For any organic compound to be volatile it must have low molar mass and low polarity. Low polarity is also expressed as lipophilicity (fat solubility). As a progression, volatile organic compounds are dissolved into and transverse human skin layers (Cal, 2006), due to the phospholipid membranes of squamous cells and the phospholipid bilayer of the extracellular matrix. Lipophilic compounds with moderate polar head space, such as by having a keto or hydroxyl group, travel through the dermis faster than carbures (hydrocarbons), however even α-pinene can follow the transdermal route, albeit fluxing at a slower rate than components such as linalool or terpinen-4-ol (Cal, 2006). Nevertheless, because essential oil components are penetration enhancers of other drugs (Okabe et al., 1990; Chen et al., 2016), it is feasible that they are also enhancers of other components in an essential oil, meaning that carbures in combination with moderately polar components (i.e., terpinene-4-ol or linalool) will have more efficient transdermal penetration. Unfortunately, not much is known about the differences of absorption with whole essential oils compared to individual components.

Hence, topical application of essential oil components and transdermal penetration is more efficient than expected by non-specialists, but an encapsulation vehicle, such as a pressed oil (i.e., rosehip oil) is sometimes necessary to augment this effect, particularly to slow the rate of evaporation of the essential oil from the skin. For example, 97% of topical linalool was evaporated if applied with ethanol onto the skin (Green, 2007), but if mixed with a fixed oil ‘carrier’ most of it is absorbed (Jäger et al., 1992). Furthermore, East Indian Sandalwood essential oil (Santalum album) was topically applied onto candidates who wore a face mask to prevent inhalation of the aroma and resulted in statistically significant physiological changes, such as blood pressure, pulse rate and ‘alertness’ compared to the control (Hongratanaworakit et al., 2004). Inhaled essential oils can also become systemic and lead to changes in metabolic pathways associated with anxiety related behaviour, which has been demonstrated to occur in rats (Wu et al., 2012).

Topically applied, ingested or inhaled essential oils, or aromatic extracts, release components into the body that rapidly ‘sink’ into fat tissue, while some components are transported around the body in the vehicle of blood albumin. Compounds with keto groups (carbonyls) bind to blood albumin and are circulated throughout the body but are thereafter eliminated in metabolism or sunk into adipose tissue or the phospholipid membranes of some cells, like keratinocytes. Components that are trans-dermally absorbed (via lungs or skin) enter capillaries and the blood stream, where they are detected within 20 min and for as long as 90 min (Jäger et al., 1992) before sinking and/or eliminated in metabolism. Lipophilic compounds cross the blood brain barrier, and can create psychoactive effects, such as the phenylpropanoid elemicin (Beyer et al., 2006), the terpene incensole acetate (Moussaieff et al., 2008) or the phytocannabinoids (Griffin et al., 1999).

The transdermal route greatly slows the metabolism of compounds by avoidance of the ‘first pass’ effect that occurs in digestion of orally administered matter, where metabolites entering portal circulation from the intestines are circulated directly to the liver (Sadgrove and Jones, 2019). However, in some cases, the oral route to the absorption of volatile organic compounds is more convenient. For example, although d -limonene does not have a keto group, plasma levels reached as high as 1.65 μM with lemonade drinking and over the course of 4 weeks accumulated in adipose tissues to levels nearly 200 fold greater than maximum plasma concentration (Miller et al., 2010). Alternatively, via the oral route linalyl acetate is immediately converted into linalool in the digestive process (Nölder et al., 2011), and linalool concentrations peak in blood plasma at 1915 ng ml⁻¹ (Shi et al., 2016). Thus, in the case of linalyl acetate, topical application is better, i.e., topical application of lavender essential oil to a human abdomen resulted in maximum plasma concentrations of >250 ng ml⁻¹ essential oil, made up by 100 ng ml⁻¹ linalool and 121 ng ml⁻¹ linalyl acetate (Jäger et al., 1992). Linalool accumulates in organs and fat at concentrations many folds higher than peak plasma concentrations (Nölder et al., 2011). Similarly, in the ‘Karoo’ of South Africa a lamb that forages on Pentzia incana (Thunb.) Kuntze reputably acquires an artemisia flavour to its meat, known as the ‘Karoo lamb’, which is a consequence of volatile organic compounds accumulating in its adipose tissues (Hulley et al., 2018).

