Who tracks skin allergies? How patch-test data is collected

When you hear that a preservative or a fragrance ingredient is a “common allergen”, someone had to count. That counting happens in dermatology clinics, one patch test at a time, and it’s pooled across clinics so trends show up. Here’s who does it, how, and how their numbers end up changing what’s allowed in your shampoo.

Where the numbers come from

A patch test is how a dermatologist finds out what your skin is allergic to. Small amounts of common allergens are taped to your back for a couple of days, and the skin is checked for a reaction over the following days. (We explain the test itself in our article on patch testing.)

One clinic’s results only tell you about that clinic’s patients. The useful picture comes when many clinics test in the same way and put their results together. That pooled data is the main early-warning system for skin allergies.

Who does the tracking in North America

In Canada and the US, the best-known group is the North American Contact Dermatitis Group, or NACDG. It’s a research group of dermatologists at specialist clinics, and its members include dermatologists in Toronto, Ottawa and Québec City. They test patients the same way, with the same screening series of allergens, and publish combined results about every two years.

Their most recent full report, published in 2025, covers 2021 and 2022:

• 12 centres across North America took part.
• Each patient was tested with a screening series of 80 allergens, prepared and applied in a standard way.
• 3,056 patients were tested.
• 72% had at least one positive reaction, and 46.6% were diagnosed with allergic contact dermatitis (an itchy rash caused by an allergy to something touching the skin) as their main problem.

The most common positives were nickel (a metal), the preservative methylisothiazolinone, hydroperoxides of linalool (a fragrance-related allergen), and cobalt, another metal.

What this kind of data is good at

Europe’s Scientific Committee on Consumer Safety (SCCS), which advises the European Commission on cosmetic safety, summed up what clinic patch-test data offers. It gives:

• the first sign that exposure to a substance is causing allergy in the population;
• a way to compare how important different allergens are, by how often they cause reactions;
• a way to follow allergy trends over time.

Trends are the real strength. Because the same allergens are tested the same way year after year, a rise or fall stands out, even if the absolute numbers aren’t typical of everyone.

What it can’t tell you

The same SCCS opinion is just as clear about the limits. Clinic data doesn’t prove which products caused someone’s allergy, doesn’t show how much exposure it took, and doesn’t say what level would be safe.

A few other cautions are worth knowing:

• These aren’t general-population rates. The people tested were sent to a specialist because they had a rash that looked allergic. So “11.5% reacted to methylisothiazolinone” means 11.5% of patients tested in these clinics, not 11.5% of Canadians.
• The patients aren’t a mirror of the population. A 2026 NACDG analysis of more than 32,000 patients tested between 2007 and 2020 found that 86.5% were white. The authors listed this referral population, with uneven representation of racial and ethnic groups, as a limitation. They also found some allergens were more common in certain groups, likely because people’s everyday exposures differ.
• The screening series misses things. In 2021–2022, 21.1% of patients had at least one relevant reaction to something that wasn’t in the 80-allergen series. Clinics add extra allergens based on a person’s job and products.
• The test material itself can shift the numbers. One NACDG study found that positive reactions to propolis (a bee product used in some natural cosmetics) jumped to 14.7% in 2020, when the test material came from a different source, then fell to 2.2% in 2021–2022. The authors concluded that the switch in source explained the jump, and that unexpected changes should be investigated before anyone treats them as a real trend.

How the data changed the rules: the methylisothiazolinone story

The clearest example of patch-test data changing regulation involves methylisothiazolinone, or MI, a preservative that was widely used in cosmetics, including wet wipes.

In the early 2010s, clinics in Europe noticed a sharp rise in MI allergy. In its 2013 opinion (revised in 2014), the SCCS called the rise in reported cases, as detected by diagnostic patch tests, “unprecedented in Europe”. It concluded that the increase was mainly driven by growing consumer exposure to MI from cosmetics, and said an obvious increase in allergy among consumers showed “a failure of risk assessment and/or management of the risk.” It found no safe level for leave-on products, including wet wipes, had been shown.

North American clinics saw the same wave. In Canada, Health Canada acted through its Cosmetic Ingredient Hotlist, the official list of ingredients that are banned or restricted in cosmetics. In 2018, it prohibited MI in leave-on products and cut the amount allowed in rinse-off products, stating the change was “due to the risk of sensitization” (sensitization means the immune system learning to react to a substance, the first step toward an allergy).

And the data shows the change working. The NACDG’s 2021–2022 report says MI positivity has kept falling since its peak in 2017–2018, and describes the epidemic of MI allergy in North America as continuing its descent. It’s still among the most common positives, but the trend is heading down.

What this means for you

Tracking data helps explain why certain ingredients keep showing up on “avoid” lists for sensitive skin, and why some have disappeared from products over time. It doesn’t mean everyone should avoid every listed allergen. Most people won’t react to most of them.

What matters is your own skin. If you have a rash that keeps coming back where products touch your skin, such as your hands, eyelids, face or scalp, a patch test is the way to find out what you’re actually allergic to. If it isn’t better after two weeks of gentle, fragrance-free care, see your doctor or pharmacist and ask whether a referral for patch testing makes sense.

What to remember: skin allergy trends are tracked by pooling patch-test results from specialist clinics, including Canadian ones. The data is strongest at spotting rising allergens and following trends, and weakest at telling you how common an allergy is in everyone. It flagged the methylisothiazolinone epidemic, regulators in Europe and Canada restricted it, and the numbers have been falling since.