Light Shed on Relationship Between Environmental Factors and Breast Cancer Incidence

Breast cancer is the most common and second most fatal invasive cancer in the U.S. female population. Modern scientists have long studied the relationships between breast cancer risk and genetic and environmental factors, forming a general consensus that both types of factors play a role. 

A recent study conducted by Duke University researchers, entitled Cumulative environmental quality is associated with breast cancer incidence differentially by summary stage and urbanicity, examined the relationship between cumulative environmental factors and the incidence of different stages of breast cancer. Researchers found a direct correlation between poor environmental quality and increased rates across various stages of the disease.

Background: The Challenges 

Many past studies have demonstrated links between breast cancer risk and specific environmental factors – such as income, location, inhalation of tobacco smoke, pesticides, bisphenol-A (BPA) food contamination, and more. 

The issue is, most of these studies have only focused on one of the aforementioned elements, rendering them ineffective in clarifying the combined impact of multiple environmental factors (plus social, biological, and genetic factors) on breast cancer risk. This seemingly small shortcoming may cause scientists to underestimate the true implications.

The EQI and Breast Cancer

In 2014 the US Environmental Protection Agency (EPA) created the EQI, Environmental Quality Index: a standardized method to measure county-level cumulative environmental exposure. The EQI’s purpose is to help researchers better understand the relationship between health/disease and cumulative environmental factors. It has been used to study environmental impacts on birth defects, asthma, infant mortality, end-stage renal disease, obesity, and more. 

The practicality of the EQI is that it includes:

  • Quantities of accumulated data
  • The stratification of factors into different environmental domains
  • The stratification by county urbanicity

A study conducted in 2017 by Jagai, J.S. et al showed a significant association between breast cancer incidence and poor environmental quality (measured by the EQI). However, the study came up short, only using the total amounts of breast cancer incidence without taking into consideration the different stages of the disease. Therefore, a correlation between environmental factors and specific stages was unable to be determined. 

This is problematic as breast cancer is a diversified disease with risk factors that differ by stage and hormone receptor subtype at the time of diagnosis. There are five stages of breast cancer, ranging from 0 to IV. Each stage is determined by diagnostic tests performed on the tumor, nodes, and metastasis. 

Duke Researchers Study Breast Cancer Incidence Based on EQI

In 2020, Duke researchers studied whether environmental factors, according to the EQI, are associated with an increased incidence of different breast cancer stages. Their goal was to determine whether environmental quality and urbanicity are related to the initiation of cancer formation and the development of more advanced tumors.

North Carolina was an excellent choice for the study location as it has a diverse population of more than 10 million people, spread across rural and urban areas. 

The study found that: 

  • Urban counties in North Carolina had a higher overall incidence of breast cancer than rural counties in the state.
  • Higher mammography screening rates were associated with lower regional breast cancer incidence rates.
  • Higher incidence rates for later-stage disease and total breast cancer in counties with higher populations of black women.
  • Total breast cancer incidence was higher by 10.82 cases per 100,000 persons in counties with poor environmental quality compared to those with good environmental quality. Specifically, there were higher incidences of stage 0 breast cancer, localized breast cancer, and total breast cancer in counties with poor land quality.

What Do the Findings Mean?

The study’s findings signify that in counties with poor overall environmental quality, the incidence of breast cancer is higher than in counties with higher levels of environmental quality. 

The study also highlights the need for future research to assess cumulative environmental exposures with regard to the different stages of breast cancer. Further research of this type will enable scientists to develop measures to reduce disease incidence in communities with poor overall environmental quality.