Essential oil components can also have prooxidant effects that are a negative consequence of higher than safe levels (Bakkali et al., 2008). This is of relevance to phenylpropanoids and other phenolics that demonstrate pronounced in vitro radical scavenging abilities. As previously mentioned, lipophilic compounds dissolve into the phospholipid walls of human cells. The concentration determines if a positive or negative effect occurs, wherein a wide concentration range for positive therapeutic effects is available. Volatile organic compounds increase the permeability of phospholipid membranes, not just in cell walls but also in the walls around organelles. Permeabilization of the mitochondrial membrane can potentially interfere with the electron transport chain, leading to the upregulation of radical oxygen species that oxidise cellular contents. If phenolic compounds are present, their oxidation will generate significantly more reactive species (Bakkali et al., 2008). However, studies that report on prooxidant effects are still describing concentrations that are high, such as 30–90 μg ml⁻¹ (200–600 μM) of carvacrol (Liang and Lu, 2012). While such concentrations may seem unrealistic, they are frequently reported as peak plasma concentrations in mice studies. For example, a pharmacokinetic study of borneol and menthol demonstrated peak plasma concentrations of 20 and 70 μg ml⁻¹ respectively, which were metabolised in one and 3 hours respectively (Xu et al., 2011).

In considering pharmacokinetic studies wholistically, plasma concentrations peak instantly with intravenous administration, or after oral administration anywhere from 20 min to 3 h. The peak plasma concentrations are dependent upon dose, but in vivo rat models have demonstrated as high as 300 μg ml⁻¹ in plasma, present mostly bound to plasma proteins (Dawidowics and Dybowski, 2014). Peak plasma concentrations are usually lower than tissue plasma concentrations, but tissue plasma concentrations are usually not measured in studies.

Essential oil components are either metabolised or absorbed into adipose tissues and organs, lowering plasma levels to baseline within 1–3 h for low doses, but at high doses plasma concentrations are maintained for several days as the components are buffered from the body’s tissues. Hence, during metabolism they are slowly removed from adipose tissue and the organs, which is highest in fat, followed by the liver, then kidneys and lowest in cerebrospinal fluids and brain. After 24–72 h after a single dose the components are still present in adipose tissue, where they persist for some time. This is corroborated by evidence presented in animal studies (Serrano et al., 2007).

In metabolism, essential oil components are oxidised by phase 1 and 2 enzyme mediated reactions in the liver or other tissues (Zehetner et al., 2019) creating polar derivatives in phase 1, then sulphate, glutathione or glucuronide conjugates in phase 2. A list of metabolic derivatives of common essential oil components is given by Kohlert et al. (2000).

After the xenobiotics are metabolised by phase 1 or 2 processes they are then eliminated via urination or secreted into the bowel for microbial fermentation. For example, during a pharmacokinetic study of menthol, participants received an oral dose of >500 mg pure menthol, yet peak plasma concentrations did not exceed 160 ng ml⁻¹ whereas menthol glucuronides were as high as 7 μg ml⁻¹ (Valente et al., 2015). Although these glucuronides are created to facilitate the removal of menthol from the system, they may be recycled for therapeutic effects by deconjugation when in contact with the enzyme glucuronidase, which is upwardly expressed in the inflamed tissues of the body (Shimoi and Nakayama, 2005). This encourages us to think of the conjugated forms of essential oil components as quasi-bioavailable.

The phase 2 β-glucuronide metabolite is characterised by a glucuronide moiety O-linked to the xenobiotic (the compound). As previously mentioned, once these products reach this higher level of polarity, they have short half-lives because they are efficiently eliminated by the kidneys. However, high amounts of these polar conjugates can be dissolved in blood plasma and transported to any extracellular space in the body, reaching sites of infection or inflammation. Higher amount of β-glucuronidase alluded to above is common in cancers as well as inflamed tissues. When the glucuronide moiety is removed, the xenobiotic is returned to a much more hydrophobic intermediate, commonly a derivative that was formed in an earlier metabolic step, if not the pre-metabolised form. This causes the xenobiotic to lose solubility and accumulate on-site, potentially enacting biological effects locally (Sperker et al., 2001). Current research on the pharmacokinetics of natural products ignores this latter observation in the context of rational in vivo translation of in vitro outcomes (Sadgrove and Jones, 2019).

