Table 6 Variations in the definitions of smoking exposures
From: Modifiable risk factors in women at high risk of breast cancer: a systematic review
Age at smoking initiation | Never vs. < 20yo vs. 20 + yo | Never vs. before or < 5 years after menarche vs. 5 + years after menarche | Unspecified |  |  |
Cigarettes/day | Never vs. 1–4 vs. 5 +  | Unspecified |  |  |  |
Duration (years or pack-years) | Never vs. 1-9y vs. 10 + y | Never vs. 20 + y | 0y vs. < 11y vs. 11-20y vs. 21-30y vs. > 30y | Never vs. ≤ 30py vs. > 30py | Unspecified |