Incidence rates tend to be highest in urban areas
FREMONT, CA (March 17, 2015) - Scientists know that geographic location is one of the strongest predictors of breast cancer incidence and that rates tend to be highest in urban areas. Interestingly, less than half of these cases can be attributed to traditional risk factors, such as diet and lifestyle differences associated with urban living. Could the environment be to blame?
CPIC Senior Scientist Peggy Reynolds and her colleagues are particularly interested in the role urban environmental exposures, like air pollution, might play in the development of breast cancer. Animal studies have shown a link between breast tumors and certain chemicals, known as mammary gland carcinogens, which are present in the air at varying levels. The team at CPIC took a close look at these levels, examining the differences in breast cancer incidence for women living in neighborhoods with higher concentrations of pollutants thought to be associated with breast cancer, and women living in neighborhoods with lower levels of these chemicals.
The study, “Hazardous air pollutants and breast cancer risk in California teachers: a cohort study,” was published January 30, 2015 in the journal Environmental Health and included more than 112,000 women living throughout California.
Since 1996, the Environmental Protection Agency (EPA) has provided estimates of the levels of up to 180 hazardous air pollutants in neighborhoods throughout the United States. From this list, the researchers selected 24 chemicals that had been identified in prior animal studies as mammary gland carcinogens. The research team examined the relationship between breast cancer incidence and levels of these 24 chemicals present in the air in neighborhoods where study participants lived.
While a direct connection between the 24 hazardous air pollutants and breast cancer in general was not found, the team did find significant associations between certain of these chemicals and specific breast cancer subtypes. For example, levels of carbon tetrachloride are higher in urban areas and appeared to be associated with ER+/PR+* breast cancers. Vinyl chloride and its byproduct ethylidene chloride, present in consumer products as well as in the air, also showed an association with ER+/PR+ breast cancer, particularly among post-menopausal women who were current or former users of hormone replacement therapy. And, finally, findings suggest a link between neighborhood levels of the chemical benzene and the rarer ER-/PR- breast cancer subtype. Vehicle emissions are the primary source of benzene exposure; thus, this contaminant is closely associated with urban environments.
This CPIC study is the first to examine the relationship between exposure to these kinds of harmful air pollutants at the neighborhood level and risk of breast cancer incidence, evaluating the chemicals as individual compounds. Other studies have looked only at summary levels. CPIC’s findings suggest that women living in neighborhoods where higher levels of dangerous chemicals are present in the ambient air may be at greater risk of developing both common and rare types of breast cancer.
The strengths of this study include the large and well-followed group of women in the California Teachers Study (CTS), who have provided extensive personal health and behavior information since 1995. The careful assessments of air pollutants by the EPA over time allowed the team to conduct detailed analyses, as well. Further study is warranted to evaluate the chemicals noted to be of greatest concern, the researchers suggest.
*ER+/PR+ = estrogen-receptor positive / progesterone-receptor positive breast cancers accounted for 77% of all breast cancers in this study population.
About the Cancer Prevention Institute of California
The Cancer Prevention Institute of California (CPIC) is the nation’s premier organization dedicated to preventing cancer and to reducing its burden where it cannot yet be prevented. CPIC tracks patterns of cancer throughout the entire population and identifies those at risk for developing cancer. Its research scientists are leaders in investigating the causes of cancer in large populations to advance the development of prevention-focused interventions. CPIC’s innovative cancer prevention research and education programs, together with the work of the Stanford Cancer Institute, deliver a comprehensive arsenal for defeating cancer. For more information, visit www.cpic.org.
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