Contact dermatitis is an inflammation of the skin caused by direct contact with an irritant or allergen. Irritant dermatitis, the most common type of contact dermatitis, involves inflammation resulting from contact with acids, alkaline materials such as soaps and detergents, solvents, or other chemicals. The reaction usually resembles a burn.
Allergic contact dermatitis, the second most common type of contact dermatitis, is caused by exposure to a substance or material to which you have become extra sensitive or allergic. The allergic reaction is often delayed, with the rash appearning 24-48 hours after exposure. The skin inflammation varies from mild irritation and redness to open sores, depending on the type of irritant, the body part affected, and your sensitivity.

Irritant contact dermatitis may affect anyone with 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, seborrheic dermatitis, atopic dermatitis,Irritant dermatitis,Contact dermatitisdressers and food handlers. The dermatitis is usually confined to the site of contact with the irritant, but if the dermatitis is prolonged or severe it may spread to previously unaffected areas.
Sometimes it is easy to recognise irritant contact dermatitis and no specific tests are necessary. The rash usually heals once the irritant is removed and, if necessary, special treatment is applied. While there are some tests that can provide an indication of the irritant potential of substances, there are no specific tests that can reliably show what the effect of an irritant will be in each individual case. Irritant dermatitis in any case is usually the result of the cumulative effect of multiple irritants. Patch tests may be used to confirm allergic contact dermatitis and identify the allergen(s), but they do not exclude irritant contact dermatitis as the two may coexist.

Though the configuration of the rash aids in diagnosis, it's not so easy to determine whether an allergy or irritant is involved. The skin reaction produced by either, especially when mild, frequently looks the same. Redness or an itchy rash may be the first sign: however, blisters that weep or form a crust, along with swelling, are more likely to appear in allergic dermatitis such as poison ivy. As the inflammation lessens, the skin may scale and become temporarily thickened. When the dermatitis becomes chronic, the skin becomes dry, thickened and cracking. If the inflammation mild irritants continues for a long time, the original irritation disappears because the skin becomes hardened.

A person may know what caused the inflammation--for example, recent hand contact with a corrosive, such as an oven cleaner--or may not have a clue. The patient's hobbies, diet, occupation, sports activities, clothing and cosmetics, as well as medications that are taken internally should all be taken into consideration.