The 3 main types of dermatitis -- atopic, contact allergy and contact irritant -- have different causes. While not everyone agrees about the causes of atopic dermatitis, there is general agreement that it is a genetic disorder that is influenced by environmental factors. The causes and processes involved in contact dermatitis (whether allergic or irritant) are much clearer, although little is known about what makes people susceptible to allergic dermatitis.

The cause of atopic dermatitis

The importance of inheritance in atopic dermatitis was shown by a Danish study which analyzed the brothers and sisters (siblings) of affected children. They found that siblings were more likely to have atopic dermatitis if they were identical twins than if they were non- identical twins, not twins at all. Other studies have shown that two thirds of patients with atopic dermatitis have a family history of this condition - atopy.

Atopy is an inherited tendency to develop such conditions as asthma, hay fever (allergic rhinitis), urticaria, and atopic dermatitis or eczema. In atopic people, the external surface of the body -- the skin -- and some internal surfaces -- those of the nose and the pulmonary tree - are hypersensitive to substances found in the environment. The nature of these substances is unfortunately only known in a minority of patients: they are allergens, wool acrylic, soaps and detergents. Additionally, the following environmental conditions have been shown to act as skin irritants in patients with atopic dermatitis: decreased humidity, very high, or very low outside temperatures. On an individual level, excessive sweating, anxiety, and stress can also play a role.

As an example, Nicola , the baby in our case studies, has flare-ups during extreme weather conditions and when she has an upper respiratory tract infection. Gregory has flare-ups in the cold, and when exposed to normal bath soaps and woolen clothes. Christina's mother has to vacuum her mattress to minimize her exposure to dust mites.

Of course not everyone born in a family with atopy will develop the tendency, but atopy is more likely to be seen in a family member of a patient with atopic dermatitis.

The causes of allergic and irritant contact dermatitis

Allergic contact dermatitis occurs when a highly reactive chemical substance comes into direct contact with the skin. The skin reaction, which can occur days after the initial exposure, occurs predominantly at the site of contact. Only minute quantities of the substance are needed to cause the reaction.

The principle sites of allergic contact dermatitis and the likely causes are given in the following table:

Site	Likely Cause
Face	Cosmetics, perfumes, soaps, nickel in ear rings, spectacle frames
Scalp	Hair dyes (para-phenylene-diamine - PPD - a black dye), lotions and tonics containing Balsam of Peru
Neck	Nickel in jewelry
Wrist Hands	Nickel in jewelry, PPD, chromates, cosmetics, soaps
Body	Nickel clips or rubber material used in underwear
Feet	Dyes used in socks and shoes, glues, chromates in shoes

As you can see, allergic contact dermatitis tends to be caused by substances in everyday use, but will also affect people who use and manipulate these substances in the manufacturing industry.

Irritant contact dermatitis can potentially occur in anyone. The skin erupts when it encounters certain irritants, such as chemicals that directly damage the superficial layer of the skin. The damage occurs more readily in people with constantly wet hands. Common skin irritants are soaps and detergents, disinfectants and cleaning chemicals, shampoos, dyes, foodstuffs and cutting oils. As a consequence, irritant contact dermatitis is often job-related; hairdressers, cooks and caterers, dish-washers, machine-tool operators, nurses and homemakers are all at risk.