Allergic contact dermatitis is an itchy skin condition caused by an allergic reaction to material in contact with the skin. It arises some hours after contact with the responsible material, and settles down over some days providing the skin is no longer in contact with it.

Contact dermatitis should be distinguished from contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours. The allergic reaction to latex is the best known example of allergic contact urticaria.

Allergic contact dermatitis is also distinct from irritant contact dermatitis, in which a similar skin condition is caused by excessive contact with irritants. Irritants include water, soaps, detergents, solvents, acids, alkalis, and friction. Irritant contact dermatitis may affect anyone, providing they have had enough exposure to the irritant, but those with atopic dermatitis are particularly sensitive. Most cases of hand dermatitis are due to contact with irritants.

Allergy is the term given to a reaction by a small number of people to a substance (known as the allergen) which is harmless to those who are not allergic to it. Only small quantities of allergen are necessary to induce the reaction. Contact allergy occurs predominantly from the allergen on the skin rather than from internal sources or food. The first contact does not result in allergy; often the person has been able to touch the material for many years without adverse reaction.

Irritant contact dermatitis occurs when chemicals or physical agents damage the surface of the skin faster than the skin is able to repair the damage. The dermatitis or eczema is often well demarcated with a glazed surface but there may be redness, itching, swelling, blistering and scaling of the damaged area. This may be indistinguishable from other types of dermatitis. The severity of the dermatitis is highly variable and depends on many factors including:

  • Amount and strength of the irritant
  • Length and frequency of exposure (eg. short heavy exposure or repeated/prolonged low exposure)
  • Skin susceptibility (eg. thick, thin, oily, dry, very fair, previously damaged skin or pre-existing atopic tendency)
  • Environmental factors (eg. high or low temperature or humidity)


Irritants include such everyday things as water, detergents, solvents, acids, alkalis, adhesives, metalworking fluids and friction. Often several of these act together to injure the skin. Irritants damage the skin by removing oils and moisture from its outer layer, allowing the irritants to penetrate more deeply and cause further damage by triggering inflammation.

Irritant contact dermatitis may affect anyone, given sufficient exposure to irritants, but those with atopic dermatitis are particularly susceptible. 80% of cases of occupational hand dermatitis are due to irritants, most often affecting cleaners, hairdressers and food handlers.

Allergic contact dermatitis may look similar to irritant contact dermatitis, but it is caused by an immune response following skin contact with an allergenic substance. Tiny quantities may be sufficient to cause allergy, whereas a certain minimum exposure is necessary for irritant contact dermatitis. Irritant and allergic contact dermatitis may coexist.

Contact dermatitis is an inflammation at skin and is caused by direct contact with an irritant or an allergen. Symptoms may include mild redness and swelling, blistering , itching and scaling and temporary thickening of skin. The most severe reaction is at the contact site.

Eczema—also called atopic dermatitis—is a skin disorder in which an allergic response especially to foods has a role in some people, especially children. Eczema is most common in infants and children. Although all eczema is not allergy-related, people with eczema usually have a family history of one or more allergic conditions such as asthma, hay fever, or allergic rhinitis (runny, stuffy nose). Adults are affected by allergic contact dermatitis more than young children or the elderly.

Irritant contact dermatitis is triggered by exposure to a chemical that is poisonous (toxic) or irritating to human skin. It is not an allergic reaction. In children, the most common form of irritant contact dermatitis is “diaper dermatitis,” a skin reaction in the diaper area caused by prolonged contact with the natural chemicals found in urine and stool. In adults, irritant contact dermatitis is often an occupational illness triggered by exposure to strong soaps, solvents or cutting agents. It is especially common among health care workers, homemakers, janitors, mechanics, machinists and hairdressers, but it can occur in anyone whose household chores or hobbies involve exposure to irritating chemicals.

Allergic contact dermatitis is an immune reaction that occurs only in people who are naturally oversensitive to certain chemicals. With allergic contact dermatitis, the inflammation may not develop until 24 to 36 hours after contact with the substance (allergen). The skin develops sensitization. It starts with the penetration of allergenic substances into the outer layer of the skin. The process lasts from four days to three weeks. During this period there are no signs of skin damage.

