If you think you aren't allergic to anything, try rubbing poison ivy on your bare skin.
While 99 percent of people will suffer the typical rash and itch, a few will escape the encounter with the natural botanical product unscathed, because they aren't allergic to it.
But poison ivy is not like most other allergens. People who suffer from other skin allergies will also develop a rash from contact with the allergen, but they don't have the vast majority of people who share in their discomfort. And those substances can range from metals such as nickel and chromium, to chemicals such as formaldehyde that can be found in everyday products.
A variety of substances can be allergy culprits, and these allergens can affect you even if you don't inhale or eat them.
Dr. Clifford Bassett, the chair of public education for the American Academy of Allergy, Asthma and Immunology, recalled one young boy who had hives, the source of which turned out to be a his father's moisturizer and shaving cream, which contained nuts. It aggravated the boy's nut allergy when the two embraced.
"It's not just what you put in your mouth, but it may also be contact," he said, noting that allergens can be found in products one would never expect. "You need to be a label detective."
Of course, not all skin itches are due to allergies.
For example, some people think they are allergic to Ivory soap, which is supposed to be moisturizing but can give some a rash. That is likely dry skin, not an allergy, said Dr. Sharon Jacob, assistant professor of medicine and pediatrics at the University of California at San Diego.
"It actually tends to be fairly drying for the skin," she said, noting that she recommends that patients with eczema and dermatitis avoid that brand of soap.
"Like we might say with a medication, it's more of a side effect," said Jacob.
While allergists test patients for foods they eat and allergens they may inhale, dermatologists are often the ones who look at contact allergies.
As opposed to the testing often done by allergists, which involves pricking patients' skin, dermatologists typically conduct a patch test, in which a patch containing the allergen is stuck to the skin for 48 hours and checked again after 72 hours to test for contact allergies.
"We're not testing for foods and those types of things," explained Dr. Bryan E. Anderson, an associate professor of dermatology at Penn State Milton S. Hershey Medical Center. He said this separate test can confuse patients who think they have been already tested for all of their allergies.
While only a dermatologist can tell you if you have a contact allergy, see if you should ask about one of these nine culprits.
Women who regularly use eye makeup may find that it can suddenly cease to agree with their eyelids.
"Very commonly they may use the same makeup for years, and then something changes and now they're getting an itchy, flaky red rash, usually on their eyelids," said Dr. Andy Nish, president of Allergy and Asthma Care Center in Gainesville, Ga.
Over time, the effect of the eye makeup can lessen or get worse, which can make it a hard allergy to detect.
"Sometimes, even though they have the allergy, it sort of cycles," said Nish. "It's not always obvious."
Further complicating matters is that women can get puffiness around their eyes from allergies aside from eye makeup, said Bassett.
Other things women may put on, such as nail polish, may remain on their hands and, when rubbing their eyes, can cause a reaction.
"What's on the skin ... can affect other body areas, depending on where you touch things," said Bassett.
Hair dyes can also present a potential problem, which leads to scalp irritation, so salons may take precautions.
"If you have your hair treatment, sometimes they will give you a chemical to put on your skin," said Bassett.
Ultimately, getting to an allergist or dermatologist will often help get to the root of your skin problems.
While you might describe your jewelry as being silver or gold, it probably contains some nickel as well.
"Nickel is an alloy that's used frequently," said Bassett, noting that many women discover their nickel allergies when they get their ears pierced.
The ear piercing itself may also be the source of the allergy, noted Anderson, who said that nickel allergies are more common in women.
The likely reason, he said, is because the piercing leads them to be sensitized to the metal.
Anderson noted that in Europe, there has been a concerted effort to reduce the amount of nickel in jewelry for this reason.
For patients who have a nickel allergy, Nish said that the amount of nickel will depend on the karat of the silver or gold used, and so he recommends that his patients with nickel allergies wear jewelry that is sterling silver or 18 karat gold or above.
Simple skin irritation under a ring may not be a sign of a nickel allergy -- or of an allergy at all.
Anderson noted that some people get soap residue on the skin under rings, and because it is trapped under the ring it stays in contact with the skin for an extended period and is not toweled off.
"You could have an allergic reaction to the soap but it's more likely to be an irritant reaction," he said.
There are many everyday objects that will irritate a nickel allergy, but jeans can be a daily problem.
"One very common place is in snaps and the buttons that hold the pants, if they're metal," said Nish.
In this case, he said, the nickel allergy often "shows up as a fairly significant, itchy red rash on the lower abdomen.
"I probably see that once every few weeks."
He said this type of allergy often shows up among children and teenagers who wear blue jeans that have the metal snap on them.
