Fragrances are found in a variety of commonly used perfumes, skin products, household products, foods, cosmetics and other substances. Many people use fragrances regularly without any ill effect, but for sensitive people, fragrances can cause a range of other reactions, including contact dermatitis (an itchy and inflamed skin rash), asthma, nasal congestion, cough, hives (raised bumps that appear on or beneath the skin) or eczema (inflammatory skin disease with lesions that usually appear very dry, thickened or scaly).

Contact dermatitis often occurs on and near the site where the product has been applied, such as the face, ears and neck. The hands and eyes may also be affected. Fragrances can be inhaled or ingested, and therefore can affect the lungs, nose, throat and mouth.

A reaction to fragrances does not necessarily mean a person is allergic to the product. Most cases of contact dermatitis (the most common reaction) are the result of a chemical reaction and not an allergic reaction involving the immune system. However, contact allergy to fragrances is increasing worldwide. Fragrances are the second most common contact allergens (after nickel). Contact allergens are substances that trigger allergic skin reactions in some people when they come in contact with the skin.

Fragrances are made from mainly synthetic organic chemicals. Little is known of the impact of fragrances on human health. The development of fragrances is both competitive and complex, and manufacturers are reluctant to disclose specific ingredients (or production methods) to protect their trade secrets. Furthermore, claims of adverse reactions to fragrances can be difficult to prove or to link to particular chemicals.

Irritant reactions to fragrances differ from 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 histamines and certain other chemicals. These chemicals then trigger allergy symptoms.

Unlike most allergens, fragrance materials are not protein-based, and they are generally 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, thus triggering a reaction.

Some fragrance materials are absorbed intact into the body and remain that way. Others are broken down into compounds. The reaction may be to the entire material, or to one or more of its compounds. Sometimes, light breaks down these materials, resulting in a photosensitive reaction due to the light exposure.

Once a person has developed fragrance irritation it is likely that the sensitivity will increase with time and with repeated exposure. Certain chemicals may be sensitizers at high levels of exposure and can result in sensitivity to the chemical at much lower levels after initial exposure.

Fragrance irritation potentially affects not only the person wearing the fragrance, but anyone who comes into contact with that person and is exposed to the substance.