On 2015 May 18, Tony Larkman commented:

Recent advances in the detection of adulteration in tea tree oil (TTO) published by Wong et al in May 2015 (Enantiomeric distribution of selected terpenes for authenticity assessment of Australian Melaleuca alternifolia oil: http://www.sciencedirect.com/science/article/pii/S0926669015000680) show a significant number (>50%) of commercial samples varying markedly from the ∼68.5% expected value found for (+) terpinen-4-ol. It is notable that of the 15 commercially sourced samples from the European Union, 73% of these showed significant differences in chiral abundances indicating widespread incidence of this fraudulent practice.

In this article Santesteban Muruzábal et al state that the incidence of allergic contact dermatitis was "...until recently...infrequent in our setting" and "Further, three of the five patients also reacted to oxidized d-limonene". While limonene is present in pure TTO at ~1%, if the TTO is correctly stored the level of oxidation of not only the limonene but all other components remains low. More detail on this is available in a paper by Brophy et al: Gas chromatographic quality control for oil of Melaleuca terpinen-4-ol type (Australian tea tree) http://pubs.acs.org/doi/abs/10.1021/jf00089a027

In Australia, and other countries, when pure TTO steam distilled from Melaleuca alternifolia is correctly handled and stored to minimize degradation through oxidation the incidence of allergic contact dermatitis appears to be far less than that experienced in the European Union.

It is therefore not unreasonable to expect that the reactions noted may be caused by the use of adulterated material masquerading as TTO - this material is often manufactured using a poor quality (highly oxidized) product diluted with industrial waste sourced from 'normalising' other essential oils such as Pine and Eucalyptus. This industrial waste is uncontrolled and a diverse range of substances of unknown origin have also been detected when analyzed (personal observation: 2013 - 15).

It would be interesting to know if the enantiomeric ratios for the material used in this study conformed to the expected ratios as published by Wong et al.

On 2015 May 18, Tony Larkman commented:

Recent advances in the detection of adulteration in tea tree oil (TTO) published by Wong et al in May 2015 (Enantiomeric distribution of selected terpenes for authenticity assessment of Australian Melaleuca alternifolia oil: http://www.sciencedirect.com/science/article/pii/S0926669015000680) show a significant number (>50%) of commercial samples varying markedly from the ∼68.5% expected value found for (+) terpinen-4-ol. It is notable that of the 15 commercially sourced samples from the European Union, 73% of these showed significant differences in chiral abundances indicating widespread incidence of this fraudulent practice.

In this article Santesteban Muruzábal et al state that the incidence of allergic contact dermatitis was "...until recently...infrequent in our setting" and "Further, three of the five patients also reacted to oxidized d-limonene". While limonene is present in pure TTO at ~1%, if the TTO is correctly stored the level of oxidation of not only the limonene but all other components remains low. More detail on this is available in a paper by Brophy et al: Gas chromatographic quality control for oil of Melaleuca terpinen-4-ol type (Australian tea tree) http://pubs.acs.org/doi/abs/10.1021/jf00089a027

In Australia, and other countries, when pure TTO steam distilled from Melaleuca alternifolia is correctly handled and stored to minimize degradation through oxidation the incidence of allergic contact dermatitis appears to be far less than that experienced in the European Union.

It is therefore not unreasonable to expect that the reactions noted may be caused by the use of adulterated material masquerading as TTO - this material is often manufactured using a poor quality (highly oxidized) product diluted with industrial waste sourced from 'normalising' other essential oils such as Pine and Eucalyptus. This industrial waste is uncontrolled and a diverse range of substances of unknown origin have also been detected when analyzed (personal observation: 2013 - 15).

It would be interesting to know if the enantiomeric ratios for the material used in this study conformed to the expected ratios as published by Wong et al.