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Hair dye and chemical straightener use and breast cancer risk
in a large US population of black and white women
Carolyn E. Eberle
1
, Dale P. Sandler
2
, Kyla W. Taylor
3
and Alexandra J. White
2
1
Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, NC
2
Epidemiology Branch, National Institute of Environmental Health Sciences, NIH, Research Triangle Park, NC
3
Office of Health Assessment and Translation, National Toxicology Program, NIH, Research Triangle Park, NC
Many hair products contain endocrine-disrupting compounds and carcinogens potentially relevant to breast cancer. Products used
predominately by black women may containmore hormonally-active compounds. In a national prospective cohort study, we examined
the association between hair dye and chemical relaxer/straightener use and breast cancer risk by ethnicity. Sister Study participants
(n=46,709), women ages 35–74, were enrolled between 2003 and 2009, and had a sister with breast cancer but were breast cancer-
free themselves. Enrollment questionnaires included past 12-month hair product use. Cox proportional hazards models estimated
adjusted hazard ratios (HRs) and 95%confidence intervals (95% CIs) for the association between hair products and breastcancer;
effect measure modification by ethnicity was evaluated. During follow-up (mean = 8.3years), 2,794 breast cancers were identified.
Fifty-five percent of participants reported using permanent dye at enrollment. Permanent dye use was associated with 45% higher
breast cancer risk in black women (HR = 1.45,95%CI:1.10–1.90), and7% higher risk in whitewomen (HR = 1.07,95%CI:0.99–1.16;
heterogeneity p=0.04). Among all participants, personal straightener use was associated withbreast cancer risk (HR = 1.18,95%CI
0.99–1.41); with higher risk associated with increased frequency (pfor trend = 0.02). Nonprofessional application of semipermanent
dye (HR = 1.28,95%CI1.05–1.56) and straighteners (HR = 1.27,95%CI0.99–1.62) to others was associated with breast cancer risk.
We observed a higher breast cancer risk associated withany straightener use and personal useof permanent dye, especially among
black women. These results suggest that chemicals in hair products may play a role in breast carcinogenesis.
Background
In the United States, breast cancer incidence remains high for all
women, and appears to be increasing for non-Hispanic black
women to the point of possible convergence with non-Hispanic
white women.
1,2
Despite the similar trends in incidence, racial
disparities persist with black women more likely to be diagnosed
with aggressive tumor subtypes and to die after a breast cancer
diagnosis.
3–6
Understanding potentially modifiable breast cancer
risk factors, especially those that may contribute to racial dispar-
ities, is central to identifying potential targets to reduce risk.
Hair dye use is very common; it has been estimated that more
than one-third of women above the age of 18 in the United States
use hair dye.
7
Hair products contain more than 5,000 chemicals,
7,8
including some with mutagenic and endocrine-disrupting proper-
ties such as aromatic amines.
9–11
Dye constituents, such as
2,4-diaminoanisole sulfate and para-phenylenediamine, have been
found to induce tumors in the mammary gland of rats.
12,13
Other
aromatic amines including 4-aminobiphenyl-(ABP) can reach
the breast tissue; women who used hair dye in the past year were
eight times more likely to have ABP-DNA adducts detected in
breast ductal epithelial cells.
9,14
Chemical treatments used to per-
manently or semi-permanently straighten or relax hair (hence-
forth referred to as straighteners) contain a mixture of chemicals,
including formulations in which the carcinogen formaldehyde is
an active ingredient.
15,16
Prior findings on the association between hair dye use and breast
cancer have been inconsistent; while a few studies have reported a
positive association,
17–20
many concluded that there was no elevated
risk.
10,17,20–26
However, recently published case–control studies
reported risk increases in excessof25%forhairdyeuseinassocia-
tion with breast
18,19
and bladder cancer.
27
Fewer studies have con-
sidered the risk associated with straighteners, which are used
Additional Supporting Information may be found in the online version of this article.
Key words: breast cancer, hair dye, chemical straighteners, hair products, personal care products
Abbreviations: ABP: aminobiphenyl; BMI: body mass index; CI: confidence interval; DCIS: ductal carcinoma in situ; ER: estrogen receptor;
HR: hazard ratio; WCHS: Women’s Circle of Health Study
Conflict of Interests: None declared.
