Contact dermatitis

Identifying 'abnormal' skin can be straightforward but identifying the cause is more complex. It is helpful for health and safety and medical professionals to work together to establish whether the condition is work related.

The majority of work-related skin diseases are dermatitis. So, the first step is to establish whether the skin problem is dermatitis. This can be quite challenging for medical professionals since the appearance of both acute and chronic dermatitis can vary and it can be presented to the doctor at different stages. Also, there are some common skin conditions, which are confused with dermatitis: psoriasis, pustulosis of the palms and soles, tinea, scabies, lichen planus.

If the condition is dermatitis, the next question is whether it is constitutional or if it arises from contact with an external agent, ie contact dermatitis. An important clue is the site of the area affected. If it is the hands, contact dermatitis should always be suspected.

The final question is whether the 'contact' arises from work or from home. Medical practitioners will take a range of factors into consideration when making a diagnosis, including the site and spread of rash, history of onset, work done by the patient, hobbies and medications.

Some helpful clues to recognise a work-related cause are:

• if it is primarily on the hands and face;
• if the condition improves away from work and relapses on return;
• if more than one person affected in same work area or handling same materials.

There are two main types of work-related contact dermatitis, irritant contact dermatitis and allergic contact dermatitis.

Irritant contact dermatitis

An irritant directly damages cells if in contact with the skin in sufficient concentration and for sufficient time. Most irritants cause dermatitis by gradually overwhelming the skin's barrier and repair mechanisms. Mild irritants such as detergents will wash out the stratum corneum lipids and if exposure exceeds the capacity of the skin to regenerate those lipids, dermatitis will result. Powerful irritants ‑ such as caustic soda ‑ produce an immediate effect. These cause direct damage to keratinocytes. Dermatitis induced by mild irritants is called chronic or cumulative irritant contact dermatitis.

People vary in their susceptibility to irritants. For reasons not yet understood, certain groups of people are more susceptible to irritants:

• those with constitutional dermatitis/eczema (known as 'atopics');
• those with very dry skins;

In general, irritant contact dermatitis is more common occupationally than allergic contact dermatitis. There are a number of occupations and materials associated with irritant contact dermatitis. It is important to realise that contamination of the skin from a 'dirty job' rarely causes irritant dermatitis, it is more commonly the cleansers used to remove the contamination. The effect is exacerbated in winter by environmental factors (wind, cold) which cause drying of the skin. It is not unusual to see more cases of hand dermatitis in the winter and it is particularly important for employees at risk to know how to protect themselves during the winter months.

Allergic contact dermatitis

Allergic contact dermatitis is caused by contact with a 'sensitiser' (allergen) that causes a type IV or 'delayed hypersensitivity' reaction. A sensitiser is a substance that can induce an 'over-reaction' of the body's immune system.

A sensitiser must first penetrate the skin (most contact sensitisers are small molecules with molecular weights below 1000). Next the sensitiser is combined with skin immune cells (Langerhans cells) which then leave the skin and travel to lymph glands nearby (known as induction). Here, they react with another type of immune cell (T-lymphocytes or T-cells), which reproduce and produce 'memory' cells that can remember that particular sensitiser.

Once sensitisation has occurred, subsequent contact causes T-cells to recognise the sensitiser and multiply. This induces the release of substances such as histamine that bring about the features of inflammation (known as elicitation). This second phase can happen hours or days following contact hence its name 'delayed hypersensitivity'. Very small quantities of the sensitiser can trigger a response once sensitised.

There is a range in potency for sensitisers. The initiation of sensitisation may occur at the very first contact or it may not happen until there has been repeated contact for months or even years. The process of sensitisation produces no visible change in the skin.

Generally, only a small proportion of an exposed occupational group becomes sensitised. It depends on the concentration of sensitiser and the degree and duration of skin contact ‑ as well as the sensitising potential of the sensitiser.

Sensitisation is specific to one substance or to a group of substances that are chemically similar. Once sensitised a person is likely to remain so for life.

Both irritant and allergic contact dermatitis can occur together (particularly on the hands) and either may co‑exist with constitutional dermatitis. It is common for exposure to occur to more than one irritant and more than one allergen at any one time. Such exposures may give rise to a cumulative irritant and cumulative allergic response. An irritant contact dermatitis may also develop first, rendering the skin more susceptible to penetration by sensitisers. It is also possible that an original allergic contact dermatitis might be later sustained by an irritant.

There are a number of occupations and materials associated with allergic contact dermatitis. Some substances can act both as irritants and sensitisers, for example cement; its wetness, alkalinity, grittiness and hygroscopicity make it an irritant. It contains the sensitisers chromate and cobalt.