Terpinenes are a group of naturally occurring hydrocarbons belonging to the monoterpene family, commonly found in essential oils of various plants, such as tea tree oil and cumin. They are known for their fresh, herbaceous, and slightly citrusy scent and are widely used in cosmetics, personal care products, and aromatherapy due to their antioxidant, antimicrobial, and anti-inflammatory properties. Terpinenes are also valued for their ability to soothe irritated skin and protect against environmental stressors.

Chemical Composition and Structure
Terpinenes are unsaturated hydrocarbons with a C10H16 molecular structure, typically classified into three isomers: α-terpinene, γ-terpinene, and β-terpinene. These isomers differ in the placement of their double bonds, which affects their chemical properties and reactivity. They are commonly found in essential oils as volatile compounds that contribute to both the fragrance and therapeutic effects of the oils.

Physical Properties
Terpinenes are typically clear, colorless to pale yellow liquids with a fresh, herbaceous aroma and a slightly citrusy undertone. They are volatile and lipophilic (oil-soluble), making them suitable for oil-based formulations and products where they can evaporate and leave behind a pleasant fragrance. Their volatility and lightness make them ideal for aromatherapy and topical applications, as they are easily absorbed by the skin.

Production Process
Terpinenes are generally obtained through steam distillation or cold-pressing of plants that contain these compounds, such as tea tree, cumin, and marjoram. The production process includes:

1. Harvesting: Plants rich in terpinenes are harvested, typically when the essential oil content is highest.

2. Steam Distillation: The plant material undergoes steam distillation to extract the essential oil containing terpinenes and other aromatic compounds.

3. Separation and Purification: The terpinenes are separated and purified from the essential oil through additional distillation or fractionation techniques.

4. Formulation: The purified terpinenes are then incorporated into various cosmetic, skincare, and personal care formulations.

Applications

• Skincare: Terpinenes are used in skincare products for their antimicrobial and anti-inflammatory properties. They help soothe irritated skin, reduce redness, and prevent bacterial growth, making them ideal for acne-prone skin. They are often found in creams, lotions, and cleansers.

• Aromatherapy: Due to their fresh and calming scent, terpinenes are commonly used in aromatherapy products such as diffusers, oils, and candles. They are known to promote relaxation and reduce stress.

• Haircare: In haircare formulations, terpinenes are included for their ability to cleanse the scalp, reduce excess oil, and provide a fresh scent. They are also used in anti-dandruff treatments and scalp tonics.

• Cosmetic Fragrance: Terpinenes are often used as a fragrance component in cosmetic products, offering a fresh and uplifting scent to creams, lotions, and perfumes.

Environmental and Safety Considerations
Terpinenes are generally considered safe for use in cosmetic and personal care products. However, as with other essential oil components, they should be used in recommended concentrations to avoid skin irritation, particularly for individuals with sensitive skin. In higher concentrations, terpinenes can cause allergic reactions or skin sensitization. 

It is important to note that some terpinenes, particularly when exposed to sunlight, can become photosensitizers, which may increase the skin's sensitivity to UV light.

From an environmental perspective, terpinenes are derived from renewable plant sources, making them sustainable when harvested responsibly. However, care should be taken to ensure that the extraction and production methods do not contribute to overharvesting or environmental degradation.

References__________________________________________________________________________

Maquera Huacho PM, Rodriguez Herrero E, Verspecht T, Pauwels M, Marcantonio E Jr, Palomari Spolidorio DM, Teughels W. Terpinen-4-ol and carvacrol affect multi-species biofilm composition. Biofouling. 2019 May;35(5):561-572. doi: 10.1080/08927014.2019.1630386.

Abstract. The aim of this study was to investigate the cytotoxic activity and inhibitory effect of terpinen-4-ol (T4ol) and carvacrol against single- and multi-species biofilms. The toxicity of each compound was tested on oral keratinocytes and evaluated by XTT assay. Inhibition and eradication of single-species biofilms were analyzed by crystal violet assay and the effect on multi-species biofilm composition was evaluated by qPCR. T4ol and carvacrol did not affect the epithelial cell viability, in contrast to chlorhexidine, which showed a high cytotoxic effect. Inhibition and eradication of single-species biofilms treated with T4ol and carvacrol were observed. The same inhibitory effect was observed for multi-species biofilms, especially on periodontal pathogens. In conclusion, specific concentrations of T4ol and carvacrol without toxicity towards the epithelial cells reduced the numbers of periodontal pathogens in single- and multi-species biofilms.

