Hair dye is used to color your hair. Hair relaxers are used to make your hair straight. Both hair dye and hair relaxers can hurt your skin, hair, and eyes, especially if you are not careful. Some of these problems can be prevented. Get the facts before using hair dyes and hair relaxers.

Tips to Keep You Safe

• Follow all directions in the package.

• Do a patch test on your skin before using dye on your hair. Rub a small amount of dye on your skin. Let it dry for 48 hours. If you get a rash, do not use the dye on your hair.

• Wear gloves when applying hair dye or relaxers.

• Do not dye your eyebrows or eyelashes at home. This can hurt your eyes and cause permanent discoloration. You might even go blind. The FDA has provided for the safe use of silver nitrate as a color additive, in professional-use only cosmetics, to color eyebrows and eyelashes. Use is restricted to up to 4 percent silver nitrate (by weight) in a viscous gel product. These silver nitrate containing cosmetic products are not intended for use on persons under the age of 16 or for application for longer than 1 minute. Furthermore, these products are not for distribution or direct sale to consumers.  

• Do not leave the product on longer than the directions say you should.

• Rinse your scalp well with water after using hair dye or relaxers.

• Keep hair dyes and relaxers out the reach of children.  

Hair dyes use chemicals to change the color of the hair. There are three main types of hair dye: permanent, semi-permanent, and temporary. 

Permanent hair dyes, which make up about 80% of currently marketed hair dye products, use colorless dye intermediates and dye couplers. In the presence of hydrogen peroxide, the intermediates and couplers react with one another to form pigment molecules. Darker colors are formed by using higher concentrations of intermediates. 

The other types of hair dyes, known as semi-permanent and temporary hair dyes, do not involve such chemical reactions. Instead, they include colored compounds that stain hair directly.

Some of the chemicals in hair dye products have been reported to be carcinogenic (cancer-causing). These include aromatic amines, which were used as dye intermediates in early permanent hair dye formulations. 

In the mid- to late 1970s, manufacturers changed the components in permanent hair dye products to eliminate some of the carcinogenic chemicals used as dye intermediates. It is not known whether any of the chemicals still used in hair dyes are carcinogenic. 

People who use hair dyes frequently as part of their job, such as professional hairdressers, likely have higher exposures to potentially harmful chemicals in these products than people who have their hair dyed in a salon or who dye it at home (that is, “personal users” of hair dye). Thus, studies have been conducted to understand both exposure to hair dyes in the occupational setting and exposure from personal use. 

It has been estimated that between 50% and 80% of women in the United States, Japan, and the European Union have used hair dyes. Given the widespread use of hair dye products, if these products had even a small effect on cancer risk that could translate into a considerable public health impact.

In 2010, a report from the International Agency for Research on Cancer (IARC) concluded that some of the chemicals professional hairdressers are exposed to are “probably carcinogenic to humans”. This conclusion is based on many studies that have found an increased risk of bladder cancer in hairdressers and barbers who were occupationally exposed to hair dye. A 2010 meta-analysis of 42 studies found that the longer someone had worked as a hairdresser, the more likely they were to have developed bladder cancer. Those who had been hairdressers for at least 10 years were nearly twice as likely to develop bladder cancer as those who had never worked as a hairdresser.

One study looked at risks among Swedish hairdressers over different time periods. That study found no increased risk in recent decades, suggesting that professional use of modern hair dyes, which no longer contain aromatic amines, may not be associated with bladder cancer risk.

Epidemiologic studies that follow groups of people over time (cohort studies) or that study people with and without specific cancers (case–control studies) have examined possible associations between personal use of hair dyes and risks of several cancers. Based on a review of existing evidence, the IARC Working Group considers personal use of hair dyes overall as “not classifiable as to its carcinogenicity to humans”.

Bladder cancer. A 2014 analysis of data pooled from 17 studies found no evidence of an increased risk of bladder cancer associated with personal hair dye use (9). However, for personal use of permanent hair dyes specifically, the evidence of an association with bladder cancer is mixed. Some studies found no association, whereas others found an association in specific subgroups of users. 

