Exogenous factors - Irritants:
Irritants are harmful chemical or physical agents with a direct cytotoxic effect after single or repeated application, which cause a non-immunological inflammatory reaction, the so-called irritant contact dermatitis (ICD). ICD is the most common occupational skin diseas and affects exposed areas of the skin, especially the hands. The stratum corneum is a determining barrier for penetration of the skin. Susceptibility to ICD appears to result from a thin, permeable stratum corneum. Therefore, regions like the face or neck with a thin stratum corneum are more damageable as hands or feets. Irritants may remove surface lipids, denature epidermal keratins or damage cell membranes. They may also have cytotoxic effects or induce the release of inflammatory mediators like cytokines. ICD seems to be a very complex reaction, although, still the pathogenesis is poorly understood.
Two major types of ICD are distinguished, acute and cumulative.
Exposure to potent irritants like chemical acid or alkali solution induces an acute eczematous dermatitis. Further more very hurtful substances may even induce chemical burns. Typical irritants are sodium and potassium hydroxides. Acute ICD occurs in any person if duration and concentration of the contact with irritants is sufficient.
The intensity of cumulative ICD depends on several endogenous and exogenous factors. Mostly it is caused by additive contact to mild irritants like soap, water, detergents, cement, metal salts, phenol-solvents etc. One single exposure is subthreshold and elicits no skin changes, but the repeated exposure induces eczema. Predisposing factors include heat or cold, occlusion and humidity. Susceptibility to irritants is more common in winter. Often several factors affect the skin simultaneously. The most important endogenous cofactor is atopy. Individuals with a history of atopic eczema are more susceptible to mild irritants and should avoid professions with intensive exposure to irritants.