Struggling With Persistent Itchy Rashes? A Patch Test Could Reveal their Source

If you have sensitive skin and you tend to react dramatically to new skincare or haircare products – red, itchy skin or a rash that can take weeks to clear up – you might benefit from a patch test, the gold standard for uncovering both common and rare skin allergies.

Over the next couple blog posts, I’ll explain what happens during a patch test, who’s a good candidate for one, and what happens after the test. Before I get into that, I want unpack skin allergens and highlight the most common ones found in beauty products.

What are skin allergens, exactly?

A skin allergen refers to any kind of chemical that, when it comes into contact with your skin, causes your immune system to think it’s under attack. In response, your immune system produces antibodies to combat the allergen. The result of that process is a red, typically itch rash called contact dermatitis.

There are two types of contact dermatitis, irritant and allergic. Irritant contact dermatitis refers to a skin irritation that occurs minutes or hours after coming into contact with the likes of a harsh cleaning product. In these instances, burning, stinging, or pain are the most common reactions. Allergic contact dermatitis is a rash that results from an allergic reaction to a chemical that touched your skin. Pinpointing the offending chemical can be difficult because the rash can develop days after contact.

An itchy rash may sound like a relatively mundane concern at first, but over days and weeks, it can erode the quality of your sleep and begin to affect your work performance and overall happiness, making a diagnosis all the more critical.

What are the most common culprits in beauty products?

For those with sensitive skin, it’ll likely come as little surprise to learn that fragrance ingredients and preservatives are among the most common skin allergens in beauty, skincare, haircare, and personal-care products.

More specifically, methylisothiazolinone (MI) and paraphenylenediamine (PPD) are two of the most frequent culprits. The latter is widely used in permanent hair dyes, although it can trigger a severe allergic reaction all over the body, not just the scalp.

We tend to have a false sense of security with natural fragrances, believing that only the synthetic ones should be avoided. But essential oils can be allergy-inducing, too. 

As I mentioned, pinpointing the source of allergic contact dermatitis can be a challenge, with the reaction often lagging days behind contact. And that’s not the only complicating factor. Allergies to certain ingredients aren’t always instantaneous. In fact, some people can develop an allergy to a product after months or even years of use. It’s a phenomenon called cumulative exposure.

Before you start to think that all your beauty, skincare, haircare, and personal-care products pose a threat to your skin, talk with a board-certified dermatologist to see if a patch test is right for you. To that end, I’ll detail what you can expect to happen during and after one in my next post.