Glutathione conjugates are less commonly described as a metabolic product of essential oil components. When the glutathione conjugates were observed in earlier studies, they were thought to be non-enzymatic phase 2 reactions that were initiated by a phase 1 oxidation (Thompson et al., 1990). However, the understanding of glutathione S-transferases and their role in conjugation of glutathione to xenobiotics (Sheehan et al., 2001) changed this view. Several studies describe glutathione conjugates of essential oil components, such as cinnamaldehyde (Choi et al., 2001), pulegone (Lassila et al., 2016) and eugenol (Thompson et al., 1990), just to name a few. Conjugation by S-transferases typically creates an S-linked glutathione but in some cases N-linked conjugates are non-enzymatically formed, which can occur when furans form reactive aldehydes that react in a Schiff-base fashion with the free glutamyl amine on the glutathione reactant, which happens to menthofuran (Lassila et al., 2016). Essential oils are known to upregulate the expression of glutathione S-transferase in the liver (Banerjee et al., 1994; Abd El-Moneim et al., 2012), but minimal study has been dedicated to the P isoform that is upregulated in cancers (Tew et al., 2011). It is unclear if upregulation of glutathione S-transferase in cancers by essential oils is a positive or negative outcome because chemotherapeutic drugs are metabolised faster, which is a negative, but so are carcinogens, which is a positive. Furthermore, the biological effects of glutathione conjugates of essential oils have minimal research, but they should be examined in the context of cancers as part of the growing body of research dedicated to glutathione S-transferase prodrugs (Townsend and Tew, 2003). Finally, many xenobiotics are not conjugated to glutathione (Kohlert et al., 2000), and because there are minimal reports of this occurring in essential oil components, it may be considered less common.

While essential oil components are usually metabolised by both phase 1 and 2 processes in the liver, there is some evidence that more is ‘sunk’ into adipose tissues and organs than is eliminated, i.e., one study reported in humans that with 1 mg oral dose of thymol the peak plasma concentration reached 0.093 μg ml⁻¹, but only about 16% was eliminated as thymol sulphate or glucuronide, suggesting accumulation in organs and fat (Kohlert et al., 2002).

Minimal studies are available to determine peak plasma or organ concentrations before toxic effects may be considered in people. A single study was found that examined the human maximum tolerance dose of d -limonene and quantities administered ranged from 0.5 to 12 g m² orally. It was determined that the safe dose was 8 g m² i.e., 12–16 g oral dose, which could be sustained for 11 months with no adverse effects. Despite such a high oral dose, the peak plasma concentrations were 2.12 μg ml⁻¹ compared to tissue plasma concentration of 5.52 μg ml⁻¹, and the major phase 1 metabolic products are perillic acid, perillic acid isomers, perillyl alcohol and limonene-diol derivatives. Hence, peak plasma concentrations of limonene combined with its oxidised forms were >14 μg ml⁻¹ (Vigushin et al., 1998). Some biological roles of these metabolic products have been demonstrated, including immunomodulation, anti-inflammatory and antiproliferative effects against pancreatic cancer cells (da Silveira e Sá et al., 2013).

In mice there are several studies that push the limits in terms of safety. For example, intravenous and oral doses of 12.5 mg geraniol in mice produced peak plasma concentrations of approximately 250–300 μg ml⁻¹ by both routes, which was metabolised or ‘sunk’ within 2 h. To test for toxicity the authors then administered a 10-fold higher concentration of 120 mg day⁻¹ for 4 weeks and demonstrated no apparent toxic effects (Pavan et al., 2018). This indicates that mammals can experience plasma concentrations familiar to many of the in vitro studies, whilst remaining several orders of magnitude below possibly toxic concentrations. Evidently, toxicity is dependent on the functional groups of individual essential oil components, so they will need to be considered on an individual basis.

Unfortunately, there are limited studies that focus on the possible biological effects of the metabolic conjugates or phase 1 metabolites of essential oil components, i.e., phase 1 oxidised or phase 2 conjugated sulphate, glutathione or glucuronide forms. Although this has been visited to an extent in the cases of limonene metabolites (da Silveira e Sá et al., 2013), it is worth considering other leads in future studies.

Volatile organic compounds can also influence the expression and activity of cytochrome P450 enzymes and transferases involved in metabolism (Sadgrove and Jones, 2019), which can influence the nature of its own metabolism or the metabolism of other drugs in the system, either slowing down or speeding up the rate of metabolism and changing the drug’s half-life (Zehetner et al., 2019). There is a growing body of knowledge of the metabolism of essential oil components when administered in pure form (Zehetner et al., 2019), but less is known about component metabolism when ingested as part of parent plant material that also includes components that modulate cytochrome p450 (CYP) isozymes and change the rate of metabolism of specific components relative to others. In several cases plant material has CYP isozyme inhibitors that increase the peak plasma concentrations of the metabolite (Ashour et al., 2017). Furthermore, interactions of essential oil components with drugs should be taken into consideration if candidates use pharmaceuticals. A comprehensive guide to the safety of essential oils is given by Tisserand and Young (2013).