Lam NSK, Long XX, Griffin RC, Chen MK, Doery JC. Can the tea tree oil (Australian native plant: Melaleuca alternifolia Cheel) be an alternative treatment for human demodicosis on skin? Parasitology. 2018 Oct;145(12):1510-1520. doi: 10.1017/S0031182018000495. 

Abstract. Australian tea tree oil (TTO) and its extract terpinen-4-ol (T4O) are found to be effective in moderating demodex-related diseases. Their possible effects are lowering the mite counts, relieving the demodex-related symptoms and modulating the immune system especially the inflammatory response. This review summarizes the topical treatments of TTO and T4O in human demodicosis, their possible mechanism of actions, side-effects and potential resistance in treating this condition. Although current treatments other than TTO and T4O are relatively effective in controlling the demodex mite population and the related symptoms, more research on the efficacy and drug delivery technology is needed in order to assess its potential as an alternative treatment with minimal side-effect profile, low toxicity and low risk of demodex resistance.

Martella N, Colardo M, Sergio W, Petraroia M, Varone M, Pensabene D, Russo M, Di Bartolomeo S, Ranalli G, Saviano G, Segatto M. Lavender Essential Oil Modulates Hepatic Cholesterol Metabolism in HepG2 Cells. Curr Issues Mol Biol. 2023 Jan 3;45(1):364-378. doi: 10.3390/cimb45010026. 

Abstract. Cholesterol is an essential lipid that guarantees several biological processes in eukaryotic cells. Its metabolism is regulated by a complex protein network that could be significantly influenced by numerous exogenous sources, such as essential oils (EOs). For instance, it has been speculated that monoterpenoid and sesquiterpenoid compounds contained in lavender essential oil (LEO) may exert important hypocholesterolemic activities. However, the molecular mechanisms by which LEO influences cholesterol homeostasis are not characterized. In this work, we evaluated the ability of LEO to regulate the protein network that controls cholesterol metabolism in the HepG2 cell line. The main findings indicate that LEO administration increases intracellular cholesterol content. Concurrently, LEO affects the expression of proteins involved in cholesterol uptake, biosynthesis, and trafficking. These effects are partially mediated by terpinene-4-ol, one of the most abundant compounds in LEO. These results demonstrate that LEO modulates cholesterol metabolism in hepatic cells.

Pincigher L, Valenti F, Bergamini C, Prata C, Fato R, Amorati R, Jin Z, Farruggia G, Fiorentini D, Calonghi N, Zalambani C. Myrcene: A Natural Compound Showing Anticancer Activity in HeLa Cells. Molecules. 2023 Sep 21;28(18):6728. doi: 10.3390/molecules28186728.

Abstract. γ-terpinene, α-terpinene, p-cymene, and myrcene are monoterpenes found in many essential oils extracted from a variety of plants and spices. Myrcene also occurs naturally in plants such as hops, cannabis, lemongrass, and verbena and is used as a flavoring agent in food and beverage manufacturing. In this research, the biological efficacy of γ-terpinene, α-terpinene, p-cymene, and myrcene was studied in human cell lines (HeLa, SH-SY5Y, and HDFa). Cytotoxicity, cell proliferation, cell migration, and morphology assays were performed to obtain detailed information on the anticancer properties. Our results show that myrcene has potential biological activity, especially in HeLa cells. In this cell line, it leads to an arrest of proliferation, a decrease in motility and morphological changes with loss of sphericity and thickness, and DNA damage. In addition, the interaction of γ-terpinene, α-terpinene, p-terpinene, and myrcene with calf thymus DNA (ct-DNA) was studied by UV-visible spectrophotometry. DNA binding experiments show that only myrcene can interact with DNA with an apparent dissociation constant (Kd) of 29 × 10-6 M.