Irritant and Allergic Contact Dermatitis

Notes in Dermatology, Skin concerns

Notes in Dermatology

I see a lot of rashes in in the skin clinic; itchy, painful, annoying, worrying, long term or short lived. So many variations, and so many ways of dealing with them. To help to answer some of your questions, I thought I would share somee basic notes in dermatology, some background information about skin dilemmas. 



Contact dermatitis (also called contact eczema) is a  rash which occurs when the skin comes in direct contact with a substance (ie fibre) or chemical (ie a soap or detergent) which causes a reaction. The reaction will be a rash made up of fine red papules, sometimes blisters, with drying, peeling and cracking of the skin. It may be itchy, or sore. It will develop on the part of the body where the contact happens, so frequently this is on hands or arms or face and neck, but it can be on any exposed skin.




Contact dermatitis usually clears up completely when the trigger substance is removed. (However it can sometimes be difficult to recognise that it is a rash in response to contact with an agent and to identify the specific agent.) The symptoms can be made to feel better by using emollient creams, mild topical steroids (hydrocortisone), or for more severe symptoms, oral steroids.

Dermatitis is a non-infectious inflammation of the skin, seen as a rash, that is – redness, sometimes blistering, crusting, scaling, flaking. Sometimes itchy, sometimes painful.

The cause of this reaction can be irritant, or allergic



Allergic contact dermatitis happens when the skin develops an allergy to a specific chemical or substance, which can happen after a single exposure, or many. An itchy rash then develops when skin comes in contact with that substance again, typically as a delayed reaction, developing 12 to 72 hours after contact. Not everyone reacts to allergens, and some may become allergic to something which they were previously tolerant of for years.


 How does it work: when the skin first comes in contact with the new allergen, it sends a piece of the allergen to the immune system for analysis. The new allergen is stored in the immune system memory, but does not cause a reaction. This process is called sensitisation. When the skin comes in contact with the substance again, the immune system recognises it, and it may trigger a response. This trigger takes a few days to occur; it is called a delayed type IV hypersensitisation reaction.


Common sources of allergic contact dermatitis are metals (such as nickel, seen for example in earrings), rubber, latex, fragrances (in soaps, lotions), cosmetics, topical antibiotics, preservatives, sunscreens.

The top allergens are Nickel, gold, resin, neomycin (topical antibiotic), perfume, formaldehyde preservative (in paint, cleaners, cosmetics, medications), cobalt chloride (in dyes, metal plating, antiperspirant), chromate (in cement, plaster, building products), bacitracin (topical antibiotic), quaternium (preservative in cosmetics, self-tanners, sunscreens).




Irritant contact dermatitis is a more immediate reaction when vulnerable skin is exposed to an irritating substance – either to a strong irritant (such as acid, solvent, alkali) or repeatedly to even a mild irritant such as a detergent. Amount and duration of contact are important factors.

How does it work? The skin barrier is broken by the irritant, and this triggers inflammation, redness, dryness, peeling flaking, pain and itching. The skin barrier is outermost layer of the epidermis, the stratum corneum, comprising lipids and corneocytes – together they make a tough impermeable layer, to protect the skin from the loss of moisture. Disturbance of this barrier is recognised as an important factor in the development of many skin diseases, and conversely, protecting the barrier with emollients is vital to healthy skin.


Substances such as detergents, soaps can irritate the skin by wearing down the natural protective surface of the skin. People who work with chemicals, soaps etc are more prone to developing this common and uncomfortable skin disorder.

The treatment is the avoidance of the irritant, hydrating of the skin and repair of the skin barrier, and the use of mild topical steroids such as hydrocortisone.


Allergic and irritant contact dermatitis can co-exist and they can be difficult to distinguish. Irritant dermatitis can happen to all of us, especially those whose work or lifestyle exposes us to contact with chemicals, however mild. Those of us who are more ‘atopic’ (which is have a tendency to allergic reactions) may more rapidly develop an irritant rash after exposure.





A persistent rash on the chin of a client had not responded to medication, (the location of the rash suggested perioral dermatitis, and had been treated with a topical antibiotic (topical steroids can sometimes be the trigger for perioral dermatitis). Moisturisers calmed the rash slightly, but it would not go away. Distressing. Some weeks later, while on holiday, she noticed the rash disappeared, but returned as soon as she was home. This suggested it was an irritant in her home environment or daily activities that was causing the dermatitis. The trigger was a scarf which she had been using every morning when she took the dog on the beach. Replacing the scarf resolved the problem.




Images courtesy of PCDS and Dermnet NZ

Ale, I.S. and Maibach, H. (2018) ‘Diagnostic Approach in Allergic and Irritant Contact Dermatitis’ Medscape [Online] Available at: Accessed on 18 September 2018.

Cunliffe, T (2018) ‘Eczema contact allergic dermatitis including latex and rubber allergy’ PCDS [Online] Available at Accessed on 25 September 2018

Oakley, A. (2012) ‘Contact Dermatitis’ Dermnet NZ [Online] Available at Accessed on 18 September 2018