The ability of essential oil components to modulate CYP isozymes may in part be related to their affinity for the pregnane X receptor (Šadibolová et al., 2019), but recent evidence has not been conclusive. Nevertheless, a comprehensive summary of the enzymes that are modulated in relation to the essential oil component is provided by Zehetner et al. (2019), where induction, inhibition and metabolizing enzymes are listed.

It is common for studies to demonstrate interesting biological effects from crude extracts of plant organs, but to fail to identify an active ingredient after following a bioassay guided fractionation approach (Sadgrove and Jones, 2019). In admitting defeat, authors of these types of studies often speculate that synergism is responsible for irreproducibility of their earlier outcome. Unsurprisingly, it is indeed true that synergisms occur, but research has only recently started to explain these synergisms and essential oil components are repeatedly demonstrated to be significantly involved (Sadgrove N. J., 2020; Langat et al., 2021; Nsangou et al., 2021). However, during the fractionation process volatile components are often lost when removing solvents, making it difficult for researchers to recognise synergistic effects.

The effects of drug or compound combinations are classified according to the four categories, additive, synergistic, antagonistic or potentiator. In the context of antimicrobial studies, these categories are usually determined by testing combinations and calculating summed fractional inhibitory concentrations (ΣFIC). This is calculated using the minimum inhibition concentration (MIC) of individual components or essential oils and comparing to combinations. The MIC assay for antimicrobial testing is elucidated by Sadgrove and Jones (Sadgrove and Jones, 2015), but briefly the protocol uses a serial two-fold dilution of the test substance in agar that is then inoculated with the bacterial organism, so that a range of concentrations are tested. The minimum concentration that can create inhibition is regarded as the MIC. Hence, to calculate the ΣFIC, several MIC values are required, then the calculations follow Eqs. 1–3.

In Eq. 1 the concentration of A (Con.A) in the mixture of drugs A+ B at the combined MIC concentration, is divided by the MIC of drug A alone (MIC-A) to give FIC-α. In Eq. 2 the concentration of B (Con.B) in MIC A+ B is divided by the MIC of drug B alone to give FIC-β. In Eq. 3 FIC-α is combined with FIC-β (α+β) to give the ΣFIC value (Sueke et al., 2010). For example, if the MIC value of A + B is 0.5 mg ml⁻¹ at a ratio of 1:4 of A:B, then Con. A is 0.1 and Con. B is 0.4 mg ml⁻¹. If the MIC value of A alone is 1.5 mg ml⁻¹ and B alone is 1.0 mg ml⁻¹ then FIC-α is 0.1/1.5 = 0.07 and FIC-β is 0.4/1.0 = 0.4; then the ƩFIC value is 0.07 + 0.4 = 0.47. F I C ‐ α = C o n . A   ( i n   M I C   A + B ) M I C ‐ A (1) F I C ‐ β = C o n . B   ( i n   M I C   A + B ) M I C ‐ B (2) Σ F I C = F I C ‐ α + F I C ‐ β (3)

When the ΣFIC is ≤ 0.5 it is synergistic, if > 0.5–1.0 it is additive, if > 1.0 – ≤4.0 it is noninteractive and >4.0 makes it antagonistic (Van Vuuren and Viljoen, 2011).

The distinction between synergism and potentiation needs to be made. Synergism is defined by the increase of activity by a combination of two ‘active’ compounds. In synergism, enhancement of the activity is greater than the sum of the two effectors (A+ B) giving 1 + 1 ≠ 2 or 1 + 1 = >2. Alternatively, a subset of synergism is potentiation, which occurs when a non-active compound enhances the activity of the active compound (i.e., 1 + 0 = >1). The distinction is usually only made in the context of defeating resistance mechanisms, i.e., if one component blocks an efflux channel to augment the effect of an antibiotic, it is regarded as the potentiator (Mandeville and Cock, 2018).

While in vivo synergism can be caused by a wider range of system interactions, such as by changing a xenobiotic’s pharmacokinetics by slowing its metabolism, in vitro synergism has a narrower range, which is commonly the outcome of targeting two different mechanisms to achieve an enhanced outcome, including resistance mechanisms of pathogenic microbes. This means that by testing against a single target, such as a single enzyme, synergism is not possible, i.e., synergism requires at least a whole cell to manifest. Because most essential oil components confer effects to cell walls of bacteria and eukaryotes, their synergistic effects when combined with compounds that have specific targets, are caused by destabilising the walls of target cells.

