Contact dermatitis

Prevalence and incidence
The majority of occupational dermatoses consist of contact dermatitis. Irritant contact derma titis is extremely common; it is reported to be as high as 55% in people with certain occupations (e.g. health care workers and those working in catering, cleaning and hairdressing). Contact aller gic dermatitis affects 1–2% of the general UK population (English, 1999).

Risk and trigger factors
These include:

• atopic eczema (current or a history of childhood atopic eczema)
• occupations with a high exposure to allergens
• occupations where repeated hand washing is essential

Irritant dermatitis
Irritant dermatitis is inflammation of the skin resulting from a non-immunological reaction to external stimuli due to excessive contact with an irritant (e.g. detergents, soap, acids, alkaline, cement, solvents, chemicals and food). Acute irritant contact dermatitis is often the result of a single overwhelming exposure or a few brief exposures to the irritant or causative agent. Chronic or cumulative contact dermatitis occurs following repeated exposure to irritants (Bourke et al., 2001).

Allergic contact dermatitis
Allergic contact dermatitis is an allergic reaction which only occurs in people whose skin has been exposed to and previously sensitised by an allergen. Subsequent contact with the antigen elicits a specific cell-mediated sensitisation (e.g. nickel, cosmetics, perfumes, hair dye, dyes and plants) of the immune system to a specific allergen/ s with resulting dermatitis or exacerbation of pre- existing dermatitis. Phototoxic, photoallergic and photo-aggravated contact dermatitis occur when allergens are photo-allergens but it is not always easy to distinguish between photo-allergic and phototoxic reactions (Bourke et al., 2001). Phototoxic reactions result from direct damage to tissue caused by light activation of the photosensitising agent. Photoallergic reactions are a cell-mediated immune response in which the antigen is the lightactivated photosensitising agent. Some patients with atopic dermatitis and other chronic inflammatory skin conditions become photosensitive (DermNetNZ, 2009).
Acute dermatitis will follow a single exposure to an irritant or an allergen, and the allergic and eczematous manifestations observed in the skin are defined by Burns et al. (2004). The manifestations are as follows:

• Vasodilatation of the dermis causes inflammation;
• Peri-vascular infiltrate of lymphocytes and polymorphs;
• Intra-epidermal vesicles form by accumulation of fluid in cells, and may coalesce to form bullae;
• Vesicles and bullae will rupture to oozing, crusting, scaling and healing.

Patients with allergic contact dermatitis usually present with acute dermatitis at the body site where the allergen has been in direct contact with the skin. Severe allergic contact reactions may extend outside the contact area or it may become generalised.