Understanding Allergic Contact Dermatitis

Fragrance is one of the most common causes of ACD worldwide, affecting up to 16% of eczema patients. A single perfume or scented product can contain 10 to 1,000 different fragrance chemicals. At least 80 substances have been identified as fragrance allergens. The most sensitizing exposures come from products left on the skin — deodorants, lotions, and fine fragrances — rather than rinse-off products like shampoo.

Where fragrance allergy typically shows up: armpits, face, eyelids, hands, neck, lips, and genital area. It can also trigger or worsen rosacea.

Hidden sources you may not expect: toys, diapers, wet wipes, pet shampoos, wound healing creams, antifungal preparations, hemorrhoid preparations, and even topical corticosteroids.

Isothiazolinones are preservatives used in cosmetics, paints, baby products, and household cleaners. MI allergy prevalence has been rising sharply in North America — it is now the second most common positive patch test result in the US (13.8% of patch-tested patients). Europe saw the same epidemic but reduced rates after banning MI in leave-on cosmetics in 2013. No equivalent ban exists in the US yet.

Key sources: lotions, shampoos, conditioners, wipes, liquid soaps, wall paints, polishes, and water-based household products. MI can also trigger airborne reactions from freshly painted walls — some patients need to repaint with MI-free paint.

Formaldehyde is a preservative found in cosmetics, skin care, and some topical medications. Many products don't contain formaldehyde directly — instead they use formaldehyde releasers that slowly release small amounts over time. Common releasers include quaternium-15, DMDM hydantoin, imidazolidinyl urea, and diazolidinyl urea.

Key sources: moisturizers, shampoos, liquid soaps, nail hardeners, some vaccines (in trace amounts), and industrial products like glues and resins.

If you test positive to formaldehyde, your allergist will help you identify which products are safe based on how much formaldehyde the releaser actually generates — there is a spectrum, and not all releasers behave the same way.

Nickel affects approximately 14–18% of the general population — the single most frequent cause of ACD worldwide. Sensitization often starts with ear piercing. Rashes typically appear where metal touches skin: earlobes, wrists, belly button, belt buckle line, and fingers. Widespread rash and chronic hand eczema can also result from systemic nickel ingested through food.

Surprising sources: stainless steel (which contains 8–35% nickel), coins, keys, zippers, jeans buttons, eyeglass frames, cell phones, and even some cardiovascular stents and orthopedic implants.

Gold: A controversial allergen — positive patch tests are common (up to 20%) but true clinical relevance is low. Most important in eyelid dermatitis, facial rash, and suspected dental metal allergy. Reactions can be delayed up to 3 weeks after patch test application.

Rubber gloves — including latex gloves — contain chemicals called vulcanization accelerators (thiurams and carbamates) added during manufacturing. These are common occupational allergens for healthcare workers, food handlers, and others who wear gloves regularly. Rash typically appears on the hands and wrists, inside the glove contact area.

What to use instead: If you react to rubber accelerators, ask for accelerator-free nitrile gloves or polyvinyl chloride (PVC) gloves. Standard latex gloves always contain accelerators. Dithiocarbamates (increasingly used as alternatives in some gloves) may also cross-react — discuss with your allergist.

2-Hydroxyethyl methacrylate (HEMA) is the most important acrylate allergen and is now found in gel manicures, acrylic artificial nails, and long-lasting nail lacquers available at most salons. This is a rapidly growing allergen — hairdressers and nail technicians have a ninefold increased risk of sensitization compared to the general public. In Europe, HEMA in nail products is restricted to professional use only.

Also watch for acrylates in: dental fillings and bonding agents, adhesives for glucose sensors (like Dexcom), insulin pump adhesives, eyelash extension glues, and industrial adhesives.

Important: Once sensitized to acrylates, future dental procedures or medical adhesive devices may trigger reactions. Tell your dentist and any treating physicians about this allergy.

Neomycin and bacitracin — both found in popular OTC triple antibiotic ointments (like Neosporin) — are among the most common topical allergens in the United States, largely because these products are so widely used. Allergy rates in the US are significantly higher than in Europe, where neomycin requires a prescription.

Neomycin: 6.3% of patch-tested patients in the US are positive. Also present in trace amounts in some vaccines (varicella, measles, rabies, influenza) — vaccination is generally safe even with neomycin allergy. Cross-reacts with related antibiotics including gentamicin, tobramycin, and streptomycin.

Bacitracin: 5.1–5.5% of patch-tested patients positive. Can also cause anaphylaxis in rare cases — not just a skin reaction.

It may seem counterintuitive, but topical steroids — used to treat rashes — can themselves cause contact allergy in 0.5–5% of patients who use them. Suspect this if your rash fails to respond to treatment, worsens with application, or quickly returns after stopping. People with chronic hand eczema, stasis dermatitis, or perianal or genital dermatitis are at higher risk.

Corticosteroids are grouped (A through D2) based on cross-reactivity patterns. Reacting to one does not mean you react to all — your allergist can identify safe alternatives within other groups using targeted patch testing.

Systemic reactions: In rare cases, taking oral or injected steroids can trigger widespread rash (sometimes called "baboon syndrome") in people sensitized to topical steroids.

Patch testing is not the same as the skin prick test used for environmental or food allergies. In patch testing, small amounts of common allergens are applied to your upper back on adhesive panels and left in place for 48 hours. Readings are taken at 48 hours and again at 96 hours — or 7–10 days later for metals and corticosteroids, which may react late.

Before your test: Avoid applying topical steroids to your back for at least 1–2 weeks beforehand, as they suppress reactions. Avoid sun exposure on the test area. Do not get your back wet while patches are applied.

What a positive result means: A positive patch test confirms allergy to that substance. Your allergist will explain which products and exposures to avoid and provide resources for reading ingredient labels. Not all positive reactions are clinically relevant — your doctor will help you sort out which ones explain your current symptoms.