Fast Five Quiz: Common Rashes and Skin Irritations

William James, MD


November 19, 2021

Contact dermatitis after a single exposure or multiple exposures may be irritant or allergenic — it may be difficult to clinically differentiate between these processes. Irritant contact dermatitis is caused by direct tissue damage after a single exposure or multiple exposures to a known irritant. By contrast, in allergic contact dermatitis, tissue damage by allergic substances is mediated through immunologic mechanisms.

Onset of symptoms occurs within minutes to hours of exposure in simple acute irritant contact dermatitis. Acute delayed irritant contact dermatitis is characteristic of certain irritants, such as benzalkonium chloride (eg, Zephiran, a preservative and disinfectant), which elicits a deferred (8-24 hours after exposure) inflammatory reaction. With allergic contact dermatitis, lesions typically appear 24-72 hours after exposure and peak at around 72-96 hours.

Allergic contact dermatitis is also slower to improve than the irritant form and tends to recur quickly with reexposure. Although no clinical signs and symptoms can differentiate between irritant and allergic contact dermatitis, pruritus is more common in the allergic form.

Patients with a history of atopic dermatitis are at increased risk of developing nonspecific hand dermatitis and irritant contact dermatitis. They are at lower risk for allergic contact dermatitis to poison ivy.

Learn more about the presentation of irritant contact dermatitis.


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