Non-Hodgkin lymphoma. A number of studies have investigated the relationship between personal use of hair dyes and the risk of non-Hodgkin lymphoma (NHL), with conflicting results. A pooled (combined) analysis of four case–control studies found that women who began using hair dye before 1980 (before formulations were changed to remove carcinogenic compounds) were 1.3 times as likely to develop NHL as women who had never used hair dye, whereas no increase in risk was seen for women who began using hair dye after 1980. A large 2020 prospective cohort study reported no association between personal use of hair dye and the risk of non-Hodgkin lymphoma.

Breast cancer. Many studies have examined possible associations between hair dye use and breast cancer, with mixed and inconsistent results. However, more recent studies that have looked at larger populations of women with longer follow-up periods and more detailed characterization of hair dyes and other breast cancer risk factors have found that certain kinds of exposures may be associated with risk. 

For example, a large US cohort study that included mostly White women reported an increased risk of breast cancer overall, and of hormone receptor–negative breast cancers, with increasing exposure to hair dye. Another prospective study of US women who used dark or permanent hair dyes found that risks of breast cancer were higher in those who reported any use of hair dye than in those who reported no use. When the authors analyzed the data by racial group, they found that among non-Hispanic White women, hair dye users were 1.07 times as likely to develop breast cancer as non users, and among Black women users were 1.45 times as likely to develop breast cancer as nonusers.

Some studies have found that individuals who apply permanent hair dye themselves at home have a higher breast cancer risk than those who have it applied by a professional. 

Other cancers. Although some individual studies have shown associations between hair dye use and the risk of leukemia, a meta-analysis of 20 studies showed a small association that was not statistically significant. In addition, a large cohort study found no association between ever use of hair dye and the risk of leukemia but very slightly increased risks of basal cell carcinoma (particularly in women with naturally light hair) and ovarian cancer.

Treatments used to straighten or relax hair contain a mixture of chemicals. Some formulations contain the carcinogen formaldehyde as an active ingredient. Several studies have found associations between the use of hair straighteners or relaxers and the subsequent development of breast cancer.

• In a large case–control study in New York and New Jersey, White women who had ever used chemical hair relaxers were 1.7 times as likely as nonusers to develop breast cancer (20), but no such association was seen in Black women.
• In another large case–control study, Ghanaian women who had ever used these products were 1.6 times as likely as nonusers to develop breast cancer.
• A large prospective study in the US found that women who used hair straighteners at least once per month in the previous 12 months were 1.3 times as likely to develop breast cancer as nonusers. In that same cohort of women, frequent use of straighteners and perms during adolescence was associated with a higher risk of premenopausal breast cancer.
• A nationwide prospective study of Black women found evidence that heavy users of lye-containing hair relaxers were more likely to develop estrogen receptor–positive breast cancers than never or light users.

Recently, a large cohort study found that women who had used any straightening products in the previous 12 months had 1.8 times the risk of developing uterine cancer as those who had never used such a product.

Because hair straighteners and relaxers contain varying compositions of chemicals, future studies will try to narrow down which chemicals have specific health impacts.

If you decide that you want to make your hair less frizzy or straighter, you may choose to go to a professional stylist at a hair salon for a hair straightening or smoothing service. Most hair smoothing or straightening products release formaldehyde gas, a known human carcinogen as classified by the International Agency for Research on Cancer, into the air during the hair straightening or smoothing process. Formaldehyde exposure can cause both short- and long-term effects that may impact your health.

What Happens During the Hair Smoothing Process?

Your stylist usually applies a hair straightening or smoothing solution to your hair and follows it with a heat processing step—typically performed with a flat iron device that seals the solution into the strands of the hair. When the solution is heated, the formaldehyde in the product is released into the air as a gas. If the salon is not properly ventilated, you, your stylist, and others in the salon are at risk of inhaling the released formaldehyde.

Why Exposure to Formaldehyde is Dangerous

When formaldehyde is released into the air and is present in the air at levels exceeding 0.1 ppm, it can cause serious irritation of your eyes, nose, and lungs. It can also cause skin sensitivity or allergic dermatitis.

