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.. If you only have one volume covering the safe use of aromatic chemicals, this should be it.

Irritation

This is the most common type of reaction, and is characterized by various degrees of dermal inflammation. It is caused when a substance reacts with the mast cells of the skin, which then releases Histamine and other inflammatory substances. the reaction is localized and does not involve the immune system. It is classified in four general stages:

Local flare at the site of contact (e.g., stinging nettle rash)
Generalized urticaria with angio-edema(capillaries leaking and tus causing swelling, inflammation and pain, (e.g., a bee sting).
Urticaria with bronchial asthma
Urticaria and Anaphylactic shock.

Allergic Sensitivity

also known as Contact Allergic or Sensitivity Reaction. This reaction involves the immune system and is classified as a cell-type or delayed hypersensitivity involving Tcells and macrophages. To start this type of reaction, first the substance has to enter the skin, bind with lymphatic tissues, and then cause the Tcells to become sensitized. If, at any time in the future, the same, or a chemically related substance is once more introduced to the skin or possibly any other part of the body the immune system will react to this substance perceiving it as a threat and causing a similar reaction as above. The resulting rash may be blotchier than with irritation. The reaction may be much quicker and 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 system contacting and binding to the skin. or from systemic absorption followed by a reaction of the skin cells. If the skin is then exposed to sunlight or other types of ultra violet light. this reaction can range from a mild color change through deep burns.

Systemic Allergic Sensitization

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