In many synergism studies, essential oils and volatile organic compounds are regarded as non-active participants in combination with pharmaceuticals, so they are described as potentiators. While other researchers require stronger effects from antimicrobials, most researchers consider an MIC at <1 mg mL⁻¹ as active (Van Vuuren and Holl, 2017), which is common in essential oils research. Consequently, the terms synergistic and potentiation are often used at the discretion of the authors in the published literature.

The most common potentiating effects described for volatile organic compounds or essential oils in the literature is focused on combinations with antibiotics from ‘big pharma’, i.e., essential oils from Thymus vulgaris L synergistically enhance the antibiotic cefixime (Jamali et al., 2017). In the pharmaceutical world the use of volatile organic compounds on their own to enact antimicrobial outcomes is not feasible for economic reasons. The concentrations must be many orders of magnitude higher to be comparable to microbially derived antibiotics (Sadgrove and Jones, 2019), which raises the cost of production to beyond reasonable, and limits the range of applications to topical use only (inhalation, topical dermal or gastro/intestinal epithelial). Hence, rather than being antimicrobial per se, volatile organic compounds are appropriately thought of as antiseptic compounds (Kon and Rai, 2012), with only general specificity in the mechanism of action. However, synergistic or potentiation effects are still of interest to pharma, by antagonising resistance mechanisms in pathogenic strains. The most commonly cited potentiation effect ascribed to plant metabolites is the attenuated effects of efflux ‘pumps’ (Khameneh et al., 2019). Prokaryotic efflux pumps are bacterial or viral membrane bound channels called ‘transport proteins’ that promote the disposal of cellular waste or toxins. Gene modulation effects by volatile organic compounds also occur in the prokaryotic cells of pathogenic microbes, which involves the downregulation of resistance associated genes (Chovanová et al., 2016), leading to the potentiation of other antimicrobial metabolites or antibiotics. Furthermore, volatile organic compounds have also shown the ability to downregulate expression of genes responsible for pathogen toxin secretion (Khoury et al., 2016), which attenuates virulence.

Normally the excretion of antimicrobial drugs via efflux pumps does not antagonise drug efficacy, but with the new trends involving overexpression of multidrug resistance efflux pump genes (Blanco et al., 2016), antibiotics are becoming less efficacious. Inhibiting this mechanism causes the accumulation of the antimicrobial drug in the bacteria’s cytoplasm, which enables an active concentration of the drug to be reached (Bambeke et al., 2003). While there are no efflux pump inhibitors in wider clinical use, volatile organic compounds are known to have this effect (Mikulášová et al., 2016). For example, the sesquiterpenes epi-cubenol and 15-copaenol were able to produce ΣFIC values in the range of 0.03–0.26 in combination with standard antibiotics against strains of Staphylococcus aureus that overexpress the NorA gene for the NorA efflux pump. These potentiating effects are attributed to efflux pump inhibition at a concentration of sesquiterpene that is less than 0.25 μg ml⁻¹ (Espinoza et al., 2019), a concentration that is feasible in blood plasma alone, discounting accumulating effects in the body’s tissues.

Essential oil components may also augment the efficacy of other drugs by enhancement of their penetration (Aqil et al., 2007; Chen et al., 2015). The mechanism is thought to be related to disruption of the highly ordered structure of the stratus corneum lipid, leading to an increase in the intercellular diffusivity. This is established by the observation of a shift from ‘trans to gauche’ conformation in the methylene carbons along the alkyl chain of lipids (Chen et al., 2015). In the case of paracetamol, penetration enhancement values for each of the essential oils correlate to the skin permeation profiles of the individual essential oils. Hence, penetration enhancement can be predicted from the flux (μg.cm⁻². h⁻¹) or Q48 (μg.cm⁻²) of the individual essential oil. Essential oils with pronounced permeation values were produced by clove, angelica and chuanxiong and the most represented components were ligustilide and eugenol (Chen et al., 2015), which have the character of delocalised electrons (aromatic rings).

