Environment and Breast Cancer: Science Review

Evidence From Humans
Print this page
A case-control study of breast cancer risk in nurses from Northeastern Ontario, Canada
Santi, S. A., Meigs, M. L., Zhao, Y., Bewick, M. A., Lafrenie, R. M., Conlon, M. S. Cancer Causes Control. 2015. 26:10, 1421-8.
Topic area
Environmental pollutant - Occupation
Study design
Population-based case-control
Funding agency
Northern Cancer Foundation
Study Participants
Menopausal Status
The menopausal status of women included in this study is listed here.
No analysis based on menopausal status
Number of Controls
Controls: 775
Participant selection: Inclusion and exclusion criteria
Criteria used to select participants in the study.
This study compiled previously collected data from two studies in Northeastern Ontario: Study #1 was a population-based case-control of active/passive smoke exposure and breast cancer. This study included 347 women aged 25-75 years diagnosed with primary invasive breast cancer between 2002 and 2004 and 775 population controls. Cases were identified from the Ontario Cancer Registry. Controls were selected using a list of randomly selected telephone numbers from the study area. Study #2 evaluated breast cancer care outcomes at Northeast Cancer Centre from 2005-2011. The study identified 855 women aged 25-75 recently diagnosed with histologically confirmed breast cancer. About half of these women consented, filled out a questionnaire, and provided a saliva sample. Cases were excluded from this analysis due to incomplete questionnaires or missing data, poor saliva sample quality, or if they were > age 75 at enrollment.
Comment about participation selection
Selection bias may be a concern given low response rates for the second study (51.8% for questionnaires and 49.1% for saliva samples). Additionally, all controls were recruited 2002-2004 while cases were recruited (across the two studies) from 2002-2011; controls may thus not be representative of the population from which cases arose.
Exposure Investigated
Exposures investigated
Ever vs never worked as a nurse, identified by self-reported lifetime work history (work or job title, industry, duties). Among nurses, duration, "intensity" (full-time, part-time) and occupational setting (hospital, office) were also evaluated.
How exposure was measured
Questionnaire, self-administered
Exposure assessment comment
Classification of work history was performed blinded to case-control status. Occupations were primarily classified into 2011 National Occupation Classification categories; 25 secondary categories were also assigned. Nursing work classified as "managerial" was not included in duration/intensity calculations. The following were considered indicators of working in an "office" setting: (1) worked in a doctor's office (2) worked in "office- or daytime based" jobs such as occupational health nurse, public health nurse, or clinical coordinator.
Breast cancer outcome investigated
Primary incident breast cancer
Confounders considered
Other breast cancer risk factors, such as family history, age at first birth, and hormone replacement therapy use, that were taken into account in the study.
Age, family history, education, oral contraceptive use, alcohol consumption, number of births, and age at first menarche.
Genetic characterization included
If the study analyzed relationships between environmental factors and inherited genetic variations, this field will be marked “Yes.” “No”, if not.
Strength of associations reported
Breast cancer risk in ever- vs. never-nurses: aOR 1.39 (95% CI 0.93-2.07)

Duration of years worked vs. never-nurses:
1-14 years: aOR 1.10 (95% CI 0.67-1.80)
15-30 years: aOR 1.60 (95% CI 0.87-2.93)
>30 years (14 cases, 6 controls): aOR 1.87 (95% CI 0.71-4.95)

Occupational setting as a nurse vs. never nurses:
Nurses who only worked in a hospital setting: aOR 1.65 (95% CI 1.04-2.62)
Nurses who worked in both a hospital and office setting: aOR 0.74 (95% CI 0.39-1.42)
Results Comments
General population, including both workers and non-workers, may not be an appropriate comparison group. Further, the occupational make-up of the reference group is not described.
Author address
Epidemiology, Outcomes and Evaluation Research, Northeast Cancer Centre, Health Sciences North, Sudbury, ON, P3E 5J1, Canada. ssanti@hsnsudbury.ca. Epidemiology, Outcomes and Evaluation Research, Northeast Cancer Centre, Health Sciences North, Sudbury, ON
Reviewers Comments
Authors note among nurses who had "shift type" information available (participants in study #2), many jobs were classified as "rotating shift" or "night shift", suggesting possible role for light at night.