Essential oils are mixtures of many components. When they oxidize through exposure to air, substances different from the original components may form. Can contact allergy to oxidized lavender oil therefore be detected by testing only its main ingredients?
A study published in 2016 by Lina Hagvall and Johanna Bråred Christensson examined this question in patients with dermatitis. It compared skin reactions to oxidized lavender oil with reactions to separately oxidized samples of its main components, investigating whether some reactions might otherwise be missed in diagnosis.
- Testing the essential oil separately from its main components
- The participants were 1,693 patients investigated for dermatitis
- Forty-seven patients tested positive to oxidized lavender oil
- Testing the main components alone missed some reactions
- Why might someone react only to the whole essential oil?
- Standardizing oxidation matters for diagnosis too
- What this study cannot tell us
- Summary
- References
Testing the essential oil separately from its main components
Linalool and linalyl acetate are among the major components of lavender oil. They can react with oxygen in air to form oxidation products called hydroperoxides. Earlier research had shown that these products can contribute to skin sensitization.
Sensitization means that the immune system becomes capable of reacting to a substance. When a sensitized person encounters that substance again, delayed dermatitis may occur. The patch tests in this study were diagnostic tests for this type of contact allergy.
The researchers exposed lavender oil and linalyl acetate to air for 45 weeks, and linalool for 25 weeks. Each oxidized material was mixed into petrolatum at a concentration of 6% for testing. The study did not simply test fresh essential oil as used at home.
The oxidized materials and patch-test preparations were also chemically analyzed to determine the amounts of major oxidation products present. An equal nominal concentration of 6% does not mean that every oxidation product in each mixture was present at the same concentration.
The participants were 1,693 patients investigated for dermatitis
The study included 1,693 patients tested at Sahlgrenska University Hospital in Sweden during 2008–2010: 517 men and 1,176 women, with a mean age of 45.7 years. They had suspected allergic contact dermatitis and also received the routine baseline series of tests.
Samples were applied to the upper back for 48 hours, and skin reactions were assessed on day 3 or 4 and on day 7. This was testing in a medical setting under defined conditions and criteria, not a method of applying essential oils at home to see what happens.
Forty-seven patients tested positive to oxidized lavender oil
Of the 1,693 patients, 47, or 2.8%, had positive reactions to oxidized lavender oil. Thirty of the positive reactions were weak, and 17 were stronger.
The 2.8% is the positive-test rate among patients being investigated for dermatitis. It does not mean that members of the general public have a 2.8% chance of developing an allergy when using lavender oil. Nor is it a count of newly induced allergies caused by the test.
A further 55 patients, or 3.2%, had doubtful reactions that could not clearly be classified as positive. Given the frequency of these reactions, the authors discussed the need for caution about misclassifying irritant or other reactions as positive. The doubtful reactions were not added to the 47 positive cases.
Testing the main components alone missed some reactions
The central issue was not simply the positivity rate, but how reactions to the different materials overlapped. Among the patients positive to oxidized lavender oil, 44 could be compared across all three materials. Of these, 19 reportedly reacted to neither oxidized linalool nor oxidized linalyl acetate.
In this patient group, therefore, testing only oxidized samples of the two main components did not capture every reaction to the oxidized whole oil. Conversely, some patients reacted to oxidized components without reacting to the whole oil. The three tests were not fully interchangeable.
The researchers also examined overlap with fragrance-related markers in routine testing. Of the 44 patients available for comparison, 23 were positive to at least one marker. The authors proposed that a standardized oxidized-lavender-oil preparation might assist diagnosis alongside these tests.
Why might someone react only to the whole essential oil?
One possible explanation is that the presence of several substances together enhances the skin response. Another is that components react with one another within the complex mixture, generating substances absent from separately oxidized samples of the two major components.
However, the study did not identify every alternative causative substance in patients who did not react to the two major components. Mixture effects and unidentified products were hypotheses proposed by the authors to explain the results.
Standardizing oxidation matters for diagnosis too
An essential oil’s contents may change over time even though its name remains the same. The authors noted that, without knowing the oxidation state of oils used for testing, it becomes difficult to compare results between centers or time periods.
It is therefore necessary to use appropriate analytical methods to establish which oxidation products are present and in what quantities, rather than simply obtaining a measure suggesting oxidation. Developing test preparations with standardized composition and improving methods to detect oxidation products in cosmetics and other products were important recommendations of the paper.
What this study cannot tell us
The study did not individually assess clinical relevance: whether each positive patient’s usual dermatitis had actually been caused by exposure to lavender oil. A positive patch test and identification of the cause of current symptoms are not the same thing.
Nor was this a trial comparing fresh and oxidized oil in the same patients, or comparing naturally and synthetically sourced versions of the same component under otherwise identical conditions. These results alone cannot quantify the extent of oxidation in everyday products or the exposure concentrations that would cause symptoms.
The authors declared no conflicts of interest. The sample concentrations and oxidation conditions were for research and cannot serve as guidance for recreating the tests without medical supervision.
Summary
Testing in dermatitis patients found positive reactions to oxidized lavender oil, including reactions not detected by separately oxidized samples of its two main components. The findings illustrate the value of studying essential oils as complex mixtures whose composition changes with oxidation.
Rather than judging solely by labels such as natural or synthetic, the authors call for detailed knowledge of the substances people encounter and the materials used to test them. Diagnostic preparations with standardized oxidation states, together with analytical methods that identify oxidation products, are important challenges in improving the investigation of contact allergy.
References
Hagvall, L., & Bråred Christensson, J. (2016). Patch Testing with Main Sensitizers Does Not Detect All Cases of Contact Allergy to Oxidized Lavender Oil. Acta Dermato-Venereologica, 96, 679–683. DOI: 10.2340/00015555-2319 / Full original article. Original article license: CC BY-NC 4.0.
