Active smoking, household passive smoking, and breast cancer: evidence from the California Teachers Study.
BACKGROUND
There is great interest in whether exposure to tobacco smoke, a substance containing human carcinogens, may contribute to a woman's risk of developing breast cancer. To date, literature addressing this question has been mixed, and the question has seldom been examined in large prospective study designs.


METHODS
In a 1995 baseline survey, 116 544 members of the California Teachers Study (CTS) cohort, with no previous breast cancer diagnosis and living in the state at initial contact, reported their smoking status. From entry into the cohort through 2000, 2005 study participants were newly diagnosed with invasive breast cancer. We estimated hazard ratios (HRs) for breast cancer associated with several active smoking and household passive smoking variables using Cox proportional hazards models.


RESULTS
Irrespective of whether we included passive smokers in the reference category, the incidence of breast cancer among current smokers was higher than that among never smokers (HR = 1.32, 95% confidence interval [CI] = 1.10 to 1.57 relative to all never smokers; HR = 1.25, 95% CI = 1.02 to 1.53 relative to only those never smokers who were unexposed to household passive smoking). Among active smokers, breast cancer risks were statistically significantly increased, compared with all never smokers, among women who started smoking at a younger age, who began smoking at least 5 years before their first full-term pregnancy, or who had longer duration or greater intensity of smoking. Current smoking was associated with increased breast cancer risk relative to all nonsmokers in women without a family history of breast cancer but not among women with such a family history. Breast cancer risks among never smokers reporting household passive smoking exposure were not greater than those among never smokers reporting no such exposure.


CONCLUSIONS
Our study provides evidence that active smoking may play a role in breast cancer etiology and suggests that further research into the connection is warranted, especially with respect to genetic susceptibilities.
Reynolds
Peggy
P
California Department of Health Services, Environmental Health Investigations Branch, Oakland, CA 94612, USA. preynold@dhs.ca.gov
Hurley
Susan
S
Goldberg
Debbie E
DE
Anton-Culver
Hoda
H
Bernstein
Leslie
L
Deapen
Dennis
D
Horn-Ross
Pamela L
PL
Peel
David
D
Pinder
Richard
R
Ross
Ronald K
RK
West
Dee
D
Wright
William E
WE
Ziogas
Argyrios
A
D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology*; D002140:California / Q000453:epidemiology; D016001:Confidence Intervals; D015986:Confounding Factors, Epidemiologic; D005191:Family Characteristics; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D016017:Odds Ratio; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology; D011446:Prospective Studies; D012107:Research Design; D018570:Risk Assessment; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects* / Q000453:epidemiology; D013663:Teaching; D014028:Tobacco Smoke Pollution / Q000009:adverse effects* / Q000706:statistics & numerical data
D014028:Tobacco Smoke Pollution