Patch testing describes a procedure to help identify what ingredients might irritate a person’s skin. Many skin care products contain chemicals that can cause an allergic reaction when a person applies them to their skin. By performing a patch test at home, a person can determine if a product contains ingredients that may cause a skin reaction.
The American Academy of Dermatology (AAD) states that more than 15,000 substances can cause an allergic skin reaction.
A 2021 study found that personal care products caused irritant contact dermatitis in 28.8% of males and 39.5% of females.
Performing a patch test at home can help people identify cosmetic products or ingredients that may cause an allergic skin reaction.
This article discusses the importance of patch testing, ingredients to look out for, and how to perform a patch test.
Learn more about allergic reactions on the face here.
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.
The AAD recommends the following steps for patch testing a new skin care product:
- 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.
- 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.
- 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.
- 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.
- 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.
Skin care products may contain many different ingredients. Some of these ingredients
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 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:
- benzalkonium chloride
- tretinoin and tetra ethylene glycol diacrylate
- 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
Additionally, the American Academy of Dermatology notes that fragrances in skin care products can also result in ACD.
Fragrances commonly found in skin hygiene products and moisturizers include:
Common preservatives include:
- 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
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.
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
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.