Merck Manual

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Contact Dermatitis


Mercedes E. Gonzalez

, MD,

  • University of Miami Miller School of Medicine
  • Florida International University Herbert Wertheim College of Medicine
  • Pediatric Dermatology of Miami

Last full review/revision Aug 2019| Content last modified Aug 2019
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Contact dermatitis is acute inflammation of the skin caused by irritants or allergens. The primary symptom is pruritus. Skin changes range from erythema to blistering and ulceration, often on or near the hands but occurring on any exposed skin surface. Diagnosis is by exposure history, examination, and sometimes skin patch testing. Treatment entails antipruritics, topical corticosteroids, and avoidance of causes.


Contact dermatitis is caused by irritants or allergens.

Irritant contact dermatitis (ICD)

ICD accounts for 80% of all cases of contact dermatitis. It is a nonspecific inflammatory reaction to substances contacting the skin; the immune system is not activated. Numerous substances are involved, including

  • Chemicals (eg, acids, alkalis, solvents, metal salts)

  • Soaps (eg, abrasives, detergents)

  • Plants (eg, poinsettias, peppers)

  • Body fluids (eg, urine, saliva)

Properties of the irritant (eg, extreme pH, solubility in the lipid film on skin), environment (eg, low humidity, high temperature, high friction), and patient (eg, very young or old) influence the likelihood of developing ICD. ICD is more common among patients with atopic disorders, in whom ICD also may initiate immunologic sensitization and hence allergic contact dermatitis.

Phototoxic dermatitis (see Chemical photosensitivity) is a variant in which topical (eg, perfumes, coal tar) or ingested (eg, psoralens) agents generate damaging free radicals and inflammatory mediators only after absorption of ultraviolet light.

Allergic contact dermatitis (ACD)

ACD is a type IV cell-mediated hypersensitivity reaction that has 2 phases:

  • Sensitization to an antigen

  • Allergic response after reexposure

In the sensitization phase, allergens are captured by Langerhans cells (dendritic epidermal cells), which migrate to regional lymph nodes where they process and present the antigen to T cells. The process may be brief (6 to 10 days for strong sensitizers such as poison ivy) or prolonged (years for weak sensitizers such as sunscreens, fragrances, and glucocorticoids). Sensitized T cells then migrate back to the epidermis and activate on any reexposure to the allergen, releasing cytokines, recruiting inflammatory cells, and leading to the characteristic symptoms and signs of ACD.

In autoeczematization (id reaction), epidermal T cells activated by an allergen migrate locally or through the circulation to cause dermatitis at sites remote from the initial trigger. However, contact with fluid from vesicles or blisters cannot trigger a reaction elsewhere on the patient or on another person.

Multiple allergens cause ACD (see Table: Causes of Allergic Contact Dermatitis), and cross-sensitization among agents is common (eg, between benzocaine and paraphenylenediamine). Cross-sensitization means that exposure to one substance can result in an allergic response after exposure to a different but related substance. The Toxicodendron species of plant (eg, poison ivy, poison oak, poison sumac) account for a large percentage of ACD, including moderate and severe cases. The offending allergen is urushiol.


Causes of Allergic Contact Dermatitis



Airborne substances

Ragweed pollen, insecticide spray

Chemicals used in shoe or clothing manufacturing

Particularly agents used in leather and rubber processing, tanning agents in shoes, rubber accelerators and antioxidants in apparel (eg, gloves, shoes, underpants), formaldehyde in durable-press finishes


Depilatories, nail polish, deodorant


Paraphenylenediamines (hair and textile dyes)


Various compounds

Ubiquitous in toiletries, soaps, and scented household products

Industrial agents

Many compounds, including acrylic monomers, epoxy compounds, vat dyes, rubber accelerators, and formaldehyde (in plastics and adhesives)

Ingredients in topical drugs

Antibiotics (eg, bacitracin, neomycin)

Antihistamines (eg, diphenhydramine)

Anesthetics (eg, benzocaine)

Antiseptics (eg, thimerosal, hexachlorophene)

Stabilizers (eg, ethylenediamine and derivatives)


Latex gloves, condoms, catheters, balloons

Metal compounds





Numerous occupational exposures

Personal items (eg, belt buckles, watch buckles, jewelry)


Poison ivy, oak, and sumac; ragweed; primrose; cashew shells; mango peel

ACD variants include photoallergic contact dermatitis and systemically induced ACD. In photoallergic contact dermatitis, a substance becomes sensitizing only after it undergoes structural change triggered by ultraviolet light. Typical causes include aftershave lotions, sunscreens, and topical sulfonamides. Reactions may extend to non–sun-exposed skin. In systemically induced ACD, ingestion of an allergen after topical sensitization causes diffuse dermatitis (eg, oral diphenhydramine after sensitization with topical diphenhydramine).

