Tea tree oil
Tea tree oil (TTO), or melaleuca oil, is an essential oil with a fresh camphoraceous odor and a colour that ranges from pale yellow to nearly colourless and clear. It is taken from the leaves of the Melaleuca alternifolia, which is native to Southeast Queensland and the Northeast coast of New South Wales, Australia.
Tea tree oil is toxic when taken by mouth, but is widely used in low concentrations in cosmetics and skin washes. Tea tree oil has been claimed to be useful for treating a wide variety of medical conditions. It shows some promise as an antimicrobial. Tea tree oil may be effective in a variety of dermatologic conditions, including dandruff, acne, lice, herpes, and other skin infections. However, the quality of the evidence is low,  and tea tree oil is not recommended for treating fungal infections or for use on children.
Tea tree oil is not recommended for the treatment of athlete's foot.
Tea tree oil is not recommended for treating head lice in children because it could cause skin irritation or allergic reactions, because of contraindications, and because of a lack of knowledge about the oil's safety and effectiveness.
A 2006 review of the toxicity of tea tree oil concludes that it may be used externally in its diluted form by the majority of individuals without adverse effect (provided oxidization is avoided). Tea tree oil is poisonous when taken internally.
Tea tree oil is a commercially refined composition of several naturally occurring chemical compounds and is hazardous if misused. Available literature suggests that tea tree oil can be used topically in diluted form by the majority of individuals without adverse effects. Topical application of tea tree oil can cause adverse reactions at high concentration. Adverse effects including skin irritation, allergic contact dermatitis, systemic contact dermatitis, linear immunoglobulin A disease, erythema multiforme like reactions, and systemic hypersensitivity reactions.
Tea tree oil is toxic when swallowed. According to the American Cancer Society, ingesting tea tree oil has been reported to cause drowsiness, confusion, hallucinations, coma, unsteadiness, weakness, vomiting, diarrhea, stomach upset, blood cell abnormalities, and severe rashes. It should be kept away from pets and children. Tea tree oil should not be used in or around the mouth. There is at least one case of poisoning reported in medical literature.
Exposure of tea tree oil to air and light results in oxidation of some of its components. Oxidized tea tree oil should not be used. Some people experience allergic contact dermatitis as a reaction to dermal contact with tea tree oil. Allergic reactions may be due to the various oxidation products that are formed by exposure of the oil to light and/or air.
In vitro testing of tea tree oil shows that it contains chemicals which are weakly estrogenic, causing particular concern for use with children. However, in tests, the chemicals which show this effect failed to show absorption into the skin, and evidence of a hormonal effect is therefore considered implausible by an EU scientific committee.
In dogs and cats, death or transient signs of toxicity (lasting 2 to 3 days), such as depression, weakness, incoordination and muscle tremors, have been reported after external application at high doses. In rats the median lethal dose (LD50) is 1.9–2.4 ml/kg.
Undiluted tea tree oil can cause some hearing loss when used in the ears of non-human animals. However, a 2% concentration has not been shown to have any lasting effect. It is not known whether the same is true for humans.
Composition and characteristics
Tea tree oil is defined by the International Standard ISO 4730 ("Oil of Melaleuca, Terpinen-4-ol type"), which specifies levels of 15 components needed to define an oil as "tea tree oil." The oil has been described as having a fresh, camphor-like smell.
Tea tree oils come in six different chemical combinations: a terpinen-4-ol type, a terpinolene type, and four 1,8-cineole types. These various oil types contain over 98 compounds, with terpinen-4-ol the major component responsible for antimicrobial and anti-inflammatory properties. A second component 1,8-cineole, is likely responsible for most adverse reactions to TTO products. Adverse reactions diminish with minimization of 1,8-cineole content. In commercial production, TTO is prepared as a terpinen-4-ol type.
History and extraction
The name tea tree is used for several plants, mostly from Australia and New Zealand, from the family Myrtaceae, related to the myrtle. The use of the name probably originated from Captain James Cook's description of one of these shrubs that he used to make an infusion, to drink in place of tea.
The commercial tea tree oil industry originated in the 1920s when Arthur Penfold, an Australian, investigated the business potential of a number of native extracted oils; he reported that tea tree oil had promise, as it exhibited powerful antiseptic properties.
Tea tree oil was first extracted from Melaleuca alternifolia in Australia, and this species remains the most important commercially. Several other species are cultivated for their extracted oil: Melaleuca armillaris and Melaleuca styphelioides in Tunisia and Egypt; Melaleuca leucadendra in Egypt, Malaysia and Vietnam; Melaleuca acuminata in Tunisia; Melaleuca ericifolia in Egypt; and Melaleuca quinquenervia in the United States. Similar oils can also be produced by water distillation from Melaleuca linariifolia and Melaleuca dissitiflora.
Studies using petri dishes originally suggested that tea tree oil kills methicillin-resistant Staphylococcus aureus (MRSA) in nasal or extra-nasal (topical) colonization studies, but, as of 2005, there appeared to be insufficient evidence to recommend it for use. A 2008 article from the American Cancer Society says that studies have previously suggested a possible role for the topical application of tea tree oil as an antiseptic, but that, "despite years of use, available clinical evidence does not support the effectiveness of tea tree oil for treating skin problems and infections in humans."
The National Center for Complementary and Integrative Health say there is tentative evidence for the use of tea-tree oil in additional-to-standard treatments of MRSA, but notes that the evidence is from small studies, and that there is no evidence involving humans. Other uses that have been researched include applications for nail fungus, dandruff, acne, and athlete's foot, but the evidence is of poor quality. Gingivitis is mentioned as another potential use.
A 2012 review of head lice treatment recommended against the use of tea tree oil on children because it could cause skin irritation or allergic reactions, because of contraindications, and because of a lack of knowledge about the oil's safety and effectiveness.
- Cajeput oil — derived from Melaleuca leucadendra
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