Cytotoxicity using Human Cell Lines

All of the previous assessments of tea tree oil have been acquired from animal and human testing. Just recently, however, cytotoxic testing of the oil has been carried out. The use of cytotoxic (toxic to cell tissue) in vitro (test tube) testing has made testing of substances that are quite toxic, more animal friendly and thus more acceptable. The other advantage is that the same tissues can be used to test many substances and provide a more accurate comparison of the relative toxicity of different substances. This approach is also being used for predictive testing for potential allergens (Krasteva et al. 1996). According to Soderberg et al. (1996), tea tree oil when tested at 100,ag/ml had a cytotoxic effect on epithelial and fibroblast cells for only the first hour of a 24 hour test duration.

More recently, Hayes et al. (1997) investigated tea tree oil cytotoxicity with respect to the major oxygenated monoterpenoid constituents terpinen-4-ol, 1,8-cineole and a-terpineol. The oil tested contained terpinen-4-ol 40%, 1,8-cineole 4%, a-terpineol 4% and results showed that a-terpineol>tea tree oil>terpinen-4-ol>cineole for overall cytotoxicity. When tea tree oil was compared with controls mercuric chloride (highly toxic) and aspirin (very low toxicity) on the same cell types (liver, skin, lymph and bone marrow), ratings were mercuric chloride>tea tree oil>aspirin. The authors state that these cytotoxicity results support the use of tea tree oil in topical applications but not for ingestion purposes.

CINEOLE AND OTHER MELALEUCA SPECIES' OILS

According to Tisserand and Balacs (1995), 1,8-cineole is non-toxic, non-irritant and non-sensitising. This being the case, one would expect that high cineole content oils would be similar. However, as mentioned earlier, eucalyptus oil, although fitting into this category, is still regarded as potentially dangerous to small children.

Commercial cajuput oil, presumably the "cajeput" (Melaleuca leucadendron) of Tisserand and Balacs (1995) and now known as Melaleuca cajuputi (see Chapter 14), is normally a high cineole (50-60%) oil. Tisserand and Balacs (1995) state that this "cajeput" oil has an LD50 between 2-5g/kg, is non-irritant and has negligible risk of skin sensitivity.

Niaouli oil, from Melaleuca quinquenervia, also a cineole type oil, has been assessed by Aboutabl et al. (1996) using ethanol extracts as having an LD50 of 147.5mg/kg while

Tisserand and Balacs (1995) indicate that niaouli oil is safe to be used in aromatherapy and is considered non-irritant and non-sensitizing. Linalool and nerolidol, the major constituents in some chemotypes of Melaleuca quinquenervia, are also considered to be safe for aromatherapy use (Tisserand and Balacs 1995) and hence non-irritant and non-sensitising (see Chapter 15).

The only Melaleuca oil that is currently considered as a potential carcinogen is the oil of Melaleuca bracteata which contains approximately 80% methyl eugenol. According to Randerath et al. (1984) and Tisserand and Balacs (1995) this oil should be avoided as currently the oil is considered carcinogenic to rodents. Until such time as it is assessed as non-carcinogenic to humans, this advice should be heeded.

CONCLUSION

Although not unduly toxic when ingested, tea tree should continue to be used mainly for topical applications. It would appear that tea tree oil (Melaleuca alterntfolia) is non-irritating even when used up to 10% (Altman 1991) or 25% (Southwell et al. 1997) on unabraded healthy skin. It is not recommended that it be used on sensitive, dermatitis affected or abraded skin for extended periods. Some people do become sensitized to tea tree oil on rare occasions especially those that have a history of allergic responses. Generally, however, as there was little irritation or sensitization with tea tree oil in formulations up to 25%, products of that concentration or less are recommended.

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