such as hydrocarbons, aldehydes, and alcohols [29]. Sesquiterpenes are responsible for anti-microbial, anti-fungal, anti-tumor, and antiinflammatory activities [30]. Many factors influence the chemistry of EOs, including the plant organ, genetic factors, geographical variations, environmental conditions, type of species the plant belongs to, mode of production, storage conditions, etc. [31][32][33]. Many components, such as citral, myrcene, ocimene, menthol, D-limolene, α-pinene, α-thujone, β-thujone, farnesol, α-bisabolol, humelene, etc., are present in EOs and have been proven to be responsible for the effects of EOs. Any small change in these components may change the aroma properties of EOs. The chemical structure of EO components is given in Table 1. oterpenes. Monoterpenes are the major component of EOs, responsible for aroma and flavor, and they act as important ingredients in the agricultural, pharmaceutical, cosmetic, and food industries [27]. They include different types, such as nerol, linalool, citronellol, citronellal, and citral [28]. Sesquiterpenes usually contain 15 carbons in their skeleton structures with different functional groups such as hydrocarbons, aldehydes, and alcohols [29]. Sesquiterpenes are responsible for anti-microbial, anti-fungal, anti-tumor, and antiinflammatory activities [30]. Many factors influence the chemistry of EOs, including the plant organ, genetic factors, geographical variations, environmental conditions, type of species the plant belongs to, mode of production, storage conditions, etc. [31][32][33]. Many components, such as citral, myrcene, ocimene, menthol, D-limolene, α-pinene, α-thujone, β-thujone, farnesol, α-bisabolol, humelene, etc., are present in EOs and have been proven to be responsible for the effects of EOs. Any small change in these components may change the aroma properties of EOs. The chemical structure of EO components is given in Table 1. oterpenes. Monoterpenes are the major component of EOs, responsible for aroma and flavor, and they act as important ingredients in the agricultural, pharmaceutical, cosmetic, and food industries [27]. They include different types, such as nerol, linalool, citronellol, citronellal, and citral [28]. Sesquiterpenes usually contain 15 carbons in their skeleton structures with different functional groups such as hydrocarbons, aldehydes, and alcohols [29]. Sesquiterpenes are responsible for anti-microbial, anti-fungal, anti-tumor, and antiinflammatory activities [30]. Many factors influence the chemistry of EOs, including the plant organ, genetic factors, geographical variations, environmental conditions, type of species the plant belongs to, mode of production, storage conditions, etc. [31][32][33]. Many components, such as citral, myrcene, ocimene, menthol, D-limolene, α-pinene, α-thujone, β-thujone, farnesol, α-bisabolol, humelene, etc., are present in EOs and have been proven to be responsible for the effects of EOs. Any small change in these components may change the aroma properties of EOs. The chemical structure of EO components is given in Table 1.  [29]. Sesquiterpenes are responsible for anti-microbial, anti-fungal, anti-tumor, and antiinflammatory activities [30]. Many factors influence the chemistry of EOs, including the plant organ, genetic factors, geographical variations, environmental conditions, type of species the plant belongs to, mode of production, storage conditions, etc. [31][32][33]. Many components, such as citral, myrcene, ocimene, menthol, D-limolene, α-pinene, α-thujone, β-thujone, farnesol, α-bisabolol, humelene, etc., are present in EOs and have been proven to be responsible for the effects of EOs. Any small change in these components may change the aroma properties of EOs. The chemical structure of EO components is given in Table 1.  [29]. Sesquiterpenes are responsible for anti-microbial, anti-fungal, anti-tumor, and antiinflammatory activities [30]. Many factors influence the chemistry of EOs, including the plant organ, genetic factors, geographical variations, environmental conditions, type of species the plant belongs to, mode of production, storage conditions, etc. [31][32][33]. Many components, such as citral, myrcene, ocimene, menthol, D-limolene, α-pinene, α-thujone, β-thujone, farnesol, α-bisabolol, humelene, etc., are present in EOs and have been proven to be responsible for the effects of EOs. Any small change in these components may change the aroma properties of EOs. The chemical structure of EO components is given in Table 1.

