Contact dermatitis is inflammation of the skin caused by something it has been in contact with. It comes in two forms: irritant, where a substance damages the skin directly, and allergic, where the immune system has learned to react to a specific substance. Telling them apart changes what happens next.
What is actually happening in the skin?
Two quite different processes share the name contact dermatitis, and the difference between them is the most useful thing on this page. Dermatitis itself just means inflammation of the skin; the qualifier carries all the information.
Irritant contact dermatitis is direct damage. A substance strips the lipids that hold the outermost layer together, or injures the cells beneath, and the skin becomes inflamed as a result. No immune memory is involved. Given sufficient exposure the same thing would happen to anybody, which is why it is so common on the hands of people who wash them constantly at work. Frequency usually matters more than strength here: mild detergent many times a day does more than something harsh once.
Allergic contact dermatitis works differently. The immune system has previously encountered a specific substance, recognised it, and retained that recognition. On a later exposure, immune cells in the epidermis respond to it, and inflammation follows. Only people who have become sensitised react, which is why one member of a household can wear a particular metal quite happily while another cannot.
The delay is characteristic. Allergic reactions of this type typically appear a day or two after contact rather than immediately, which scatters the evidence: by the time the skin reacts, the substance has long since been forgotten. That delay is responsible for a great deal of misattribution.
How would a dermatologist describe it?
Sensitisation is the term for the immune system learning to recognise a substance. Elicitation is what happens on subsequent exposures. An allergen is the substance itself; an irritant is something that causes damage without immune involvement.
Distribution and shape are described with unusual care in this dermatitis, because they carry more information here than almost anywhere else in dermatology. A sharply bounded rectangle of inflammation says something a diffuse patch does not. Clinicians pay attention to whether the edges are geometric, whether the pattern matches an item of clothing or jewellery, whether it follows where a liquid ran.
Eyelid involvement is worth its own mention. Eyelid skin is among the thinnest on the body and it frequently reacts to substances carried there on the fingers, so a reaction there may have nothing to do with anything applied to the eyes.
You may also hear photocontact or photoallergic used, describing reactions that require both a substance and light exposure to occur. That combination explains patterns that otherwise look inexplicable, appearing only on skin that was uncovered.
What usually happens at a consultation?
The history is a piece of detective work and it is longer than most. Expect questions about your occupation and what you handle at work, hobbies, what you apply to your skin and hair, jewellery, clothing, gloves, cleaning products, and what changed in the weeks before the skin did. Bringing the actual products, rather than their names, matters more here than almost anywhere.
The examination concentrates on pattern, boundary and site. A clinician is reading the shape of the reaction as evidence, and a photograph taken when it was at its worst can be genuinely decisive if the skin has since settled.
Where allergic contact dermatitis is suspected, patch testing may be arranged. It is a multi-appointment investigation rather than something done on the day, and it is the recognised way of identifying which substances are involved. What it entails, practically, is covered elsewhere on this site.
What is worth asking?
Ask whether the clinician thinks your dermatitis is irritant, allergic, or not yet distinguishable. That single answer determines whether investigation is likely to follow, and it reframes what you should be paying attention to at home.
Ask what the pattern on your skin suggested to them. Contact dermatitis is one of the few conditions where the shape of the thing is a large part of the reasoning, and hearing that reasoning teaches you what to photograph next time.
Enquire about your work specifically, if hands are involved. Occupational exposure is a substantial part of contact dermatitis and it comes with practical considerations that a clinician deals with routinely.
Where patch testing is proposed, find out what you need to do beforehand and how long the whole process takes. It involves more than one visit and some restrictions during it, and knowing that in advance avoids an unwelcome surprise.
What this page does not do
It does not identify what your skin reacted to, and no page can: that is what a history and, where needed, patch testing are for. Nor does it tell you whether what you have is contact dermatitis at all. No product, no avoidance list and no treatment is recommended here.
Common questions
What is the difference between irritant and allergic contact dermatitis?
Irritant contact dermatitis is direct damage to the skin by a substance, and enough exposure will do it to anybody. Allergic contact dermatitis involves the immune system recognising a specific substance it has met before, so it affects only people who have become sensitised to it.
Why did a product I have used for years suddenly cause a reaction?
Allergic sensitisation can develop at any point, including to something used without trouble for a long time. That is a recognised pattern rather than an oddity, and it is one reason a familiar product is not automatically ruled out when a clinician takes a history.
What is patch testing?
It is an investigation in which small amounts of standard substances are applied to the skin under dressings and read after a set interval, to see which produce a reaction. It is used to identify allergic contact dermatitis, and it takes place across more than one appointment.
Can contact dermatitis appear somewhere I did not touch anything?
Yes. Substances travel on hands to the face and eyelids, which are thinner-skinned and often show a reaction more readily than the hands themselves. Airborne exposure and transfer from another person are also recognised routes, which is why the pattern is examined carefully.
Does contact dermatitis go away once the substance is avoided?
Skin generally settles once exposure stops, though it may take longer than people expect and the underlying sensitisation, in the allergic form, does not go away. Identifying the substance is therefore the part that carries lasting value.
Sources
- Contact dermatitis — conditions — NHS (United Kingdom) · accessed
- Patient information on skin conditions — British Association of Dermatologists (United Kingdom) · accessed
Last updated . How this page was written.