Contact dermatitis occurs when the skin reacts to a particular substance and is divided into irritant and allergic forms. Irritant contact dermatitis results when the substance directly damages the outer skin barrier, whereas allergic contact dermatitis occurs when the immune system mounts a delayed hypersensitivity response to an allergen. These mechanisms can produce similar-looking rashes, but they differ fundamentally.
Irritant dermatitis can occur after sufficient exposure in a person without prior sensitisation, while allergic dermatitis generally requires an initial sensitisation process before subsequent exposure produces an immune reaction. Both can cause itching, burning, redness, dryness, cracking, swelling and blistering. The hands and face are commonly affected because they frequently contact products and environmental substances.
Irritant contact dermatitis is particularly associated with repeated exposure to weak irritants. Soaps and detergents are common examples because they remove lipids from the stratum corneum and disrupt barrier function. Frequent exposure to water, especially prolonged wet work, can itself be irritating because repeated wetting and drying damages the skin barrier. Stronger chemicals such as solvents, acids, alkalis, disinfectants and some industrial substances can cause dermatitis after shorter exposure. Mechanical friction, heat, cold and low humidity can worsen the effects. Occupational irritant dermatitis is important among cleaners, hairdressers, healthcare workers, cooks, agricultural workers, construction workers, mechanics and people who routinely handle chemicals or perform wet work.
Allergic contact dermatitis is a form of delayed-type hypersensitivity. During the initial sensitisation phase, an allergen penetrates the skin and is processed by immune cells, leading to the development of allergen-specific T-cell responses. Subsequent exposure can activate these memory T cells and produce inflammatory cytokine signalling, usually with symptoms developing hours to days later rather than immediately. Common allergens include nickel and cobalt in some metals, fragrances, preservatives, hair-dye chemicals, rubber components, textile dyes and resins, strong adhesives and some topical medicines. A person can become sensitised after repeated exposures or after seemingly limited exposure, and once sensitised, reactions can recur with small amounts of the allergen.
Product exposure is a frequent practical cause because cosmetics, toiletries and household products can contain numerous potential irritants or allergens. Perfumes, preservatives, shampoos, moisturisers, soaps, cosmetics, nail products and hair dyes can cause dermatitis in susceptible individuals. The relevant ingredient is not necessarily the product's headline ingredient; preservatives, fragrances and other formulation components can be responsible. A rash that repeatedly occurs around the eyelids, face, neck or hands may reflect exposure to a cosmetic, fragrance, hair product or transferred allergen.
The distribution can sometimes be indirect, such as dermatitis on the eyelids caused by an allergen transferred from the hands or airborne exposure to a product. Careful product and ingredient review can therefore be more useful than simply switching between brands without identifying the responsible chemical.
Occupational exposure can create chronic contact dermatitis when protective measures are inadequate or the skin is repeatedly exposed to irritants. Hairdressers may encounter dyes, shampoos and repeated water exposure; healthcare workers may encounter disinfectants and frequent handwashing; cleaners may handle detergents and chemicals; construction workers may contact cement; mechanics may contact oils and solvents. Gloves can reduce exposure but can also cause sweating, friction or allergic reactions to glove materials. Appropriate occupational assessment may therefore involve identifying the specific substance, improving work practices, using suitable protective equipment, taking breaks from wet work and applying emollients. Where work exposure is responsible, documentation can be important because persistent exposure may prevent the condition from resolving.
Atopic eczema increases susceptibility to irritant contact dermatitis because an already impaired skin barrier is more vulnerable to external substances. This does not mean that every flare of atopic eczema is contact allergy. Similarly, an individual can have both atopic eczema and allergic contact dermatitis, making diagnosis more complicated. Irritant dermatitis tends to develop directly in exposed areas and can appear relatively quickly, whereas allergic contact dermatitis may show delayed onset and can spread beyond the exact contact site. Clinical history therefore considers the timing between exposure and symptoms, whether the same product repeatedly causes disease, whether symptoms improve away from work or holidays, and whether other body sites are involved.
Patch testing is the principal investigation for suspected allergic contact dermatitis. Small quantities of standardised allergens are applied under patches, commonly to the back, and the skin is examined after the patches are removed and again later because allergic reactions are often delayed. Patch testing differs from prick testing, which is generally used for immediate-type allergies such as some food or inhalant allergies.
There is no equivalent single test for irritant contact dermatitis because irritation depends on exposure intensity, duration, concentration and individual barrier susceptibility. Diagnosis is therefore primarily based on history and clinical pattern. If the responsible substance cannot be identified or symptoms do not respond to appropriate measures, dermatology referral is appropriate.
The most effective treatment is identifying and reducing exposure to the responsible substance. If the irritant or allergen is successfully avoided, the skin can often improve substantially or clear. Emollients help restore and protect the barrier, while topical corticosteroids may be prescribed to reduce significant inflammation. In severe widespread cases, short courses of systemic corticosteroids are occasionally used under medical supervision. Continued exposure can make otherwise appropriate treatment appear ineffective. Preventive measures can include protective clothing, appropriate gloves, substitution of less irritating products, careful rinsing and regular emollient use. Contact dermatitis is not contagious, and controlling exposure is more important than attempting to eliminate microorganisms from the skin.