Grant sponsor: National Institute of Environmental Health Sciences; Grant number: Z01-ES044005
DOI: 10.1002/ijc.32738
History: Received 13 Aug 2019; Accepted 1 Oct 2019; Online 3 Dec 2019
Correspondence to: Alexandra J. White, E-mail: alexandra.white@nih.gov
International Journal of Cancer
IJC
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
predominately by women of African descent.
18,28,29
Hair product
constituents vary depending on whether they are marketed to
blackorwhitewomen;studiessuggest that products designed for
use by black women may contain more endocrine-disrupting
chemicals.
30–32
Additionally, personal care product use patterns
vary by ethnicity and thus differences in exposure to chemicals
through hair products may contribute in part to racial disparities in
breast cancer incidence.
30,33–37
In our study, we evaluated the association of hair dye and
straightener use with breast cancer in a large, prospective cohort
of US women. We hypothesized that risk would be higher among
women who used hair dye and straighteners and would vary by
ethnicity, with black women having higher risk.
Methods
Study population
The previously described prospective Sister Study cohort enrolled
50,884 women aged 35–74 living in the United States (including
Puerto Rico) from 2003 to 2009.
38
Womenwereeligibletopartic-
ipate if they had no history of breast cancer but had at least one
sister who had been diagnosed with breast cancer. Participants
answered structured questionnaires and computer-assisted tele-
phone interviews at the time of enrollment. A trained examiner
measured height and weight during a home visit. Participants
provided annual updates to document health changes and com-
pleted detailed follow-up assessments every 3 years. Response
rates have remained over 90% throughout study follow-up.
39
All study participants provided written informed consent.
The Sister Study was approved by the institutional review
board of the National Institute of Health. This report includes
follow up through September 15th, 2016 (Data Release 6.0).
Breast cancer outcome ascertainment
Participants reported incident breast cancer diagnoses in
annual health updates, during follow-up surveys or by calling
the Sister Study helpline. After self-reported diagnoses, cases
granted the release of medical records to confirm the diagno-
sis and ascertain further details, including estrogen receptor
(ER) status and staging. Estrogen receptor responsiveness was
available for 87.4% of invasive breast cancers. Tumors with
positive or borderline results were classified as ER-positive.
Medical records were available for over 80% of cases. When
medical record data was not available, we used self-reported
data. Agreement between self-reported tumor characteristics
and those from the medical record is high; the positive predic-
tive value for ER-positive breast cancer is 99.1%.
40
Cases were defined as women diagnosed with an invasive
breast cancer or ductal carcinoma in situ (DCIS). Women diag-
nosed with breast cancer prior to completion of all required
enrollment activities were excluded (n=62). We considered
whether associations varied by the extent of the tumor (invasive
vs. DCIS), ER status, or menopausal status at diagnosis. When
considering ER status, we limited to invasive cases as ER status
was less commonly determined for in situ disease.
Exposures and covariate assessment
Current hair dye and straightener use, defined as use in the
12 months before enrollment, was assessed by questionnaire and
was completed by 47,650 participants. Participants reported their
frequency of personal use of permanent hair dye, semi-permanent
hair dye, temporary dye and straighteners in the 12 months before
enrollment. Frequency was reported as “Did not use,”“1–2times
per year,”“every 3–4months,”“every 5–8weeks,”“once a month”
and “more than once a month”.Additionally,participantsreported
their frequency of non-professional application of permanent dye,
semipermanent dye and straighteners to others in the 12 months
before enrollment. Current hair dye and straightener exposures
were summarized as dichotomous variables—“did not use in the
past 12 months”vs.“any use in the past 12 months”—and based on
reported frequency of use—“did not use in the past 12 months,”
“used 1–2or3–4timesinthepast12months”or “used every
5–8 weeks or once a month or more in the past 12 months”.
Women that reported current permanent and semiperma-
nent dye use were asked whether they had used dark colors
(black, brown, auburn/dark red), light colors (blonde, light
red) or both. All participants reported duration of permanent
and semi-permanent hair dye personal use as “did not use,”
“less than 5 years,”“5–9 years”or “10 or more years”; this
was collapsed into <5 vs. 5 or more years.
Covariate information, including demographics, socioeco-
nomic status and reproductive history, was obtained from partic-
ipants during the enrollment telephone interview. Menopausal
status and age at menopause were assessed at enrollment and
updated during follow-up by asking about the timing of their last
menstrual period and history of hysterectomy or oophorectomy.