Formaldehyde Exposure: Potential Short-and Long-Term Effects

The greater the exposure to products that contain formaldehyde in terms of both length of time and concentration, the higher the potential health risks. Individuals who have experienced formaldehyde exposure have described reactions such as eye problems or irritation, nervous system problems (for example, headaches and dizziness), respiratory tract problems (sore or scratchy throat, cough, wheezing), nausea, chest pain, vomiting, and rashes. Chronic effects associated with formaldehyde can include an increased prevalence of headaches, asthma, contact dermatitis (a red, itchy rash caused by direct contact with a substance or an allergic reaction to it as a result of sensitization) and possibly cancer.

It’s important to note that some people are very sensitive to formaldehyde, whereas others have no reaction to the same level of exposure. If you are sensitive to formaldehyde, you should not use this type of product.

Steps to Take If You’re Thinking of Using Hair Smoothing Products

1. Don’t do it yourself. Don’t apply them yourself at home. Go to a licensed hair professional in a salon. Hair professionals should be trained to take precautions such as wearing gloves and safety glasses, as well as making sure that there is proper ventilation when applying hair smoothing products.
2. Read the list of ingredients. While the FDA discourages consumers from buying these products for home use, if you are considering purchasing a product from a store or online to apply yourself, read the required list of ingredients. If there isn’t one, don’t buy the product. If the hair smoothing product lists one of these ingredient names on the packaging or company website: formaldehyde, formalin, or methylene glycol, then it means the product contains formaldehyde or will release formaldehyde. Of note, not all hair smoothing products contain formaldehyde, which is why you need to read the label to see what is in the product.
3. You may also want to ask your salon professional these questions: Does the product contain formaldehyde? Is there an ingredient list available for this product that I could read? Would it be possible for me to review the Safety Data Sheet for this product? Have you been trained to apply this product, and do you know the necessary safeguards to minimize exposure to formaldehyde? May I see your training certificate from the manufacturer and the directions for product use? Does the salon have proper ventilation? Do you periodically test the air for adherence to the Occupational Safety and Health Administration’s limits for formaldehyde? Do you have an alternative hair smoothing product that does not release formaldehyde when heated?
4. Report bad reactions. If you experience a bad reaction to a hair smoothing product, stop using the product and report your symptoms to your health care provider. You can also report your symptom to the FDA by contacting the Consumer Complaint Coordinator in your area or by reporting them online at MedWatch, the FDA’s safety information and adverse reporting program. You also may call MedWatch at 1-800-332-1088 to request a reporting form by mail.

People who dye their hair should follow these safety precautions:

• Follow the directions in the package. Pay attention to all "Caution" and "Warning" statements.

• Do a patch test before using dye on your hair. Here's how: Rub a tiny bit of the dye on the inside of your elbow or behind your ear. Leave it there for two days. If you get a rash, don't use the dye on your hair. You should do the test each time you dye your hair. (Salons should also do the patch test before dyeing your hair.)

• Never dye your eyebrows or eyelashes at home. This can hurt your eyes and cause permanent discoloration. You might even go blind. The FDA has provided for the safe use of silver nitrate as a color additive, in professional-use only cosmetics, to color eyebrows and eyelashes [21 CFR 73.2550]. Use is restricted to up to 4 percent silver nitrate (by weight) in a viscous gel product. These silver nitrate containing cosmetic products are not intended for use on persons under the age of 16 or for application for longer than 1 minute. Furthermore, these products are not for distribution or direct sale to consumers.

• Keep hair dyes out of the reach of children.

• Don't leave the dye on longer than the directions say you should.

• Rinse your scalp well with water after dyeing.

• Wear gloves when you apply the hair dye.

• Never mix different hair dye products. This can hurt your hair and scalp.

