Study: Do hair dyes or straighteners increase breast cancer risk?

IN-DEPTH REVIEW OF RESEARCH

Study background:

On average, 13 percent of women in the U.S. will develop breast cancer in their lifetime. These rates can be substantially higher if women have a family history of breast cancer due to inherited mutations in breast cancer genes. Non-Hispanic black women have a lower rate of breast cancer (12.4 percent lifetime) than non-Hispanic white women (13 percent lifetime).  However, black women are more frequently diagnosed with more aggressive forms of cancer. Mortality rates are higher in black women than in white women. Differences in screening and treatment delays, whether due to provider bias, insurance availability or other socioeconomic factors are known to contribute to differences in mortality rates.

Other controllable factors may contribute to breast cancer risk. The researchers in this study note that about one-third of women in the U.S. over age 18 use hair dyes. Do hair dyes contribute to breast cancer risk? Prior research has been unclear; several studies have shown no increase in breast cancer risk, while a few studies have found a slight increase in breast cancer risk. Most of these studies did not look at the use of hair straighteners and most did not focus on black women. There have been a few smaller studies that suggested some effect of hair dyes in black women (e.g. Llanos et al., reviewed by XRAYS here in 2017). The researchers of this study hypothesized that women who use hair dyes and/or straighteners would have a higher rate of breast cancer and that black women would have a higher risk of breast cancer because of their use of hair dyes and straighteners.

Researchers of this study wanted to know:

Whether using hair dyes or hair straighteners increases breast cancer risk.

Populations looked at in this study:

The 46,709 participants in this study were part of the Sister Study, a national research project that recruited women who were ages 35 to 74 from 2003 to 2009 and who had one or more sisters with breast cancer.

• 39,261 (84%) were non-Hispanic white women.
• 4,087 (8.7%) women were non-Hispanic black women.
• 2,141 (4.5%) were Hispanic women.
• 1,220 (2.6%) were women who indicated other ethnic or racial identity.

Women who self-identified as non-black Hispanic or other were not included in the statistical analysis because there were too few women to reach a statistically sound conclusion.

At the time of enrollment in the Sisters Study, the participating women had not had breast cancer. The genetic mutation status of participants was not reported.

Each woman was asked questions about their personal and medical history at the time of their enrollment, including about their use of hair products in the prior 12 months. After enrollment, participants were asked annually about their medical history and whether they had been diagnosed with breast cancer (average follow-up time was just over eight years) and had a more detailed follow-up every three years after enrollment.

Diagnosis of breast cancer was self-reported, and then confirmed by examining medical records to determine ER-status and staging of tumors. Women with confirmed cases of ductal carcinoma in situ (DCIS) or invasive cancer were included; women with lobular carcinoma in situ (LCIS) were excluded. Women who had worked professionally in a hair salon were excluded.

Study design:

Participants were asked by questionnaire whether and how often they had used permanent, semi-permanent or temporary hair dyes or straighteners in the 12 months before enrollment. Their options for answering included:

• did not use
• 1-2 times per year
• every 3 to 4 months
• every 5 to 8 weeks
• once a month
• more than once a month

Participants were also asked how long they used these hair products with options of:

• did not use
• less than 5 years
• 5 to 9 years
• more than 10 years

Other questions included whether participants applied hair dyes or straighteners on other people non-professionally, their demographic information (race/ethnicity and age), socioeconomic status, reproductive history, menopausal status and age at menopause, as well as any history of uterine or ovary removal.

Data analysis was performed using the Cox proportional hazard model as a statistical method to determine the risk of breast cancer by age for different variables. Confounding factors (characteristics that depended on the tested factor) were identified and hazard ratios were adjusted for age and identified confounding factors.

Study findings:  

55% of women reported using hair dye in the 12 months prior to enrollment in this study:

• Women who used permanent dye tended to be younger, less educated, more likely to be smokers and to have used oral contraceptives.
• Black women were less likely than white women to have used permanent dyes, as were postmenopausal women or those with fewer children.

Among the 46,709 participating women, 2,794 women developed breast cancer (almost six percent):

• 2,402 white women were diagnosed with breast cancer during this study.
• 208 black women were diagnosed with breast cancer during this study. All subsequent analysis of breast cancer risk in black women in this study is based on this small group of patients.