Symptoms and Signs

Irritant contact dermatitis

ICD is more painful than pruritic. Signs range from mild erythema to hemorrhage, crusting, erosion, pustules, bullae, and edema.

Allergic contact dermatitis

In ACD, the primary symptom is intense pruritus; pain is usually the result of excoriation or infection. Skin changes range from transient erythema through vesiculation to severe swelling with bullae, ulceration, or both. Changes often occur in a pattern, distribution, or combination that suggests a specific exposure, such as linear streaking on an arm or leg (eg, due to brushing against poison ivy) or circumferential erythema (under a wristwatch or waistband). Linear streaks are almost always indicative of an external allergen or irritant.

Any surface may be involved, but hands are the most common surface due to handling and touching potential allergens. With airborne exposure (eg, perfume aerosols), areas not covered by clothing are predominantly affected. The dermatitis is typically limited to the site of contact but may later spread due to scratching and autoeczematization (id reaction). In systemically induced ACD, skin changes may be distributed over the entire body. The eruption usually begins within 24 to 48 hours after exposure to the allergen.

Pearls & Pitfalls

  • Lesion shape or pattern (linear streaks are almost always indicative of an external allergen or irritant) can help differentiate contact dermatitis from other forms of dermatitis.


  • Clinical evaluation

  • Sometimes patch testing

Contact dermatitis can often be diagnosed by skin changes and exposure history. The patient’s occupation, hobbies, household duties, vacations, clothing, topical drug use, cosmetics, and spouse’s activities must be considered. The “use” test, in which a suspected agent is applied far from the original area of dermatitis, usually on the flexor forearm, is useful when perfumes, shampoos, or other home agents are suspected.

Patch testing is indicated when ACD is suspected and does not respond to treatment, suggesting that the trigger has not been identified. In patch testing, standard contact allergens are applied to the upper back using adhesive-mounted patches containing minute amounts of allergen or plastic (Finn®) chambers containing allergen held in place with porous tape. Thin-layer rapid use epicutaneous (TRUE TEST®) patch testing is a simple, easy-to-use kit with the most common contact allergens that can be applied and interpreted by any health care practitioner. Skin under the patches is evaluated 48 and 96 hours after application. False-positive results occur when concentrations provoke an irritant rather than an allergic reaction, when reaction to one antigen triggers a nonspecific reaction to others, or with cross-reacting antigens. False-negative results occur when patch allergens do not include the offending antigen. Definitive diagnosis requires a positive test result and a history of dermatits in the area where the tested agent contacted the skin.


Common Allergens Used in Patch Testing




In topical antibiotic preparations

Balsam of Peru (myroxylon)

A flavoring agent for drinks and tobacco, as well as a fixative and fragrance in perfumes; also occurs in many topical drugs, dental agents, and other products

Chief allergens: Esters of cinnamic and benzoic acid, vanillin

Cross-reactions with colophony (rosin) and balsam of Tolu, cinnamates, benzoates, styrax, and tincture of benzoin

Probably also some phototoxicity

Black rubber mix

In rubber

May cross-react with hair dyes


A preservative found in cosmetics, shampoos, and skin care products; also in some detergents and cleaning agents


Screens for class B corticosteroid allergy. A corticosteroid drug, used to treat various diseases; can be found in creams and ointments used to treat eczema; in eye drops or inhaled medications

Caine mix

Contains 3 topical anesthetics: Benzocaine, dibucaine hydrochloride, and tetracaine hydrochloride

Often used in dentistry but also widely found and used in topical preparations to reduce itching, pain, and stinging and widely used in hemorrhoidal preparations and cough syrups

Carba mix

Used as an accelerator in rubber, rubber glues, vinyl, and some pesticides

Cl+ Me– isothiazolinone and methylisothiazolinone

Occur in cosmetics and skin care products, some drugs, household cleaning products, and certain industrial fluids and greases

Cobalt dichloride

Occurs in some paints, cement, metal, and metal-plated objects

Coactivity with nickel (which is not cross-sensitivity)

Colophony (rosin)

Used by string players (violinists are especially prone to rosin allergy), baseball players, and bowlers

Derived from several conifer species

Occurs in cosmetics, adhesives, lacquers, varnishes, soldering fluxes, paper, and many other industrial products

Diazolidinyl urea

A preservative with broad-spectrum application found in cosmetics, shampoos, skin care products, and cleaning agents

Disperse blue 106

A dark blue textile dye found in fabrics colored dark blue, brown, black, purple, and some greens

Epoxy resin

A low molecular weight (340) epoxy based on bisphenol A and epichlorohydrin

Is a sensitizer only when uncured or incompletely cured


Used as an emulsifier and stabilizer in certain topical drugs, eye drops, some industrial solvents, curing agents for certain plastics, and anticorrosion agents

Formaldehyde and formaldehyde releasers

Released by quaternium-15, a germicidal agent, and occasionally by imidazolidinyl urea

