The importance of patch testing skin care products

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A patch test refers to a diagnostic exam that people can use to determine whether specific products result in skin irritation or an allergic reaction. Many different substances can cause a skin reaction, chemicals, preservatives, perfumes, and cosmetics.

Patch testing involves applying a small amount of a substance or product to the skin and leaving it on to see if a reaction develops.

This differs from allergy tests, such as a skin prick test. Typically, an allergist will perform an allergy test to identify what is causing the symptoms, which may provide immediate reactions. However, skin allergies from a patch test can take days to develop.

A dermatologist or another doctor may offer patch testing in their office. However, people can perform a patch test at home to trial a cosmetic or skin care product before fully incorporating it into their routine.

Learn about RAST tests here.

The AAD recommends the following steps for patch testing a new skin care product:

1. Apply the product to a small patch of skin where a person is unlikely to accidentally wash or rub it away. Good areas may include the inside of the arm or bend of the elbow.
2. Apply the product to a quarter-sized patch of skin. A person should apply the product as thickly as they would when using it regularly.
3. Leave the product on the patch of skin for as long as it would normally be on the skin. If a person is testing a product that they would usually wash off, such as a cleanser, they should keep the patch on for 5 minutes or as long as the instructions advise.
4. Repeat the patch test twice a day for between 7–10 days. A reaction may not happen immediately, so it is important to continue applying the product for this length of time.
5. If a person’s skin reacts to the product, they should wash it off as soon as possible and stop using it. A person can use a cool compress or petroleum jelly to relieve the skin if needed.

Learn how to treat an allergic reaction here.

Skin care products may contain many different ingredients. Some of these ingredients can irritate a person’s skin or even cause an allergic reaction. By performing a patch test on a small area of skin, a person can see how their skin tolerates a product before committing to using the product over a larger area.

Irritation from an ingredient may result in dermatitis or inflammation of the skin. When exposure to a substance irritation to a person’s skin, it is known as contact dermatitis, which can be either irritant contact dermatitis (ICD) or allergic contact dermatitis (ACD).

Contact dermatitis is a delayed hypersensitivity reaction, which can take a few days to appear. Therefore, it is important that a person does a patch test for at least 7 days before using new products fully. However, if a person has an allergy to a new substance, they might have a delayed response, which means they might not develop any symptoms straight away. However, if a person has previously encountered an irritating substance, a reaction can develop immediately.

ICD occurs when an external substance damages the skin’s epidermal cells, triggering an inflammatory response. Evidence suggests that 80% of people with contact dermatitis have ICD. The irritation can worsen due to low or high temperatures, high humidity, and a person’s skin type.

ACD occurs when a person has an allergic reaction following skin contact with an allergen. An allergic reaction occurs when a person’s immune system mistakes a harmless substance as harmful and attacks it. This immune response leads to inflammation and irritation.

Learn more about the differences between contact dermatitis and atopic dermatitis here.

Skin care products contain various chemicals that can potentially cause a reaction.

ICD often results from continuous exposure to mild irritants. Common causes of ICD can include soaps, acids, solvents, and alkalines.

Research suggests that typical irritants can include:

• anthralin
• benzalkonium chloride
• tretinoin and tetra ethylene glycol diacrylate
• dithranol
• epichlorohydrin
• podophyllin
• propane sulphone

Other research has shown that propylene glycol, an emulsifier commonly found in cosmetics, such as skin creams and lotions, is the cause of many instances of contact dermatitis.

Learn about other causes of allergic reactions and rashes here.

The Food and Drug Administration (FDA) classifies the common allergens that cosmetic products may contain into five groups: natural rubber, fragrances, preservatives, dyes, and metals.

Additionally, the American Academy of Dermatology notes that fragrances in skin care products can also result in ACD.

Other evidence suggests that fragrances and preservatives in cosmetic products are responsible for many allergic reactions.

Fragrances commonly found in skin hygiene products and moisturizers include:

• linalool
• limonene
• citronellol

Common preservatives include:

• methylisothiazolinone
• methylchloroisothiazolinone
• formaldehyde and formaldehyde-releasing ingredients

Balsam of Peru, a liquid from the Myroxolon balsamum tree, which many manufacturers use in perfumes, creams, and cosmetics, is another potential cause of ACD.

Dyes or chemicals in dyes to look out for include coal tar dyes, or synthetic organic dyes, which derive from petroleum, and the chemical paraphenylenediamine (PPD), found in permanent hair dyes, which can cause ACD in some situations.

Learn more about natural hair dyes here.

To avoid potential allergic reactions to skin care products, search for labels that say fragrance-free. Some products may have labels stating that they are unscented, but these may still contain fragrances.

Labels that say hypoallergic, organic, or suitable for sensitive skin do not necessarily guarantee the ingredients will not cause a reaction. Instead, people may want to look for products labeled free and clear.

Always read the list of ingredients on products carefully and see if they recognize any known allergens.

Learn more about moisturizers for sensitive skin here.

The symptoms of a skin reaction can vary depending on the product, where a person applies it, how often a person uses it, and how long it remains on the skin. A skin reaction can cause:

• a rash which may present as dry, dull, and scaly
• itching
• burning
• blistering
• swelling

A 2018 study found that reactions occurred most often on the face (94.8%) followed by the neck (22.4%). The most common causes were soaps (53%) and face creams (24%).

A 2019 study notes that many people experience reactions to cosmetics on their eyelids. They also found that the face is subject to reactions due to people directly applying cosmetics or through someone’s hands touching their face.

If a person experiences skin irritation, it is advisable to contact a dermatologist to help manage their symptoms. Particularly if they experience a severe reaction and do not get relief with a cool compress or petroleum jelly. Additionally, a person may consider seeing an allergist if they suspect ACD.

A person should seek medical assistance if they:

• develop a rash which is bleeding or oozing yellow pus, which may be a sign of infection
• think they have a skin allergy
• have a rash that has not improved after 2–3 weeks.
• develop hives all over the body
• a severe allergic reaction, known as anaphylaxis
• breathing difficulties

Skin care products are a common cause of contact dermatitis. A person can predict whether a product will irritate their skin or cause an allergic reaction by performing a patch test before using the product fully.

This can help guide what products are suitable and what ingredients to avoid.