Based on the cause, doctors divide contact dermatitis into two overall categories:

  • Irritant contact dermatitis is the most common type. This reaction occurs when a substance damages your skin's outer protective layer. Common offenders include harsh soaps, chemical solvents, and cosmetics or skin products, including deodorant. Exposure produces red, dry, itchy patches, often on your hands, fingers or face.

    The severity of irritant contact dermatitis usually depends on how long you're exposed and the strength of the irritating substance. Some strong irritants, such as bleach or certain acids, can cause a reaction after a single exposure.

  • Allergic contact dermatitis occurs when a substance to which you're sensitive (allergen) triggers an immune reaction in your skin. Allergic contact dermatitis produces a red rash, bumps and sometimes blisters when severe. Common allergens include natural rubber, metals such as nickel, costume jewelry, perfume, cosmetics, hair dyes and plants, including poison ivy.

    You may become sensitized to a strong allergen like poison ivy after a single exposure. Weaker allergens may require multiple exposures over several years to trigger an allergy. Once you develop an allergy to a specific substance, however, you'll be allergic for life. Exposure to even a small amount of the allergen will cause a reaction.

    Some substances cause allergic contact dermatitis only after you've applied them and sunlight then strikes your skin (photoallergic contact dermatitis). Common offenders include certain ingredients in sunscreens and some ointments containing nonsteroidal anti-inflammatory drugs (NSAIDs). Some oral drugs — for example, hydrochlorothiazide, which is prescribed as a water pill (diuretic) or as a blood pressure medication — may also cause a skin reaction triggered by sunlight. This reaction may look similar to photoallergic contact dermatitis, but it's usually called "photosensitivity." It's caused by a different mechanism than is contact dermatitis.

Some airborne substances, such as ragweed and insecticide sprays, can also cause contact dermatitis.

These overall irritant and allergic categories are not hard and fast. Some substances are both allergens and irritants. Common allergens and irritants implicated in contact dermatitis include:

  • Nickel, a metal widely used in earrings and other costume jewelry, watchbands, zippers and clothing fasteners, hair curlers and eyelash curlers, and coins
  • Poison ivy, oak or sumac, which contain a strongly allergenic oil (urushiol)
  • Cashew nuts, which contain a substance chemically similar to the urushiol found in poison ivy
  • Antibiotics, antihistamines or antiseptics you apply to your skin as lotions or creams (topical medicines)
  • Fragrances or flavorings, including those that contain balsam of Peru, an oil from a Central and South American tree
  • Strong detergents or soaps
  • Skin cleansers
  • Makeup and other cosmetics
  • Deodorant
  • Clothing or shoes
  • Household cleaning products
  • Formaldehyde and other chemicals
  • Natural rubber (latex)
  • Jewelry

Occupational contact dermatitis can occur when you're exposed to allergens or irritants on the job. Frequent exposure to water, friction, chemicals, oils, fuels, dyes, cleaning agents, wet cement, industrial solvents or dust (for example, cement dust, sawdust or paper dust) can lead to occupational contact dermatitis.

Jul. 30, 2011