Furthermore, the cell and organelle wall permeabilising effects of lipophilic compounds can potentiate the influx of other exogenous compounds that confer gene modulatory effects. Although not a great deal of research has focused on this, the whole range of compounds in extracts from aromatic species may have synergistic or potentiation effects. Extracts from aromatic species have a range of compounds in their extracts, which include fixed (non-volatile) and volatile metabolites. As an example, the aromatic species Elytropappus rhinocerotis (L.f) Less. is used in South Africa as a treatment against foot odours and infections. The extract includes volatile organic compounds and non-volatile labdane diterpenes, both of which antagonised the growth of food fungal pathogens and odour causing bacteria (i.e., Brevibacterium agri) (Hulley et al., 2019).

The possibility for synergism between essential oil components and cannabinoids has been explored in theory (Russo, 2011). However, limited research has been conducted to specifically address these questions. The dominant sesquiterpene β-caryophyllene has been the focus of many studies, including as a synergist in antimicrobial outcomes. A recent study of Vepris gossweileri I. Verd., discovered several antimicrobial synergisms with β-caryophyllene in an extract of the leaves, which was defined as a ‘multi-layered’ synergism (Langat et al., 2021). Synergism against a model yeast and Gram-positive organism was demonstrated between β-caryophyllene and minor alkaloids with a ΣFIC value of 0.02. However, the synergistic effects were augmented by the chlorophyll derivative pheophytin A. In a follow-up study (results unpublished) it was realized that chlorophyll, pheophytins or pheophorbides are potent antimicrobial synergists in combination with β-caryophyllene. Hence, antimicrobial effects of green plant extracts, such as cannabis or CBD oil, are likely to be the result of a synergism between β-caryophyllene and the chlorophyll derivatives.

Another study of synergism argued that essential oil components in roots of Citrus x limon (L.) Osbeck synergised with specific methoxylated flavonoids released out of decomposing leaf litter at the base of the tree, providing protective effects against pathogenic root fungi (Nsangou et al., 2021). A hypothetical that is inspired by this research outcome involves antimicrobial synergisms against ruminant organisms that reside in the gut of herbivorous insects. This has not yet been examined in detail but stands out as a high possibility. Otherwise, antimicrobial synergisms may be considered as an important part of plant defence and the wider scientific community is encouraged to examine this in more detail, because there are both ecological and health-related implications from such research, i.e., there are many ways that such synergisms can be utilised in human health as prebiotics for the gastrointestinal tract.

While it is necessary for volatile organic compounds to reach high concentrations to confer ‘contact’ antimicrobial effects, gene-modulation can occur at concentrations that are many folds’ lower. Mammalian cells have diverse super-families of transcription factors that have lipid binding domains that inevitably become the target of lipophilic compounds. One study that focused on the dermal fibroblast demonstrated upregulation of hundreds of genes associated with (anti)inflammation, metabolism and proliferation, which occurs at a concentration of less than 100 μg g⁻¹ (0.01%) by a variety of volatile compounds (Han and Parker, 2017). The genes are modulated uniquely by each of the compounds tested, at concentrations that are low enough to be feasible in topical applications or tissue bioaccumulation, without cytotoxic effects (2017).

In the context of human (eukaryotic) cells, modulation of the expression of peroxisome proliferator-activated receptor (PPAR) genes can occur in response to essential oil components (Goto et al., 2010). For example, limonene at 10 μM (1.5 μg g⁻¹) increased phosphorylation of Akt leading to enhanced glucose uptake in adipocytes, promoted adipocyte differentiation and also allegedly the expression of PPAR-γ genes (Soundharrajan et al., 2018). Linalool appears to do the opposite, by inhibiting adipocyte differentiation (Cheng et al., 2018), which may be useful in obesity control. Nevertheless, both compounds can reach systemic concentrations of 2 μg ml⁻¹ in humans, by topical and oral application, with no adverse effects.

