In recent years there have been several reports in dermatological journals suggesting that tea tree oil causes contact dermatitis. Contact dermatitis is usually caused by sensitization to a substance and at a later stage coming in contact with the same substance. Tea tree oil has been regarded as a non-sensitizing substance (Tisserand and Balacs 1995) and this is certainly the case with guinea pigs. Bolt (1989e) carried out a skin sensitization potential trial with guinea pigs (OECD method 406) using tea tree oil as reported in the Australian Tea Tree Oil Toxicology Data Sheet No. 5 (Tea Tree Oil Growers of Australia 1989):
Groups of 20 albino guinea pigs (HA strain) were tested for sensitization potential using the method of Magnusson and Kligman (1969). The induction procedure consisted of 2
intradermal injections and an epidermal induction application under occlusion for 48 hours. Two weeks after the induction applications the animals were challenged with 30% tea tree oil in petroleum jelly. No sensitization reactions were noted. Tea Tree Oil could therefore be recommended in hypoallergenic preparations.
Yet, there appears to be evidence, in humans at least, that tea tree oil can cause varying allergic responses in certain individuals. Some case studies and clinical trial results follow:
(a) A maximization test was conducted on 22 volunteers with a 1% tea tree oil in a petrolatum formula and produced no sensitization reactions (Ford 1988).
(b) A 43-year-old woman and 10-year-old boy were confirmed by patch challenge testing to have contact dermatitis from tea tree oil (Apted 1991). Individual components of the oil were not tested for allergic response.
(c) A 45-year-old man, with long standing atopic dermatitis, applied tea tree oil which exacerbated the condition after a time (de Groot and Weyland 1992). Although the oil components were listed, no concentrations were given. The constituents listed were more consistent with tea tree oil of the cineole genotype. This genotype was once reported to be readily available in Europe and caused allergic responses on other users as well (van der Valk et al. 1994). The patient gave a positive patch response when tested with cineole (eucalyptol) in contrast with the findings of Opyke (1975b), Knight and Hausen (1994) and Southwell et al. (1997). This chemical variety is not normally used commercially and would not have conformed to ISO 4730:1996E (International Standards Organisation 1996).
(d) Two occupational contact dermatitis subjects, one a chiropodist, the other a beautician were said to be allergic to tea tree oil, however no challenge patch test were reported, (de Groot and Weyland 1993).
(e) A 33-year-old woman who had used neat tea tree oil previously with no side effects, developed contact dermatitis confirmed by patch challenge testing with tea tree oil. (Selvaag et al. 1994).
(f) Knight and Hausen (1994) conducted a trial on seven patients who were treating preexisting skin conditions with neat tea tree oil and were challenge patch tested with tea tree oil at 1%. All seven reacted positively. Further to this, they tested the major components of tea tree oil at 1%. Six of seven patients reacted to limonene, 5 to a-terpineol and aromadendrene and one to each of terpinen-4-ol, ^-cymene and a-phellandrene. However, according to Opdyke (1975a), limonene proved negative on 23 subjects tested for sensitization. Furthermore, Knight and Hausen (1994) found tea tree oil to be a weak sensitizer in guinea pigs as only 3 of 10 responded at 30% challenge after 48 hr. It is interesting to note here that the terpinen-4-ol sensitized human had a previous sensitivity to Peruvian Balsam. These authors acknowledged that they were dealing with a population of tea tree oil sensitised patients with damaged skin and also suggested that "other tea tree oil products containing lower concentrations of the oil and used-on healthy skin may cause no sensitivity reactions."
(g) A 53-year-old patient suffered allergic airborne contact dermatitis from several essential oils after extensive use of the oils in wet dressings, baths and room aerosols (Schaller and Korting 1995). Although the oils used and tested included tea tree oil, only lavender, rosewood and jasmine gave positive patch test responses.
(h) Recent investigations (Southwell et al. 1997) looked at the skin irritancy of tea tree oil using 28 human subjects with occlusive tea tree oil 25% patch testing over 21 days. Oils of differing cineole (15-28.8%) and terpinen-4-ol (22.6-38.8%) showed no irritancy except for 3 subjects who showed allergic responses to all the different types of tea tree oils used. These 3 subjects were further tested with the individual constituents or constituent blends at concentrations equivalent to their concentration in a 25% formulation. None of the 3 subjects reacted to the monoterpenoids except for one panellist who reacted to a-terpinene only. However, the sesquiterpene hydrocarbon enriched blends caused reactions in all three subjects. This is consistent with Knight and Hausen (1994) who found aromadendrene, a sesquiterpene hydrocarbon, an allergen. Still more research is needed here.
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