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Authors: Vanessa Ngan, Staff Writer, 2002; Updated: Elena Redl, Medical Student, Medical University of Vienna, Austria; Dr Martin Keefe, Dermatologist, Christchurch, New Zealand. Copy edited by Gus Mitchell. May 2021
Cobalt is one of the most common metals to cause contact allergy via a delayed hypersensitivity reaction. It is also called cobalt dichloride hexahydrate, cobalt blue, cobaltous chloride hexahydrate, and cobalt hexahydrate. The name comes from the German word ‘Kobold’ meaning goblin or elf.
Cobalt was the American Contact Dermatitis Society’s 2016 Contact Allergen of the Year.
Cobalt allergy affects about 2% of the general population and can develop at any age, including in children. It shows a female predominance (2:1) and is commonly associated with atopic dermatitis.
One study reported an increased prevalence of cobalt allergy in people with skin of colour.
Cobalt is commonly used as a binding agent in the manufacture of ‘hard’ metals with increased wear resistance such as drills and cutting tools.
It is also found in:
Allergic contact dermatitis due to cobalt is diagnosed on patch testing with 1% cobalt chloride hexahydrate. A positive reaction to nickel is likely to be due to dual sensitisation rather than cross-reactivity.
Cobalt naphthenate used in the polyester resin and plastics manufacturing industries can also cause an allergic contact dermatitis, but this may not be detected by the standard patch test with cobalt chloride.
Purpuric patch test reactions are peculiar to cobalt due to a poral reaction to cobalt accumulation in the eccrine glands and are not indicative of cobalt allergy.
Rarely, intradermal tests or oral challenge tests may be appropriate.
Positive patch test to cobalt
Cobalt allergy is treated by identifying and avoiding the source of exposure.
Identify potential sources of exposure at work using material safety data sheets. Read product labels. The presence of cobalt in a metal object can be confirmed using a commercial 2-nitroso-1-naphthol-4-sulfonic acid spot-test. A clue to cobalt-containing jewellery is a dark silver rather than shiny appearance.
To minimise cobalt exposure:
Contact dermatitis treatment includes:
Allergic contact dermatitis due to cobalt will settle if contact with cobalt can be avoided.
CAS number: 7791-13-1
Appearance: Silvery grey, shiny, hard metal
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