Contact dermatitis

Although they can look alike, the two types have quite different mechanisms:

• Irritant contact dermatitis — the more common of the two, caused by direct chemical or physical damage to the skin's barrier from repeated or prolonged exposure to a substance, such as water, soap, or detergent. It doesn't involve the immune system in the way an allergy does, can develop in anyone given enough exposure, and typically appears in the exact area of contact, often worsening gradually with repeated exposure rather than appearing suddenly.
• Allergic contact dermatitis — a true immune-mediated allergic reaction to a specific substance (allergen), which only develops in people who have become sensitised to that substance, sometimes after years of previously tolerating it without any problem. Once sensitised, even a small amount of the allergen can trigger a reaction, and the rash can sometimes spread slightly beyond the exact area of contact.

Because the two can look very similar, and can also occur together, identifying which type — and what the specific trigger is — usually requires a proper assessment, and sometimes patch testing.

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Certain factors increase the likelihood of developing contact dermatitis:

• Frequent hand-washing or wet work — including healthcare, hospitality, cleaning, and hairdressing occupations
• A history of atopic dermatitis — due to an inherently more vulnerable skin barrier
• Frequent use of cosmetics, skincare, or hair products
• Certain occupations — with regular exposure to chemicals, solvents, or specific allergens
• A history of previous contact dermatitis — a sensitised allergy can persist for life once established

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Yes — there's an important relationship between contact dermatitis and atopy (the tendency toward eczema, asthma, and hay fever). People with atopic dermatitis have a naturally more vulnerable skin barrier, which makes them significantly more prone to developing irritant contact dermatitis, since irritants can penetrate and disrupt an already compromised barrier more easily. This is a common reason eczema on the hands can flare or become harder to control — an additional irritant or allergen exposure is aggravating it on top of the underlying eczema.

The relationship with allergic contact dermatitis is more complex — some research suggests atopic skin may be less prone to certain types of allergic sensitisation, but people with atopic dermatitis are frequently exposed to a wide range of topical products in managing their skin, which increases their chances of developing a genuine allergy to an ingredient over time. Because of this overlap, patch testing is often considered in people with atopic dermatitis whose eczema isn't responding as expected, to check whether a superimposed contact allergy is part of the picture.

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Diagnosis and examination, focusing on the pattern of the rash and what the affected area has been exposed to. Where allergic contact dermatitis is suspected, patch testing is used to confirm the diagnosis and identify the specific allergen.

Treatment generally involves:

• Identifying and avoiding the trigger — the single most important step, particularly for allergic contact dermatitis, where ongoing exposure will keep triggering flares
• Topical corticosteroids — to settle active inflammation
• Emollients — to help repair and protect the skin barrier, particularly important for irritant contact dermatitis and in those with an atopic background
• Practical measures — such as glove use for wet work or known irritants, and switching to fragrance-free products where relevant

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