Allergic contact dermatitis is a common skin condition that results in an itchy and weeping rash, usually a few days after direct skin contact. Common causes include nickel, chemicals, plants, cosmetics, perfumes and ointments.
Allergic contact dermatitis is common
Allergic contact dermatitis is a common skin condition which causes an itchy and weeping rash localised to the area in contact with the allergic trigger (allergen). It usually develops two or more days after contact with the allergen. It lasts as long as contact continues and for a short time afterwards, typically one to two weeks.
Nickel metal is the most common cause of allergic contact dermatitis
Approximately 8% of the population is allergic to nickel, and the prevalence is higher among females than males. Nickel may be found in the metal of costume jewellery, watch straps, bra clips and jeans studs. The ears are often affected by wearing of non-gold earrings. The usual precipitant is piercing of the ears. Two days after contact with the metal, an itchy rash develops which may blister and weep. This rash will continue for several days or longer, if further contact with the metal continues.
Plants may also cause allergic contact dermatitis
Some plants can trigger allergic contact dermatitis. Rashes often appear on exposed areas of the body after being oudoors. Allergic rashes may be triggered by direct contact with the plant, or a combination of sunlight plus contact, so called "photo-contact dermatitis". Examples of plants in Australia that may cause contact dermatitis are chrysanthemums, primula, tomato plants, grevillea, english ivy and occasionally rhus trees. Lantana or vegetables such as parsnip or celery may cause photo-contact dermatitis.
Other causes include chemicals, creams and ointments
Other common causes of allergic contact dermatitis include chemicals in rubber, leather and dyes. Contact dermatitis to shoes is common, and any one of these chemicals could be the cause. Creams and ointments used on the skin may also cause allergic contact dermatitis.
The cause may be the base ingredient (such as lanolin), or the active agent (such as neomycin, an antibiotic). The first time the cream or ointment is applied, the rash will generally not appear for 10 to 14 days. If it is applied again, it will usually take only two days to develop. Perfumes, either on their own, or in cosmetics, can also cause allergic contact dermatitis. Even sunscreens can cause problems.
Patch testing can identify the cause of allergic contact dermatitis
There are many causes of allergic contact dermatitis, particularly where individuals are working in industry and in contact with chemicals. If the cause of allergic contact dermatitis is not obvious, patch testing can be helpful. This is usually performed by a medical specialist (dermatologist or clinical Immunology/allergy specialist), and involves placing a sample of the substance on the back, and seeing if a reaction occurs after a few days. While it is not always easy to find the cause of contact allergy, intensive patch testing usually provides an answer.
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Updated May 2019