Common Irritants, Allergens, and How Patch Testing Works

When skin reacts to a product, people call it an allergy. Usually it is not. Telling the two apart changes what you should do next.

Irritation versus allergy

Irritant contact dermatitis is direct damage to the skin barrier. It is dose-dependent, it can happen the first time you use something, and it tends to appear where the product sat, with burning or stinging more than itch. Strong acids, high-percentage retinoids, harsh surfactants, and overuse of otherwise reasonable actives all cause it. Most people who “react to everything” are irritated, not allergic, and are often using too many actives at once.

Allergic contact dermatitis is an immune response. It needs prior exposure to develop, then appears roughly one to three days after contact, and it is itchy. It can spread past the area of contact. Once you are sensitized, even a small amount can set it off, and the sensitivity is usually permanent.

Ingredients that come up most often

These appear repeatedly in dermatology testing data. None of them are dangerous to the general population, and most people use them without trouble.

• Fragrance materials, including fragrance mixes and balsam of Peru. The most common cosmetic allergen group.
• Preservatives, particularly methylisothiazolinone and methylchloroisothiazolinone, and formaldehyde-releasing preservatives such as DMDM hydantoin, quaternium-15, and imidazolidinyl urea.
• Essential oils and botanicals. Limonene and linalool form sensitizing compounds as they oxidize with air. Tea tree oil is a frequent offender. Natural does not mean gentle.
• Lanolin, used in many rich balms.
• Cocamidopropyl betaine, a cleanser surfactant.
• Propylene glycol, more often an irritant than an allergen.
• Paraphenylenediamine (PPD) in permanent hair dye, which causes some of the most severe cosmetic reactions.
• Sunscreen filters, mostly the chemical ones, which occasionally cause photoallergic reactions.

Testing a product yourself

An at-home test is a reasonable screen for a new product, though it is not a substitute for clinical testing.

1. Pick a discreet area with thin skin: the inner forearm near the elbow crease, or behind the ear.
2. Apply a small amount of the product as you would normally use it.
3. Repeat twice daily for seven to ten days. One application is not enough, because allergy is delayed and often needs repeated exposure to show.
4. Watch for redness, itch, bumps, or scaling. If nothing appears, move to a small area of the face before full use.

Test one product at a time. If you start three new things in one week, a reaction tells you nothing about which one caused it.

Formal patch testing

Clinical patch testing is a different procedure. A clinician applies standardized panels of allergens to your back, they stay in place for about 48 hours, and readings are taken at removal and again a few days later, because reactions develop late. You can also bring your own products to be tested. The result is a specific list of substances to avoid, which is far more useful than guessing.

Ask about it when you have a rash that keeps returning, when the reaction is spreading or persistent, when your job exposes you to the same materials daily, or when eliminating suspects has not worked. A dermatologist or allergist runs the test.

Reducing your odds

Simplify your routine, introduce one product at a time, choose fragrance-free where you have the option, replace oil-containing products before they oxidize, and keep the packaging of anything that causes a reaction so a clinician can read the list. Before any professional service, tell your provider about past reactions. The intake form at a reputable clinic offering professional skin treatments in Knoxville, or anywhere else, asks about this for a reason.