Contact Dermatitis: Causes, Testing and Treatment

Contact dermatitis is inflammation of the skin caused by something touching it. That sounds simple enough to diagnose by observation, and it usually is not, because the reaction people most want to identify appears one to three days after the contact that caused it.

By then they have used a dozen other products, changed washing powder, and settled on the wrong culprit. Understanding that delay is the single most useful thing in this subject, and it is what separates the two forms of the condition.

⚖️ Two different mechanisms

The permanence of allergic sensitisation matters. Once someone has become allergic to nickel or to a fragrance chemical, that allergy is lifelong. It does not get outgrown and it does not resolve with avoidance — avoidance controls the symptoms while the sensitivity remains.

The two also overlap constantly. Damaged skin absorbs allergens more readily, so irritant dermatitis of the hands frequently leads to allergic sensitisation on top of it, producing a mixed picture that is harder to unpick.

🗂️ What causes it

A specific development worth knowing about is the rise in allergy to methylisothiazolinone, a preservative introduced widely into cosmetics, wet wipes and household products in the 2000s. It produced a documented epidemic of contact allergy, particularly facial dermatitis from wipes and shampoos, before regulators restricted its concentration. Anyone with unexplained facial or hand dermatitis should look at wet wipes and liquid soaps specifically.

👋 Hands, and work

Hand dermatitis is the commonest presentation and among the most disabling, because hands cannot be rested and are washed constantly.

Occupations with high rates are consistent across studies: hairdressers, exposed to dyes, bleaches, shampoos and constant wet work; healthcare workers, from hand hygiene and gloves; catering and cleaning staff, from wet work and detergents; construction workers, from cement, which is both alkaline and a source of chromate allergy; mechanics, from oils and solvents; and florists and gardeners, from plants.

The mechanism in most is the same: repeated wetting and drying strips the skin barrier, cracks develop, irritants penetrate more easily, and allergens gain access through damaged skin. What starts as irritant dermatitis becomes mixed.

Two practical points frequently go unmentioned. Occupational skin disease is reportable and often compensable in many countries, and employers have obligations regarding protective equipment and substitution of hazardous substances. And gloves are not straightforward — occlusive gloves worn for long periods trap sweat and worsen irritant dermatitis, while rubber accelerators in them cause allergy in their own right. Cotton liners under gloves, taking gloves off periodically, and choosing accelerator-free options where allergy is suspected all help.

🧪 Patch testing

This is the test that identifies allergic contact dermatitis, and it is different from the skin prick testing used for immediate allergies.

How it works. Small amounts of standardised allergens are applied to the back under adhesive chambers and left in place. It takes three visits across roughly a week: application, removal and first reading at forty-eight hours, and a second reading at ninety-six hours or later. That final reading matters, because some reactions only appear then and reading too early misses them.

What to expect. The back must stay dry for the whole period, so no showering or swimming. Sunbathing beforehand and topical steroids on the back must be avoided, since both suppress reactions. Oral steroids and immunosuppressants also interfere and usually need discussing beforehand.

What it can and cannot do. A positive result shows sensitisation to that substance. Whether it explains the current rash requires matching it to actual exposure, which is why a good history matters as much as the result. A negative test does not exclude allergy to something not on the panel — testing with the person own products is sometimes added for this reason.

Patch testing is worth requesting where dermatitis is persistent, recurs without explanation, affects the hands or face, is occupational, or has not responded to treatment. It is not needed for a straightforward irritant reaction with an obvious cause.

💊 Treating it properly

Removing the cause is the only thing that resolves it rather than suppressing it. Everything below manages the inflammation while that happens.

Topical corticosteroids are the mainstay for active dermatitis, and how they are used determines both whether they work and whether they cause problems.

Three practical points determine success.

Use enough. The standard measure is the fingertip unit — the amount squeezed from the tube onto the last segment of an adult index finger, which covers an area about the size of two flat adult palms. Under-application is extremely common and is the main reason a correctly chosen steroid appears not to work.

Use it properly for long enough, then stop. Typically once or twice daily until the skin is clear, then stopping rather than tapering into indefinite intermittent use.

Do not stop too early out of fear. Anxiety about steroid creams is widespread and leads to under-treatment, which prolongs the dermatitis and paradoxically results in more total steroid use over time. The genuine risks — skin thinning, stretch marks, absorption — come from potent steroids used continuously for long periods on thin skin, not from a properly conducted course.

Where the skin is broken, weeping or secondarily infected, a combination of a corticosteroid with an antibacterial is used, such as betamethasone with gentamicin. Scratched hand and flexural dermatitis becomes infected readily, and treating inflammation alone in that situation does not clear it. These combinations are for short courses — prolonged use of a topical antibiotic drives resistance and raises the risk of becoming sensitised to the antibiotic itself. For uncomplicated dermatitis with intact skin, a plain corticosteroid is preferred.

