Table 1.

Incidence of breast cancer in relation to hair relaxer use in the Black Women's Health Study

Duration of hair relaxer use (y)
Nonuse*1-45-910-1415-19≥20
Cases53525063109247
Person-years20,00918,54126,91752,64958,44489,737
IRRReference1.171.020.851.191.03
95% CI0.79-1.710.69-1.500.59-1.230.85-1.670.76-1.39

Frequency of hair relaxer use (times per year)

Nonuse*
1
2
3-4
5-6
≥7
Cases532543164149139
Person-years20,0097,20817,93069,89871,70277,263
IRRReference1.440.981.031.061.04
95% CI0.89-2.320.65-1.460.75-1.400.77-1.460.75-1.44

Age at first hair relaxer use (y)

Nonuse*
<10
10-19
20-29
≥30

Cases531322320180
Person-years20,00913,366140,30472,34419,394
IRRReference0.961.071.011.04
95% CI0.52-1.790.78-1.470.74-1.380.73-1.48

No. hair relaxer burns

Nonuse*
0
1-2
3-4
5-9
≥10
Cases531001481077094
Person-years20,00941,50869,04352,61633,45848,654
IRRReference0.951.021.051.201.14
95% CI0.68-1.320.74-1.400.75-1.460.84-1.730.81-1.61

Type of hair relaxer used

Nonuse*
Lye
No-lye
Unknown
Cases538035672
Person-years20,00939,386173,94526,757
IRRReference1.161.071.05
95% CI0.82-1.650.80-1.430.74-1.49
  • * Never used or <1 year of use.