Used widely in formulation of plastics, resins for clothing, glues, and adhesives

Fragrance mixes

Can contain alpha amyl cinnamic alcohol, cinnamic aldehyde, cinnamic alcohol, oak moss absolute, hydroxycitronellal, eugenol, isoeugenol, geraniol, citral, citronellol, coumarin, farnesol, hexyl cinnamal, hydroxyisohexyl-3-cyclohexene, and carboxaldehyde

Occurs in many toiletries, soaps, aftershave lotions, shampoos, and scented household products and in many industrial products (eg, cutting fluids)

Gold sodium thiosulfate

Found in gold or gold-plated jewelry or dental restorations

Hydrocortisone 17-butyrate

A corticosteroid found in creams or lotions used to treat inflammatory skin diseases; also present in some ear and eye drops

Imidazolidinyl urea

A preservative with broad-spectrum application found in cosmetics, shampoos, skin care products, and cleaning agents


Occurs in rubber, adhesives, and coolants

Mercapto mix

Occurs in rubber, glues, coolants, and other industrial products


Found in paints, adhesives, and oils

Neomycin sulfate

Found in topical antibiotics, first-aid creams, ear drops, and nose drops; possible delay (about 4‒5 days) in patch test reaction (so reading should be done at 7 days when possible)

Nickel sulfate

Occurs in jewelry, dentures, scissors, razors, eyeglass frames, silverware, and foods (eg, canned foods, foods cooked in nickel utensils, herring, oyster, asparagus, beans, mushrooms, onions)

Paraben mix

Five parabens: Methyl, ethyl, propyl, butyl, and benzyl parahydroxybenzoates, which are the most common preservatives used worldwide and occur in numerous creams and cosmetics and in some industrial oils, fats, and glues


A sesquiterpene lactone naturally present in the traditional medicine herb, feverfew (Tanacetum parthenium); found in natural medicine and cosmetics

Cross-reacts with other genera in Compositae and Magnoliaceae

Potassium dichromate

Occurs in cement (in minute amounts), in tanning solutions for leather, and in safety matches

Used in photography, electroplating solutions, many anticorrosives, paints, glues, pigments, and some detergents

p-Phenylenediamine (PPD)

Occurs in hair dyes, some inks, photo developers, and textile dyes

p-Tert-butylphenol formaldehyde resin

A resin formed by condensation between p-tert-butylphenol and formaldehyde

Occurs in leather finishes (especially shoes), paper, fabrics, rockwood, furniture, and certain glues


Common preservative occurring in cosmetics and in some household cleaners and polishes

Quinolone mix

Contains clioquinol and chlorquinaldol

Antimicrobials occurring in certain medicated creams and ointments, medicated bandages, and veterinary products


Preservative in contact lens solutions, certain cosmetics, nose and ear drops, and injectables

Source often not identified

Thiuram mix

Common rubber allergen

Also occurs in adhesives, certain pesticides, and drugs (eg, disulfiram)


Screens for class A corticosteroid allergy; found in buccal, nasal, throat, and rectal but not topical corticosteroid preparations

Wool alcohols

Part of lanolin; found in many cosmetics, ointments, sunscreens, and prescription and over-the-counter topical medications


Resolution may take up to 3 weeks. Reactivity is usually lifelong. Patients with photoallergic contact dermatitis can have flares for years when exposed to sun (persistent light reaction).


  • Avoidance of offending agents

  • Supportive care (eg, cool compresses, dressings, antihistamines)

  • Corticosteroids (most often topical but sometimes oral)

Contact dermatitis is prevented by avoiding the trigger; patients with photosensitive contact dermatitis should avoid exposure to sun.

Topical treatment includes cool compresses (saline or Burow solution) and corticosteroids; patients with mild to moderate ACD are given mid- to high-potency topical corticosteroids (eg, triamcinolone 0.1% ointment or betamethasone valerate cream 0.1%). Oral corticosteroids (eg, prednisone 60 mg once a day for 7 to 14 days) can be used for severe blistering or extensive disease. Systemic antihistamines (eg, hydroxyzine, diphenhydramine) help relieve pruritus; antihistamines with low anticholinergic potency, such as low-sedating H1 blockers, are not as effective. Wet-to-dry dressings can soothe oozing blisters, dry the skin, and promote healing.

Key Points

  • Contact dermatitis can be caused by irritants (eg, plants, soaps, chemicals, body fluids), comprising 80% of cases, or by allergens, comprising 20% of cases.

  • Symptoms can include predominantly pain (for irritant contact dermatitis) or pruritus (for allergic contact dermatitis).

  • Diagnosis is usually clinical.

  • Patch testing is helpful when allergic contact dermatitis is suspected and the trigger has not been identified.

  • Treatments commonly include cool compresses, topical corticosteroids, and systemic antihistamines as needed for pruritus.

Drugs Mentioned In This Article

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