Statistical analysis
We conducted descriptive analyses evaluating participant charac-
teristics by current permanent dye use. We used Cox proportional
What’s new?
Products such as hair dye and hair straightener are widely used. These products contain numerous chemicals, some of which
may contribute to cancer risk. The present study evaluated the relationship specifically between breast cancer risk and hair
dye and chemical straightener use in a prospective cohort of women ages 35-74. Hair dye use was reported by more than half
of study participants. Nearly three-quarters of black women reported use of hair straightener in the past year. Straightener use
and personal permanent hair dye use were associated with elevated breast cancer risk. Associations for hair dye were
strongest among black women.
384 Hair products and breast cancer
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
hazards models to estimate hazard ratios (HRs) and 95% Confi-
dence Intervals (CIs) for the association between dye and straight-
ener use and breast cancer risk. For the Cox model, age was the
timescale with follow-up beginning at enrollment and person-
time accrued until breast cancer diagnosis or censoring event
(defined as age of last follow-up or death). We tested for a linear
trend for frequency of use with a chi-square test for the ordinal
characterization of the variable. Participants diagnosed with lobu-
lar carcinoma in situ were censored in all analyses. The propor-
tional hazards assumption was assessed using a likelihood ratio
test to compare models with and without interaction terms
between each covariate and time with an α= 0.05. There was no
evidence of time-variant associations.
Confounders were identified using a directed acyclic
graph.
41
All models were adjusted for age at menarche (con-
tinuous), enrollment menopausal status (premenopausal, post-
menopausal), ethnicity (Non-Hispanic White, Black, Hispanic
or Other), educational attainment (high school degree or
equivalent or less, some college, bachelor’s degree or higher),
body mass index (<25, 25–30 and >30 kg/m
2
), smoking his-
tory (never, former or current smoker), ever oral contracep-
tive use, parity (0–1, 2, 3 or more births) and age at first birth
(nulliparous, <23, 23–27, >27). All women who self-reported
black ethnicity, regardless of ethnicity, were classified as black.
There was limited missing covariate data (<2%) so we con-
ducted a complete case analysis limiting to those without
Table 1. Characteristics of the study population stratified by permanent hair dye use in the 12 months before enrollment, Sister Study
2003–2009
No permanent hair dye use (n= 20,822) Any permanent hair dye use (n= 25,887)
Participant characteristics n%n%
Age at baseline (years), mean SD 56.9 9.3 54.7 8.5
Age at menarche (years), mean SD 12.6 1.5 12.7 1.5
Ethnicity
Non-Hispanic White 17,368 83.4% 21,893 84.6%
Black 2,388 11.5% 1,699 6.6%
Hispanic 553 2.7% 1,588 6.1%
Other 513 2.5% 707 2.7%
Highest education
High School or less 2,979 14.3% 4,191 16.2%
Some college 6,412 30.8% 9,241 35.7%
Bachelor’s degree or more 11,431 54.9% 12,455 48.1%
Body mass index
Normal/underweight 7,998 38.4% 9,900 38.2%
Overweight 6,460 31.0% 8,398 32.4%
Obese 6,364 30.6% 7,589 29.3%
Smoking Status
Never 12,281 59.0% 14,029 54.2%
Past 7,133 34.3% 9,519 36.8%
Current 1,408 6.8% 2,339 9.0%
Enrollment Menopause status
Premenopausal 6,249 30.0% 9,408 36.3%
Postmenopausal 14,573 70.0% 16,479 63.7%
Oral Contraceptive Use
Never 3,820 18.4% 3,611 14.0%
Ever 17,002 81.7% 22,276 86.1%
Parity
0–1 child 7,132 34.3% 8,047 31.1%