Citation: Eberle CE, Sandler DP, Taylor KW, White AJ. Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women. Int J Cancer. 2020 Jul 15;147(2):383-391. doi: 10.1002/ijc.32738. Epub 2019 Dec 3. PMID: 31797377; PMCID: PMC7246134. [Abstract]

1. What is the background for this study?
   Hair products such as hair dyes and chemical straighteners contain many different chemicals that may act as carcinogens or endocrine disruptors and thus may be important for breast cancer risk. There has been longstanding interest in understanding the relationship between hair dye and breast cancer. The previous research on both hair dye and chemical straighteners has been inconclusive.

2. What are the main findings?
   Women who used permanent hair dye and chemical straighteners were at a higher risk of breast cancer compared to women who did not use those products. The association with permanent hair dye differed by race; black women had a higher risk associated with the use of permanent hair dye compared to the risk for white women.

   • Overall, use of permanent hair dyes in the year before study enrollment was associated with a 9 percent increase in risk of breast cancer. For non-Hispanic white women, the associated increase in risk was 7%; for black women it was 45%.
   • Breast cancer risk was increased 60% for black women who used permanent dyes every 5-8 weeks or more.
   • Frequent use of chemical straighteners was associated with a 31% increase in breast cancer risk.
   • Personal use of semi-permanent or temporary dyes was not associated with increased breast cancer risk.

3. How large was the study and how long were women followed?
   The study included 46,709 women who are participants in the Sister Study, a prospective nationwide study of the environmental and genetic risk factors for breast cancer for breast cancer. During an average follow-up of 8.3 years, 2,794 women were diagnosed with breast cancer.

4. What should readers take away from your report/what are the implications of this research?
   These findings suggest that women should consider their use of hair products in light of the fact that the chemicals in hair dye and chemical straighteners may influence their risk of developing breast cancer. However, the overall risk is not large and chemical hair products are just one of many factors that may influence a woman’s chances of getting breast cancer. Our study estimated relative risks, comparing the risk of breast cancer among women who did or did not use specific hair products. We did not estimate the added number of breast cancers that could be due to use of these products. More research is needed before firm recommendations can be made.
   
   Healthcare experts could consider sharing this evidence with others to inform their decisions regarding hair product use.

5. What recommendations do you have for future research as a result of this study?
   More research is needed to confirm these findings and to identify the specific constituents in permanent dye and straighteners that may be contributing to this higher risk.

6. What could explain the difference in the associations of hair products by race?
   The association for chemical straighteners did not vary by race; women who used hair straighteners at least every 5-8 weeks, regardless of race, were 31% more likely to get breast cancer. However, 74% of black women and 3% of white women reported using chemical straighteners. Therefore, chemical straighteners may be a more important contributor to breast cancer in black women.
   
   For hair dye, the risk estimate for the association with breast cancer was higher in black women although frequency of use was not very different (56% of white women, 42% of black women used in the past 12 months). It is possible that type of dye or application of dye is different for black women compared to white women. It is also possible that hair texture plays a role in how much dye is needed or absorbed. It is important to keep in mind that the results for black women, especially for high frequency users, were based on a much smaller number of study participants.

7. Are there specific chemicals in permanent dyes and straighteners that are associated with breast cancer risk? Are these chemicals absent in temporary hair dyes?
   There are many different formulations of both hair dye and straighteners which have changed over time. This makes it very challenging to pin down individual compounds that may increase risk of developing breast cancer. Additionally, the application of these products varies by the type of dye. Differences in formulation or application may explain the differing associations for permanent dye and semi-permanent or temporary dyes. Our study did not ask participants about the specific dyes used and we were unable to evaluate individual components of the products in our study.

8. Did you find a difference for light compared to dark dye?
   There were no meaningful differences by dye color.

9. What about women from other race/ethnicity groups, such as Hispanic women or Asian women?
   Unfortunately, we did not have enough women in these groups to evaluate the associations in either Hispanic women or Asian women. We hope to be able to address this in the future.

10. Is this the first conclusive evidence from a study to state that women who use permanent hair dye and chemical hair straighteners have a higher risk of developing breast cancer than women who don't use these products?
    This study is the first to observe a positive association between chemical straighteners and breast cancer in a prospective cohort setting. These results confirm recent published findings from studies that ask women who have already been diagnosed with breast cancer about their hair product use. However, such studies cannot rule out the fact that women who have breast cancer may be more likely to recall their use of hair products compared to women who do not have breast cancer. A strength of our study is that we obtained information on hair product use before women developed breast cancer. More research is needed before we can say that the findings are conclusive.

