Litigation Guides

An Attorney's Guide to Chemical Hair Straightening Products & Cancer

Exploring the evolving risks of hair straightening products from Garrett Augustus Morgan's accidental discovery in 1909 to a 2022 NIH study linking them to uterine cancer.


A Timeline of Hair Straightener Risks

1909: Garrett Augustus Morgan accidentally found a hair straightening cream when trying to find a solution to ease friction on sewing machines in his tailor shop.

2010: The Center for Research on Occupational and Environmental Toxicology (CROET) received complaints of difficulty breathing, nose bleeds, and eye irritation from stylists in one salon who claimed to have used a hair treatment as directed.

2010: The FDA announced that it was working with other organizations, to determine whether the products or ingredients in a Brazilian hair straightening product would be likely to cause health problems.

2022: A large cohort study by the NIH showed the association between hair straightener use and uterine cancer.


What are Hair Straightening Products?

woman at salon

Hair straightening products (HSPs) are chemicals that change the structure of hair to make it straighter or smoother. These come in various forms like shampoos, conditioners, sprays, gels, and moisturizers. HSPs fall into two main types:1

  • Permanent HSPs: These straighten hair for several months, typically by altering its bonds.
  • Temporary HSPs: These last for shorter periods, usually a few days, by coating or relaxing the hair strands.

The Department of Toxic Substances Control (DTSC) is raising concerns about potentially harmful chemicals in HSPs. They are specifically looking at the following ingredients:

  • Formaldehyde
  • Sodium hydroxide
  • Cyclosiloxanes
  • Parabens
  • Diethanolamine
  • Phthalates
  • Benzophenone-3
  • Triclosan

Scientific studies suggest a link between these chemicals and various health risks, including1:

  • Respiratory problems: difficulty breathing, coughing, wheezing
  • Hormone disruption: interference with hormone balance, potentially affecting fertility
  • Developmental and reproductive toxicity: harm to developing fetuses and children and fertility problems
  • Skin problems: irritation, rashes, and allergic reactions
  • Environmental exposure concerns: persistence in the environment and accumulation in living organisms, including human communities

What Contaminants are in Chemical Hair Straighteners?


Some professional HSPs contain high levels of formaldehyde, a highly dangerous chemical. Despite being banned in certain contexts, formaldehyde became popular in professional hair straightening due to its lower cost and desired results.4

Formaldehyde poses significant health risks to both users and stylists. It can cause serious problems in the upper respiratory tract, including:4

  • Cancer: Formaldehyde exposure has been linked to cancers of the mouth, nose, lungs, and blood, particularly with long-term exposure.
  • Short-term irritation: Even brief exposure can cause eye and nose irritation, as well as allergic reactions.

Sodium hydroxide

Hair relaxers often use sodium hydroxide (lye) to straighten hair by breaking its bonds, changing its curl pattern, and straightening it. While convenient, this chemical can be harmful:1

  • High levels can cause burns, irritation to skin, eyes, and lungs, and even temporary hair loss.
  • While safety data suggests most hair straighteners contain lower levels of sodium hydroxide (2.5% or less), they still pose a risk of adverse effects for both salon clients and workers.


Glutaraldehyde, a chemical used in some hair straighteners after formaldehyde's ban, poses even greater DNA and nasal tissue damage risks than formaldehyde.4

  • Studies show it can cause mutations and breaks in DNA, potentially increasing cancer risk
  • It can irritate and inflame nasal tissues, leading to breathing difficulties.


Parabens, common preservatives in cosmetics, raise concerns about hormone disruption and potential health risks. Here are some additional details regarding parabens:1,2

  • Used in shampoos, lotions, and more, parabens prevent bacteria growth and extend product shelf life.
  • Exposure happens through skin contact and dust inhalation.
  • Classified as endocrine disruptors, they may interfere with hormone function.
  • Studies suggest potential links to reproductive problems and breast cancer, although more research is needed.


Cyclosiloxanes are commonly used in various personal care products, including hair relaxers, for their antistatic and hair-conditioning properties.5 However, these chemicals raise concerns due to their potential for:1

  • Environmental persistence: They remain in the environment for a long time, potentially harming ecosystems and human communities.
  • Bioaccumulation: They build up in living organisms, potentially causing harm to wildlife and humans.
  • Reproductive toxicity: Studies suggest they may harm reproductive health.

