Types of contact dermatitis

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There are two types of contact dermatitis - irritant and allergic.

Irritant contact dermatitis?

This is caused by direct contact with a substance which irritates the skin. This is a common problem and causes about 8 in 10 cases of contact dermatitis. It most commonly affects the hands. Irritant substances are those that can cause inflammation in almost everyone if they are in contact for long enough, repeatedly enough, and in strong enough concentration. For example:

Detergents (washing-up liquid, soaps, bleach, etc). People who do a lot of cleaning are prone to irritant contact dermatitis.
Solvents such as petrol.
Oils and other chemicals used in various places of work.
Acids and alkalis, including cement.
Powders, dust, and soil.
Certain plants (for example, ranunculus, anemone, clematis, helleborus, mustards).

You can develop irritant contact dermatitis quickly from a single exposure to a strong irritant. For example, from contact with a strong chemical in a work situation. However, you may also develop irritant contact dermatitis because of repeated exposures to weaker irritants. For example, from a detergent that you use when washing up dishes regularly.

There is often a 'vicious circle'. For example: a patch of skin may become sore after being in contact with an irritating substance. This causes some skin damage. Once damaged, the skin is more easily affected by irritants. So, further contact even with small amounts of the substance may cause further inflammation and damage, and so on.

Allergic contact dermatitis

This occurs when your immune system reacts against a specific substance. The substance is then called an 'allergen'. You only need a small amount of allergen in contact with your skin to cause the rash.

You are not born with this type of allergy - you must have previously come in contact with the allergen which has 'sensitised' your immune system. Once sensitised, your skin reacts and becomes inflamed when it comes into further contact. This is why you can suddenly develop a skin allergy to something you have come into contact with many times before. It is not clear why some people become allergic to some substances, and most people do not.

There are many substances that can cause an allergic contact dermatitis. Common ones include:

Nickel. This is the most common cause. Nickel occurs in many types of metal. For example: jewellery, studs in jeans and other clothes, bra straps, etc. So it is common to develop itchy red patches on the skin next to such things.
Cobalt. Traces of this metal may occur in jewellery
Cosmetics - particularly perfumes, hair dyes, preservatives, and nail varnish resins.
Additives to leather and rubber (in shoes, clothes, etc).
Preservatives in creams and ointments.
Plants - the most common culprits being chrysanthemums, sunflowers, daffodils, tulips, and primula.

Sometimes the cause is not clear and you may need tests to find the cause.

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