Emollients are the least glamorous and most consistently useful part of treatment. Applied generously and frequently they restore the barrier, reduce irritant penetration, relieve itch and reduce the amount of steroid needed. Ointments are more effective than creams and creams more than lotions, at the cost of feeling greasier. Fragrance-free is essential here for obvious reasons.

🤷 Why antihistamines do little

This is worth stating clearly because antihistamines are routinely recommended for contact dermatitis and are of limited value in it.

The itch in urticaria is driven by histamine, which is why antihistamines work well there. The itch in contact dermatitis is driven by a delayed cell-mediated immune reaction with other mediators involved, and blocking histamine addresses little of it.

Where a sedating antihistamine helps at all it is by improving sleep in someone kept awake by scratching, which is a real benefit and a different mechanism from treating the rash. Non-sedating antihistamines add very little in this condition, which is the reverse of their role in urticaria and angioedema.

Topical antihistamine creams should be avoided entirely. They are poorly effective and are themselves a recognised cause of contact allergy, which is a particularly unhelpful combination in someone already reacting to something applied to their skin.

🧴 When it becomes infected

Broken, scratched dermatitis is readily colonised and infected, most often by staphylococci.

Signs are increasing pain rather than itch, spreading redness with warmth, yellow crusting, weeping or pus, and sometimes fever. Small clustered blisters with punched-out erosions suggest herpes simplex infection of the dermatitis, which is a different and more urgent problem requiring antiviral treatment.

Localised infection is treated with a topical antibacterial or a steroid-antibacterial combination; more extensive infection needs oral antibiotics. Reducing scratching, keeping nails short and using emollients generously all reduce the risk in the first place.

🛡️ Preventing recurrence

Identify and avoid is the whole of the cure. That means reading ingredient lists, which is tedious and effective. Patch test clinics usually provide a written list of what to avoid and the many names a single allergen appears under.

Reduce wet work where possible — less frequent washing, lukewarm rather than hot water, thorough drying especially between fingers, and using an emollient after every wash.

Use gloves thoughtfully. Wear them for wet work and chemicals, keep the duration short, use cotton liners, and avoid latex if there is any suspicion of latex allergy, which is a separate and potentially more serious condition.

Barrier creams have modest evidence and are a supplement to gloves rather than a replacement for them.

Simplify the routine. Fewer products means fewer possible culprits and easier identification if something does react. Fragrance-free everything is a reasonable default for anyone with a history of contact dermatitis.

Address it early at work. Occupational hand dermatitis becomes chronic and job-limiting if it continues, and early substitution of the offending substance, better protection or a change of task prevents that far more effectively than treating repeated flares.

❌ Common misconceptions

A reaction to something used for years does not exclude allergy. Sensitisation requires prior exposure by definition, so allergy typically develops to things already in regular use. The familiar phrase — I have used this for years, it cannot be that — is exactly backwards.

Natural products are not safer. Plant extracts and essential oils are among the more frequent sensitisers found on patch testing.

Contact dermatitis is not contagious and cannot be passed to anyone else.

The rash is not always where the contact was. Allergens are transferred by fingers to the face, eyelids and neck, and eyelid dermatitis from nail varnish is a classic example that confuses people entirely.

Steroid creams are not something to fear at appropriate strength and duration. Under-treatment prolongs the condition and results in more total steroid over time.

🧑‍⚕️ When to see someone

• The rash is not settling after a week or two of appropriate treatment and avoidance
• It keeps recurring without an identifiable cause — this is the main indication for patch testing
• It affects the hands or face, where the impact and the diagnostic difficulty are both higher
• It appears related to work, since early intervention prevents a chronic occupational problem
• Signs of infection — increasing pain, spreading redness, crusting, pus or fever
• Clustered blisters with punched-out erosions, suggesting herpes infection of the dermatitis
• It worsens on treatment, which suggests allergy to the treatment itself
• It is widespread or severe

Topical treatment for dermatitis with broken or infected skin is stocked in the skin care category, including Diprogenta (betamethasone with gentamicin). Combination products are for short courses where infection is present or likely; uncomplicated dermatitis with intact skin is better treated with a plain corticosteroid chosen for the site.

❓ Frequently asked questions

📑 Sources and editorial

• British Association of Dermatologists and European Society of Contact Dermatitis guidance on the diagnosis and management of contact dermatitis
• European baseline series for patch testing and its periodic revisions
• Literature on the methylisothiazolinone contact allergy epidemic and subsequent regulatory restriction
• Evidence on occupational skin disease incidence by industry and on glove-related dermatitis
• Fingertip unit methodology for topical corticosteroid quantification
• Reports of contact allergy to topical corticosteroids and to topical antibiotics
• Related reading: allergies explained
• Related products: skin care category
• RXshop Editorial Team — reviewed by Emily Chen, MD, MPH, Internal Medicine