title: Age of smoking initiation and risk of breast cancer in a sample of Ontario women.
abstract: OBJECTIVE
  To examine the association between time of smoking initiation and both the independent and joint effects of active and passive tobacco smoke exposure and the risk of breast cancer in a sample of Ontario women.
  
  
  METHODS
  Data from two large population-based case-control studies conducted among Ontario women aged 25-75 years were combined for analysis (n = 12,768).
  
  
  RESULTS
  Women who had ever smoked and were exposed to passive smoke had a significant increased risk of breast cancer (OR 1.13, 95%CI 1.01-1.25). A significant increased risk was also observed among women who initiated smoking: at age 26 or older (OR 1.26, 95%CI 1.03-1.55); more than five years from menarche (OR 1.26, 95%CI 1.12-1.42); and, after their first live birth (OR 1.25, 95%CI 1.02-1.52).
  
  
  CONCLUSIONS
  The results suggest that women who initiate smoking at an older age are at an increased risk of breast cancer.
authors.affiliation: Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada. scott.leatherdale@cancercare.on.ca.
authors.forename: Erin
authors.initials: E
authors.lastname: Young
authors.forename: Scott
authors.initials: S
authors.lastname: Leatherdale
authors.forename: Margaret
authors.initials: M
authors.lastname: Sloan
authors.forename: Nancy
authors.initials: N
authors.lastname: Kreiger
authors.forename: Andriana
authors.initials: A
authors.lastname: Barisic