Methodology
The keywords 'aromachology', 'aromatherapy', and 'aromatherapy and phytoncides' were used to search the scientific databases PubMed, Google Scholar, Medline, and PsycINFO. The search for aromatherapy returned 3296 results consisting of both reviews and research articles spanning the choice of aromatherapy for dementia, depression, stress, cognitive dysfunction, anxiety, cardiovascular diseases, etc.
The search for aromatherapy and phytoncides yielded less than a hundred articles, highlighting forest bathing, phytochemicals, essential oils, and volatile organic compounds, and their effects on the immune system and against certain viral infections. The total number of search results for aromachology was nil in PubMed. Hence, it was searched in Google Scholar. Approximately 671 results related to aromachology were found in Google Scholar. The abstracts of these articles were read and sorted according to their relevance to health benefits in humans. Non-relevant articles were excluded, and relevant studies were used for writing the manuscript. Only research articles and abstracts in English were considered. Reports published up until December 2021 were collected. This combinational review is a collective compilation of information on aromatherapy and aromachology, encompassing all the necessary details about these fields to date. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) chart explains the selection criteria of the collected articles (Figure 1). their relevance to health benefits in humans. Non-relevant articles were excluded, and relevant studies were used for writing the manuscript. Only research articles and abstracts in English were considered. Reports published up until December 2021 were collected. This combinational review is a collective compilation of information on aromatherapy and aromachology, encompassing all the necessary details about these fields to date. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) chart explains the selection criteria of the collected articles (Figure 1).

How Does Aroma Work with the Brain?
Aromas are inhaled into the body through the nose, where they pass the blood-brain barrier and affect the central nervous system [34,35], the autonomic nervous system, and the endocrine system [2]. Aromas can elicit quick emotional changes in humans [36]. When the aroma from essential oils is inhaled, the volatile molecules bind to the olfactory receptors in the cilia cells. Hence, the electrochemical message is transmitted through the olfactory tract and olfactory bulb to reach the brain's olfactory regions, which stimulates autonomic function and a strong emotional response regarding the received aroma stimuli [37]. The aroma elucidates different pathways: orthonasal (odor passes through the nose) and retronasal (odor enters the nostrils through the oral cavity) pathways. Both possess different sensory experiences [38,39]. Generally, the perception of aromas through these two different routes excites two different sites in the human brain. Likewise, the different odorant types elicit different responses that depend upon the routes through which the odorants are administered [40]. Retronasal perception occurs only when the food has been chewed. The inhaled aroma is influenced by factors such as temperature and water-soluble and non-volatile components of the oral cavity [41,42]. Hummel and Heilmann stated that the perception response was larger when an odor was presented retronasally [43].
The aroma enters through the orthonasal or retronasal routes, reaches the olfactory receptors in the olfactory epithelium of the nasal cavity, and spreads over the olfactory bulb cells. There, it enters the primary olfactory cortex, which comprises the anterior olfactory nucleus, piriform cortex (PC), peri-amygdaloid, and entorhinal cortices (EC) [44]. From the PC, subcortical projections spread out to the thalamus [45], hippocampus, and orbitofrontal cortex (OFC) [46]. The entorhinal cortex leads to the hippocampus [47] and thalamus nuclei. Furthermore, the thalamus extends its projections into the OFC and insular cortex [48][49][50]. This network of nerve fibers connects the PC, thalamus, OFC, EC, amygdala, hippocampus, insular cortex, and the olfactory bulb [51] and controls the incoming olfactory inputs, producing quick signals (Figure 2).
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]. Woo 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, α-and βpinene, β-thujene, bornyl acetate, and 3-carene [99]. Amongst these terpenoids, α-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 α-pinene showed enormous biological activities, such as anxiolytic, sleep-enhancing, anti-microbial, antinociceptive, 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 anxi- Depending upon the odor delivered through the ortho-or retronasal passages, the responses of the neurons may vary, which were recorded by functional magnetic resonance imaging (fMRI). In our study, neural responses were received from the insula/operculum, thalamus, hippocampus, amygdala, and caudolateral cortex during the orthonasal delivery of the odors. In contrast, responses were received from the perigenual cingulate and medial orbitofrontal cortex during the retronasal delivery of the odors [52]. Any VOCs that enter the bloodstream through the nasal or lung mucosa diffuse into the olfactory nerves and the brain's limbic system [53,54]. Odors induce perception, emotional learning, belief, cognitive, behavioral, and other associated emotions [55,56]. Any pleasant or unpleasant odors can elicit olfactory emotions [57][58][59] and can produce positive or negative moods, which eventually result in behavioral changes. A positive mood increases productivity and tends to help [60][61][62]. A negative mood suppresses prosocial behavior [63]. If any odor triggers anxiety, it creates a fear associated with that odor [64]. Thus, the olfactory efferent is wired to the brain to produce a sequence of psychological-emotional responses resulting in emotions, memory processing, and mind-body interactions [65].

