Skin Sensitisation

Certain cobalt substances are classed as skin sensitisers based on human data and/or animal data.

What is skin sensitisation?

Skin sensitisation, also termed allergic contact dermatitis (ACD) in humans, is a rash or eczema on the skin of people who are allergic to cobalt. It is a local immune response, in which first contact with an “allergenic” substance activates the immune system, and subsequent contact produces a local effect in the skin (e.g. redness, swelling, itchiness). This local effect at point of contact can also spread to skin on other parts of the body.

Potentially allergenic substances must penetrate through the barrier layer of the skin until they reach a viable (‘living’) skin layer, after which they bind to skin proteins and immune cells1. For an allergenic substance to penetrate the skin it must be sufficiently small and possess favourable properties (i.e. not too fat-soluble and not too water-soluble). Allergenic substances that are bound to immune cells can then be translocated to an area in which they interact with T-lymphocytes (another type of immune cell), which leads to expansion of this cell population and concludes the first phase of sensitisation (termed “induction”).

In a sensitised individual, the next contact with an allergenic substance then results in a wide-spread elicitation of the immune system (i.e. allergic contact dermatitis) due to a reaction between the allergen-specific T-lymphocytes circulating in the body and the allergen at the site where it has entered the viable skin1. The symptoms of allergic dermatitis can include a rash or bumps, itching, blisters and swelling. The entrance and contact of the allergic substance in viable skin can also physically damage skin cells, an action which can also lead to an immune response (e.g. localised inflammation)[1].

Clinical and patch test data from over 19,000 patients in ten European countries who were suspected of having allergic contact dermatitis revealed a prevalence of contact allergy to cobalt chloride of 6.2-8.8%[2]. However, cases of allergic shock / anaphylaxis following dermal exposure to cobalt are considered extremely rare[3].

Human T cell
In an already sensitised individual, contact between an allergen-specific T-lymphocyte and the allergen results in the elicitation of a wide spread immune response.

Cobalt-related ACD can be controlled

Symptoms of cobalt-related ACD can be avoided by limiting exposure to items that release cobalt, such as jewellery containing significant levels of cobalt. Cobalt-related ACD in the workplace can be controlled using good occupational hygiene practices and personal protective equipment.

Classification of sensitisers under UN GHS

The UN Globally Harmonized System of Classification and Labelling of Chemicals (UN GHS) defines a skin sensitiser as “a substance that will lead to an allergic response following skin contact”[4].

A substance can then be assessed and classified as a skin sensitiser either based on human data showing a sensitisation response in a substantial number of persons or positive results from an appropriate animal test3. In addition to human data and animal evidence, newly developed in vitro tests can assist in identifying skin sensitisers, however these tests do not provide as much information on the pathway to skin sensitisation as animal tests.

Cobalt and Skin Sensitisation

The following cobalt substances have legal classifications in the EU and elsewhere as skin sensitisers, based on data on human exposure in the workplace[5]:

Substance Name Classification
Cobalt metal Skin Sens. 1; H317
Cobalt carbonate Skin Sens. 1; H317
Cobalt dichloride Skin Sens. 1; H317
Cobalt sulphate Skin Sens. 1; H317
Cobalt dinitrate Skin Sens. 1; H317
Cobalt diacetate Skin Sens. 1; H317
Cobalt sulphide Skin Sens. 1; H317
Cobalt monoxide Skin Sens. 1; H317

Animal data are available for classification of further cobalt compounds, and the remaining cobalt substances have been grouped for skin sensitisation either based on similarities to the cobalt substances that meet the criteria for classification or those which are not classified.


This summary is intended to provide general information about the topic under consideration. It does not constitute a complete or comprehensive analysis, and reflects the state of knowledge and information at the time of its preparation. This summary should not be relied upon to treat or address health, environmental, or other conditions.

[1] Barker, J.N.W.N., Mitra, R.S., Griffiths, C.E.M., Dixit, V.M. and Nickoloff, B.J. 1991. Keratinocytes as initiators of inflammation. The Lancet 337:211-215.
[2] Uter et al. 2009. The European baseline series in 10 European countries, 2005/2006- Results of the European Surveillance System on Contact Allergies (ESSCA).Contact Dermatitis 61:31-38.
[3] Krecisz, B., Kiec-Swierczynska, M., Krawczyk, P., Chomiczewska, D., Palczynski, C. 2009. Cobalt-induced anaphylaxis, contact urticaria, and delayed allergy in a ceramics decorator. Contact Dermatitis 60(3):173-174.
[4] United Nations, 2013. Globally Harmonized System of Classification and Labelling of Chemicals (Fifth revised edition).
[5] Aguilar-Bernier, M., Bernal-Ruiz, A.I., Rivas-Ruiz, F., Fernandez-Morano, M.T. and de Troya-Martin, M. 2012. Actas Dermosifiliogr. 103(3): 223-228.