The following information is drawn with permission from Martin Watt’s Plant Aromatics: A Data and Reference Manual on Essential Oils and Aromatic Plant Extracts. 

Irritation

This is the most common type of reaction, and is characterized by various degrees of dermal (skin) inflammation. It is caused when a substance (in this case, an essential oil) reacts with the mast cells of the skin, which then release histamine and other inflammatory substances. The reaction is most often localized and does not involve the immune system. It is classified in four general stages: 

  • Local flare at the site of contact (e.g., rash from brushing up against a stinging nettle plant)
  • Generalized urticaria (hives) with angioedema (capillaries leak tissue fluid, causing swelling, inflammation and pain, (e.g., a bee sting)
  • Urticaria with bronchial asthma
  • Urticaria and anaphylactic shock

Allergic Sensitivity

Also known as Contact Allergy or Sensitivity Reaction. This reaction involves the immune system and is classified as a cell-type or delayed hypersensitivity involving T-cells (specialized lymphocytes) and macrophages. To start this type of reaction, a substance first has to enter the skin and then bind with lymphatic tissues, which causes T-cells to become sensitized. At any time in the future, if the same or a chemically related substance is again introduced to the skin (or possibly any other part of the body), the immune system will now react to this substance as it perceives it as a threat. It can cause one or more of the four stages of allergic reaction listed above. Any resulting rash may be blotchier than with simple irritation. The reaction may be much quicker and more severe in nature, leading to anaphylaxis unless quick treatment with adrenalin or anti-histamines is available.

Phototoxicity

This can occur as the result of a photosensitive essential oil (for instance, distilled lime) being applied to the skin and then the skin is exposed to ultraviolet light within 12 or so hours. Simply put, this causes a reaction of the skin cells. This reaction can range from a mild color change of the underlying skin all the wayup to deep burns.

Systemic Allergic Sensitization

This rare reaction can occur after internal administration (ingestion) of a sensitizing substance, or following dermal sensitization. Once the sensitization has occurred, further exposure of ANY part of the body to the same or similar substance may bring about a reaction from the immune system.