Understanding the causes of skin disease

The skin is a complex active organ, if any of its functions fail there can be serious consequences. The skin's ability to act as a barrier is particularly important for occupational health.

One way to understand the barrier function of the stratum corneum is to consider it as a brick wall. The corneocytes (made of tough protein) form the bricks and between these a double layer of lipids (fatty materials) and water make up the mortar. Some lipids have a hard crystal-like structure and are impermeable to water. Others lipids do not have this structure and they allow water to percolate through. So, the barrier is semi-permeable.

The elasticity, firmness and correct functioning of the stratum corneum depends on its moisture content. Retention of water is aided by substances in the skin called natural moisturising factors (NMFs). If the moisture content is too high or too low, it can affect the skin's barrier properties.

If the skin becomes overhydrated, for example from prolonged contact with water or from wearing gloves that prevent sweat from evaporating, it causes NMF production to stop.

If the skin dehydrates, for example in an air-conditioned environment with low humidity, the corneocytes are not shed as normal and the skin becomes rough, thickened and flaky, eventually leading to cracking because of loss of elasticity.

The 'surface film' on the epidermis also acts as a barrier, to prevent bacteria and other contaminants from penetrating the skin. The film is slightly acidic and can help to neutralise the contaminants that are typically alkaline in nature. Excessive use of harsh alkaline soaps can destroy the acidity of the film and hence the protection it offers.

Problems occur when the skin's barrier is breached. This can happen when:

• a material/agent penetrates the barrier layer or alters it so other materials/agents can penetrate it;
• a material/agent enters sweat ducts or hair follicles, by-passing the barrier layer.

How does the skin react to 'breaches'?

The skin has a limited range of protective responses. The most common one is inflammation. This is known as dermatitis or eczema. It is characterised by redness and heat from dilation of local blood vessels, swelling and blistering from plasma leaking from the vessels to the surrounding tissue and itch caused by stimulation of nerve fibres. Secondary changes due to infection and scratching include crusting, ulcers and thickening of the skin.

Dermatitis caused by a malfunction or disorder within the body is termed endogenous or constitutional dermatitis. Work-related dermatitis  is due to contact with some external agent. This is known as exogenous dermatitis or more commonly contact dermatitis.

External agents tend mostly to come into contact with the hands and forearms, so around 95% of work-related skin diseases occur in this area. The majority of the remainder are on the face. Most work-related skin diseases are contact dermatitis. External agents are involved in an important minority of other work-related skin diseases. These include contact urticaria, acnes, cancers, leucoderma (vitiligo) and skin infections.