2 children 7,255 34.8% 10,002 38.6%
3+ children 6,435 30.9% 7,838 30.3%
Age at first birth
Nulliparous 4,208 20.2% 4,232 16.4%
<23 years old 6,408 30.8% 8,415 32.5%
23–27 years old 4,786 23.0% 6,188 23.9%
27+ 5,420 26.0% 7,052 27.2%
Eberle et al.385
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Table 2. Hair dye and straightener exposure in the 12 months prior to enrollment and incident breast cancer, Sister Study, 2003–2009
Person-years Events Age-adjusted HR (95% CI) Adjusted HR
1
(95% CI)
2,794
Permanent hair dye
Personal use in the 12 months before enrollment
No use
2
173,213 1,235 Ref Ref
Any use 213,125 1,559 1.07 (1.00, 1.16) 1.09 (1.01, 1.17)
<4 times per year 89,931 634 1.07 (0.97, 1.17) 1.08 (0.98, 1.19)
Every 5–8 weeks, 1+ per month 123,194 925 1.08 (0.99, 1.18) 1.09 (1.00, 1.19)
p for trend 0.05
Dye color
None 173,213 1,235 Ref Ref
Light colors only 92,847 713 1.10 (1.00, 1.21) 1.12 (1.02, 1.23)
Dark colors only 94,908 683 1.07 (0.97, 1.18) 1.08 (0.98, 1.19)
Light and dark colors 23,009 144 0.95 (0.80, 1.14) 0.96 (0.81, 1.14)
Years of personal use
None 135,033 965 Ref Ref
<5 63,995 458 1.07 (0.96, 1.20) 1.07 (0.96, 1.20)
5 or more 182,209 1,340 1.04 (0.96, 1.13) 1.05 (0.96, 1.14)
Applied to others
No use
2
357,754 2,606 Ref Ref
Any use 28,585 188 0.97 (0.84, 1.13) 0.99 (0.85, 1.15)
<4 times per year 24,012 160 0.99 (0.85, 1.17) 1.01 (0.86, 1.19)
Every 5–8 weeks, 1+ per month 4,573 28 0.86 (0.59, 1.25) 0.88 (0.61, 1.28)
pfor trend 0.7
Semi-permanent dye
Personal use in the 12 months before enrollment
No use
2
315,173 2,311 Ref Ref
Any use 71,165 483 0.96 (0.87, 1.05) 0.96 (0.87, 1.06)
<4 times per year 43,032 295 0.99 (0.88, 1.12) 0.99 (0.88, 1.12)
Every 5–8 weeks, 1+ per month 28,133 188 0.91 (0.78, 1.05) 0.91 (0.78, 1.06)
pfor trend 0.3
Dye color
None 315,173 2,311 Ref Ref
Light colors only 20,860 145 0.94 (0.79, 1.11) 0.95 (0.80, 1.13)
Dark colors only 42,236 282 0.95 (0.84, 1.08) 0.96 (0.84, 1.09)
Light and dark colors 3,869 21 0.79 (0.51, 1.21) 0.79 (0.51, 1.21)
Years of personal use
None 270,992 2,026 Ref Ref
<5 54,574 355 0.90 (0.80,1.01) 0.90 (0.80,1.01)
5 or more 52,540 356 0.89 (0.80,1.00) 0.90 (0.80,1.00)
Applied to others
No use
2
373,614 2,689 Ref Ref
Any use 12,724 105 1.24 (1.02, 1.51) 1.28 (1.05,1.56)
<4 times per year 10,794 88 1.24 (1.00, 1.53) 1.27 (1.03,1.58)
Every 5–8 weeks, 1+ per month 1,930 17 1.26 (0.78, 2.04) 1.31 (0.81, 2.12)
pfor trend 0.02
Temporary dyes (rinses)
Personal use in the 12 months before enrollment
No use 356,833 2,573 Ref Ref
Any use 29,506 221 1.04 (0.90,1.19) 1.05 (0.91,1.21)
(Continues)
386 Hair products and breast cancer
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
missing information on the a priori confounders. The final
sample size was n= 46,709.
Effect measure modification on the multiplicative scale was
assessed using a likelihood ratio test to compare models with
and without a cross-product term for ethnicity and the expo-
sure variables. Stratum-specific estimates were obtained for
black and non-Hispanic white participants. Stratum-specific
estimates were not estimated for women classified as non-
black Hispanic or other ethnicity due to small sample size and
limited power.
When considering associations by ER status, women diag-
nosed with another type of breast cancer (e.g., ER−when the
outcome of interest was ER+) were censored at the time of
diagnosis. When considering associations by menopausal sta-
tus at diagnosis, person-time was stratified by menopausal sta-
tus. Women became at risk for postmenopausal breast cancer
at enrollment or age at menopause, whichever was later.
