While research on treatment for breast cancer often relies on human studies that, for instance, compare a group that receives a drug with another that does not, such studies are rarely possible when researching the connection between toxic exposures and breast cancer. Because so many of these exposures are nearly ubiquitous in the population, there is no way to find an unexposed control group. And scientists certainly can’t purposefully expose anyone to chemicals or radiation suspected of harm.
Instead, we use a diversity of approaches to scientific research to help us better understand the links between environmental exposures and breast cancer risk. Overlaying results from studies using methods ranging from human, animal and cell studies to computer models can provide strong evidence that a given exposure is of serious concern. Combining animal and human studies provides a particularly strong model for understanding environmental risks for breast cancer, because the strengths of each can address some of the limitations of the other (IBCERCC, 2013).
Below is an overview of the various scientific approaches used in researching the link between breast cancer and environmental exposures.
Environmental epidemiological studies explore the link between toxic exposures and illness by examining exposures and health outcomes of groups of people with suspected high exposures or high rates of disease. These studies seek to understand aspects of the historical, social and environmental conditions of disease in these groups. Two kinds of epidemiological studies are cohort studies (which follow a group that shares common exposures or characteristics) and case-control studies (which compare individuals who have developed an illness with those who have not in an effort to determine differences between the groups).
Studies of accidental and occupational exposures are epidemiological studies that focus on people who have been exposed to catastrophically high exposures (such as the atomic bombs on Hiroshima and Nagasaki; the Chernobyl disaster; or dioxin exposure in Seveso, Italy), or on those who endure frequent, repeated toxic exposures in the workplace.
Community-based participatory research (CBPR) is an approach to epidemiological study that fully engages participants in the study design, the reporting of results, and often the application of findings to influence policy change. Some CBPR studies are initiated by communities seeking research assistance, while in other cases researchers approach communities to build partnerships.
Environmental health tracking measures toxic exposures and the occurrence of disease by location and over time, usually in a large population. These studies allow for long-term surveillance of exposures and disease, as well as the cross-referencing of large bodies of data.
Tools and Techniques for Human Studies
Environmental monitoring allows researchers to measure chemicals in outdoor environments (like air and water), and in indoor environments (like household air and dust).
Biomonitoring is the process of measuring chemicals in people’s bodies. Developing ways to measure specific chemicals accurately can be a complex process, but scientists have developed methods to accurately and rapidly measure more than 300 chemicals in human blood and urine, which has deepened our understanding of how much environmental toxicants get into the body and how long they remain there.
Biomarker monitoring measures internal effects of exposures from the outside environment. This strategy looks at abnormal changes in cells, genes or biological processes that often result from toxic exposures but that precede the development of a disease.
Geographic information systems (GIS) provide an opportunity to explore the spatial relationships among environmental, demographic (race, class, age), historical and health-related data, even those reported at the individual level. Data from all of these sources can be overlaid in maps, which help tell a visual story and can prompt further study.
Experimental studies in animals (in vivo) allow scientists conduct experiments to compare exposed and non-exposed animals. Some studies also compare different doses of exposures to better understand effects of chemicals or radiation at various exposure levels. These studies allow for control and careful monitoring of exposure levels, but since species vary in their susceptibility to exposures, it often requires multiple, repeated studies to determine if an effect applies to humans. There is also debate about the ethics of animal experimentation.
Mammary gland changes are not assessed in standard animal studies of toxic exposures, which has created a significant data gap with regard to environmental exposures and breast cancer. Mammary and breast tissue may be uniquely susceptible to some exposures, which is overlooked if the tissue is not routinely examined in ways that allow scientists to see subtle, early developmental changes.
Experimental studies in cells (in vitro) look at the effects of toxic exposures on colonies of identical cells that have been grown from either tumors or normal tissue. These cell lines allow scientists to rapidly screen chemicals for specific qualities, such as the capacity to behave like estrogen. Cell studies cannot capture the context of normal cells that live in tissues with feedback loops, but they do serve as early indicators of concern.
Genomic studies allow scientists to look at specific genetic variations and mutations across a vast number of genetic samples, all at once. Through these studies, scientists can determine whether a particular toxic exposure repeatedly results in the same kinds of genetic mutations, if similar exposures reliably turn some genes off and others on, or if different types of tumors (as they are understood in clinical medicine) also have different genetic profiles.
In Genome-wide Association Studies (GWAS), scientists study the full genome (the entire sequence of human DNA) to find genetic variations that are associated with diseases, including breast cancer. This approach can often find associations between genes that have subtle effects on the risk for a particular disease.