Phytoncides in Dental Care
Phytoncides have been used for treating periodontal disease and against bad breath [94]. Microbial attacks on dentures cause denture stomatitis and aspiration pneumonia [116]. Candida albicans (C. albicans) is responsible for this effect on dentures [117]. Lee and his co-workers used a phytoncide-incorporated polymethyl methacrylate (PMMA) biocompatible dental polymer [118]. Their results confirm that the fungal attachment on the dental surface was reduced significantly compared with the control. Phytoncides-incorporated PMMA material can be used as a dental base resin to execute anti-fungal effects and reduce oral biofilm deposition. Eugenol is widely used in dental treatments because of its antibacterial and anesthetic properties. Eugenol has been used in root canal sealers, temporary fillings, and paste for pulp capping. It is also helpful in tooth canal treatment [119]. Incorporating phytoncide within microcapsules (PTMC) into the denture base resin can combat denture stomatitis. Phytoncide is released from PTMC at an acidic pH and inhibits the growth of C. albicans [120]. 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]. Woo 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, αand βpinene, β-thujene, bornyl acetate, and 3-carene [99]. Amongst these terpenoids, αpinene 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 α-pinene showed enormous biological activities, such as anxiolytic, sleep-enhancing, anti-microbial, 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].

Anti-Cancer Activity of Phytoncides
3-carene has a sweet and pungent odor and has been used as raw material in perfumes and cosmetics [109,110]. Like α-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 α-pinene and 3-carene prolongs GABAergic inhibitory postsynaptic signaling. Thus, phytoncides enhance sleep with fewer side effects than conventional hypnotics [98].

Phytoncides in Dental Care
Phytoncides have been used for treating periodontal disease and against bad breath [94]. Microbial attacks on dentures cause denture stomatitis and aspiration pneumonia [116]. Candida albicans (C. albicans) is responsible for this effect on dentures [117]. Lee and his coworkers used a phytoncide-incorporated polymethyl methacrylate (PMMA) biocompatible dental polymer [118]. Their results confirm that the fungal attachment on the dental surface was reduced significantly compared with the control. Phytoncides-incorporated PMMA material can be used as a dental base resin to execute anti-fungal effects and reduce oral biofilm deposition. Eugenol is widely used in dental treatments because of its antibacterial and anesthetic properties. Eugenol has been used in root canal sealers, temporary fillings, and paste for pulp capping. It is also helpful in tooth canal treatment [119]. Incorporating phytoncide within microcapsules (PTMC) into the denture base resin can combat denture stomatitis. Phytoncide is released from PTMC at an acidic pH and inhibits the growth of C. albicans [120].

Anti-Cancer Activity of Phytoncides
About 50 different terpene components are found in pine tree essential oil [121]. Thirty-seven monoterpenes have been reported for their anti-cancer properties [122]. αpinene possesses anti-cancer activities against various cancer types, including human hepatocellular carcinoma [123,124] and prostate cancer [125]. α-pinene induces apoptosis in human ovarian cancer [126] and suppresses the expression of matrix metalloproteinase-9 in human breast cancer cells, thus inhibiting cancer invasion [100]. These anti-cancer activities of α-pinene are possible through increased CD56 and CD107a, resulting in enhanced NK cell activation. The activated NK cells release perforins, which easily attach to the cell membranes of cancer cells and induce pores on its surface, leading to the diffusion of granzyme B proteins, resulting in the apoptosis of cancer cells [89]. Aroma components in the oils of sweet orange (Citrus aurantium var. dulcis; C. aurantium var. dulcis), grape (Citrus paradise; C. paradise), and lemon (C. limon) were also found to induce apoptosis in human leukemic (HL-60) cancer cells [127]. Blood orange (C sinensis) essential oil inhibits the vascular endothelial growth factor (VEGF), prevents cell proliferation, and induces apoptosis in colon cancer cells [128].