Women who were premenopausal at enrollment were cen-
sored at time of menopause for premenopausal breast cancer.
Sensitivity analyses included excluding women who had ever
worked in a hair salon, mutually adjusting the associations for
straightener and permanent hair dye use, including additional
adjustment for history ofhormone replacement therapy and alco-
hol use and testing for effect measure modification by body mass
index (BMI). All analyses were completed using SAS 9.4 (SAS
Institute, Cary, NC).
Data availability
Data used in this analysis may be requested through the Sister Study
data management system; information on requesting data can be
found at https://sisterstudy.niehs.nih.gov/English/data-requests.htm.
Results
There were 2,794 incident breast cancer cases reported in 386,338
person-years. The average length of follow up was 8.3 years. Per-
manent hair dye use was common with 55% of women reporting
use in the 12 months before enrollment. Compared to women
who did not use permanent dye, women who used permanent
dye tended to be younger, had fewer years of education, were
more likely to currently smoke cigarettes and more likely to have
used oral contraceptives (Table 1). Black women, postmeno-
pausal women, and those with fewer children were less likely to
have used permanent dye. Permanent dye use did not notably dif-
fer based on BMI or age at menarche.
Permanent dye use was related to breast cancer risk in the total
sample. Compared to nonuse, the hazard ratio for breast cancer was
1.09 for any current permanent dye use (95% CI: 1.01–1.17)
(Table 2). This association did notvarybyfrequencyofuse.Com-
pared to nonuse, use of light-colored dye (HR = 1.12, 95% CI:
1.02–1.23) and dark-colored dye (HR = 1.08, 95% CI: 0.98–1.19)
were associated with higher breast cancer risk. Associations did not
vary with years of use. While semi-permanent dye use was not asso-
ciated with risk, there was an association with nonprofessional
application of semipermanent dye to others (HR = 1.28, 95% CI:
1.05–1.56). This risk was positively related to increasing frequency
(pfor trend = 0.02). Temporary dye use was not associated with
breast cancer risk. Associations for all exposures were similar for
invasive breast cancer and DCIS (data not shown).
Associations with hair dye use tended to vary by ethnicity,
especially for permanent dye (heterogeneity p= 0.04; Table 3). In
black women, any permanent dye use in the 12 months before
enrollment was associated with a 45% higher breast cancer risk
(HR = 1.45, 95% CI: 1.10–1.90). Increased frequency of use was
Table 2. Hair dye and straightener exposure in the 12 months prior to enrollment and incident breast cancer, Sister Study, 2003–2009
(Continued)
Person-years Events Age-adjusted HR (95% CI) Adjusted HR
1
(95% CI)
Chemical straighteners
Personal use in the 12 months before enrollment
No use
2
351,502 2,543 Ref Ref
Any use 34,837 251 1.09 (0.95,1.24) 1.18 (0.99,1.41)
<4 times per year 16,279 107 0.98 (0.81, 1.19) 1.07 (0.86, 1.34)
Every 5–8 weeks, 1+ per month 18,558 144 1.18 (0.99, 1.39) 1.31 (1.05, 1.63)
pfor trend 0.02
Applied to others
No use
2
376,147 2,716 Ref Ref
Any use 10,192 78 1.20 (0.96,1.51) 1.27 (0.99,1.62)
<4 times per year 7,619 62 1.28 (0.99,1.65) 1.35 (1.03,1.77)
Every 5–8 weeks, 1+ per month 2,573 16 0.97 (0.59, 1.59) 1.03 (0.62,1.70)
pfor trend 0.2
1
models adjusted for age, ethnicity, education, ever oral contraceptive use (OC), parity, age at first birth, smoking status, BMI, age at menarche and
menopausal status.
2
For hazard ratio estimates associated with personal use in the 12 months before enrollment and application to others in the 12 months before enroll-
ment, “No use” is the referent category for “Any use.” For frequency of use, “No use” is also the referent category for “<4 times per year” and “Every
5 weeks, 1+month” and the pfor trend is provided.
