Tea Tree Oil

What is tea tree oil?

Tea tree oil is a clear to pale yellow essential oil distilled from the foliage of Melaleuca alternifolia (R), a hardwood tree native to the northeastern area of New South Wales, Australia. The plant has also been cultivated in other states, including Queensland and Western Australia, as well as in other countries (R).

During World War 2, the Australian Army issued tea tree oil to soldiers to use as a disinfectant (R). Today, it is included as an ingredient in skin care and other over-the-counter products such as soaps, bath additives, shampoos and conditioners (R) mostly at concentrations of 2% and below, except in nail care products where it is typically included at a concentration of 20% (R). Undiluted tea tree oils are also available.

Tea tree oil is a mixture of almost 100 compounds, but the most abundant are terpinen-4-ol (40.1%), gamma-terpinene (23.0%), alpha-terpinene (10.4%) and eucalyptol (5.1%) (R). 6 distinct oil phenotypes have been identified to date, of which 1 is dominated by terpinen-4-ol, 1 by terpinolene and the remaining 4 by eucalyptol. Although all chemotypes are present throughout the distribution range of the tea tree, there is a connection between the different oil types and geographic location. Terpinen-4-ol types predominate in and around the Bungawalbin basin in northern New South Wales, eucalyptol types predominate slightly further south around Grafton, and terpinolene types predominate in southern Queensland (R, R). The terpinen-4-ol chemotype is the one used for commercial tea tree oil production (R).

Does tea tree oil get absorbed?

Several studies have evaluated the uptake of tea tree oil and some of its components (terpinen-4-ol, eucalyptol, linalool, alpha-pinene and beta-pinene) into the skin. All the studied components are known to be absorbed in the viable epidermis and dermis, with terpinen-4-ol showing the fastest penetration (R). Terpinen-4-ol in 5% tea tree oil formulations has also been shown to accumulate in the follicles of bovine udder skin (R), as well as to be retained in human skin tissue following topical application (R, R).

The type of dermatological vehicle employed affects the absorption kinetics of tea tree oil compounds. Terpinen-4-ol and linalool for instance penetrate the skin better from a hydrogel than from oily solutions or oil-in-water emulsions (R, R). A semisolid oil-in-water emulsion containing 5% tea tree oil, however, led to greater flux across the skin than similar preparations in white petrolatum or an ambiphilic cream (R).

Excipients too can influence the cutaneous uptake of tea tree oil, with oleic acid in particular observed to enhance the absorption of terpinen-4-ol by perturbing the stratum corneum barrier, whereas isopropyl myristate, polyethylene glycol 400 and Transcutol only caused a weak enhancement effect (R).

The penetration of tea tree oil components through human skin appears limited -- only 1-2% and 2-4% of the dose respectively penetrated into or through human epidermis when a 20% tea tree oil solution and pure tea tree oil were applied, though partial occlusion of the application site did increase the epidermal penetration of pure tea tree oil to approximately 7% (R). Tea tree oil patches prepared using the methacrylic copolymer Eudragit E100 have also demonstrated high retention and low permeation of terpinen-4-ol (R).

Unabsorbed tea tree oil is probably removed from the skin through evaporation. In an experiment on filter paper, 98% of the oil evaporated within 4 hours (R).