Contact dermatitis guide: irritant vs. allergic, patch testing, and treatment

Contact dermatitis is inflammation of the skin caused by direct contact with a substance — either through direct chemical damage (irritant) or an immune-mediated allergic reaction (allergic). It is extremely common — accounting for 90–95% of all occupational skin disease — and is frequently misdiagnosed or mistaken for eczema, seborrheic dermatitis, or psoriasis. The distinction between types is critical because they have different mechanisms, implications, and management approaches.

Irritant contact dermatitis (ICD) vs. allergic contact dermatitis (ACD)

| | Irritant CD (ICD) | Allergic CD (ACD) |
|---|---|---|
| Mechanism | Direct chemical damage to skin cells | Type IV (delayed) hypersensitivity immune reaction |
| Immune system required | No | Yes (T-cell sensitization) |
| Who gets it | Anyone (with sufficient exposure) | Only sensitized individuals |
| First exposure reaction | Yes (dose-dependent) | No (requires prior sensitization) |
| Onset | Minutes to hours after exposure | 12–72 hours after re-exposure |
| Distribution | Exactly at contact site | Contact site; may spread beyond |
| Dose-dependence | Yes — higher dose = worse reaction | No — even trace amounts trigger reaction in sensitized |
| Management | Reduce/eliminate exposure; barrier repair | Identify and eliminate the specific allergen |

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Irritant contact dermatitis

Mechanism

ICD occurs when a substance damages keratinocytes directly — bypassing immune sensitization. The damage releases cytokines (IL-1α, TNF-α) that drive inflammation without T-cell involvement. Common mechanisms:
Alkali damage: Detergents, soaps, cement — saponify lipids and denature proteins
Acid damage: Acids disrupt the acid mantle and barrier integrity
Oxidative damage: Bleach, hydrogen peroxide
Desiccation: Repeated hand washing removes barrier lipids → dry, fissured hands
Mechanical friction: Repetitive physical contact without chemical insult

Common ICD exposures

Wet work (healthcare, foodservice, hairdressing): Repeated wetting + drying + soaps → chronic hand dermatitis — the most prevalent occupational skin disease globally
Diapers (infants): Urine + feces + moisture → perianal ICD (diaper rash)
Saliva (lip-licking dermatitis): Amylase and lipase in saliva disrupt perioral skin
Cleaning products: Quaternary ammonium compounds, bleach, surfactants

Presentation

Erythema, scaling, dryness, cracking at contact sites
Stinging and burning (more common than itch in ICD)
Chronic ICD: lichenification (skin thickening), fissures, hyperkeratosis
No vesicles in chronic ICD (contrast with ACD)

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Allergic contact dermatitis

Mechanism (Type IV delayed hypersensitivity)

Sensitization phase (first exposure):
Hapten (small allergenic molecule) penetrates skin → binds to protein to form hapten-protein complex → uptake by Langerhans cells (antigen-presenting cells) → migration to lymph nodes → antigen presentation to naïve T cells → clonal expansion of hapten-specific T cells → systemic memory T cells circulate

Elicitation phase (subsequent exposures):
Same hapten contacts sensitized skin → memory T cells recognize hapten → local T-cell activation → IL-2, IFN-γ release → mast cell degranulation + keratinocyte damage → clinical dermatitis 12–72 hours after contact

Key implication: First exposure never causes allergic reaction. A reaction suggests prior sensitization — which may have occurred months or years earlier from the same or cross-reacting substance.

