Table 2.

Risk of breast cancer in relation to hair relaxer use according to age (<45, ≥45) in the Black Women's Health Study

Age <45 y
Duration of hair relaxer use (y)
Nonuse*1-45-910-1415-19≥20
Cases121611255562
Person-years8,8759,25017,40940,45043,77336,099
IRRReference1.280.600.661.100.98
95% CI0.60-2.700.26-1.360.33-1.320.59-2.070.53-1.82
Frequency of hair relaxer use (times per year)

Nonuse*
1
2
3-4
5-6
≥7
Cases12610415953
Person-years8,8753,6318,70836,78244,23752,417
IRRReference1.230.830.841.080.90
95% CI0.46-3.270.36-1.930.44-1.600.58-2.000.48-1.68

Age at first hair relaxer use (y)
Nonuse*
<10
10-19
20-29
≥30

Cases126113482
Person-years8,87511,952104,75627,8191,960
IRRReference0.580.970.980.45
95% CI0.22-1.550.53-1.750.52-1.840.10-2.00

No. hair relaxer burns
Nonuse*
0
1-2
3-4
5-9
≥10
Cases122741342838
Person-years8,87519,25039,75531,95322,35133,181
IRRReference1.060.800.871.060.97
95% CI0.54-2.090.42-1.530.45-1.690.54-2.090.51-1.86

Type of hair relaxer used
Nonuse*
Lye
No-lye
Unknown
Cases123011917
Person-years8,87526,199106,23711,456
IRRReference1.031.011.21
95% CI0.53-2.020.56-1.820.58-2.53

Age ≥45 y
Duration of hair relaxer use (y)
Nonuse*
1-4
5-9
10-14
15-19
≥20
Cases4136393854185
Person-years11,1349,2919,50812,19814,67253,638
IRRReference1.111.230.971.191.04
95% CI0.71-1.740.79-1.910.62-1.510.79-1.800.74-1.47

Frequency of hair relaxer use (times per year)
Nonuse*
1
2
3-4
5-6
≥7
Cases4119331239086
Person-years11,1343,5779,22233,11627,46524,846
IRRReference1.511.021.091.021.10
95% CI0.87-2.600.64-1.620.77-1.560.70-1.490.76-1.61

Age at first hair relaxer use (y)
Nonuse*
<10
10-19
20-29
≥30

Cases41711015378
Person-years11,1341,41335,54844,52517,434
IRRReference1.661.091.021.10
95% CI0.74-3.720.75-1.590.72-1.440.75-1.61

No. hair relaxer burns
Nonuse*
0
1-2
3-4
5-9
≥10
Cases4173107734256
Person-years11,13422,25829,28920,66311,10815,473
IRRReference0.911.111.121.251.21
95% CI0.62-1.330.77-1.600.76-1.650.81-1.930.80-1.83

Type of hair relaxer used
Nonuse*
Lye
No-lye
Unknown
Cases415023755
Person-years11,13413,18767,70815,301
IRRReference1.231.101.01
95% CI0.82-1.870.79-1.520.68-1.51
  • * Never used or <1 year of use.