Exposure to cyclosiloxanes can occur through:1

  • Direct skin contact: This results from using products containing them.
  • Ingestion of breast milk: This results from a nursing mother using cyclosiloxane-containing products
  • Inhalation: These chemicals are volatile and can evaporate into the air.

Benzophenone-3 (BP-3)

Benzophenone-3 (BP-3), also known as oxybenzone, is a common ingredient in hair relaxers and sunscreens.1,2 While it protects these products from sun damage, concerns are rising about its potential health and environmental risks. Key concerns about BP-3 include the following:1,6

  • Potential carcinogenicity: Animal studies are ongoing to assess its cancer-causing potential.
  • Endocrine disruption: Studies suggest it may interfere with hormone function, raising concerns about reproductive and developmental effects.
  • Environmental impact: BP-3 can enter waterways through various means, putting a greater number of humans at risk through drinking water.

Contaminants Banned by California

California's new Toxic-Free Cosmetics Act bans several harmful chemicals in hair straightening products starting in 2025. These chemicals include:1

  • Diethylhexylphthalate (DEHP): a suspected endocrine disruptor linked to reproductive problems.
  • Formaldehyde and methylene glycol (formaldehyde releasers): carcinogens associated with respiratory problems and cancer.
  • Isobutylparaben and isopropylparaben: parabens linked to potential endocrine disruption and breast cancer.

This move highlights growing concerns about the health risks associated with certain chemicals in hair straightening products.


What are Potential Dangers of Hair Straightening Products?

Damage to the Scalp and Hair and Allergic Responses due to Hair Products

Repeated use of professional chemical hair straighteners for over a year can lead to various health problems, including:4

  • Scalp irritation: itching, burns, and even scarring on the scalp
  • Hair damage: weakened and thinned hair shafts, discoloration, and hair loss
  • Allergic reactions: sensitivity to chemicals in the products affecting the skin, eyes, and lungs

Gynecological Cancers and Hair Products

According to the National Cancer Institute, uterine cancer rates have surged in the U.S. over the past two decades, with estrogen imbalance identified as a significant risk factor.7,8,9 Earlier research connected hair product usage to increased risks of hormone-sensitive cancers like breast and ovarian cancer.9,10

  • The Sister Study (2003-2009) by White and colleagues conducted on 40,000 women specifically found that hair product use, including hair relaxers and possibly permanent hair dye, may be linked to a higher risk of ovarian cancer.10
  • A recent 2022 NIH study involving 34,000 women from the Sister Study revealed a notable association between frequent use of hair straighteners/relaxers and a higher risk of uterine cancer. The risk increased by a factor of 2.55 for those with more than 4 times yearly use compared to those that never used.8

Breast Cancer and Phthalates

A study in Mexico explored a potential link between phthalates, commonly found in hair relaxers and other products, and breast cancer risk in women. Researchers compared urine samples from 233 women with breast cancer to 221 healthy women and made the following findings:11

  • Higher levels of diethyl phthalate (DEP), a specific phthalate, were found in breast cancer patients.
  • This DEP broke down into a metabolite called monoethyl phthalate (MEP).
  • Women with the highest levels of MEP had more than double the odds of having breast cancer compared to those with the lowest levels.
  • The link was even stronger in premenopausal women, with over four times the odds of breast cancer for those with the highest MEP levels.
    • This suggests a potential association between specific phthalates and breast cancer risk, especially in premenopausal women.
    • However, more research is needed to confirm these findings and understand the underlying mechanisms.

