The ingredients most likely to cause a skin allergy

Thousands of ingredients go into the products we put on our skin, but only a handful cause most of the allergic reactions seen in clinics. Dermatologists know which ones because they patch test people with rashes year after year and count the results. Here are the ingredients that top the list, and where they turn up in everyday Canadian life.

Where the numbers come from

The best data for our part of the world comes from the North American Contact Dermatitis Group, a network of dermatologists in Canada and the United States who patch test people with suspected allergic contact dermatitis (an itchy rash caused by the immune system reacting to something that touched the skin). In their 2019–2020 report, 4,121 people were tested at 13 centres with a standard series of 80 allergens.

About 70% of those tested had at least one positive reaction, and just over half were diagnosed with allergic contact dermatitis as their main problem. One thing to keep in mind: these are people who were sent for testing because they already had a rash. The percentages below describe that group, not the general population, where allergies are much rarer.

Nickel: the metal that never leaves the top spot

Nickel was positive in 18.2% of people tested, the highest of any allergen. DermNet describes it as one of the most common causes of allergic contact dermatitis, more often in women, probably because of ear piercing.

Nickel isn’t a cosmetic ingredient, but it’s everywhere else. DermNet’s list of everyday sources includes earrings, watches, bracelets and rings; zips, buttons, bra hooks and glasses frames; mobile phones, keys, razors, scissors and cutlery; and silver-coloured coins, which are made of a copper-nickel alloy. The classic rash sites give the game away: the earlobes from earrings, the wrists from a watch strap, and the lower belly from a jeans stud.

If you suspect nickel, DermNet notes that a nickel-testing kit (dimethylglyoxime) can be obtained from a dermatologist, pharmacist or online supplier: rub a drop on the metal with a cotton bud, and pink means nickel. Stainless steel, silver and solid gold of 12 carats or more are usually safe choices for jewellery.

Methylisothiazolinone: the preservative behind an epidemic

Methylisothiazolinone, usually shortened to MI, was positive in 13.8% of people tested. It’s a preservative, an ingredient added to water-based products to stop bacteria and mould from growing. According to DermNet, it moved from industrial uses into everyday consumer products around 2005, and within a few years dermatologists were seeing so many new cases that they called it an epidemic. The American Contact Dermatitis Society named it Allergen of the Year in 2013.

Canada acted. Health Canada’s Cosmetic Ingredient Hotlist, the list of ingredients that are prohibited or restricted in cosmetics, was changed in December 2015 to prohibit the combination of methylchloroisothiazolinone and methylisothiazolinone in leave-on cosmetics. In June 2018, MI on its own was also prohibited in leave-on products, and the total allowed in rinse-off products was capped at 0.0015%, or 15 parts per million. So in Canada, a moisturizer or sunscreen shouldn’t contain MI anymore, but a shampoo or hand soap still can.

That helps, but cosmetics aren’t the only place MI lives. DermNet’s list of sources includes wet wipes and baby wipes, laundry detergents and fabric softeners, household cleaners, paints, adhesives and printing inks. None of those are cosmetics, so the Hotlist doesn’t apply to them. The good news in the 2019–2020 data is that, for the first time, the rate of MI allergy didn’t rise between reporting periods.

A related preservative, benzisothiazolinone, was positive in 10.4% of people, and its rate was rising. Reading labels for anything ending in “-isothiazolinone” covers the family.

Fragrance: the one you can smell coming

Fragrance mix I, a standard blend of eight common fragrance ingredients used in patch testing, was positive in 12.8% of people tested. DermNet ranks fragrance allergy second only to nickel as a cause of allergic contact dermatitis, and estimates it affects about 1% of adults and around 1.8% of children and teenagers.

The eight ingredients in fragrance mix I are oakmoss absolute, isoeugenol, eugenol, cinnamyl alcohol, cinnamaldehyde, hydroxycitronellal, geraniol and amyl cinnamaldehyde. Some are plant extracts and some are synthetic, which is a reminder that “natural” fragrance is still fragrance: a 2011 European review counted 82 established fragrance allergens, 28 of them natural extracts. A second mix of six more ingredients is also tested.

The fastest riser in the North American data is a group called hydroperoxides of linalool, positive in 11.1% of people, with hydroperoxides of limonene at 3.5% and both increasing. Linalool and limonene are two of the most widely used fragrance ingredients, and both are on the European list of fragrance allergens that must be named on labels. Their hydroperoxides are the forms that most often cause reactions.

DermNet’s list of where fragrance hides is long: personal care products, household cleaners, food flavourings and even some medicines. It also notes that “unscented” products may contain a masking fragrance to cover the smell of the base. If you have a fragrance allergy, “fragrance-free” is the label to look for, and an ingredient list beats either word. Typical rash sites are the hands, face and neck, and the armpits in both men and women.

You can browse products reviewed for the fragrance-free tag if you want a shortlist.

Cocamidopropyl betaine: the foam that isn’t as gentle as its name

Cocamidopropyl betaine is a surfactant, the kind of ingredient that makes a cleanser foam and lift dirt. It’s derived from coconut oil and is often used in products sold as mild. Yet DermNet reports a diagnosis rate of 6% in an Australian group of people suspected of having allergic contact dermatitis, more often in women.

You’ll find it in shampoo, conditioner, body wash, hand soap, makeup remover, shaving cream, some toothpastes, contact lens solutions and household cleaners. Reactions typically show up on the face and hands, and DermNet notes that eye irritation, redness and itching can come with them, which makes sense for something that runs over your eyes in the shower. It’s blended during manufacturing with two related chemicals, dimethylaminopropylamine and amidoamine, which is why those names sometimes appear in patch-test results. The practical advice is simply to avoid products that list it, under that name or its alternatives.

Two more names worth knowing

Lanolin, the wax from sheep’s wool used in many ointments, and propylene glycol, a common humectant (an ingredient that pulls water into the skin), also appear in patch-test series. Each gets its own explainer on this site.

What to do with this list

If you have a rash that keeps coming back, check your products for nickel-releasing metal, anything ending in “-isothiazolinone”, “parfum” or “fragrance”, and cocamidopropyl betaine. Keep the packaging of anything that seems to set it off.

But don’t self-diagnose. The 2019–2020 data found that about one in five people reacted to an allergen that wasn’t even in the standard series, and a rash can have several causes at once. If a rash isn’t better after two weeks of avoiding the likely culprit, see your doctor or pharmacist, and ask about patch testing. It’s the only way to turn a guess into a personal list of ingredients to avoid.

The short version: nickel, methylisothiazolinone and fragrance are the three ingredients most likely to be behind a contact allergy in North America, followed by oxidized fragrance ingredients, a related preservative and the foaming agent cocamidopropyl betaine. They live in jewellery and coins, wipes and cleaners, shampoo and scented everything. Canada has pulled MI out of leave-on cosmetics, but it can still be in rinse-off and household products, so the ingredient list remains your best tool.