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Contact dermatitis



Dermatitis due to contact with an external agent. It cannot develop without exposure to substances in the environment.




Three types:

  1. Irritant - Not caused by the immune system, but appears to be caused by a direct chemical reaction with the skin (basically, no one knows why).
  2. Allergic - Delayed type IV hypersensitivity reaction. More likely to be regular: exposure to allergen will often cause similar reaction.
  3. Phototoxic - irritants that are made worse, or activated by, sunlight.

Risk Factors

Common Irritants

  • Water
  • Soaps & Detergents
  • Acids & Alkalis
  • Solvents
  • Friction
  • Cold, heat
  • Jeremy Clarkson

Common Allergens

  • Hair dye
  • Makeup
  • Nail polish
  • Nickel
  • Chromate

Clinical Features

Often seen acutely, with an erythema in the area where contact was: for example in a nickel sensitivity reaction to a nickel watch, there will often be a red rash encircling the wrist.

Since hands are often used to touch things, irritants are a common cause of chronic hand eczema.


Usually it becomes apparent why the dermatitis has occurred, often due to the pattern of erythema.

Patch testing

However, if it has not been identified, or needs confirmation, patch testing is the gold standard for identifying allergy.

Generally irritants will be differing from allergens by their reaction:

Allergen Irritant
Decrescendos - slowly disappears after 48 hours. Crescendo - persists after 48 hours.
Once off (usually) Repeatable
Reduces with dilution Similar reaction on dilution
Sore Itchy


The treatment is to identify the cause, and then avoid it.


Sometimes changing job is required, if contact with irritants is not avoidable.