In 2024, three dermatologists, Rachel Lin, Deborah Lin, and Andrea Maderal, published a review in the journal Dermatitis with a title built to make you flinch: "Toxic Ingredients in Personal Care Products." It runs through the usual suspects of the worried bathroom cabinet: bisphenols, phthalates, parabens, the "forever chemicals" (PFAS), a hair-dye chemical called p-phenylenediamine, and formaldehyde (Lin et al., 2024).
It is a generous, well-organised piece of work, the kind of review that gathers scattered evidence into something a non-specialist can navigate, and the authors are admirably honest throughout about where the evidence is strong and where it thins out. We'd encourage anyone curious to read the original. What we want to borrow from it is its most useful and least-quoted lesson: the case against each of these ingredients is wildly different in strength, and "toxic" is almost never a yes or no.
The evidence ladder, in plain terms
When scientists ask whether an ingredient causes harm, not all studies carry equal weight. Picture a ladder.
↑ Studies following large numbers of real people over time — strongest evidence
Controlled human experiments
Animal studies
↓ Cell tests in the lab — useful for spotting mechanisms, but isolated cells are not a person and doses often far exceed real-world exposure
Lin and colleagues, read carefully, place their own ingredients up and down this ladder. So let's follow them, honestly, in both directions.
Where the evidence is genuinely strong, and we won't soften it
Two of the review's ingredients sit near the top of the ladder, with real human evidence. They deserve to be stated plainly.
p-Phenylenediamine (PPD), in permanent hair dyes, is a well-documented contact allergen. The authors report it ranking among the top five allergens in patch testing, with reactivity that has risen sharply over the past two decades (Lin et al., 2024). For a sensitised person this is not theoretical: reactions range from eczema to, rarely, dangerous facial swelling. This is a real, evidence-backed concern, and we say so without hedging.
Formaldehyde carries the heaviest label of all, and it earns a careful, unflinching section of its own.
Formaldehyde: same molecule, very different exposures
The International Agency for Research on Cancer classifies formaldehyde as a known human carcinogen. The review is precise about the basis for that classification: it is driven by inhalation, breathing the vapour, linked in occupational settings to cancers of the nose, throat, and lungs (Lin et al., 2024).
That detail matters, because it means the danger depends heavily on the exposure. And the review points at one genuinely concerning exposure that a comforting "it's only a trace in a lotion" would dishonestly skip: hair-smoothing and keratin treatments. That eye-watering smell in the salon is formaldehyde vapour, at high concentration, often heated, in an enclosed room, the closest a personal-care product comes to recreating the occupational, inhaled exposure behind the cancer classification. The authors specifically report that hair straightening with formalin — an aqueous formaldehyde solution of roughly 40% — was associated with a threefold increase in the odds of frontal fibrosing alopecia, a scarring hair loss (Lin et al., 2024). This is not the place for reassurance. A high-concentration, heated, inhaled formaldehyde exposure is a real hazard, and the evidence says so.
That is different from a formaldehyde-releasing preservative present at a fraction of a percent in a rinse-off shampoo, a far smaller, cooler, mostly-not-inhaled exposure. Same molecule; different product, dose, and route; honestly different level of concern. Holding those apart is the point, not collapsing them in either direction.
Where the evidence is genuinely weaker, and we'll say that too
The ingredients that generate the most fear often have the least certain evidence, and the authors are candid about it.
On bisphenol A (BPA), the review calls it a "weak" hormone mimic, binding the body's estrogen receptors a thousand to ten-thousand times less tightly than the real hormone, and notes that its links to skin conditions get weaker once other co-measured chemicals are accounted for, with the prenatal evidence "mixed" (Lin et al., 2024).
On phthalates, it notes the proposed mechanism is "unknown," that a US study of children found no eczema link where Asian studies did, and that the headline melanoma finding came from cells in the lab, with, in the authors' own words, no human studies on phthalates and melanoma to date (Lin et al., 2024).
On parabens, perhaps the most maligned of all, the review reports systemic exposure is low, toxicity studies show no acute harm by any route, and — the detail nobody quotes — in 2019 the American Contact Dermatitis Society named paraben mixes its "non-allergen of the year" (Lin et al., 2024).
For these, the honest verdict is "studied, plausible in the lab, unproven in people," and saying so is not complacency. Over-stating a weak risk and under-stating a strong one are the same failure; we're trying to do neither.
Where New Zealand has already drawn the lines
Here's the local payoff, and it reads almost like the evidence ladder turned into law. New Zealand's rulebook is explicitly built on the European Union's cosmetics regulation, the strictest benchmark going (NZ Cosmetic Products Group Standard 2020, cl. 8(1)), and watch how finely it slices the very list above:
That is not "ban everything that sounds scary." It is proportionate: hard bans for the strongest cases, limits for the dose-dependent ones, and chemical cousins judged one at a time. A regulator, working from the same science Lin and colleagues summarise, reached the same nuanced conclusion the review keeps gesturing toward.
The takeaway you can carry to the shelf
The next time a label or an app flashes a scary ingredient at you, the useful question isn't "is it on a list." It's the one this review, read honestly, teaches: what kind of evidence, at what dose, by what route, and has the regulator already drawn a line? Sometimes the honest answer is "this concern is overblown." Sometimes, as with a heated formaldehyde hair treatment in a closed room, it is "take this one seriously." Telling those two apart, plainly and in both directions, is the entire job.
Our thanks to Lin, Lin and Maderal, whose careful review made this translation possible.
References
– Lin, R.R., Lin, D.A., & Maderal, A.D. (2024). Toxic Ingredients in Personal Care Products: A Dermatological Perspective. Dermatitis, 35(2), 121–131. DOI: 10.1089/derm.2023.0215
– New Zealand Environmental Protection Authority. Cosmetic Products Group Standard 2020 (HSR002552, effective 1 January 2026). Schedule 4 (prohibited): Bisphenol A (1176); isopropyl-, isobutyl-, phenyl-, benzyl-, pentylparaben (1374 to 1378); formaldehyde, paraformaldehyde, methylene glycol (1577 to 1579); quaternium-15. Schedule 7 (allowed preservatives): DMDM hydantoin, capped at 0.6%. Formaldehyde-releaser 0.1% w/w threshold; PFAS phase-out (cl. 4); EU basis (cl. 8(1)).
This is a translation of one review and one regulation, not medical advice. We reach no verdict on any brand and name none. The Ingredient Lab takes no advertising and holds no affiliate links. Spotted something we've got wrong? science@theingredientlab.org