There are many types of rashes, including those caused by toxins (such as poison ivy) or illnesses (such as roseola or chicken pox). If you get a red, itchy rash with no known cause, it could be allergic contact dermatitis. 

This itchy skin rash can result from exposure to a chemical or compound that causes a response from the skin’s immune system. The rash will arise at the point of contact with the allergen. The area is usually a pink or red color and feels itchy. Contact dermatitis may appear flat or raised, and in severe cases, blisters filled with clear fluid may result.

Although some people react more quickly than others, these rashes tend to take time to develop and don't occur with the first exposure. 

“Allergic contact dermatitis is something that's what we call a delayed type hypersensitivity,” says Keith Choate, MD, PhD, a Yale Medicine dermatologist. “Someone will be, for example, exposed in the garden on Sunday, and then start noticing that they have an eruption on Monday. So it's not instantaneous. It takes time.”

Yale Medicine dermatologists have highly specialized expertise in treating complex skin disorders, including skin testing to determine the specific cause of a person's allergic contact dermatitis. "Our physicians are really dedicated to getting answers," Dr. Choate says.

Allergic contact dermatitis can be caused by a wide variety of allergens and requires a minimum of two separate exposures. The first exposure sensitizes the person to the agent in question, while the second exposure brings on the rash.

According to Dr. Choate, allergic contact dermatitis is a condition that causes progressively more serious reactions. “It typically can worsen over time. The initial rash may be relatively mild,” he says. "And each subsequent time you're exposed, it can actually get worse and worse until it reaches a maximum severity.”

There aren’t different kinds of allergic contact dermatitis, but there are many different allergens that can cause the condition. These allergens can be obvious, like poison ivy, or more subtle, like preservatives in personal care products, leather dyes or spandex. 

Allergic contact dermatitis needs to be distinguished from irritant contact dermatitis, rashes that are usually caused by repeated exposure to soaps, detergents or industrial chemicals. People can have both irritant and allergic contact dermatitis. If you have atopic dermatitis (eczema), your skin is more sensitive and susceptible to irritant reactions from a variety of exposures.

Typically a diagnosis of allergic contact dermatitis is made by a dermatologist. Often the shape of the rash will determine the diagnosis. Rashes caused by external factors such as allergens or irritants are usually asymmetrical, and those caused by internal factors—such as atopic dermatitis—are often symmetrical, meaning that they occur in both sides of the body, for example, on both arms or legs. 

But a successful allergic contact dermatitis diagnosis doesn’t always indicate what caused the reaction in the first place. If someone hasn’t been exposed to a known allergen such as poison oak, it’s necessary to perform patch testing to determine the cause. 

Patch testing involves the application of square patches that are impregnated with a variety of different allergens—typically 36 of the most common ones. The patches need to remain on the patient’s skin for about 48 hours to ensure that the skin has time to react to the allergens. Once the patches are removed, it will typically take another 24 to 36 hours for a rash to appear.

Common allergens used in patch testing might include: nickel, fragrances, potassium dichromate (a metal used in tanning leather), neomycin (an ingredient in topical antibiotics) and formaldehyde-releasing preservatives often found in shampoo and other personal-care products.

Patch testing can be extremely helpful in determining the cause of the dermatitis and equipping patients with the tools to prevent these kinds of reactions in the future. “For people who have this ongoing dermatitis that's showing up in a pattern that points to contact dermatitis, the patch test will frequently give us a result,” Dr. Choate says. “It's very helpful in terms of figuring out what to do next.”

Allergic contact dermatitis is best treated by identifying and avoiding the allergen(s) that you are sensitized to. 

For symptom relief, applying a hydrocortisone cream and taking an antihistamine can help stop itching. Your doctor may prescribe topical steroids, and if the condition is widespread and more severe, it can be treated with systemic steroids such as Prednisone, which doctors will sometimes prescribe to ensure that the condition is fully treated. It can take a week to 10 days for the rash to resolve, but patients should experience relief from itchiness shortly after treatment begins.

Fully evaluating allergic contact dermatitis through patch testing, especially when it’s recurrent, can provide a patient with substantial quality of life improvements. According to Dr. Choate, “The minute that you figure out what's causing reactions, you can have patients modify their lifestyles and everything goes away. It's really remarkable.”

Because so many things can ignite irritant or allergic reactions, contact dermatitis isn’t associated with specific, easily identifiable risk factors. But people who spend a lot of time outdoors or who work in an industrial setting are likely to experience this skin condition. 

“I think contact dermatitis happens for most weekend warriors out there who like to go into the mountains and go into their gardens and spend their time outdoors," says Dr. Choate. 

It also can occur in people who work in industrial settings. Many compounds used in industrial materials such as cement and rubber can be allergens. "There are literally hundreds of things that people can have irritant or allergic reactions to,” Dr. Choate says.

A person’s ability to develop an allergic response is thought to be determined by genetic factors and the frequency and intensity of exposure to the allergen. The potency of the allergen is another critical factor. Generally, once a person develops an allergy, the sensitivity to that chemical is lifelong.

The integrity of the skin barrier is considered to be an important element in the sensitization process. Allergens, which are small molecules that can pass through most gloves, are more likely absorbed through damaged skin. In this way, people who have irritant contact dermatitis (e.g. from frequent hand washing) may be more likely to develop an allergy to substances that they’re exposed to (e.g. chemical preservative in a skin moisturizer).

Under those circumstances, it makes sense that people who develop allergic contact dermatitis based on a new allergy might mistake it for another kind of skin condition such as chronically dry skin, says Dr. Choate.

According to Dr. Choate, one thing that sets Yale Medicine dermatologists apart is their willingness to aggressively pursue results.

“Our physicians are really dedicated to getting answers,” Dr. Choate says. "If a patient is having recurrent episodes and comes to us, we look at the rash and say, 'Gosh, that has features of allergic contact dermatitis.' Then we actually go and we find the answer to what is causing the reaction by using the patch-test approach.”