Section 2.2 Allergic Contact Dermatitis

Allergic contact dermatitis is a delayed type of allergy. Allergic contact dermatitis is an individual response i.e. a person may be allergic to something, that can another person can use with no problems.

It can occur at any time, even if you have been using the same product for many years (with no previous problems) or just a few weeks. With this form of dermatitis, the rash appears 8 to 24 hours after contact and lasts for several days, and sometimes longer. It may develop after a spill, where there is prolonged exposure to a particular allergen, or with short repeated exposures. ACD does not occur the first time an individual is exposed to a substance.


If the skin is already damaged from irritant contact dermatitis, the risk of becoming allergic to something is higher. This is because there is direct entry for the allergens to enter the body and cause allergy to develop. Once a contact allergy to something has developed, it tends to be life-long, and even the smallest amount of contact with that particular substance may cause the rash to appear again.

Example of allergic contact dermatitis

Not all chemicals cause allergy. Even if a substance causes allergy, not all people will become allergic to it. Only certain chemicals have the potential to cause allergic reactions. It is said that there are approximately 100,000 chemicals, but only about 4,000 have been reported to cause allergic contact dermatitis. Some very strong substances, such as kerosene, do not cause allergy, but are extremely irritating to the skin.

Whether people develop an allergic reaction depends on:

  • The type of chemical contacted
  • The concentration of chemical on the skin
  • Duration of skin contact
  • Individual tendency to develop allergy

Common causes of contact allergy are:

  • Hair dye
  • Nickel
  • Chromate (found in cement and leather)
  • Glues and coatings e.g. epoxy resins
  • Preservatives in skin care products such as Methylisothiazolinone
  • Fragrances used in skin cleansers and moisturisers
  • Rubber chemicals (ingredients used in manufacture or rubber gloves)
  • Some food such as garlic and onion

Sometimes causes of allergic contact dermatitis are very specific to a particular occupation and/or workplace. See the occupational section of this manual for more information.

Patch testing is used to diagnose allergic contact dermatitis.

Diagnosing allergic contact dermatitis can be challenging. Clinically, there is often a delay of days between exposure to the allergen and the development of dermatitis, which can make it difficult to link the exposure to the rash. People are often exposed to multiple allergens, particularly in the workplace. It is necessary for the clinician to have a thorough knowledge of the many potential sources of allergens found in both workplaces and in recreational or domestic exposures.

Special note: Since approx. 2012, an allergen called Methylisothiazolinone (MI) which is a preservative has been found in many products and has been causing very high rates of allergic contact dermatitis. This preservative is found in some cosmetics, sunscreens, baby wipes and make-up wipes, shampoo, conditioner, hair products, moisturisers, hand wash and shower gels and some house paints. Check the ingredient listing and avoid products that contain this allergen.

Recommended reading

https://dermnetnz.org/topics/allergic-contact-dermatitis

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