BMC Cancer 2008, 8:157 http://www.biomedcentral.com/1471-2407/8/157 Table 2: Active smoking and breast cancer among cases and controls, Delaware, 2000–2002 cases controls odds ratioa 95% confidence intervala Active smoke exposure Ever smoked 100 cigarettes No Yes Total years smoked cigarettesb <10 10–19 20–29 30–39 40–49 50–62 Average number of cigarettes/dayb <10 10–19 20–29 30–68 Total pack-yearsb <5 5–9 10–19 20–29 30–39 40–49 50–102 Years smoked at or before age 18c <5 5–10 Years smoked, before first live birthd Never smokers <5 5–9 10–14 15–39 n 124 163 23 27 34 36 27 15 58 67 25 9 37 19 37 28 10 16 12 93 16 108 38 59 12 11 % 43.2 56.8 8.0 9.4 11.9 12.6 9.4 5.2 20.5 23.7 8.8 3.2 13.1 6.7 13.1 9.9 3.5 5.7 4.2 32.4 5.6 37.6 13.2 20.6 4.2 3.8 n 161 150 26 33 26 32 21 12 49 58 29 12 36 18 26 29 12 9 18 91 15 138 36 55 27 9 % 51.8 48.2 8.4 10.6 8.4 10.3 6.8 3.9 15.9 18.8 9.4 3.9 11.7 5.8 8.4 9.4 3.9 2.9 5.8 29.3 4.8 44.4 11.6 17.7 8.7 2.9 1.00 1.43 1.36 1.13 1.80 1.31 1.50 1.21 1.60 1.53 1.12 0.84 1.45 1.65 1.83 1.24 1.06 1.92 0.75 1.34 1.33 1.00 1.25 1.37 0.69 1.99 reference 1.03–1.99 0.72–2.55 0.64–2.01 1.02–3.20 0.76–2.25 0.79–2.82 0.52–2.84 1.01–2.53 0.99–2.35 0.61–2.04 0.34–2.08 0.85–2.45 0.81–3.35 1.04–3.23 0.69–2.24 0.44–2.56 0.81–4.55 0.34–1.65 0.91–1.96 0.62–2.85 reference 0.73–2.13 0.87–2.16 0.33–1.45 0.76–5.18 a adjusted for age, education, and menopausal status b Numbers do not always total due to missing data c Excludes women who smoked only after age 18 d Excludes nulliparous women and those who smoked only after the birth of their first child hand smoke exposure at work was not associated with increased breast cancer risk, even when considering 20–45 years of exposure duration and "heavy" self-rated inten- sity of exposure (Table 3). reference group, two less stringent definitions were con- sidered, but no statistically significant associations were observed using either of these alternative reference groups (Table 4). In active smoke exposure re-analyses, only 13 cases and 14 controls comprised the most stringent reference group of ever active smokers with no secondhand smoke exposure at home or at work during their lifetimes (Table 4). Using this reference group, ever having smoked cigarettes was not statistically significantly associated with breast cancer (OR = 1.21, 95% CI = 0.55–3.32), after adjustment for age and menopausal status. There was no evidence of a dose- response relationship between smoking and breast cancer risk using this reference group, nor were exposures before 18 or before first live birth associated with breast cancer (Table 4). As mentioned, to increase the sample size of the Discussion Overall, our findings do not support exposure to active or secondhand tobacco smoke as a risk factor for breast can- cer. A statistically significant increased risk of breast cancer was observed for ever having smoked at least 100 ciga- rettes in one's lifetime. However, the magnitude of this risk was modest (OR = 1.43) and decreased rather than increased with use of more stringent reference groups excluding those exposed to secondhand smoke. Addition- ally, there was no evidence of a dose-response relation- ship between breast cancer risk and total years smoked, cigarettes per day, or pack-years. Residential exposure to Page 5 of 9 (page number not for citation purposes)