When sensitized skin are re-exposed, lymphocytes recognize the allergen and release chemical substances, causing itching, pain, redness, swelling, and the formation of small wheals or blisters on the skin. This inflammation is usually confined to the site of contact with the allergen, but in severe cases it may spread to cover large areas of the body. It usually starts within twelve hours from exposure and is at its worst after three or four days. It slowly improves in about seven days. The allergic sensitization may remain with the individual through life. If there is no further contact with the allergen, the level of sensitivity may gradually decline.

ACD represents a T helper cell Type 1 [Th1] dependent delayed-type (Type IV) hypersensitivity reaction. The instigating exogenous antigens are primarily small lipophilic chemicals (haptens) with a molecular weight less than 500 Da. (1) On direct antigen exposure to the skin or mucosa, an immunologic cascade is initiated which includes cytokines, i.e. interleukin 2 and interferon gamma , T cells and Langerhan cells. This complex interaction leads to the clinical picture of ACD.

Allergic contact dermatitis is not usually caused by things like acid, alkali, solvent, strong soap or detergent. These harsh compounds, which can produce a reaction on anyone’s skin, are known as ‘irritants’. Although some chemicals are both irritants and allergens, allergic contact dermatitis results from brief contact with substances that don’t usually provoke a reaction in most people.

Among the most common types of allergens responsible are a chemical found in poison ivy, oak and sumac; nickel and cobalt in metal jewelry, clothing snaps, zippers and metal-plated objects; neomycin in antibiotic skin ointments; potassium dichromate, a tanning agent found in leather shoes and clothing; latex in gloves and rubberized clothing; and certain preservatives, such as formaldehyde. About 20% of people in the United States are probably at risk of allergic contact dermatitis because of skin sensitivity to at least one common chemical allergen.

The most common factors contributing to the development of allergic contact dermatitis are pre-existing skin conditions such as irritant contact dermatitis (see CCOHS publication Irritant Contact Dermatitis). Cuts or scratches, into which allergenic substances can enter, also contribute to the development of allergic contact dermatitis. The chemical nature of the substance is important (for example, as are the amount and concentration that comes into contact with the skin, and the length and frequency of the exposure.

Important individual factors include the resistance of the skin, which increases with age. Hereditary factors influence the variety of reactions in different persons exposed to the same allergen.  Environmental factors play a significant role.

A skin “patch” test is used to determine the most likely substances that are causing you to have an allergic response on your skin. A complete physical and history, along with test results, will assist your doctor in making a diagnosis. To help your doctor make the most accurate diagnosis, keep a log with details related to your skin reactions.

Patch testing is a safe and quick way to diagnose contact allergies. A small amount of the suspected allergen is applied to the skin for a fixed time, usually two days. Some things like nickel, rubber, dyes, and poison ivy, poison oak and related plants are fairly common allergens.

The thin-layer rapid use epicutaneous (T.R.U.E.) test of 23 common allergens is a valuable, first-line screening tool used by many dermatologists. Although the test focuses on common allergens, frequent questions have arisen from colleagues and patients as to where a specific allergen is derived or what products patients should avoid. With this in mind, this column was developed to provide educational information about the T.R.U.E. test allergens.

Thousands of common products contain substances that can trigger an allergic response on your skin. Many times these substances can be “hidden” components of a product you are using, so be sure to read labels!

Sensitized workers should avoid further exposure to the allergen. This alone is an effective remedy. Allergic contact dermatitis may be treated with anti-inflammatory drugs, and with ointments and skin cleansers. In general, the affected skin should be protected from physical trauma, excessive sunlight, wind, and rapid temperature changes while the dermatitis is active.

Depending on your pattern of skin symptoms, the doctor will ask about your personal and family allergy history, your history of exposure to irritating chemicals at work or at home, or your contact with poisonous plants. In some cases, your doctor also may need to know the names of specific ingredients found in products that you routinely apply to your skin or hair, especially cosmetics, shampoos, hair dyes, skin lotions, nail polish or antibiotic skin ointments.