Permanent hair dyes

Among all participants, a 9 percent increase in breast cancer risk was associated with use of permanent hair dye. Compared to the 13 percent risk of breast cancer in the general population, the risk of breast cancer among women who also use permanent hair dye would be 14.2 percent.

• This risk did not differ with frequency of use or how long a woman had used permanent hair dye.

Among white women, use of permanent dye in the 12 months before enrollment was associated with a 7 percent increased risk of breast cancer. Compared to 13 percent risk of breast cancer in the general population, the risk of breast cancer among women who also use permanent hair dye would be 13.9 percent:

• 1,064 women (2.7% of all white women in the study) had breast cancer and did not use permanent hair dye.
• 1,399 women (3.4% of all white women in the study) had breast cancer and used permanent hair dye.
• The confidence interval is large and overlaps the baseline, suggesting that this finding may have resulted from chance rather than because permanent dyes increase breast cancer risk.

Among black women, use of permanent dye in the 12 months before enrollment was associated with a 45 percent increased risk of breast cancer. Compared to 13 percent risk of breast cancer in the general population, the risk of breast cancer among women who also use permanent hair dye would be 19 percent.

It should be noted that the numbers of cases in this sub-group are small:

• 106 women (2.6% of all black women in the study) were diagnosed with breast cancer and did not use permanent hair dye.
• 102 women (2.5% of all black women in the study) were diagnosed with breast cancer and used permanent hair dye.
• For women who used permanent hair dye less than 4 times per year, the  confidence interval is large and overlaps the baseline, suggesting that this finding may have resulted from chance rather than because permanent dyes increase breast cancer risk.

Semi-permanent and temporary hair dyes

No increase in breast cancer risk was associated with a woman’s use of semi-permanent or temporary hair dyes.

An increased risk of breast cancer was associated with applying semi-permanent dyes to others. This group included few women, making the result uncertain.

• 69 white women and 15 black women with breast cancer applied semi-permanent dye to others—these numbers are very small. The large variability shown by the data suggests that this finding may have resulted from chance rather than because semi-permanent dye application increases breast cancer risk.

Straighteners

Among all of the participants, an 18 percent increase in breast cancer risk was associated with use of hair straighteners after adjusting for confounding factors. Compared to the 13 percent risk of breast cancer in the general population, the risk of breast cancer among women who also use permanent hair dye would be 15 percent.

• 74% of black women and 8% of white women use hair straighteners.
• An increased risk of breast cancer was associated with more frequent use of straighteners in the 12 months prior to enrollment in this study.
• A similar increase in the risk of breast cancer associated with use of hair straighteners was seen in white and black women. However, the confidence interval is large and overlaps the baseline for both groups, suggesting that this finding may have resulted from chance rather than because hair straighteners increase breast cancer risk.

Limitations:

• This is an observational study that cannot show the direct cause of breast cancer.
  This is an observational study, meaning that researchers observe participants and what occurs during the study, but are not testing an intervention. Correlations between breast cancer and other factors can be observed, but these results do not clarify whether factors like hair dye use actually cause breast cancer.
• Confounding factors may account for the data observed.
  The researchers tried to control for some confounding factors that may correlate with hair product used but there are many factors that they did not account for. Other unknown confounding factors that may also correlate with hair dye use (e.g. premature graying or cosmetic use) or with participants race (e.g., dietary differences or genetic mutation status) remain untested. It is possible that these unknown or untested factor may be associated with breast cancer rather than hair product use.
• The population tested may not be representative of the general population.
  The percentage of women using hair dyes or straighteners in this study was 55% - substantially higher than estimates of one-third of women in the U.S. This observation suggests that the study participants may not be representative of women in the U.S. in general. It is unclear how this difference would affect the study results.
• Genetic mutation status was not accounted for in this study.
  A major limitation of this study is that all participants had a sister with breast cancer and are therefore not representative of women in the general population. Because of their family history of breast cancer, some of the women in this study are likely to have an inherited mutation in a breast cancer gene. This means that the data is not generalizable to the broader population, and may be more representative of women with a family history of breast cancer.

