Environment and Breast Cancer: Science Review

Evidence From Humans
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Hair relaxers not associated with breast cancer risk: evidence from the black women's health study
Rosenberg, L., Boggs, D. A., Adams-Campbell, L. L., Palmer, J. R. Cancer epidemiology, biomarkers & prevention : a p. 2007. 16:5, 1035-7.
Topic area
Environmental pollutant - Consumer product chemicals
Study design
Prospective cohort
Funding agency
National Cancer Institute
Study Participants
Menopausal Status
The menopausal status of women included in this study is listed here.
No analysis based on menopausal status
Number in Cohort
Cohort: 48,167
Cohort participation rate
≥ 80% through 2003
Participant selection: Inclusion and exclusion criteria
Criteria used to select participants in the study.
The Black Women's Health Study is a prospective cohort of 59,000 Black women from across the US who were age 21-69 in 1995 and responded to a mailed health questionnaire. Follow-up questionnaires are mailed every 2 years. The 1997 questionnaire included the first questions about hair relaxer use, so this analysis followed women for incident breast cancer reported on follow-up questionnaires between 1997 and 2003. A portion of self-reported breast cancers have been verified with medical records; all 1,069 women self-reporting incident breast cancer were eligible for this analysis, except women who had had diagnosis disconfirmed by medical records review. Women with any prevalent cancer in 1997 were excluded from the analysis, as were women who did not complete the questions about hair relaxers or did not complete any follow-up questionnaires after 1997.
Exposure Investigated
Exposures investigated
Use of chemical hair straighteners, including frequency (times per year), duration of use, age at first use, number of burns and type (formulation) used. Each variable analyzed as categories of use vs nonuse.
How exposure was measured
Questionnaire, self-administered
Exposure assessment comment
Because the study mails out repeated waves of questionnaires to increase the response rates, 1,060 women ended up returning duplicate questionnaires. Comparison of duplicate responses from the same woman indicated good reproducibility.
Early life exposures considered
Yes, age at first use included categories for <10 years old, 10-19 years old, and 20-29 years old
Breast cancer outcome investigated
Primary incident breast cancer
Ethnic groups with separate analysis
If this study provided a separate analysis by ethnic or racial group, the groups are listed here.
Confounders considered
Other breast cancer risk factors, such as family history, age at first birth, and hormone replacement therapy use, that were taken into account in the study.
Age, age at menarche, age at first birth, parity, family history of breast cancer, benign breast disease, alcohol use, physical activity, education, and geographic region
Genetic characterization included
If the study analyzed relationships between environmental factors and inherited genetic variations, this field will be marked Yes. No, if not.
Strength of associations reported
For use of hair relaxers ≥ 7 time a year for ≥ 20 years vs non-users:
Among women < 45 years old: aIRR 0.84 (95% CI 0.42-1.71)
Among women ≥ 45 years old: aIRR 1.03 (95% CI 0.68-1.56)

Age at first hair relaxer used, compared to non-users (never or < 1 year)

Among women < 45 years old:
<10 years old: aIRR 0.58 (95% CI 0.22-1.55)
10-19 years old: aIRR 0.97 (95% CI 0.53-1.75)
20-29 years old: aIRR 0.98 (95% CI 0.52-1.84)
≥ 30 years old: aIRR 0.45 (95% CI 0.10-2.00)

Among women ≥ 45 years old:
<10 years old: aIRR 1.66 (95% CI 0.74-3.72)
10-19 years old: aIRR 1.09 (95% CI 0.75-1.59)
20-29 years old: aIRR 1.02 (95% CI .072-1.44)
≥ 30 years old: aIRR 1.10 (95% CI 0.75-1.61)
Results Comments
Overall breast cancer incidence rate was not significantly associated with any category of duration or frequency of hair relaxer use, age at first use, number of burns during relaxer use, or type of relaxer used. Stratifying analyses by age (<45 and ≥45) did not appreciably change these findings.
Author address
Slone Epidemiology Center at Boston University, Boston, Massachusetts 02215, USA. lrosenberg@slone.bu.edu