Cancer and Formaldehyde

A 2006 report by the International Agency for Research on Cancer (IARC) definitively classified formaldehyde as a carcinogen, primarily due to its strong link with nasopharyngeal cancer, a type of cancer that affects the upper part of the throat behind the nose.12

While relatively few new studies have directly investigated this association since then, several evaluations and analyses of existing research have reaffirmed the earlier findings.12

A study on 84 anatomy lab workers exposed to formaldehyde and 87 unexposed individuals found that those exposed (average exposure 0.38 parts per million) had significantly higher levels of:13

  • Chromosomal aberrations: abnormalities in the structure or number of chromosomes, potentially leading to health problems and cancer
  • DNA damage: breaks or other changes in the genetic material, potentially increasing cancer risk

This suggests that even relatively low levels of formaldehyde exposure can harm chromosomes and DNA, raising concerns of increased cancer risk for workers in environments with this chemical.


Who's at highest risk?

Studies suggest Black women and children use certain hair products more often than other groups. These products include hormone-disrupting hair and skin products, hair straighteners, and moisturizers.2,3

Recent research shows that hair products often contain chemicals of concern, including2:

  • Endocrine Disrupting Chemicals (EDCs): These chemicals can interfere with hormones and potentially harm health.
  • Asthma-associated chemicals: These chemicals have been linked to an increased risk of asthma.
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Strength of Available Evidence

Chemical Hair Straightening Products & Gynecological Cancer

Strength of evidence: HIGH

  • Recent scientific evidence indicates a link between HSPs and the risk of specific gynecological cancers, along with other health concerns.
  • Despite these findings, uncertainties persist in accurately predicting the definite health risks associated with exposure to these chemicals.

Works Cited


Department of Toxic Substances Control, California Environmental Protection Agency. Work Plan Implementation: Chemicals in Hair Straightening Products Background Document. Published 2021. Accessed January 23, 2024.


Helm JS, Nishioka M, Brody JG, Rudel RA, Dodson RE. Measurement of endocrine disrupting and asthma-associated chemicals in hair products used by Black women. Environmental Research. 2018;165:448-458. doi:


Taylor KW, Baird DD, Herring AH, et al. Associations among personal care product use patterns and exogenous hormone use in the NIEHS Sister Study. Journal of exposure science & environmental epidemiology. 2017;27(5):458-464. doi:


Miranda-Vilela AL, Botelho AJ, Muehlmann LA. An overview of chemical straightening of human hair: technical aspects, potential risks to hair fibre and health and legal issues. International Journal of Cosmetic Science. 2013;36(1):2-11. doi:


European Chemicals Agency (ECHA). Committee for Risk Assessment (RAC) Opinion on an Annex XV Dossier Proposing Restrictions on Octamethylcyclotetrasiloxane, Decamethylcyclopentasiloxane. Published online 2016.


National Toxicology Program (NTP). Technical Report on the Toxicology and Carcinogenesis Studies of 2- Hydroxy-4-methoxybenzophenone (CASRN 131-57-7) Administered in Feed to Sprague Dawley (Hsd: Sprague Dawley SD) Rats and B6C3F1/N Mice. Published 2020. Accessed January 23, 2024.


National Cancer Institute (NCI). Cancer stat facts: uterine cancer. Published November 10, 2021. Accessed January 23, 2024.


Chang CJ, O’Brien KM, Keil AP, et al. Use of Straighteners and Other Hair Products and Incident Uterine Cancer. JNCI: Journal of the National Cancer Institute. 2022;114(12). doi:


Mallozzi M, Leone C, Manurita F, Bellati F, Caserta D. Endocrine Disrupting Chemicals and Endometrial Cancer: An Overview of Recent Laboratory Evidence and Epidemiological Studies. International Journal of Environmental Research and Public Health. 2017;14(3). doi:


White AJ, Sandler DP, Gaston SA, Jackson CL, O’Brien KM. Use of hair products in relation to ovarian cancer risk. Carcinogenesis. 2021;42(9):1189-1195. doi:


López-Carrillo L, Hernández-Ramírez RU, Calafat AM, et al. Exposure to Phthalates and Breast Cancer Risk in Northern Mexico. Environmental Health Perspectives. 2010;118(4):539-544. doi:


IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Chemical Agents and Related Occupations: IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, No. 100F. International Agency for Research on Cancer; 2012. Accessed April 25, 2023.


Costa S, Carvalho S, Costa C, et al. Increased levels of chromosomal aberrations and DNA damage in a group of workers exposed to formaldehyde. Mutagenesis. 2015;30(4):463-473. doi:

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