LOST & FOUND: SENSITIZATION
“What is sensitization? – First of all it is not referring to those with “sensitive skin”, sensitive “temperaments”, or sensitive people in general. Separate from irritation, sensitization is a potentially serious immune system response to foreign molecules which can provoke a response (we call them haptens), such as pollen, dust or certain essential oil components like terpene hydroperoxides and sesquiterpene lactones. Reactions can range from a small rash to acute respiratory distress and (in extreme cases) death by anaphylactic shock in certain highly susceptible individuals. Some oils (such as Costus oil: Saussurea lappa) are KNOWN sensitizers and are not to be used on the skin –period – due to risking injury to themselves and others. Others (pine oils, citrus oils, linalool-containing oils) may develop sensitizing components on aging. If anyone knowingly uses sensitizing oils for Aromatherapy skin blends, one is practicing unethically and risks injury to themselves and others, not to mention lawsuits for injury. If one unknowingly uses them, this is worse, as there is no excuse for ignorance.” – NAHA [Feb 2006 – April 2013]
“Sensitization refers to the development of an allergic skin reaction to certain aromatic compounds present in some essential oils. Responsible compounds penetrate the epidermis, bind to skin proteins and provoke an immune reaction that leads to the production of histamine and other irritant compounds by basophils and mast cells. A skin rash or eczema is the usual outcome and subsequent exposure to even tiny amounts of the sensitizing compound can elicit the same response, as well as creating cross-sensitivities to other compounds.”
Tisserand, R. and Balacs, T. 1995 Essential Oil Safety pgs. 77-83, Churchill Livingstone Edinburgh
“exposure to allergen that results in the development of hypersensitivity.” 
So what is hypersensitivity? Is a hypersensitivity reaction the same as an allergic reaction?
Answer, yes. They are synonyms, BUT there are four different types of allergic reaction:
“a local or general reaction of an organism following contact with a specific allergen to which it has been previously exposed and sensitized; immunologic mechanisms gives rise to inflammation or tissue damage. Allergic reactions are classified into four major types: type I, anaphylactic and IgE dependent; type II, cytotoxic; type III, immune-complex mediated;type IV, cell mediated (delayed).”
For the purposes of aromatherapy safety, any essential oil can become an allergen by using it undiluted on the skin; and this risk is there for all essential oils, including lavender. So while there are certain essential oils which have a known reputation for being potential allergens or with a reputation for sensitization, using any essential oil neat (undiluted) sets the individul up for a potential allergic reaction, leading to sensitization, and forever being allergic to that essential oil.
Robert Tisserand explains:
“Yes, sensitization is the process that takes place in the body that leads to an allergic reaction. They are not the same thing, but they are not totally different either. There are 4 types of allergic reaction,  but only two are relevant to essential oils. Type 4 (delayed hypersensitivity) accounts for 90% of allergic reactions. Type 1 is immediate hypersensitivity (generally not anaphylactic) and accounts for the other 10%. You could say the risk is potentially there for all essential oils, but this is a little unfair on the majority of oils, that have never been known to cause such reactions. I don’t like to assume risk that may not exist. The less you dilute the more you increase risk, but that doesn’t mean that undiluted copaiba oil is a greater risk than 1% cinnamon bark oil. It isn’t.”
To learn more, see the video which is a sample from Webinar 4 “Irritation and Allergy” from Essential Oils and the Skin 10-Day Series by the Tisserand Institute: https://www.facebook.com/video.php?v=898998096798059
1. Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved
2. Farlex Partner Medical Dictionary © Farlex 2012