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There are two major types of contact dermatitis: irritant contact dermatitis (ICD) and allergic contact dermatitis (ACD).

ICD makes up approximately 80 percent of the cases of contact dermatitis, according to the American Academy of Allergies, Asthma and Immunology. ICD is triggered by exposure to a chemical that is naturally irritating to the human skin. Hands are the most commonly affected body area.

There are several forms of ICD:

• Acute corrosion. Caused by a single exposure to strong acids and alkalis.
  
  
• Acute irritation. Caused by a single exposure to chemicals such as strong solvents and noncorrosive acids and bases.
  
  
• Cumulative irritation. The most common form of ICD, it involves repeated exposures to substances, particularly surfactants (promote lathering) and emulsifiers (skin softener). May be difficult to distinguish from ACD.
  
  
• Phototoxicity. Caused by contact with irritating chemicals that only react when exposed to sunlight. Such substances include shaving lotions, sunscreens, sulfa ointments, some perfumes, coal tar products and oil from the skin of a lime. Symptoms usually affect those areas most susceptible to sunlight, such as the face, arms and upper chest area.

ICD may affect people who are continuously exposed to a certain substance. However, some people have skin that is more sensitive than others to the chemicals that cause ICD. Factors that determine an individual’s susceptibility to ICD include age, genetics, the environment, underlying skin diseases and exposure to other chemicals. Irritant ICD is also more likely in people with dry skin and light-colored (or “fair”) skin. People with a history of atopic dermatitis are more susceptible to ICD.

The most common form of ICD in children is diaper rash, which is a skin reaction to prolonged contact with natural chemicals found in the urine and stool. ICD also can be an occupational illness in adults who are exposed to strong soaps and solvents, such as domestic workers, janitors or hairdressers. Florists, food preparers, constructions workers and health care providers are also at risk for contracting work–related contact dermatitis. Anyone whose household chores or hobbies involve irritating chemicals may also experience ICD.

Mild cases of ICD produce reddening of the skin, but more severe cases may cause swelling or ulceration and resemble a burn. Unlike ACD, symptoms begin immediately after exposure to the harmful substance, or upon repeated exposure to it in the case of cumulative irritation.

ACD involves an immune system reaction that takes place when the skin is exposed to a substance the body perceives as dangerous. To date, more than 3,000 allergens have been identified that are known to cause this condition. ACD may not develop immediately, but instead often appears within 24 to 48 hours after contact with an allergen, or longer in some cases. Specific skin sensitivities vary from person to person, but ACD is a delayed form of hypersensitivity (excessive sensitivity) that generally takes time for the body’s immune defenses to react. ACD can develop at any age. However, adults develop the condition more often than young children or the elderly.

Contact dermatitis caused by an allergic reaction produces localized skin redness (erythema), blistering and severe itching (pruritus). The characteristics of the allergic reaction vary depending on its cause. For example, metal jewelry allergies usually produce rings of inflammation around the body area where the jewelry is worn (e.g., around the neck for a necklace or wrist for a bracelet). Poison ivy produces a pattern of tiny lines in the places where the plant leaves have contacted the skin.

Unlike most allergic reactions, the trigger in ACD is external rather than internal. The skin reacts to external physical contact with an allergen. The initial exposure to the allergen does not cause a rash. Instead, it sensitizes the skin. Once sensitized to an allergen, the body will continue to produce the symptoms of contact dermatitis with each subsequent exposure to the substance. Sensitization can occur after only one exposure to a substance, or after many exposures. In fact, a person can develop ACD after years of being exposed to a substance without a problem.

The presence of preexisting ICD can lead to the development of ACD. Cuts and other common wounds can also contribute by providing an entry point for allergens.

Overtreatment dermatitis is a form of contact dermatitis that occurs when treatment for a separate skin disorder causes ICD.