PPAR agonism is also associated with anti-inflammatory effects (immunomodulation) by inhibition of interleukin-1 induced cyclooxygenase-2 expression (Hotta et al., 2010). However, immunomodulation by essential oil components is not limited to this effect. The literature describes the modifying effects on secretion of a wide diversity of cytokines other than interleukin-1 (Valdivieso-Ugarte et al., 2019), which can occur by agonism of nuclear or membrane receptors, such as the cannabinoid receptors, but the details are often not explained. For example, systemic treatment using 50 mg kg⁻¹ essential oil from Cordia verbenacea A. DC reduced tumour necrosis factor (TNF-α) production, which interrupted the inflammatory cascade induced by carrageenan (Passos et al., 2007). Ingestion of 50 mg kg⁻¹ of the two main volatile organic compounds, α-humulene and E-caryophyllene, also reduced inflammation. E-Caryophyllene only diminished TNF-α release whereas α-humulene also interrupts interleukin-1β, cyclooxygenase-2, nitric oxide and prostaglandin E-2 (PGE(2)) (Fernandes et al., 2007). Furthermore, inflammation was greatly attenuated by oral treatment an hour before lipopolysaccharide (LPS) was used as an inducer, evidently by the same mechanism as above (Medeiros et al., 2007). In topical applications a much lower concentration is required. Hence, a commercial product named Acheflan with C. verbenacea essential oil as an active ingredient is available in Brazil as a topical anti-inflammatory. Pharmacokinetic studies of the main sesquiterpene, α-humulene, using oral and intravenous doses of 1,000 mg kg⁻¹ mouse, demonstrated that peak plasma concentrations can reach from 2–20 μg ml⁻¹ without adverse effects in the short term (Chaves et al., 2008).

Before the term ‘potentiator’ came into practice, researchers used the former term ‘entourage effect’ to describe the potentiating effects of volatile organic compounds from the marijuana variety of Cannabis sativa. The psychoactive effects from marijuana are caused by tetrahydrocannabinol, which is a potent agonist for cannabinoid receptor-1. However, as previously mentioned, the headspace of marijuana also includes the volatile sesquiterpene β-caryophyllene, which is regarded as a phytocannabinoid that is not psychoactive because it is a selective agonist of cannabinoid receptor-2 (CB2), a receptor in immune cells (Gonçalves et al., 2020). Caryophyllene is the most publicised example of a specific CB2 agonist (Francomano et al., 2019). It is known to promote wound healing in dermal skin models by following multiple routes, but the anti-inflammatory effects are likely to be the most important in this outcome. Concomitant with higher rates of re-epithelialization is the upregulated expression of hair follicle bulge stem cells, which has strong implications to hair health (Koyama et al., 2019).

But the effects of β-caryophyllene are not restricted to cannabinoid receptors. β-Caryophyllene positively regulates the pI3K/Akt/mTOR signalling pathway in tissues that express Akt3, a protein kinase B isoform important for the regulation of neuronal development. Alternatively, in liver cells and T lymphocytes this pathway is negatively regulated by the same treatment but upregulated in neuronal cells, indicating a role in tissue-specific inflammation treatment. Regulation of the pI3K/Akt/mTOR pathway is entirely dependent on Akt3, meaning that it makes sense that the JAK/STAT signalling pathway is upregulated independently. Hence, the essential oil of copaiba that is rich in β-caryophyllene confers gene regulatory effects that differ according to the tissue (Urasaki et al., 2020), i.e., copaiba essential oil can confer anti-inflammatory effects without dulling the immune response. Furthermore, it was demonstrated that by upregulation of the pI3k/Akt/mTOR pathways in the dermis, promotion of reepithelization of superficial wounds occurs (Koyama et al., 2019).

In contrast, the gingerols from Zingiber officinale allegedly downregulate the pI3k/Akt/mTOR pathway (Wang et al., 2016). The gingerols are also potent antioxidants (Mao et al., 2019). Together these effects confer protection against oxidative species generation from mitochondrial respiration. Previously it was thought that anecdotal accounts of use of ginger for hair restoration, in men living with androgenetic alopecia, were a contradiction because it slowed the growth of dermal papilla cells in vitro (Miao et al., 2013), however evidence now indicates the Akt-mTOR pathway is overactive in bald scalps and this process is associated with overproduction of reactive oxygen species (Sadgrove, 2021).

A bit like CBD oil, the anti-inflammatory effects of essential oil components may also play a role in aiding sleep. A famous sleep-inducing herbal tea is chamomile (Matricaria recutita L., Asteraceae). The blue colour of the essential oil is caused by chamazulene, which is an anti-inflammatory component that is active at 10–60 μg ml⁻¹ in vitro (Ma et al., 2020). However, chamazulene is a derivative produced in hydrodistillation by conversion of the precursor matricine, which is the version that is present in chamomile tea. Matricine is active at a lower concentration, inhibiting NF-KB activation within the margin of 3–22 μg ml⁻¹ (Flemming et al., 2015). Because NF-KB activation is associated with sleep deprivation (Irwin and Wang, 2008), the link between chamomile tea and restful sleep may be in the anti-inflammatory effects of matricine, or its combined effects with the non-volatile flavonoid component quercetin (Kambe et al., 2010). Alternatively, mice treated with essential oil from chamomile were observed to have lower plasma histamine levels than control after challenging with 2,4-dinitrochlorobenzene (Lee et al., 2010), which conveys that the essential oil may be an antihistamine and antihistamines are known to induce sleep or drowsiness.

