California Teachers Study (CTS)

Principal Investigator:
Peggy Reynolds, Ph.D., David O. Nelson, Ph.D.
Leslie Bernstein, Ph.D., Huiyan Ma, Ph.D., Yani Lu, Ph.D., James V. Lacey, Jr., Ph.D., and Sophia Wang, Ph.D. (City of Hope National Medical Center); Dennis Deapen, Dr.P.H., and Eunjung Lee, Ph.D. (University of Southern California); Hoda Anton-Culver, Ph.D. and Al Ziogas, Ph.D. (University of California, Irvine)
Funding Source:
National Cancer Institute
Funding Period:
Study Website:
The California Teachers Study (CTS) is a major long-term research study initiated by CPIC in partnership with other California universities and research institutes. The CTS is a statewide health study of over 133,000 female current and former teachers and school administrators originally recruited from teacher retirement records. The top priority of the CTS has been to find the causes of breast cancer. However, investigators also use CTS data to understand women’s health more broadly, including studying risk factors of other cancers, asthma, cardiovascular disease and death, stroke, and overall longevity.

How We Collect the Data
How We Use the Data
Some of Our Findings
Selected Research Projects
CTS Collaborators
CTS Information Sheet

How We Collect the Data

Starting in 1995, researchers at CPIC and other collaborating institutions gathered information from CTS participants using an extensive questionnaire asking about their diet, physical activity, medical history, reproductive history, and other aspects of their lifestyle. Every few years, the participants fill out a new questionnaire with updated or new information about changes in their health and lifestyle. 
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How We Use the Data

The research team follows the health of each CTS member by linking participant data with data in California statewide databases that provide information on individual cancer diagnoses and details, deaths and specific causes of death, and hospitalizations. Tracking participants’ health in this way provides a highly accurate summary, allowing for high-quality research.

Study data are being used to understand how various factors work independently or together to cause cancers. These factors include genetics, obesity, diet (especially cruciferous vegetables and plant-based estrogens), physical activity, alcohol consumption, early life exposures (including those thought to be relevant to immune system development), medications taken in adulthood (like menopausal hormone therapy and aspirin), and exposure to environmental toxins such as tobacco smoke, pesticides, and air pollution.
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Some of Our Findings

CTS data have provided numerous insights into the health of California women. Among other findings, they have revealed that teachers have different risks of cancer than women of similar age and race in California. Teachers were more likely to develop breast, endometrial, ovarian and thyroid cancer, melanoma, lymphoma, and leukemia than comparison women. Teachers, however, were less likely to be diagnosed with cervical or lung cancer. These observations have helped us focus our efforts on trying to understand the causes and prevention of cancer in women. Our targeted research studies have attracted substantial federal funding, and many scientific publications describe CTS results, helping to advance cancer prevention.
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Selected Research Projects utilizing CTS data at CPIC

Diet, Alcohol and Cancer
CPIC investigators have evaluated extensively the relationship between diet and risk of breast cancer. We found that if a woman is using hormone therapy, drinking two or more alcoholic beverages a day increases her risk of breast cancer. One drink per day - which helps reduce the risk of heart disease - did not increase breast cancer risk in teachers. A woman’s current drinking habits were shown to be more important than past drinking. The combination of alcohol consumption (2+ drinks per day) and use of hormone therapy was associated with the greatest breast cancer risk.

Through our ongoing studies, we are also researching how diet, medication use, and alcohol consumption may be related to cancer, stroke, and death. We are looking at how genetic differences may influence alcohol metabolism. Other important CTS investigations in the area of diet include studying the compounds found in particular plant foods (e.g., cruciferous vegetables such as broccoli and cauliflower) and whether they may reduce the risk of developing breast and endometrial cancers.

Cadmium and Breast and Endometrial Cancers
Cadmium is a carcinogenic metal that exhibits estrogen-like activity. The major sources of non-occupational exposure to cadmium include cigarette smoke, diet (leafy vegetables and seafood), and inhalation of contaminated indoor air. CPIC researchers are currently evaluating how dietary and environmental sources of cadmium may contribute to a woman’s total exposure to this metal and her risk of breast and endometrial cancers.

Light-at-Night, Hazardous Air Pollutants, Persistent Organic Pollutants and Breast Cancer Risk
Our research in these areas seeks to understand how these important environmental exposures may contribute to breast cancer development. The exploratory light-at-night study uses satellite imagery data, in combination with CTS questionnaire data and urinary measures of melatonin, to evaluate the hypothesis that internal body clock disruption from exposures to light at night may be related to breast cancer risk. We are gathering and testing blood samples to evaluate the risk of breast cancer associated with body burden levels of persistent organic pollutants (POPs). This study is the first large-scale cancer study conducted on polybrominated diphenyl ethers (PBDEs) and other flame retardants in humans.

Early-life exposures to microbes, farm living, and later cancer
The “hygiene hypothesis” holds that early-life exposure to microbes (bacteria and its byproducts)—by, for example, living on a farm, or with frequent exposure to animal pens or household pets—prevents asthma and allergy, perhaps by priming the immune system. In our most recent CTS questionnaire, we asked CTS participants about some early life exposures to barns, stables, pets, and children, in order to understand if these factors are associated with later risk of cancer. Our interesting preliminary results suggest that living near barns and stables may play a protective role against thyroid cancer. We are currently following up these results, as well as evaluating the role of these factors in relation to cancers of the breast, endometrium, colon, and melanoma.

Second-Hand Smoke and Cancer
The CTS represents one of the largest studies of tobacco exposures and colorectal cancer risk ever conducted. CTS data have been used to assess the relationship between lifetime exposure to second-hand smoke (i.e., "passive smoking") and breast cancer. We found that while current smoking increases the risk of breast cancer, current exposure to household second-hand smoke does not. We are currently evaluating the role of lifetime exposure to second-hand smoke, as well as exposure in the work place and social settings, in relation to breast cancer risk. We are also examining the roles active and passive smoking may play in the development of colorectal cancer.

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CTS Collaborators

The CTS is a collaborative study conducted by researchers all over the state of California, including CPIC, the City of Hope National Medical Center, the University of Southern California, and the University of California, Irvine. Funding has been provided primarily by the National Institutes of Health, but also from the state of California, and the US Department of Defense.

For more information about the California Teachers Study, please visit

© Cancer Prevention Institute of California