Eberle et al.387
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Table 3. Hair dye and straightener exposures and incident breast cancer in non-Hispanic white and all black women, Sister Study 2003–2009
Non-Hispanic White Black
Person-
Years Events
Adjusted HR
(95% CI)
Person-
Years Events
Adjusted HR
1
(95% CI)
2,402 208
Permanent hair dye
Personal use in the 12 months before
enrollment
No use
2
147,144 1,064 Ref 17,671 106 Ref
Any use 183,378 1,338 1.07 (0.99,1.16) 12,491 102 1.45 (1.10,1.90)
<4 times per year 74,915 524 1.06 (0.95,1.18) 8,218 62 1.36 (0.99,1.87)
Every 5–8 weeks, 1+ per month 108,464 814 1.08 (0.98,1.18) 4,273 40 1.60 (1.11,2.30)
pfor trend 0.1 0.006
Dye color
None 147,144 1,064 Ref 17,671 106 Ref
Light colors only 85,978 664 1.12 (1.01,1.23) 2,401 21 1.46 (0.91,2.34)
Dark colors only 74,888 529 1.04 (0.94, 1.16) 8,883 74 1.51 (1.12,2.05)
Light and dark colors 20,792 133 0.97 (0.81, 1.16) 686 3 –
Years of personal use
None 115,391 822 Ref 13,395 94 Ref
<5 51,631 376 1.10 (0.97,1.24) 7,427 53 1.08 (0.77,1.52)
5 or more 159,466 1,177 1.06 (0.97,1.16) 8,681 59 0.97 (0.70,1.34)
Applied to others
No use
2
308,822 2,265 Ref 28,087 196 Ref
Any use 21,700 137 0.94 (0.79,1.12) 2,075 12 0.88 (0.49,1.58)
<4 times per year 18,543 118 0.96 (0.80,1.16) 1,709 10 0.91 (0.48,1.74)
Every 5–8 weeks, 1+ per month 3,157 19 0.86 (0.55,1.35) 366 2 –
pfor trend 0.5
Semipermanent dye
Personal use in the 12 months before
enrollment
No use
2
277,145 2,054 Ref 19,528 131 Ref
Any use 53,377 348 0.91 (0.81,1.02) 10,634 77 1.15 (0.86,1.53)
<4 times per year 32,227 218 0.97 (0.85,1.12) 6,903 47 1.10 (0.78, 1.54)
Every 5–8 weeks, 1+ per month 21,150 130 0.82 (0.69,0.98) 3,731 30 1.24 (0.83,1.84)
pfor trend 0.04 0.3
Dye color
None 277,145 2,054 Ref 19,528 130 Ref
Light colors only 18,385 127 0.94 (0.78,1.12) 929 5 0.79 (0.32,1.93)
Dark colors only 28,895 184 0.90 (0.77, 1.05) 8,590 62 1.16 (0.85,1.57)
Light and dark colors 3,139 15 0.69 (0.41, 1.14) 363 3 –
Years of personal use
None 240,094 1,803 Ref 15,415 110 Ref
<5 43,798 289 0.90 (0.80,1.02) 6,734 40 0.88 (0.61,1.27)
5 or more 40,087 271 0.88 (0.78,1.00) 6,989 48 0.99 (0.70,1.39)
Applied to others
No use
2
321,900 2,333 Ref 28,369 193 Ref
Any use 8,621 69 1.23 (0.96,1.56) 1,794 15 1.35 (0.79,2.29)
<4 times per year 7,534 57 1.17 (0.90, 1.52) 1,495 13 1.43 (0.81,2.53)
Every 5–8 weeks, 1+ per month 1,088 12 1.61 (0.91, 2.84) 299 2 –
pfor trend 0.06
(Continues)
388 Hair products and breast cancer
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
positively associated with risk (pfor trend = 0.006); black women
who used dye at least every 5–8 weeks had a 60% higher breast
cancer risk (95% CI: 1.11–2.30). The association with permanent
dye use among black women was evident for both dark-colored
dye (HR = 1.51, 95% CI: 1.12–2.05) and, although less precise,
light-colored dye (HR = 1.46, 95% CI 0.91–2.34). Among white
women, breast cancer risk was associated with use of light-
colored permanent dye (HR = 1.12, 95% 1.01–1.23) but not dark
dye (HR = 1.04, 95% 0.94–1.16).
