The U.S. Food and Drug Administration (FDA) defines a cosmetic as a product intended to cleanse or beautify a person without altering the body’s structure or functions. This includes products ranging from hair shampoo to perfume and underarm deodorant.

An allergic reaction to cosmetics occurs when the body reacts to a substance it mistakenly perceives as a threat. It typically manifests itself as contact dermatitis, causing the skin to become red, itchy, flaky and inflamed.

A reaction to cosmetics does not necessarily mean a person is allergic to the product. Irritation, which also results in contact dermatitis, is more common than an actual allergy. It is the result of a chemical reaction that occurs when a cosmetic is applied to the skin, and not because the body perceives the cosmetic to be an antigen (a substance that causes the immune system to produce antibodies to defend the body).

Contact dermatitis often occurs on or near the site where the product has been applied, such as the face, lips, eyes, ears and neck. The hands can also be affected (usually by moisturizers or nail products).

The most common type of cosmetic allergy involves fragrances. Fragrances can be inhaled or ingested, and they can affect eyes and lungs.

Allergic reactions to fragrances differ slightly from most other kinds of allergies. In most allergic reactions, the body reacts to an allergen that it perceives as a harmful invader by triggering the cells of the body to release certain chemicals. These chemicals, such as histamines, trigger inflammation and other changes that manifest themselves in the symptoms of an allergic reaction.

However, unlike most allergens, fragrance materials are not protein–based, and they generally are too small to be detected by the body. Instead, they act as a hapten, binding with proteins in the skin. As they do so, they modify the proteins, causing the body to perceive the proteins as a foreign substance and to trigger an allergic reaction.

Some fragrance materials are absorbed intact into the body and remain that way. Others are broken down into compounds. The allergic reaction may be to the entire material, or to one or more of its compounds.

Once the process of sensitization to a fragrance material has occurred, it is likely to grow worse with time and repeated exposures. Moreover, fragrance allergies potentially affect not only the person wearing the fragrance, but anyone who comes into contact with that person and is exposed to the substance.

Some individuals are allergic to the scent of a cosmetic, and others react when a chemical ingredient is applied to the skin. Sometimes, it is the preservatives in the cosmetic that trigger an allergic reaction. Some cosmetic preservatives are also used in foods, and ingesting these substances can produce a reaction.

In addition, various fragrances tend to trigger asthma symptoms in some individuals. Researchers have not yet established whether chemicals in fragrances simply exacerbate attacks, or actually sensitize the body and cause asthma itself.

Unlike drugs, most cosmetics do not have to be tested for safety before they go on the market. The exception to this is cosmetics containing medications (e.g., some dandruff shampoos, some fluoride toothpastes), which do have to comply with FDA drug standards. Makers of cosmetics also do not have to register their formulations, test results or consumer complaints with the FDA.  Outside of color additives and a few prohibited substances, manufacturers can use whichever ingredients they like and make various claims about them.

Because the industry is self-regulated, consumers should take the safety claims on cosmetic products with a grain of salt. Experts warn that terms used by the industry to promote the safety of cosmetics – including “hypoallergenic,” “natural” and “fragrance-free” – have no objective measure and are largely meaningless. Even products billed as fragrance-free or “unscented” may contain scenting designed to cover up the cosmetic’s chemical smell.

The term "dermatologist-tested" is also not a guarantee that a product will not cause a reaction. This claim only denotes that a physician evaluated the general likelihood of the product to trigger a reaction. It does not mean that the product is safe for everyone.

Though some manufacturers may advertise their products as "allergy free," there is no such thing as a non-allergenic cosmetic. All cosmetics have the potential to cause allergic reactions.

Manufacturers are required to list the ingredients that make up a cosmetic. They also have to list them in descending order from the ingredient with the highest concentration in the cosmetic (usually water) to the lowest. The makers of cosmetics also must include a warning if a product’s safety has not been proved through testing.