A number of remedies have been tried with varying success, including tucking in a t-shirt to keep fabric between skin and the metal, or covering the area with nail polish.
He noted that there are some professional products that can be used, but he declined to endorse any of them for all patients.
He said that the popularity of blue jeans makes it hard for nickel allergy sufferers to give them up.
"Sometimes it's hard getting teenagers not to wear pants with metal snaps," he said.
He also noted that the irritation may cycle, and that in some cases the person may not realize something is wrong.
"It may not occur to them that it might be an allergy," he said.
The only way to confirm it is to see a doctor.
"If you have a rash and there's metal involved and there's some kind of pattern, you should get a test," said Bassett.
Deodorants can cause problems for a variety of reasons.
While it is rarer than a nickel allergy, the presence of aluminum can cause a reaction, as can some of the other chemicals in various deodorants.
"In general, there are always alternatives," said Bassett, noting that deodorants containing aluminum can be replaced with powders such as baby powder or cornstarch or deodorant products with rock salt.
"Of course, it's not as pleasant," he said.
While some may opt for hypoallergenic or all-natural products, those might not be the best idea.
"It doesn't mean that it's a good product or that it's the right product for an individual."
Bassett and Anderson agreed that "all-natural" doesn't mean a product won't cause a reaction.
"Just because it's natural doesn't mean you can't have an allergy to it," said Anderson, noting that aloe, lavender and peppermint, found in many all-natural products, are known to cause allergic reactions in some people.
"They're natural ingredients, but you can still be allergic to them, just like poison ivy," said Anderson.
For patients who complain of problems with laundry detergent, Bassett said that he encourages them to use "allergy-friendly type products," which means that they contain less colorants and fragrances.
"We basically ask most of our patients to reduce exposures," he said, noting that this often means discouraging fabric softeners and other products that would go on the clothing.
Anderson notes that laundry detergent allergies are less common than other contact allergies.
"The reason for that is they're wash-off products," he said. "There's very little laundry detergent left on the clothing."
He noted that soaps and shampoos are less likely to cause reactions than moisturizers and nail polishes because they are not kept on the body for long periods of time.
"If you're going to have a problem with a laundry detergent, it's typically going to be the fragrance," said Anderson, although he called that "fairly rare."
"Cell phones have been linked to nickel as well," said Jacob, noting that nickel can be found "in the metallic facing of cell phones."
Anderson noted that this is an increasing problem because of the prevalence of nickel allergies.
"Nickel, in and of itself, is the most common allergen we encounter in a dermatology clinic," he said, noting that nickel content in cell phones is not regulated and patients can come in with rashes on their cheeks, faces and ears where their faces come in contact with their phones.
He estimates that nickel allergies make up 5 percent of general dermatology practices, and about 15 percent of his own.
If your children have unexplained rashes on the backs of their legs or on their lower backs, the seats they are constantly told to sit in may be the source of the problem.
"You can get nickel exposure from the studs in the chairs," said Jacob.
And chairs may not be the only classroom problem for children.
Jacob noted in a separate interview with ABCNews.com in October that flutes can also be a problem.
Because nickel is so ubiquitous, it becomes important to find if children have an allergy to it.
"Nickel is in just about everything we interact with," Jacob said.
Formaldehyde is found in many products, added in order to give a longer shelf life. While there are significant concerns about its carcinogenic properties (it has caused cancer in laboratory rats, but similar tests could not be ethically performed in humans), formaldehyde is a definite source of allergic reactions in people.
In the case of baby wipes, a preservative known as quaternium-15 causes an allergic response in babies and adults.
"Basically how it works is it's a molecule that releases small amounts of formaldehyde over time," said Anderson.
Because he generally sees adults in his practice, he said that patients who have an allergy are usually mothers who get rashes on their hands from the baby wipes.
On children, the rash is typically on the buttocks where the wipe is used.
Jacob, who has studied the chemical's presence in other products used for babies, said it can also be found in baby shampoo and is a possible reason for an increase in allergies.
"We don't know the long-term effect of sensitizing our children and our newborns to these chemicals," she said.
While nickel may be a more common allergen, cobalt and chromium are two other metals that can present allergy problems.
Cobalt is often found in nature with nickel, and is also in blue glass, cookware and utensils.
The most common use of chromium is in the tanning of leather, according to Jacob, which can present a problem for the fashion-conscious chromium-allergy sufferer.
Both metals are also used in dyes (cobalt for blue and chromium for green), and that is the source of another chromium allergy problem.
Chromium salt is an agent used with felt in order to give it a green color, and so it can be found on pool and poker tables.
While the allergy is relatively uncommon, it does occur often enough to have its own name -- "blackjack dermatitis."
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