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.

Symptoms

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, Sun, Swimming Pools With High chlorine ), containing the irritant.