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|Classification and external resources|
Rash resulting from skin reactions.
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Contact dermatitis is a term for a skin reaction (dermatitis) resulting from exposure to allergens (allergic contact dermatitis) or irritants (irritant contact dermatitis). Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight.
Contact dermatitis is a localized rash or irritation of the skin caused by contact with a foreign substance. Only the superficial regions of the skin are affected in contact dermatitis. Inflammation of the affected tissue is present in the epidermis (the outermost layer of skin) and the outer dermis (the layer beneath the epidermis). Unlike contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours, contact dermatitis takes days to fade away. Even then, contact dermatitis fades only if the skin no longer comes in contact with the allergen or irritant. Contact dermatitis results in large, burning, and itchy rashes, and these can take anywhere from several days to weeks to heal. Chronic contact dermatitis can develop when the removal of the offending agent no longer provides expected relief.
In the Americas, the most common causes of allergic contact dermatitis are plants of the Toxicodendron genus: poison ivy, poison oak, and poison sumac. Specific plant species that can induce such contact dermatitis include Western Poison Oak, a widespread plant in the western USA.
The alkyl resorcinols in Grevillea banksii and Grevillea 'Robyn Gordon' are responsible for contact dermatitis. Bilobol, another alkyl resorcinol found in Gingko biloba fruits is a strong skin irritant.
Other common causes of irritant contact dermatitis are harsh (highly alkaline) soaps, detergents, and cleaning products.
There are three types of contact dermatitis: irritant contact dermatitis, allergic contact dermatitis, and photocontact dermatitis. Photocontact dermatitis is divided into two categories that is, phototoxic and photoallergic.
Irritant contact dermatitis 
Irritant contact dermatitis can be divided into forms caused by chemical irritants and those caused by physical irritants. Common chemical irritants implicated include solvents (alcohol, xylene, turpentine, esters, acetone, ketones, and others); metalworking fluids (neat oils, water-based metalworking fluids with surfactants); latex; kerosene; ethylene oxide; surfactants in topical medications and cosmetics (sodium lauryl sulfate); alkalis (drain cleaners, strong soap with lye residues). Physical irritant contact dermatitis may most commonly be caused by low humidity from air conditioning. Also, many plants directly irritate the skin.
Allergic contact dermatitis 
Although less common than ICD, ACD is accepted to be the most prevalent form of immunotoxicity found in humans. By its allergic nature, this form of contact dermatitis is a hypersensitive reaction that is atypical within the population. The mechanisms by which these reactions occur are complex, with many levels of fine control. Their immunology centres around the interaction of immunoregulatory cytokines and discrete subpopulations of T lymphocytes. Allergens include nickel, gold, balsam of Peru (Myroxylon pereirae), chromium and the oily coating from plants of the Toxicodendron genus: poison ivy, poison oak, and poison sumac.
Photocontact dermatitis 
Sometimes termed "photoaggravated", and divided into two categories, phototoxic and photoallergic, PCD is the eczematous condition which is triggered by an interaction between an otherwise unharmful or less harmful substance on the skin and ultraviolet light (320-400 nm UVA) (ESCD 2006), therefore manifesting itself only in regions where the sufferer has been exposed to such rays. Without the presence of these rays, the photosensitiser is not harmful. For this reason, this form of contact dermatitis is usually associated only with areas of skin which are left uncovered by clothing, and it can be soundly defeated by avoiding exposure to sunlight. The mechanism of action varies from toxin to toxin, but is usually due to the production of a photoproduct. Toxins which are associated with PCD include the psoralens. Psoralens are in fact used therapeutically for the treatment of psoriasis, eczema and vitiligo.
Photocontact dermatitis is another condition where the distinction between forms of contact dermatitis is not clear cut. Immunological mechanisms can also play a part, causing a response similar to ACD.
Allergic dermatitis is usually confined to the area where the trigger actually touched the skin, whereas irritant dermatitis may be more widespread on the skin. Symptoms of both forms include the following:
- Red rash. This is the usual reaction. The rash appears immediately in irritant contact dermatitis; in allergic contact dermatitis, the rash sometimes does not appear until 24–72 hours after exposure to the allergen.
- Blisters or wheals. Blisters, wheals (welts), and urticaria (hives) often form in a pattern where skin was directly exposed to the allergen or irritant.
- Itchy, burning skin. Irritant contact dermatitis tends to be more painful than itchy, while allergic contact dermatitis often itches.
While either form of contact dermatitis can affect any part of the body, irritant contact dermatitis often affects the hands, which have been exposed by resting in or dipping into a container (sink, pail, tub, swimming pools with high chlorine) containing the irritant.
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Self-care at home 
- Immediately after exposure to a known allergen or irritant, wash with soap and cool water to remove or inactivate most of the offending substance.