The most common allergens

Fragrances — #1 contact allergen in cosmetics:
Fragrance mix I (8 components including eugenol, isoeugenol, geraniol, citral)
Fragrance mix II (6 additional components)
Balsam of Peru — cross-reacts with many fragrances and flavors
"Fragrance-free" vs. "unscented": "Unscented" may contain masking fragrances; "fragrance-free" is the safer label

Metals:
Nickel — the most common metal allergen; jewelry, belt buckles, phone cases, jean buttons; higher prevalence in women (ear piercing)
Cobalt — often co-sensitizes with nickel; jewelry, dyes, hard metals
Chromate — cement (construction workers), leather goods, chrome-plated objects

Preservatives:
Methylisothiazolinone (MI) / methylchloroisothiazolinone (MCI/MI blend): Rinse-off products, baby wipes, industrial products; MI rate of sensitization rose sharply 2010–2016 before EU restriction
Formaldehyde releasers: DMDM hydantoin, quaternium-15, diazolidinyl urea — found in shampoos, conditioners, lotions
Parabens: Less sensitizing than commonly feared; rarely primary contact allergens

Rubber chemicals:
Thiuram mix, carba mix — latex gloves, elastic in clothing; important in healthcare workers
Accelerators in non-latex gloves also sensitize

Plants:
Toxicodendron species (poison ivy/oak/sumac): Urushiol — most potent contact allergen in North America; 85% of population sensitizes on sufficient exposure
Compositae/Asteraceae family: Chrysanthemum, chamomile, arnica — cross-reactive sesquiterpene lactones; increasingly recognized in cosmetic contact allergy

Hair dye:
p-Phenylenediamine (PPD): Oxidative hair dye; cross-reacts with benzocaine, sulfonamides, PABA sunscreens; very potent sensitizer

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Patch testing: how contact allergens are identified

Patch testing is the definitive diagnostic test for ACD. It is the only reliable way to identify which specific allergens are causing a reaction.

How patch testing works:
Standardized panels of potential allergens (typically 36–80 agents in the TRUE Test or extended series) are applied to the back on adhesive chambers
Patches remain on the back for 48 hours (patient avoids water)
Reading at 48 hours (removal) and 72–96 hours (delayed readings)
A positive reaction = ≥2mm erythema + papules/vesicles at a specific allergen's chamber

Interpreting results:
Positive test confirms sensitization — but the clinician must determine if that allergen is clinically relevant (contact with that allergen in the patient's daily life explains the dermatitis pattern)
A positive patch test result requires cross-referencing the allergen with the patient's product exposures, occupation, and hobbies

Who performs patch testing: Dermatologists with patch testing expertise; allergists. Primary care typically refers for this investigation.

Limitations: False negatives can occur (sensitization present but test negative); active dermatitis can increase background reactivity; some allergens are not in standard panels.

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Treatment

Acute ACD (e.g., poison ivy)

Topical corticosteroids (medium-to-high potency for limited area; short course 7–14 days)
Oral corticosteroids (prednisone 1 mg/kg/day tapered over 2–3 weeks) for widespread eruption or involvement of face/genitalia — standard treatment for extensive poison ivy
Oral antihistamines for itch control (sedating antihistamines at night for sleep; non-sedating for daytime)
Cool compresses for acute vesicular phase

Chronic ICD (e.g., hand dermatitis)

Barrier restoration: Frequent emollient application; avoid wet work where possible; use gloves (cotton liner + nitrile outer for wet work); switch to gentle fragrance-free soaps
Topical steroids for active flares
Barrier cream: Used before exposure to reduce future irritant contact; petroleum jelly or dimethicone-based products
Tacrolimus/pimecrolimus for maintenance in facial or genital ICD where steroid atrophy is a concern

Long-term management

The single most important step for ACD: identify and eliminate the causative allergen. This requires patch testing, review of all products, and substitution with allergen-free alternatives.

For nickel allergy: avoid nickel-containing items; use 1% dimethylglyoxime test strips to identify high-nickel items; clear nail polish over nickel surfaces as temporary barrier.

For fragrance allergy: switch all personal care products to genuinely fragrance-free formulations; check ingredient lists for "parfum," "fragrance," "aroma," and common fragrance chemicals by name.

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When to refer

Uncertain diagnosis (ACD vs. ICD vs. psoriasis vs. other)
Recalcitrant or widespread contact dermatitis
Occupational dermatitis affecting work capacity
Suspected fragrance, preservative, or occupational allergen (requires patch testing)
Positive patch test result needing clinical relevance interpretation

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