In October 2022, the Journal of the National Cancer Institute published an analysis from the Sister Study cohort. Women who used chemical hair straighteners frequently — more than four times in the previous twelve months — had a hazard ratio of 2.55 for incident uterine cancer, with a 95 percent confidence interval from 1.46 to 4.45. The cohort was 33,947 women followed for roughly eleven years.
Two-and-a-half times the risk. That figure went everywhere, almost always attached to a headline about Black women’s hair products, because 59.9 percent of the ever-users in the study self-identified as Black or African American.
- Systematic reviewWhat a verdict wants: people, randomized, followed long enough for the effect to be real rather than new.
- Randomized trialWhat a verdict wants: people, randomized, followed long enough for the effect to be real rather than new.
- Small human trialWhat a verdict wants: people, randomized, followed long enough for the effect to be real rather than new.
- Case seriesWhat the box usually cites. Real research, but a long way from your face — a reason to look, never proof.
- Animal modelWhat the box usually cites. Real research, but a long way from your face — a reason to look, never proof.
- In vitroWhat the box usually cites. Real research, but a long way from your face — a reason to look, never proof.
Here is what did not travel with it.
The cohort was 85.6 percent white. The Sister Study was not a study of Black women; it was a large study of women in general, in which Black women were a minority who happened to make up most of the straightener users. And when the researchers looked for a difference in the association by race, they did not find one. The relationship between frequent straightener use and uterine cancer looked the same regardless of who was doing the straightening.
Which is a substantially different finding from the one the coverage described. It is not a story about a product category harming Black women specifically. It is a story about a chemical exposure that affects whoever is exposed, and Black women are disproportionately exposed.
The absolute numbers, which are the ones you actually need
A hazard ratio is a ratio. It tells you how much something multiplies a risk. It tells you nothing about how large the risk was to begin with, and uterine cancer is not common.
In the same analysis: estimated risk of developing uterine cancer by age 70 was 1.64 percent among women who never used straighteners, and 4.05 percent among frequent users.
That is an absolute increase of about 2.4 percentage points, which works out to roughly one additional uterine cancer for every 42 frequent users, over a lifetime, by age 70.
Both framings are honest and they land completely differently. “Two and a half times the risk” sounds like an emergency. “About one in forty-two, over a lifetime, among frequent users” sounds like what it is: a real signal, worth acting on, not a reason to panic about a decision you made in 2009.
I am giving you both because the relative figure without the absolute one is how health reporting frightens people into inaction, and the absolute figure without the relative one is how industries talk you out of caring.
| How it is stated | The figure |
|---|---|
| Risk by age 70, never used straighteners | 1.64% |
| Risk by age 70, used more than four times in the previous twelve months | 4.05% |
| The difference between those two | about 2.4 percentage points |
| The same difference, counted in people | roughly one extra cancer per 42 frequent users |
| The same difference, as a hazard ratio | 2.55 (95% CI 1.46–4.45) |
Hair relaxers cause uterine cancer in Black women.
- Source
- Chang CJ, O'Brien KM, White AJ et al., Journal of the National Cancer Institute, 17 October 2022, Sister Study cohort
- Participants
- 33,947 women, approximately 11 years of follow-up
- Measured
- A US cohort that was 85.6% white; 59.9% of ever-users self-identified as Black or African American
- Not measured
- Brand and ingredient data — none was collected. No difference in the association was found by race.
HR 2.55 for frequent use. Absolute risk by age 70: 1.64% never-users versus 4.05% frequent users, roughly one additional case per 42 frequent users. The exposure is disproportionate; the biology, as measured, was not.
The second cohort, which is the one that should have led
A separate analysis in Environmental Research in 2023 used the Black Women’s Health Study — nearly 45,000 Black women with no prior cancer and an intact uterus. Frequent or long-duration users, meaning more than twice a year or more than five years, showed greater than 50 percent increased risk of uterine cancer compared with never or rare users, particularly postmenopausal.
That is a genuinely relevant cohort asking the relevant question in the relevant population, and it found a signal in the same direction at a smaller magnitude. It received a fraction of the attention.
Two cohorts, two designs, both pointing the same way, different effect sizes. That is what a real finding usually looks like — not a single dramatic number, but a consistent direction across studies that disagree about how much.
Nobody recorded the ingredients
The Sister Study collected no brand and no ingredient data. Neither did the Black Women’s Health Study analysis, in any usable form.
So four years after the headline, there is no published answer to the only question that would let a person do anything with it: which chemical? Formaldehyde-releasing agents? Phthalates? Parabens? Something in the neutralizing step? The exposure is presumed to work through endocrine-disrupting compounds and possibly through scalp burns increasing absorption, and that is a mechanism sketch, not an identification.
Which means the practical advice available is crude. Use them less often. Do not let it burn — scalp burns are a plausible absorption route and they are common and they are treated as normal. Ventilate. And that is about the extent of what the evidence supports, because the studies that found the signal did not write down what was in the bottle.
The regulation, still not finished
In October 2023 the FDA was reported to be preparing a proposed ban on formaldehyde and formaldehyde-releasing agents in hair smoothing and straightening products. The rule is RIN 0910-AI83.
It has missed deadline after deadline. As of January 2026, contemporaneous reporting had the agency overdue again, with the proposed rule still not published.
Three years from proposal to nothing, on a rule addressing a chemical exposure concentrated in one population, with two cohort studies and a hazard ratio behind it.
That delay is a policy choice, and it is being made at the expense of people who are still, in the meantime, having to guess which bottle.
Evidence check: Moderate
Evidence level: two independent cohorts pointing the same direction at different magnitudes. Neither recorded brand or ingredient data, so no product-level conclusion is available.
Sources reviewed September 13, 2026: Chang CJ, O’Brien KM, Keil AP, Gaston SA, Jackson CL, Sandler DP, White AJ, “Use of Straighteners and Other Hair Products and Incident Uterine Cancer,” Journal of the National Cancer Institute, 17 October 2022, n=33,947. “Hair relaxer use and risk of uterine cancer in the Black Women’s Health Study,” Environmental Research 2023, n≈45,000. FDA proposed rule RIN 0910-AI83, October 2023; status as reported January 2026.
This article is educational and is not a substitute for care from someone who can look at your skin. Anything new, changing, painful or spreading should be assessed rather than researched.