Other essential oil components that have been demonstrated as anti-inflammatory by in vitro assay of lipopolysaccharide induced cytokine release include the santalol isomers from sandalwood (Sharma et al., 2014), eugenol from clove (Saad et al., 2013), and carvacrol from thyme (Hotta et al., 2010) among others. The numbers of essential oil components associated with anti-inflammatory effects are numerous and mechanisms are often not explained but it is reasonable to hypothesise a role for PPARS.

Because essential oil components accumulate in the body’s tissues, the obstacle of bioavailability may be overcome, particularly in cancers. As previously mentioned, metabolite conjugation reduces a compound’s bioavailability and prevents it from reaching a potentially toxic concentration in normal tissue, but in cancerous tissue deconjugation reverses the phase 2 metabolism and causes a localised build-up of preconjugated xenobiotics. The prooxidant effects (Burt, 2004) that are normally not occurring in healthy tissue are enabled by this localised concentration of xenobiotics, which include a host of ingested plant-derived secondary metabolite, including essential oil components.

Generally, phase 1 metabolism makes oxidised derivatives of essential oil components and in phase 2 metabolism they are conjugated to either a glucuronide, glutathione or a sulphate moiety (Sadgrove and Jones, 2019). Although this process is thought to make the respective xenobiotic completely unavailable, it is now known that deconjugation processes return xenobiotics to their active pre-conjugated forms. These effects are well known for non-volatile plant compounds, such as curcumin, which is rapidly metabolised into a glucuronide that is regarded as unavailable, but is transformed back into the aglycone by β-glucuronidase activity in bone tissue undergoing osteoclastogenesis (Kunihiro et al., 2019). Previously several essential oils were discovered as preventors of bone reabsorption (osteoclast breakdown) (Mühlbauer et al., 2003), and the modern realization of the β-glucuronidase activity clarifies how the essential oil components can meet the required concentrations. A similar scenario was observed with the non-volatile metabolite resveratrol, which is quickly metabolised into a sulphate conjugate that is reversed by steroid sulphatases in cancer cells, inducing autophagy (Andreadi et al., 2014).

Cancers express high levels of β-glucuronidase (Su et al., 2014) and steroid sulphatases (Foster, 2021). Hence, conjugated xenobiotics may be regarded as glucuronide or sulphate prodrugs that are activated on-site to enact therapeutic effects (de Graaf et al., 2002). Although this type of research has not focused on essential oil components, the anticancer effects of phase 1 metabolites are sometimes examined by forward thinking researchers, which are the likely forms that appear in tumours or cancers when deconjugation occurs. For example, d -limonene was previously considered a worthy candidate in chemotherapy, and the phase 1 metabolites were also of interest in this context (da Silveira e Sá et al., 2013).

Research on the anticancer effects of essential oil components generally focuses on cytotoxic effects caused by various mechanisms, including overexpression of liver detoxification enzymes, changes to the membrane potential of mitochondria (causing leakage of free radicals) and whole cancer cells, prooxidant effects (mainly by arenes), inhibition of angiogenesis, and modification of tumour-inducing genes (Bhalla et al., 2013; Legards et al., 2014). Complementary, additive or synergistic effects, where a supportive role to the more conventional treatment, is also a point of interest in modern research (Legards et al., 2014). However, the insulin sensitising effect of essential oil components (Talpur et al., 2004) is a worthy consideration, because modulation of glucose metabolism in cancers has also become a point of interest in modern research (Hay, 2016).

Alternatively, lifestyles that include diets fortified with aromatic species may be preventative or antagonistic of oncogenesis. This school of thought is known widely as a chemoprevention strategy. Hence, the powers of plant-derived xenobiotics, such as essential oil components, are more than likely realized as preventative because of the anticancer subtly by comparison with conventional chemotherapy drugs. Milder nature-based chemotherapy metabolites, such as essential oil components, can be endured by the human body over the long term, meaning that cancerous cells can be antagonised before they establish as larger resilient cysts. Furthermore, anticancer research of essential oils should focus on the possible antagonism of cancer metastasis during the remission period between conventional anticancer treatments. This is a feasible initiative because it requires following cancer survivors and quantifying those who stay in remission whilst incorporating a plant-based health regime.