11. What should be done in light of these findings?
    Women should be informed of the possibility that hair product use may influence breast cancer risk in order to make their own decisions regarding their use of hair dye and straighteners. Since semi-permanent dyes were not associated with breast cancer risk in this study, women might consider switching from permanent to semi-permanent products if those products would work for them. Women who apply the dye themselves should make sure that they wear gloves and follow the directions about how long to leave the dye in their hair and carefully rinse it out as instructed.

12. The study statistical models adjusted for a lot of factors including age, menopausal status, race, ethnicity, education, BMI, smoking history, oral contraceptive use, and age of first birth. Did this affect the results?
    We adjusted for factors that could be related to both breast cancer risk and to use of hair dyes. Adjustment for these factors did make some difference in the magnitude of the associations but did not alter any conclusions. Without adjustment for these factors, permanent hair dye use and straighteners were still positively associated with breast cancer risk.

13. In the paper, it says that most past studies on this topic solely looked at white women. Why is it important to include women of all races? Is this the first study to do so?
    It is important to include women of all races because breast cancer risk and mortality tends to differ by race. For example, in the US African American women are more likely to develop more aggressive types of breast cancer. Furthermore, women of different races may use different types of hair products and at vastly different frequencies. For example, in our study 74% of black women reported using chemical straighteners in the past year whereas 3% of white women did. Although this is not the first study to evaluate these associations in both black and white women, we had a large population of women who were asked about hair product use years before their diagnosis which reduces the possibility of bias.

14. Since there are many factors that lead to the development of an oncological disease, is it correct that this observed association does not constitute a cause-and-effect connection?
    This was an observational study that investigated the association of hair dye and straightener use with breast cancer risk. Thus, we cannot conclude that these observed findings are causal. Further research is necessary to support these findings.

15. Increased breast cancer risk was observed for women who used permanent hair dyes and for women who used hair straighteners. What about women who used both?
    Unfortunately, we were not able to directly study the risks among women who used only one type of hair product or who used both, because of small numbers. However, when we looked at hair dye, adjusting for use of straighteners and vice versa, our results did not change. This suggests that both hair dye and straighteners contribute to risk.

16. Women in the study all have a sister with breast cancer, how could that impact your findings?
    The women in our study have a wide range of familial risk, with some women in our study having only a half-sister with breast cancer. Most have a single sister who had breast cancer. Only a small percentage have known breast cancer genes. We are not studying only high-risk women with extensive family histories of cancer. Women in our study are similar to women overall who participate in research studies. Therefore, we believe these results are likely generalizable to women in the US although it is possible that the magnitude of our results may differ from a population without a familial risk.

17. What is the Sister Study?
    The Sister Study, led by researchers from the National Institute of Environmental Health Sciences (which is part of the National Institutes of Health – NIH), is a prospective cohort study of 50,884 women from across the United States including Puerto Rico. Women enrolled in the study between 2003 and 2009 and have been followed annually ever since to study the incidence of breast cancer and other health conditions that are of importance to women. To be eligible for the study, women had to be between the ages of 35 and 74, could not have had breast cancer before they enrolled, and had to have a full or half-sister who had been diagnosed with breast cancer.

18. Was the age at which girls started using these products a factor in whether or not they could potentially develop breast cancer?
    In this paper, we did not consider the association based on starting age of use. It is something we hope to study in the future.

19. Were there differences in risk for women who applied the hair dye themselves or had their hair colored at a salon?
    Unfortunately, we were not able to study this using the information we collected. We reported on risk associated with having a woman’s hair dyed (personal use), regardless of who did it or where it was done. We also looked at the association between applying hair dye to others, but not as a professional. Overall, applying permanent hair dye to others, for example coloring a friend’s hair, was not associated with increased breast cancer risk.