Table 1. Pooled relative risk and corresponding 95% confidence interval for breast cancer risk for current, former, and ever cigarette smokers versus never cigarette smokers, overall and in strata of selected characteristics.
| Strata | Current smokers | Former smokers | Ever smokers | |||||||||
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| Number of studies | Pooled RR (95% CI) |
P-valuea | P-valueb | Number of studies | Pooled RR (95% CI) |
P-valuea | P-valueb | Number of studies | Pooled RR (95% CI) |
P-valuea | P-valueb | |
| Total | 90 | 1.07 (1.05–1.10) |
<0.01 | 86 | 1.08 (1.06–1.10) |
<0.01 | 135 | 1.09 (1.07–1.11) |
<0.01 | |||
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| Geographic areac | ||||||||||||
| North America | 39 | 1.10 (1.05–1.14) |
0.28 | <0.01 | 38 | 1.09 (1.05–1.12) |
0.20 | 0.01 | 54 | 1.08 (1.06–1.11) |
<0.01 | <0.01 |
| Europe | 31 | 1.05 (1.01–1.10) |
<0.01 | 29 | 1.08 (1.05–1.11) |
0.31 | 39 | 1.07 (1.02–1.12) |
<0.01 | |||
| Asia | 11 | 1.13 (0.90–1.40) |
0.02 | 10 | 1.09 (0.76–1.57) |
<0.01 | 30 | 1.32 (1.16–1.50) |
<0.01 | |||
| Oceania | 2 | 1.11 (0.83–1.48) |
0.09 | 2 | 1.05 (1.00–1.11) |
0.96 | 3 | 1.03 (0.99–1.08) |
0.60 | |||
| South America | 1 | 2.43 (1.01–5.84) |
– | 1 | 1.90 (1.14–3.15) |
– | 1 | 2.02 (1.30–3.13) |
– | |||
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| Income groupd | ||||||||||||
| High income | 83 | 1.08 (1.04–1.11) |
0.87 | <0.01 | 79 | 1.08 (1.06–1.10) |
<0.01 | <0.01 | 114 | 1.08 (1.06–1.11) |
0.07 | <0.01 |
| Middle income | 2 | 1.20 (0.33–4.35) |
0.01 | 2 | 1.96 (1.29–3.00) |
0.82 | 14 | 1.44 (1.06–1.95) |
<0.01 | |||
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| Study design | ||||||||||||
| Cohort | 44 | 1.10 (1.07–1.14) |
0.02 | <0.01 | 40 | 1.08 (1.05–1.10) |
0.69 | 0.02 | 44 | 1.08 (1.06–1.10) |
0.36 | 0.02 |
| Case-control | 46 | 1.03 (0.98–1.08) |
<0.01 | 46 | 1.08 (1.05–1.12) |
0.03 | 92 | 1.10 (1.06–1.14) |
<0.01 | |||
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| Type of controlse | ||||||||||||
| Hospital | 9 | 0.97 (0.80–1.17) |
0.44 | <0.01 | 9 | 1.08 (0.94–1.23) |
0.84 | 0.18 | 25 | 1.09 (0.95–1.24) |
0.85 | <0.01 |
| Population | 34 | 1.04 (0.99–1.10) |
<0.01 | 34 | 1.09 (1.04–1.14) |
<0.01 | 62 | 1.10 (1.06–1.15) |
<0.01 | |||
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| Endpointf | ||||||||||||
| Incidence | 34 | 1.09 (1.06–1.13) |
0.01 | <0.01 | 33 | 1.07 (1.06–1.09) |
0.82 | 0.34 | 37 | 1.08 (1.06–1.10) |
0.14 | 0.02 |
| Mortality | 11 | 1.18 (1.12–1.24) |
0.46 | 8 | 1.10 (0.90–1.33) |
<0.01 | 8 | 1.14 (1.06–1.22) |
0.29 | |||
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| Year of publication | ||||||||||||
| <2002 | 26 | 1.06 (0.99–1.13) |
0.28 | <0.01 | 25 | 1.06 (1.01–1.12) |
0.18 | 0.16 | 47 | 1.06 (1.01–1.11) |
0.33 | <0.01 |
| 2002–2011 | 29 | 1.04 (0.99–1.10) |
0.03 | 29 | 1.12 (1.07–1.17) |
<0.01 | 41 | 1.11 (1.07–1.17) |
<0.01 | |||
| ≥2012 | 35 | 1.09 (1.06–1.13) |
<0.01 | 32 | 1.07 (1.01–1.12) |
0.11 | 47 | 1.09 (1.06–1.11) |
<0.01 | |||
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| Menopausal status | ||||||||||||
| Pre- | 22 | 1.03 (0.99–1.08) |
0.02 | 0.42 | 22 | 1.09 (1.02–1.15) |
0.57 | 0.20 | 37 | 1.10 (1.04–1.16) |
0.61 | <0.01 |
| Post- | 26 | 1.13 (1.06–1.21) |
<0.01 | 25 | 1.06 (1.03–1.10) |
0.55 | 40 | 1.12 (1.08–1.17) |
<0.01 | |||
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| Estrogen Receptor | ||||||||||||
| Positive | 12 | 1.09 (1.02–1.16) |
0.39 | 0.19 | 12 | 1.09 (1.04–1.14) |
0.81 | 0.18 | 16 | 1.11 (1.05–1.16) |
0.57 | 0.02 |
| Negative | 12 | 1.04 (0.96–1.13) |
0.40 | 12 | 1.08 (0.97–1.19) |
0.10 | 16 | 1.08 (1.00–1.16) |
0.08 | |||
CI, confidence interval; RR, relative risk.
aP-value for heterogeneity across strata.
bP-value for heterogeneity within strata.
cStudies conducted in multiple countries from different geographic areas were not included. No studies from Africa.
dStudies conducted in multiple countries with different income were not included. No studies from low income countries.
eType of controls for case-control studies only. Pooled analyses considering both studies with hospital and with population controls were not included.
fEndpoint for cohort studies only. Studies providing RRs for both incidence and mortality were considered in both categories.