Benefits of Exposure to the Forest Environment
Another method of aromatherapy is forest bathing or forest therapy. Forest bathing is the inhalation of phytoncides by breathing the phytoncides-rich forest air. Smelling the aromatic phytoncides from the forest environment reverses stress-induced immunosuppression and normalizes the immune function and neuroendocrine hormone levels [129]. During stress, cortisol secretion increases through the increased activation of the sympathetic nervous system and hypothalamus-pituitary-adrenal system. Phytoncides have many positive effects, such as stress reduction, reducing cortisol levels, minimizing blood pressure, and enhancing the immune system and autonomic nervous system [130]. A handful of studies evaluated the effect of phytoncides on improving immune function [131][132][133]. The biogenic VOCs were profoundly found to regulate blood pressure and endocrine activity, reduce blood glucose, maintain mental health by relieving stress, boost immunity, treat respiratory diseases, and fight cancer [73,93,[134][135][136][137][138][139][140][141]. Additionally, forest bathing aids patients with hypertension [142], chronic obstructive pulmonary disease [143], chronic heart failure [144,145], and chronic stroke [146]. Forest bathing with phytoncides promotes brain function by producing relaxation, reducing mental stress, promoting cognitive ability, and stabilizing the mood [147]. In recent studies, more attention has been given to the physiological relaxation effects of forest environments [148,149]. The forest environment enriched with phytoncides will increase parasympathetic nervous activity (a sign of relaxation), suppress sympathetic nervous activity (a sign of reduced stress) [130,150,151], reduce blood pressure [152], reduce heart rate [153], and diminish the level of stress hormones such as salivary cortisol [148].

Essential Oils and Their Health Outcomes
The essential oils taken from plants without extracting peculiar phytoncides are used to treat stress and pain. The aroma-enriched essential oils possess some rejuvenating effects in humans. Major health disorders, such as hypertension and cardiovascular diseases, are associated with stress and anxiety. Stress disturbs cognitive function, behavior, mood, and thinking skills. Mental, physical, and emotional problems interfere with individuals' learning capabilities, where stress and anxiety presumably cause hypertension and mortality. The combination of four essential aroma oils, lavender (Lavandula angustifolia; L. angustifolia), ylang-ylang (Cananga odorata; C. odorata), marjoram (Origanum majorana; O. majorana), and neroli (Citrus aurantium; C. aurantium), can decrease systolic and diastolic blood pressure and reduce salivary cortisol in prehypertensive and hypertensive patients [154].
Chronic mental stress also initiates sudden death or myocardial infarctions [155]. Besides health disorders and irreversible diseases, stress also negatively impacts human emotions. Managing stress can address those negative consequences [156]. Aroma oils can be inhaled or massaged over the skin; the applied oil vaporizes and stimulates the olfactory system [157]. L. angustifolia aroma oil reduces mental stress and increases arousal [158]. Likewise, Yuzu essential oil (Citrus junos Sieb. ex Tanaka) reduces negative emotional stress [159,160] and inhibits platelet aggregation, which could be helpful in individuals with a high risk of cardiovascular disease [161].
Essential oils promote de-stressing effects such as relaxation and sleep. Jung and his team reported that the inhalation of ylang-ylang essential oil decreases blood pressure [162], reduces vibrations and promotes relaxation [163], and increases alertness [164]. Aromatherapy reduces blood pressure, anxiety [165], and agitation [166] in dementia patients. Aromatherapy with essential oils such as linalool, santalol, cedrol, piperonal, true lavender, and sweet orange oil improved sleep in the elderly with dementia [167]. The inhalation of essential oils seems to lessen depression and increase the sleep quality of postpartum mothers [168,169]. The essential oils of sandalwood, sweet marjoram, and lavender are purportedly used in sedation, relaxation, treating anxiety, and relieving irritability, loneliness, insomnia, and depression [170][171][172].
Marjoram lowers the activity of the sympathetic nervous system and kindles the parasympathetic nervous system, which increases vasodilation and reduces blood pressure. Neroli essential oil soothes emotions, gives comfort, and reduces vibrations resulting from shock or fear [163]. Chamine and Oken evaluated the stress-reducing effect of lavender aroma. The results show that lavender aroma promoted post-stress cognitive performance. Hence, the protective effects of lavender aroma on working memory prove that aromas protect cognitive function after stress [173]. Lavender aroma lessens cardiac excitation, reduces BP, and is effective in hypertension and palpitations [154]. The aroma from Litsea cubeba (L.cubeba) is used in treating cognitive discomfort. It was found to improve mood and reduce stress and confusion by reducing the salivary cortisol level in healthy individuals [174] ( Table 2).
Pain is another discomfort, not only caused by physical illness but also by psychological distress. In a contradictory way, psychological distress can induce progressive pain. Saunder's concept of pain explains that pain is a person's suffering in social, physical, psychological, and spiritual aspects [175]. Pain induces fear, which further deteriorates social interactions and produces anxiety, depression, and stress [176][177][178]. The aroma of agave (Polianthes tuberosa; P. tuberosa) essential oil reduced test anxiety [179]. The aroma of lavender and bergamot EO can act as an antidepressant and relaxant. Pleasant odors encourage a positive mood [170] and alleviate negative emotions [180]. Bitter orange essential oil (C. aurantium) reduces first-stage labor pain and anxiety in primiparous women [181]. Ginger (Zingiber officinale; Z. officinale) and orange essential oil (C. sinensis) reduce knee pain in older adults [182]. Bergamot (Citrus bergamia; C. bergamia) essential oil possesses anti-nociceptive and anti-allodynic properties and modulates the sensitive perception of pain [183]. Bitter orange (C. aurantium) and damask rose blossom (Rosa damascena mill L.; R. damascena) aromas, together, were found to improve the symptoms of premenstrual syndrome [184,185]. Neroli, lavender, and bitter orange EOs reduce anxiety and blood pressure in postmenopausal women [186,187].  Reduces negative emotional stress. Decrease total mood disturbance, tension, anxiety, anger, hostility, and fatigue during the premenstrual stage. [158,159] 26 Yuzu EO C. junos Inhibits platelet aggregation.

