Contact dermatitis

Two conditions, one name

Contact dermatitis means skin inflamed by something it has touched. It comes in two forms that look similar and are not the same thing.

Allergic contact dermatitis is an immune reaction to a specific substance that is harmless to anyone not sensitised to it. Only the sensitised react, and once sensitised they will react to very small amounts.

Irritant contact dermatitis is not an allergy and involves no immune reaction. It is physical damage to the skin barrier from repeated exposure to something harsh, and given enough exposure it will affect anyone. Water is one of the commonest culprits, along with soaps, detergents, solvents and friction.

Irritant is the commoner of the two, which matters, because the two have different answers. Allergic dermatitis can often be resolved by identifying one substance and avoiding it. Irritant dermatitis has no single culprit to find — it is about cumulative exposure and a damaged barrier, so the answer is protection and repair rather than detection. Looking for an allergy that is not there is a common way to spend months getting nowhere.

How allergic contact dermatitis works

This is worth being precise about, because it is routinely confused with hay fever and it does not work the same way at all.

Hay fever and food allergy are immediate reactions driven by IgE antibodies and the release of histamine, which is why they happen within minutes. Allergic contact dermatitis is a delayed, type IV reaction driven by T cells rather than antibodies. The T cells recognise the substance in the skin and release signalling molecules that produce the inflammation, and that takes time — typically 24 to 72 hours after contact.

That delay is the most practically useful fact about the condition, and it is why people so often blame the wrong thing. The rash that appears on Wednesday was caused by something touched on Monday.

It also explains why antihistamines do so little here. The reaction is not histamine-driven, so an antihistamine has nothing much to act on. A sedating one at night is sometimes used to help someone sleep through severe itching, but that is managing the sleep rather than treating the rash.

What causes it

Nickel is the commonest, found in jewellery, piercings, belt buckles, jean studs, watch straps, spectacle frames and, increasingly, phone cases and other hardware.

Fragrance is next, and it is in far more than perfume — soaps, moisturisers, cleaning products, laundry products, and plenty of items labelled unscented, which can mean a masking fragrance rather than none.

Preservatives, and here the picture has changed considerably in recent years. Methylisothiazolinone, used in wash-off hair products, cosmetics and baby wipes, caused a wave of contact allergy and characteristically produces an itchy red face. Formaldehyde and the compounds that release it are the longer-standing problem.

Hair dye, specifically paraphenylenediamine, which can cause marked swelling and blistering of the scalp, face and neck rather than a modest rash.

Rubber accelerators — the chemicals used in making rubber rather than the rubber itself — which affect people who wear gloves for work.

Others worth naming: topical antibiotics such as neomycin; cobalt; chromate, found in cement and leather tanning; acrylates, in gel and acrylic nails and in some adhesives; colophony, the rosin in adhesive plasters and some cosmetics; and plants.

Older lists of “top ten” allergens circulate widely and are worth treating with some caution. They are often drawn from American clinic data, they go out of date as product formulations change, and a substance that was significant twenty years ago may be uncommon now while something that did not exist then is causing a great deal of trouble.

Hands, and work

Hand dermatitis is where most of this lands, and most of it is irritant rather than allergic — driven by wet work and the things that strip the skin's barrier.

The occupations where it is common are predictable: hairdressing, catering and food preparation, cleaning, healthcare, construction, and anything involving frequent hand washing or repeated glove use. It is one of the commonest occupational skin conditions in the UK, and it can be serious enough to end a career if it is left to become chronic.

What helps is unglamorous: cotton liners under gloves, drying hands properly rather than leaving them damp, soap substitutes instead of detergent soaps, and emollient applied steadily rather than only when the skin is already cracked. Catching it early makes the difference between a nuisance and a chronic problem.

One specific warning

Temporary “black henna” tattoos are worth singling out. Genuine henna is reddish-brown and rarely causes trouble. The black versions sold at markets and holiday resorts are darkened with paraphenylenediamine, often at concentrations far above anything permitted in hair dye, applied directly to the skin and left on.

They can cause severe blistering reactions in the shape of the design, and they can sensitise someone to paraphenylenediamine permanently — which means a lifelong problem with permanent hair dye, and sometimes with related substances, from a holiday decoration that lasted a fortnight.

Finding the cause

Much of the work is history rather than testing: what the person does for a living, what they handle, what hobbies they have, which products they use, where on the body the rash appears and what that location suggests. An eyelid rash points somewhere different from a waistband rash.

Where contact allergy is suspected and the dermatitis is recurrent or persistent, patch testing is the definitive test. It is not the same as the skin prick testing used for hay fever and food allergy: patches carrying a standard panel of substances are worn on the back for around 48 hours and the skin is read when they come off and again a couple of days later, because the reaction is delayed. Allergy testing covers how it differs from the other tests, and why a positive result still has to be read against the history.

For nickel specifically, a spot test exists that can show whether a particular object contains it, which is useful when the suspicion is a specific watch or buckle.

Treatment

Avoidance is the only thing that resolves allergic contact dermatitis, which is why identifying the substance matters so much — and that means reading labels, knowing which related substances cross-react, and using suitable gloves where contact is unavoidable.

For the dermatitis itself: emollients are the foundation and are used generously and continuously, not just during a flare. A topical steroid treats active inflammation, and the mild 1% hydrocortisone available over the counter is often not strong enough for established contact dermatitis, particularly on the hands, where thicker skin needs a more potent preparation on prescription.

Antibiotics have a place only where the skin has actually become infected — they are not taken to prevent infection in cracked skin, and taking them that way does no good.

Severe or widespread cases may need a short course of oral steroids, and genuinely chronic hand eczema that resists everything else is managed in dermatology, where there are further options including light treatment and drugs that suppress the immune response.

Living with it

Sensitisation to a contact allergen is usually lifelong. The dermatitis settles once the substance is avoided, but the allergy does not fade, and a small re-exposure years later can bring it back.

That is worth knowing rather than discouraging. A named allergen is a manageable thing — the difficulty is almost always in finding it rather than in avoiding it once found. The general picture of how allergy works, and the fuller range of treatments, are covered elsewhere.

When to see someone

A pharmacist can advise on emollients, soap substitutes and mild topical steroids.

A GP is worth seeing where a rash keeps returning without an obvious cause, where it is not settling on over-the-counter treatment, where it is affecting the hands and the ability to work, where it is severe or blistering, or where there are signs of infection — increasing pain, spreading redness, weeping or crusting, or feeling unwell. Persistent or recurrent cases are the ones that warrant referral for patch testing, and that is worth asking about rather than waiting for.