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Medical Dermatology

Contact Dermatitis & Skin Allergies

A red, itchy rash where something has touched the skin is one of the most common reasons for a dermatology visit. The key questions are whether it is an irritant or an allergic reaction — and, if allergic, exactly what is responsible.

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At a glance

What it is
Inflammation of the skin caused by contact with an external substance — either irritant or allergic.
Who it affects
Anyone. Irritant reactions are especially common with wet work; allergies develop in sensitised individuals.
Common triggers
Detergents and frequent washing; nickel, fragrances, preservatives, hair dye and rubber.
Finding the cause
Patch testing identifies a specific contact allergen — distinct from the skin prick test for immediate allergy.
Treatment
Identifying and avoiding the trigger, repairing the skin barrier, and calming inflammation.
Understanding the condition

Two kinds of contact dermatitis

Contact dermatitis means inflammation of the skin caused by something it has touched. It comes in two forms, which look similar but behave quite differently:

  • Irritant contact dermatitis is direct damage to the skin from a substance — water and frequent hand-washing, soaps, detergents, solvents or friction. Given enough exposure it can affect anyone, and it is the more common of the two.
  • Allergic contact dermatitis is a specific immune reaction that occurs only in people who have become sensitised to a particular allergen. Once sensitised, even brief contact can trigger a flare.

The distinction matters because it shapes both the treatment and, above all, the prevention.

Sensitisation can take years Allergic contact dermatitis can appear to a product used happily for a long time. The immune system builds up sensitivity gradually until a threshold is crossed — which is why a familiar cosmetic, watch strap or ingredient can suddenly start causing a rash.
Common culprits

What tends to cause it

Some substances irritate; others provoke allergy. Many everyday items appear on both lists.

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Common irritants

  • Water and frequent hand-washing
  • Soaps, detergents and cleaning products
  • Solvents and industrial chemicals
  • Friction and repeated wet-and-dry cycles
  • Especially affects the hands in wet work
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Common allergens

  • Nickel — jewellery, watch straps, buckles
  • Fragrances in cosmetics and toiletries
  • Preservatives in skincare products
  • Hair dye (paraphenylenediamine)
  • Rubber chemicals and certain plants
Finding the cause

Patch testing for contact allergy

When an allergic contact dermatitis is suspected, patch testing is the standard way to identify the responsible allergen. Small amounts of a series of common allergens are applied to the back under patches and left in place, and the skin is examined over the following days to see which, if any, produce a reaction.

It is important to understand that patch testing is not the same as the skin prick test. Patch testing detects delayed contact allergy — reactions that build over a day or two — whereas the skin prick test detects immediate allergy such as hay fever and food allergy. The two examine entirely different immune mechanisms and answer different questions.

Why identifying the allergen matters For allergic contact dermatitis, avoidance of the specific trigger is the mainstay of long-term treatment. Pinpointing the exact allergen — rather than guessing — is what makes reliable avoidance possible.
Treatment

How contact dermatitis is treated

Treatment rests on three things: removing the cause, repairing the skin, and calming the inflammation.

  • Avoiding the trigger is the single most important step, and for many people the rash settles once it is removed
  • Restoring the skin barrier with regular emollients, particularly for hand dermatitis, helps the skin recover and protects it from further irritation
  • Anti-inflammatory treatment settles an active flare
  • Skin protection — gloves for wet work, care with known irritants — prevents recurrence where the cause cannot be entirely avoided

Where an allergen has been identified by patch testing, practical advice on how to avoid it in daily life is a central part of management.

Seeking help

When to see a dermatologist

Consider a specialist assessment if:

  • A rash keeps recurring in the same place or pattern
  • It is not settling despite avoiding the obvious culprits
  • It affects the hands, face or eyelids and is interfering with work or daily life
  • You suspect an allergy but cannot work out the cause
  • Hand dermatitis is affecting your ability to work

Patch testing to identify a specific allergen is particularly worthwhile when the diagnosis is unclear or the rash is persistent.

Dr Chen Qiping, Consultant Dermatologist, Chen Dermatology Singapore

Dr Chen Qiping

Consultant Dermatologist · MBBS, MRCP (UK), M.Sc, FAMS (Dermatology)

Dr Chen practises general adult and paediatric dermatology, including contact dermatitis and allergic skin disease, alongside surgical, laser and cosmetic dermatology. Consultations are available in English and Mandarin.

Full profile →
Common questions

Contact dermatitis — frequently asked questions

What is the difference between irritant and allergic contact dermatitis?
Both produce a red, itchy rash where something has touched the skin, but the mechanism differs. Irritant contact dermatitis is direct damage to the skin from a substance — frequent hand-washing, detergents, solvents or friction — and can affect anyone given enough exposure. Allergic contact dermatitis is a specific immune reaction that develops only in people who have become sensitised to a particular allergen, such as nickel, fragrance or a preservative, and can flare from even brief contact. Distinguishing them guides both treatment and prevention.
How do I find out what I'm allergic to?
Suspected allergic contact dermatitis is investigated with patch testing, which is different from the skin prick test used for immediate allergies. In patch testing, small amounts of common allergens are applied to the back under patches and left in place, with the skin examined over several days to see which, if any, produce a reaction. It is the standard way to identify the cause of a contact allergy so that it can be avoided.
Can I suddenly become allergic to something I've used for years?
Yes. Allergic contact dermatitis develops once the immune system has become sensitised, and this can happen after months or years of uneventful use. This is why people sometimes react to a long-familiar product, a piece of jewellery, or an ingredient that has only recently been added to something they use. It does not mean the previous exposures were harmless — sensitisation simply builds up until a threshold is crossed.
What are the most common causes of contact dermatitis?
Common irritants include water and frequent hand-washing, soaps and detergents, solvents and friction. Common allergens include nickel (in jewellery, watch straps and belt buckles), fragrances, preservatives in cosmetics and toiletries, hair dye, rubber chemicals and certain plants. Hand dermatitis in people whose work involves wet conditions is particularly common. Identifying the specific cause is the key to lasting improvement.
Will it go away if I just avoid the cause?
Avoiding the responsible substance is the single most important step, and for many people the rash settles once the trigger is removed and the skin barrier recovers. However, the skin often needs help to heal in the meantime — moisturisers to restore the barrier and anti-inflammatory treatment to calm the reaction. Where the cause cannot be fully avoided, ongoing skin protection and treatment become important.
When should I see a dermatologist about a contact rash?
Consider assessment if a rash keeps recurring in the same place or pattern, if it is not settling despite avoiding obvious culprits, if it affects the hands, face or eyelids and is interfering with work or daily life, or if you suspect an allergy but cannot identify the cause. Patch testing to pinpoint an allergen is worthwhile when the diagnosis is unclear or the rash is persistent.

References

  1. DermNet. Contact dermatitis, Allergic contact dermatitis and Patch testing. dermnetnz.org
  2. British Association of Dermatologists. Contact dermatitis patient information leaflet. bad.org.uk
  3. American Academy of Dermatology. Contact dermatitis. aad.org

Page last reviewed: 28 July 2026.

Important — This page is general information about a skin condition and is not medical advice. It cannot take the place of an individual consultation, examination and diagnosis. Treatment options, their suitability and their possible side effects should be discussed with a qualified doctor who has assessed your skin. Individual results vary. If you have concerns about your skin, please arrange a consultation.

Have a rash you think might be a skin allergy?

Arrange a consultation with Dr Chen Qiping at our Collyer Quay clinic — two minutes' walk from Raffles Place MRT. Consultations available in English and Mandarin.

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