Insight, By Dr James Seamus Fuller, Dr Francis Lai and Associate Professor Rosemary Nixon, Thursday 20 May 2025
Contact dermatitis is an inflammatory skin condition caused by direct contact with an irritant or allergen, leading to an itchy, red rash. It is sub-classified as irritant or allergic contact dermatitis, with irritant contact dermatitis (ICD) being more common.
ICD often affects the hands and is caused by repetitive exposure to skin irritants such as soaps and detergents. Even frequent exposure to water (wet work) will contribute to ICD, together with exposure to other chemicals such as solvents or oils, as well as physical factors like occlusion and heat and sweating. There is no test to diagnose it, so it is diagnosed clinically, or often after occlusion of allergic contact dermatitis (ACD).
On the other hand, ACD occurs when the immune system specifically reacts to an allergen, causing an allergic reaction on the skin. Only chemicals of a particular small size will stimulate this response, which is a delayed hypersensitivity reaction, meaning that there is a delay between skin contact with the allergen and the appearance of the rash, of 1-3 days. In addition, for people to become allergic, they need to be sensitised to a particular chemical first. The process of sensitization takes about 2-3 weeks, so people do not react to an allergen on their first exposure. Interestingly, people can develop ACD at any time of their life, often after years of being exposed to a particular chemical. Certain areas of the skin are more commonly associated with the development of ACD, such as the thin skin on the eyelids and neck. People may react to a moisturiser, fragrance or shampoo just on their eyelids, and may have never developed any skin reactions before.
Patch testing is the gold standard in diagnosing ACD, whereby allergic reactions are reproduced. It involves applying appropriately diluted allergens (external link) to the back, with the patches being removed at 48 hours. Reactions are interpreted by experienced dermatologists at 96 hours post application.
It is important to be aware that contact dermatitis can be a mixture of both ICD and ACD. There are also many endogenous forms of eczematous rashes, which include atopic eczema (also called atopic dermatitis), discoid eczema, hand eczema, lichen simplex and others. The implication here is that these rashes are occurring from “inside”, whereas contact dermatitis occurs from the “outside.” Nevertheless, contact dermatitis may complicate endogenous forms of eczema, particularly in hand dermatitis.
The Skin Health Institute (SHI) (external link) in Melbourne is the predominant centre for patch testing in Australia and also has a role in research, advocacy and education. The Australian Baseline Series (ABS) (external link) was initiated by the SHI as including the 60 most common and relevant delayed allergens, based on years of experience with patch testing. The most frequent relevant reactions to allergens in the ABS that fully diagnosed ACD include fragrance allergens, hydroperoxides of limonene/linalool, fragrance mix 1, Balsam of Peru; the preservatives methylisothiazolinone (MI) and methylchloroisothiazolinone/MI; the hair dye p-phenylenediamine (PPD) and potassium dichromate in cement and leather. Other important allergens include rubber chemicals in gloves, epoxy resins in some occupations, acrylates including in nail cosmetics and some plants.
The initial management of contact dermatitis follows the general principles of eczema therapy, however, first of all, one needs to make a specific diagnosis which will usually involve patch testing. Once an allergen is detected, it must be avoided subsequently, as there is no desensitisation available for this type of allergy. It is also important to avoid skin irritants (such as water, soaps, friction and heat), apply emollients or moisturising creams, and finally topical anti-inflammatory agents, such as corticosteroids to affected areas. ACD will usually resolve, provided that the responsible allergen has been identified and avoided.
It is necessary to identify allergens from all sources by reading product labels and using safety data sheets. A useful and simple-to-use app, CosMe has been developed by a Dermatologist at the Skin Health Institute, Dr Francis Lai, to help users find the right products to use by avoiding specific allergens. It also advises on high-risk ingredients in order to make safer, healthier choices for skin care.
Helpful Links related to this insight:
- Skin Health Institute (SHI) (external link)
- CosMe App
This insight was provided to Allergy Life Australia by Dr James Seamus Fuller, Dr Francis Lai and Associate Professor Rosemary Nixon. This Page was last updated on 29th May 2025.
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