Anyone who has wandered through the maze of cosmetics counters in their local department store, or even just hit their neighborhood drugstore, knows that thousands of products have been developed to cater to people seeking flawless skin. But in certain people, skin care products, including make-up and sunscreen, as well as hair and nail care products, can lead to an allergic reaction on the skin called contact dermatitis. Sometimes it?s even difficult for people and their doctors to discern the cause of the allergic reaction because the rash may not appear in the area where the product was applied. Unfortunately, avoiding allergic reactions isn?t as simple as choosing products labeled 'hypoallergenic'. And products deigned 'all natural' aren't any less likely to cause allergic reactions than other products, either. Below, Frances J. Storrs, MD, a professor of dermatology emerita at Oregon Health and Science University in Portland who specializes in contact dermatitis, explains how to wisely choose your skin products. What kinds of allergic reactions to skin products do people usually have? They usually develop contact dermatitis, which is the allergic reaction similar to the one you would get if exposed to poison oak or poison ivy. So someone might just have dry skin, and as they begin to use a product, their skin becomes more and more red and they might develop what we call vesicles. These are little tiny blisters on the skin that become crusty and ooze and then spread to other parts of the body. The dermatitis might spread up an arm or the whole face or the eyelids might be involved. Depending on how strong an allergen you?re dealing with, you may get an allergic eczema, which is an itchy rash. The allergic reaction just gets worse and worse until the person stops using the product. Who is most likely to have an allergic reaction to a skin product? Occasionally allergic reactions occur in people with normal skin, but more often then not, they occur in people in which the barrier has been broken so that the skin is no longer completely intact. This includes someone with a little bit of flaking on their face?something we call seborrheic dermatitis?or someone with eczema or someone who just has dry skin. What products cause allergic reactions? Lotions and creams, foundations, moisturizers, sunscreens, shampoos, salon hair care products and nail care products are most likely to cause allergic reactions. What ingredients in skin care products are most likely to cause an allergic reaction? It depends on the product you?re talking about, but the most common problems are caused by preservatives. Any agent that contains water requires a preservative to keep bacteria or funguses from growing in it. The most common preservatives associated with allergy are those that release formaldehyde. A good example of that is quaternium 15 found in various lotions or creams or even shampoos and cleansing agents. There?s also a collection of moisturizing lotions that are preserved with a chemical called methyldibromo glutaronitrile. If you are allergic to those chemicals, you will need to avoid products that contain them, so you will have to learn to look for them on the label of all skin and hair care products. Parabens are another type of preservative that are used in thousands of products, and they cause fewer allergic reactions. What ingredients other than preservatives can cause allergic reactions? In addition to the preservatives, some people think perfumes are common causes of contact dermatitis. I think fragrances are probably overrated, frankly, as a cause of allergic reactions. But fragrances are very complicated compounds that contain hundreds of chemicals that might cause an allergic reaction. An ingredient we're seeing more and more of right now are the botanicals found in products like shampoos. Botanicals are plant extracts, or the so-called natural chemicals. They may have an odor, but they are not officially designated as fragrances on the label. In my office, I?ve seen three people who were using a deodorant that contained some extracts of lichens who developed severe underarm dermatitis. Lichens are little primitive plants that grow on trees. Can people have allergic reactions to products that are too old? They can but I can honestly say I?ve never seen anybody develop a skin infection from using a product that was too old. When bacteria and fungus grow on the product because the preservative is no longer active, you can see them, and it looks awful. It?s like seeing a piece of moldy bread. If you ate the moldy bread, probably nothing would happen to you, but it?s going to taste awful and it looks awful. There is some concern about bacteria and fungus in products that are used around the eye though. So the advice for using products around the eyes, such as mascara, is usually to replace them every six months to make sure that it?s properly preserved and fresh. Why do sunscreens cause allergic reactions? In the United States, some sunscreens contain chemicals called oxybenzone and octyl dimethyl PABA, which have been associated with allergic contact dermatitis reactions. Unfortunately, the cosmetics industry does not require that those chemicals be designated by those names on the label, so they may use alternative names for these chemicals. Now there are excellent alterative sunscreens that are advertised as being what they call chemical free. Now they're not chemically free, when they say chemical free, it means they don't contain the sunscreens that have been most commonly associated with allergic contact dermatitis reactions. Instead, they contain zinc oxide or titanium dioxide, which are pretty inert substances and excellent sunscreens. Why do hair care products cause allergic reactions? Far and away the most common cause of allergic reactions to hair care products in the United States are certain kind of hair dyes. These are the two-part hair dyes, which contain a chemical called paraphenylenediamine. Fortunately, there are lots of substitutes for permanent hair dyes. So the semi-permanent hair dyes or ones that don?t last quite as long usually don?t contain that chemical. And the second most common hair product to cause allergic reactions is permanent waves that contain a chemical called glyceryl thioglycolate. These are usually three-part permanent waves, or so-called 'acid perms'. Cysteamine is a new chemical in permanent waves that can cause allergy. However, I think it?s going to be a very rare problem. And another good alternative are two-part perms, which contain ammonium thioglycolate. These are the old-fashioned cold permanent waves, and they hardly ever cause allergy problems.