Current straightener use (9.9% in total sample) varied by eth-
nicity, with 74.1% of black women reporting any use compared to
3.0% of non-Hispanic white women. Overall, straightener use in
the 12 months before enrollment was associated with 18% higher
breast cancer risk (95% CI: 0.99–1.41;Table2).Morefrequent
straightener use was associated withhigher risk (pfor trend = 0.02);
women who used straighteners at least every 5–8 weeks had a 31%
higher breast cancer risk (95% CI: 1.05–1.63). Similarly, a higher
risk was evident for nonprofessional application of straighteners
to others (HR = 1.27, 95% CI: 0.99–1.62). The risk associated with
straightener use did not notably vary by ethnicity and was evident
for both Non-Hispanic white and black women (p=0.8).
We observed little evidence of heterogeneity by menopausal
status at diagnosis. However, we did note a higher risk of
premenopausal breast cancer (HR = 1.30, 95% CI: 1.04–1.62)
associated with light dye use (Supporting Information Table S1).
The association for non-professional application of semiperma-
nent dyes (HR = 1.35, 95% CI 1.08–1.69), and personal use
(HR = 1.26, 95% CI 1.02–1.55) and application to others
(HR = 1.34, 95% CI: 0.99–1.79) of straighteners was most evident
for postmenopausal breast cancer.
Although we had limited power to detect differences by
ER-status (Supporting Information Table S2), overall the risk
associated with both permanent hair dye and straightener use
appeared to be elevated for ER−invasive breast cancer com-
pared to ER+ invasive breast cancer.
Neither excluding women who had ever worked in hair
salons (n= 1,616) nor adjusting for alcohol and hormone
replacement therapy use materially changed results. There was
no evidence of modification by BMI. Mutual adjustment for
permanent dye and straightener use did not notably change
the findings (data not shown).
Discussion
In this large prospective US-based study, we evaluated the
relationship between hair dye and straightener use with breast
cancer risk and found that women who used permanent dye
Table 3. Hair dye and straightener exposures and incident breast cancer in non-Hispanic white and all black women, Sister Study 2003–2009
(Continued)
Non-Hispanic White Black
Person-
Years Events
Adjusted HR
(95% CI)
Person-
Years Events
Adjusted HR
1
(95% CI)
Temporary dyes (rinses)
Personal use in the 12 months before
enrollment
No use 312,860 2,270 Ref 21,849 148 Ref
Any use 17,662 132 1.02 (0.86,1.22) 8,314 60 1.12 (0.83,1.51)
Chemical straighteners
Personal use in the 12 months before
enrollment
No use
2
321,146 2,334 Ref 7,783 48 Ref
Any use 9,376 68 1.16 (0.91,1.48) 22,380 160 1.20 (0.87,1.66)
<4 times per year 5,459 37 1.09 (0.79, 1.51) 8,808 56 1.05 (0.71,1.55)
Every 5–8 weeks, 1+ per month 3,917 31 1.26 (0.88, 1.80) 13,572 104 1.30 (0.92, 1.85)
pfor trend 0.2 0.1
Applied to others
No use
2
327,889 2,380 Ref 23,502 158 Ref
Any use 2,634 22 1.38 (0.90,2.10) 6,660 50 1.22 (0.87,1.70)
<4 times per year 2,039 19 1.55 (0.98,2.44) 4,900 38 1.27 (0.88,1.84)
Every 5–8 weeks, 1+ per month 595 3 – 1,760 12 1.08 (0.59,1.96)
pfor trend 0.4
Tests for interaction on the multiplicative scale: Black *permanent dye p= 0.0382; Black *relaxer/straightener: p= 0.7983.
1
models adjusted for age, education, ever oral contraceptive use (OC), parity, age at first birth, smoking status, BMI and age at menarche.
2
For hazard ratio estimates associated with personal use in the 12 months before enrollment and application to others in the 12 months before enroll-
ment, “No use” is the referent category for “Any use.” For frequency of use, “No use” is also the referent category for “<4 times per year” and “Every
5 weeks, 1+month” and the pfor trend is provided.
Eberle et al.389
Int. J. Cancer: 147, 383–391 (2020) ©2019 UICC
Cancer Epidemiology
10970215, 2020, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ijc.32738 by National Institute of Environmental Health Sciences, Wiley Online Library on [04/04/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
or straighteners, or applied straighteners to others, in the
12 months before enrollment were at a higher breast cancer
risk. The association with permanent hair dye was particularly
evident in black women, for whom we observed a 45% higher
breast cancer risk. Overall, these results support the hypothe-
sis that hair dye and straightener use, which are highly preva-
lent exposures, could play a role in breast carcinogenesis.