While the researchers tested for interactions and confounding factors that may have altered the results, they failed to identify genetic mutation status a major confounding factor. The participants were recruited based on their sisters’ being diagnosed with breast cancer. It is possible and reasonable that more than the average number of women in this study may have had an underlying inherited mutation in a gene that is associated with breast cancer. This potential confounding factor makes the reliability of data suspect.

• There may be different frequencies of breast cancer mutations in black and white participants.
  While the authors note differences between black and white participants in this study, they do not know if there is a difference in the frequency of genetic mutations that varies by race in this group of participants. There is clear data  that there can be a high frequency of inherited mutations among some black women (e.g. women of Bahamian descent described in our XRAY review here and reviewed more generally in Felix et al.). Given the historical reluctance of black women to participate in clinical trials (due in part to valid historical concerns), it is possible that participants may not be representative of black women in general and may have a stronger family history than the white participants. This issue is not addressed in the study.
• Researchers collected data on hair dye and straightener use only one time, based on past memory.
  Another limitation is that all of the data regarding hair dye use was based on participants’ recollection—data may be inaccurate in any study that depends on a person’s memory. Women were only asked about hair dye use in the 12 months prior to entering the study; they were not asked about their use between enrollment and breast cancer diagnosis or the end of study.
• The statistical confidence in this data is weak.
  Most of the data is presented as hazard ratios that compare breast cancer risk between small groups of women in the study. For much of the data, the statistics show that there is no difference between many of the groups (e.g., between women how use semi-permanent dye compared to women who do not use hair dye). For the data that the researchers indicate is significant, the statistical confidence is low suggesting that the impact may be real but not large.  Some data is highlighted as significant that may not be: e.g., researchers point to increased breast cancer risk associated with the use of straighteners but the confidence intervals suggest the breast cancer risk may not be statistically different from women who did not use straighteners. The wide range of confidence around the data points suggests that data may be less precise and that there is a large amount of variability. Further research is needed to determine if these weak associations hold up.\
• The number of women in some sub-groups are small and this limits the conclusions.
  While the number of participants who participated in the Sister Study is large, the number of women with breast cancer is not: 2,402 white women and 208 black women.
  • The absolute number of cases of breast cancer among black women who use any hair dye was a small number: 102 of 4,087 (2.5%) of black women versus 1,399 (3.4 percent) of white women; 106 of 4,087 (2.6%) of black women had breast cancer but did not use hair dye versus 1,064 of 39,261 (2.7%) of white women who had breast cancer but did not use hair dye. These rates do not differ greatly.

The small size of these populations means that when the subgroups (e.g., women who did or did not use permanent dye) are looked at there are even fewer women in each category. Some sub-groups are very small (e.g., there are only 40 black women with breast cancer who used permanent dye every 5-8 weeks). Small sample sizes result in less statistical confidence in the results.

• Taken as a whole, the data are not internally consistent.
  The study claims that breast cancer risk is increased when permanent hair dyes are used. However, there was no correlation of breast cancer risk among the whole group of participants with how frequently women used this hair product or by how long in years these women had used this product. The lack of a trend is suspicious. Most substances that cause cancer show increasingly greater risk with more use over a longer time. The lack of this trend (called a dose-response relationship) suggests that the original observation of an association between any use of permanent dyes and increased breast cancer risk might be incorrect.

 Also inconsistent is the finding that permanent dyes increased risk to a person they are used on but not when applied to others. Similarly, breast cancer risk was not elevated in women who use semi-permanent dyes (if anything the data shows a decrease in breast cancer risk) but when women apply the same dye to others they observed an apparent increase breast cancer risk. These results seems counter-intuitive and are not explained.

Inconsistency in the overall data suggests the conclusions may not be reliable.

• Questions by other researchers
  There have been concerns about this study raised by others. A critical response to this study (linked here) was recently published by the same journal that discusses the validity of these findings.

Conclusions:

This research suggests that use of permanent hair dyes and straighteners may increase breast cancer risk for women, particularly for black women. There are many limitations of this study that suggest the conclusions need to be taken cautiously. Even if true, the effect of permanent hair dye and straightener use would be an increase in breast cancer risk that may have limited real-world impact. Other lifestyle factors (drinking alcohol, obesity in post-menopausal women, exercise, age of first childbirth) have comparable or much greater effects on breast cancer risk.

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(Posted 1/29/20)