Sensitization Revisited Again

By Tony Burfield & Sylla Sheppard-Hanger

Sensitivity is a theme we keep returning to, because in our recent international travels we have both independently noticed there is little appreciation of this phenomenon worldwide. In other words not only is the message not getting through, it doesn’t have a proper vehicle for distribution! Also, as noted in the last issue, even our leading lights don’t quite get the message, so until we see this clearly spelled out in literature, course syllabus’ and store precautions, we are going to continue to harp on it. Sensitization is different from irritation and toxicity.

We are hopefully by now familiar with those essential oils which have sensitising potential: the classic list including such items as Cassia oil (Cinnamomum cassia), Cinnamon bark Sri Lanka (Cinnamomum zeylanicum), Costus (Saussurea costus), Elecampane (Inula helenium), Fig Leaf absolute (Ficus carica), Oakmoss products (Evernia prunastri), Opoponax (Commiphora erythraea), Peru Balsam (Myroxylon pereirae), Styrax (Liquidambar styraciflua), Tea absolute (Thea sinensis), Tree Moss products (Evernia furfuracea) and Verbena absolute (Lippia citriodora). Many of these substances have achieved notoriety via RIFM monographs.

However it is true to say that in the past many irritants may have been classified as sensitizers and mild sensitizers may have been grouped with moderate or severe sensitizers. In addition, anything can become a sensitizer, especially with undiluted or prolonged use. We may see some professional organisations revising these lists soon. Please note also these lists are an expression of hazard potential, as determined by predictive tests. In actual use we are dealing with risk, and this is a very different area.

Risk can be affected by concentration, length of time on skin, condition of skin and many other factors. A highly allergic substance can under certain conditions therefore not pose that great a risk (especially if there are mitigating conditions of use e.g. rinsing off afterwards). This is why one famous aromatherapy educator has advocated the use of sensitising essential oils that a perfumery company abiding by the IFRA guidelines would never endorse on safety grounds. However in spite of all the bravado, this is a very dangerous game and more importantly, one not covered by the practitioner’s insurance.

However whilst these oils and extracts have an established track record of causing sensitivity problems we also know about some of the chemicals responsible in the essential oils which cause these problems. During the sensitisation process many of the chemicals present in the essential oil will penetrate the epidermis. After biodistribution, certain of the components may be biotransformed by the xenobiotic and P-450 enzymes in the skin to haptens (which we can think of as allergens). This transformation may also be induced in certain instances by UV light. Once formed the haptens react with nucleophilic residues on proteins forming antigenic sites. The immune system then swings into action and a response is generated. It is fair to say that not every binding reaction type will produce an increase in reaction: some may produce a decrease. It is also fair to say the overall immune system reaction may be greater than the sum of the reactions generated by individual essential oil components i.e. a synergistic effect operates.

At a more complex level, such as might result from considering essential oil biotransformations, several haptens may compete for a limited number of sites on the protein (antigenic competition). The relatively higher concentration of the least strongly bound haptens may cause a feedback mechanism to operate within the enzymes producing the hapten, which may alter the sensitisation profile of the mix. The rate of release of absorbed substances from the epidermis, and the concentration at the protein site will also affect hapten-binding.

The result of these, and other phenomena which may be a little too technical to discuss in a general article, is that the sensitisation phenomena is a complex one, and one which may be affected by the mix of chemicals in the oil. Any differences in oil quality, e.g. as encountered in chemotypes may give rise to a different sensitivity profile. Blending oils together may enhance penetration of particular components, resulting in greater bioavailability, and possibly greater hapten binding, and hence modified sensitisation. So it is a very complicated picture that therapists must be aware of and understand.

Aromatherapists therefore continually have to be on the look-out for sensitisation problems and prevent them by first of all not using the KNOWN sensitizers on the skin. Clients prone to asthma, atopic skin conditions such as contact eczema, known histories of allergy or fragrance sensitization or other nonspecific respiratory allergy etc. may be more prone to sensitisation reactions. Therefore, one cannot use the known sensitizers or even the suspect ones. Ingestion of spicy food and certain flavourings can also cause allergic reactions in some people. The idiosyncratic nature of individual reactions to essential oils must also continually be expected.

Patch testing is advisable to establish client application suitability in many cases but testing with a known sensitizer can cause it. So just avoid this process with sensitizers. There are many other safe options that can be used to treat the same problem; therefore there is no reason to patch test with known sensitizers.

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