- Weak acid solutions [lemon juice, vinegar] can be used to counteract the effects of dermatitis contracted by exposure to basic irritants.
- If blistering develops, cold moist compresses applied for 30 minutes 3 times a day can offer relief.
- Calamine lotion and cool colloidal oatmeal baths may relieve itching.
- Oral antihistamines such as diphenhydramine (Benadryl, Ben-Allergin) can also relieve itching.
- For mild cases that cover a relatively small area, hydrocortisone cream in nonprescription strength may be sufficient.
- Avoid scratching, as this can cause secondary infections.
- A barrier cream such as those containing zinc oxide (e.g. Desitin, etc.) may help to protect the skin and retain moisture.
Medical care 
If the rash does not improve or continues to spread after 2-3 of days of self-care, or if the itching and/or pain is severe, the patient should contact a dermatologist or other physician. Medical treatment usually consists of lotions, creams, or oral medications.
- Corticosteroids. A corticosteroid medication similar to hydrocortisone may be prescribed to combat inflammation in a localized area. This medication may be applied to the skin as a cream or ointment. If the reaction covers a relatively large portion of the skin or is severe, a corticosteroid in pill or injection form may be prescribed.
- Antihistamines. Prescription antihistamines may be given if non-prescription strengths are inadequate.
Since contact dermatitis relies on an irritant or an allergen to initiate the reaction, it is important for the patient to identify the responsible agent and avoid it. This can be accomplished by having patch tests, a method commonly known as allergy testing. The patient must know where the irritant or allergen is found to be able to avoid it. It is important to also note that chemicals sometimes have several different names.
In an industrial setting the employer has a duty of care to the individual worker to provide the correct level of safety equipment to mitigate the exposure to harmful irritants. This can take the form of protective clothing, gloves or barrier cream depending on the working environment.
The distinction between the various types of contact dermatitis is based on a number of factors. The morphology of the tissues, the histology, and immunologic findings are all used in diagnosis of the form of the condition. However, as suggested previously, there is some confusion in the distinction of the different forms of contact dermatitis. Using histology on its own is insufficient, as these findings have been acknowledged not to distinguish, and even positive patch testing does not rule out the existence of an irritant form of dermatitis as well as an immunological one. It is important to remember, therefore, that the distinction between the types of contact dermatitis is often blurred, with, for example, certain immunological mechanisms also being involved in a case of irritant contact dermatitis.
See also 
- European Society of Contact Dermatitis. "What is contact dermatitis".
- "DermNet NZ: Contact Dermatitis". Retrieved 2006-08-14.
- C.Michael Hogan (2008) Western poison-oak: Toxicodendron diversilobum, GlobalTwitcher, ed. Nicklas Stromberg 
- Menz, J; Rossi, ER; Taylor, WC; Wall, L (1986). "Contact dermatitis from Grevillea 'Robyn Gordon'". Contact dermatitis 15 (3): 126–31. PMID 2946534.
- Matsumoto, Kazuyuki; Fujimoto, Masao; Ito, Kazuo; Tanaka, Hitoshi; Hirono, Iwao (1990). "Comparison of the effects of bilobol and 12-O-tetradecanoylphorbol-13-acetate on skin, and test of tumor promoting potential of bilobol in CD-1 mice". The Journal of Toxicological Sciences 15 (1): 39–46. doi:10.2131/jts.15.39. PMID 2110595.
- Irritant Contact Dermatitis, at DermNetNZ, http://www.dermnetnz.org/dermatitis/contact-irritant.html
- Morris-Jones R, Robertson SJ, Ross JS, White IR, McFadden JP, Rycroft RJ (2002). "Dermatitis caused by physical irritants". Br. J. Dermatol. 147 (2): 270–5. doi:10.1046/j.1365-2133.2002.04852.x. PMID 12174098.
- Kimber I, Basketter DA, Gerberick GF, Dearman RJ (2002). "Allergic contact dermatitis". Int. Immunopharmacol. 2 (2–3): 201–11. doi:10.1016/S1567-5769(01)00173-4. PMID 11811925.
- Bourke J, Coulson I, English J (2001). "Guidelines for care of contact dermatitis". Br. J. Dermatol. 145 (6): 877–85. doi:10.1046/j.1365-2133.2001.04499.x. PMID 11899139.
- "Photocontact Dermatitis". www.skinchannel.com. Retrieved 31 March 2011.
- DermNet dermatitis/contact-allergy
- Rietschel RL (1997). "Mechanisms in irritant contact dermatitis". Clin. Dermatol. 15 (4): 557–9. doi:10.1016/S0738-081X(97)00058-8. PMID 9255462.
|Wikimedia Commons has media related to: Contact dermatitis|
- American Contact Dermatitis Society
- American Academy of Dermatology
- European Contact Dermatitis Society
- eMedicine Health article on contact dermatitis
- Skin Diseases ToolkitIOSH