The strength of association observed for permanent dye use
among black women is consistent with toxicological assessments
that report higher concentrations of estrogens and endocrine-
disrupting compounds in hair products marketed to black
women.
30,31,33,42,43
Previous studies on hair dye use and breast
cancer risk, including most that found no association, have
largely been limited to white women.
10,17,20–23,26
Our findings are
consistent with those recently reported by the Women’s Circle of
Health Study (WCHS) which observed a very similar increase in
risk (~50%) for use of dark dyes and a higher risk with increased
frequency of permanent dye use among African-American
women.
18
The WCHS study assessed exposure retrospectively
and thus, could not rule out the possibility of recall bias.
We observed a 9% higher breast cancer risk for permanent
dye use in all women but little to no associated risk for semiper-
manent or temporary dye use. These findings are consistent with
biological plausibility of differences in constituents and binding
mechanisms between types of dye; namely, permanent dyes con-
tain higher concentrations of aromatic amines.
43
Despite the esti-
mated lower concentrations of oxidizing agents present in
semipermanent dyes, we did observe an association between non-
professional application of semipermanent dyes and breast can-
cer risk. At-home kits contain gloves, but potential absorption on
hands and forearms, inhalation of toxic chemicals, and residual
agents remaining on surfaces or in the air in poorly ventilated set-
tings introduce several mechanisms for exposure.
The higher breast cancer risk observed in women who used
straighteners is consistent with recent findings from both the
Ghana Breast Health Study and WCHS.
18,29
Notably, these find-
ings contrast with null association observed in the Black
Women’s Health Study, which assessed exposure in the mid-
1990s. However, this discrepancy may reflect changes in chemi-
cal formulation of popular straighteners between studies.
28
Historically, active ingredients in straighteners included sodium
hydroxide and thioglycolic acid salts, neither of which have
identified carcinogenic effects in humans.
18
In the early 2000s
Brazilian Keratin Treatments, which contain formaldehyde, a
known carcinogen,
16
or one of its derivatives that reacts with
keratin when heated, were introduced to global markets. Our
finding is the first estimate of the association between
straightener use and breast cancer from a prospective cohort
that assessed exposure after the introduction of formaldehyde-
containing straighteners to US markets.
Our findings for straightener use and ER status are
remarkably similar to those reported by Llanos et al.
18
They
also reported no association between personal straightener use
and ER+ breast cancer and higher, but not significant, odds of
ER−breast cancer in African-American women. Their expo-
sure captured ever use of any straightener reported over a
similar time period, 2002–2014. Both studies were limited in
number of ER−cases; future research that is better powered
to estimate the risk associated with ER−breast cancers should
be considered.
We were not able to evaluate the formulation of the hair dyes
or straighteners assessed, nor are they reliably documented on
labels
31
which is a limitation. By design, all study participants have
a family history of breast cancer, which may limit the generaliz-
ability of these findings. However, this would not affect the inter-
nal validity of the study, as it would not bias our estimates of the
association between hair dye and straightener use and breast can-
cer risk. Although our study was prospective, limiting the possibil-
ity of recall bias, the potential for recall error remains. Though
detailed exposure assessment was limited to the prior year to max-
imize recall, participants may have had difficulty differentiating
between types of dye (permanent, semipermanent and tempo-
rary). Further, hair product use throughout follow-up was not
considered. Future studies that are powered to robustly evaluate
the possibility of differences by ethnicity and tumor subtype are
needed. The consideration of detailed exposure information on
hair dye and straightener use in a population of both black and
white women was an important strength of our study. Further-
more, our exposure collection time period allowed for assessment
of more modern levels and types of exposure which may be partic-
ularly important given the change in popular straighteners.
In conclusion, these findings from a large, prospective
cohort with a sufficient sample size to separately evaluate
results for white and black women provide evidence to sup-
port the relationship of hair dye and straightener use with
breast cancer risk and highlight potential differences in associ-
ations by ethnicity. As hair dye and straighteners are common
exposures, these findings have the potential for substantial
public health impact.
Acknowledgements
This research was supported by the Intramural Research Program of the NIH,
National Institute of Environmental Health Sciences (Z01-ES044005).
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