Challenges and Opportunities
EOs are the secondary metabolites produced by plants to protect themselves from microbial pathogens, pests, and weeds. In addition to these effects, EOs have been used in therapies for treating insomnia, anxiety, depression, dental problems, stress, blood pressure, and cardiovascular diseases. However, the elucidation of the molecular mechanisms of the effects of EOs on stress, sleep, and depression requires more in vivo research. The mechanism behind the intracellular signaling between essential oils and higher-order brain functions remains unknown and warrants matching experiments to reveal the pharmacological effects of aroma on the human brain and physiology. EOs deserve more attention due to their traditional healing properties. The synthesis of EOs from plant organs has become more trustworthy and affordable nowadays. The anti-microbial, anti-inflammatory, antioxidant, and anti-cancer activities of EOs were well-documented through pharmacological targets. The only challenge is the insufficient number of human studies in evaluating the potential therapeutic effects of EOs. Henceforth, future research works using clinical studies might encourage the field of aromatherapy as a strong complementary treatment methodology.

Conclusions
The "back to nature" trend has increased the use of plant extracts and oils in the health care and cosmetic industries. The pleasant aroma of EOs is of use in cosmetics production and bioactive agents. Inhaling a delightful aroma can be a pleasurable experience. The aroma can be relaxing and may be able to reduce stress physically and mentally. Aromachology and aromatherapy do not show preferential differences between these modalities. Treatments using aromas have huge health benefits. The aroma of essential oils is found useful in medicinal applications, with fewer side effects. Future studies should ideally focus on the phytoncides and their substantial health effects in all aspects of treatments, ranging from anti-microbial, anti-inflammatory, anti-stress, anti-hypertensive, anti-tumor, and analgesic effects to physical, behavioral, psychological, social, and cognitive therapies. This manuscript might provide information that aids further detailed studies on essential oils and phytoncides in terms of their beneficial effects on human health and in treating or alleviating some health complications and maintaining physical, social, and mental well-being.