Environmental Epidemiological Studies
Definition: The study of the relationships between environmental exposures and incidence of disease in identified populations or communities of people.
Epidemiological studies use real-life patterns of disease and exposures to examine the associations between ambient, accidental or occupational exposures and later health outcomes. (Moeller, 2005; Friis, 2012). Two major types of environmental epidemiological studies include cohort and case-control studies.
Cohort studies identify groups of people who have been subject to particular exposure(s) and then follow the participants to examine later development of a health outcome or outcomes that are thought to be linked to the disease. Cohort studies are longitudinal in nature; that is, researchers follow the study participants over time to understand the possible association between an exposure and later consequences. Cohort studies can explore many different health outcomes including incidence of and mortality from various diseases. On the other hand, they are limited by the amount of environmental exposure information that is collected because studies are designed around known or suspected exposures to specific toxicants. Cohort studies can be prospective, which means researchers follow participants before the onset of disease, or retrospective, which relies on participants’ memories of exposures or on biomonitoring data at the time of diagnosis. Since many chemical exposures are short-lived, and because exposures at critical time periods can have profound effects on later-life health, prospective studies are particularly important for understanding environmental links to breast cancer.
Case-control studies involve participants who either have (the cases) or have not (the controls) developed a particular health outcome, e.g., breast cancer. Efforts are made to match the two groups for as many other factors as possible, including age, gender, reproductive history, education, body weight, etc. Researchers then work to determine the history of exposure to the environmental factor(s) of interest through interviews, questionnaires and testing of biological samples (commonly urine or blood). This approach allows epidemiologists to study associations of a particular disease outcome with multiple exposures of interest, although the study format is necessarily limited to inquiries about a particular disease or health outcome (Moeller, 2005; Friis, 2012). Case-control studies may also be limited by the kinds of exposure data available at the time of diagnosis, which may be years or decades after exposure. Epidemiologists often employ creative research strategies to address these limitations, as did researchers from the Child Health and Development Studies to assess the effects of early-life exposures to the pesticide DDT.
The Child Health and Development Studies are a collection of long-term studies of 15,000 Oakland-based families who participated in comprehensive interviews about their health, lifestyle and experiences that might influence the progress of pregnancy and the health of their infants.
The Nurses Health Studies, run out of the Harvard Medical School and the Harvard School of Public Health and among the most renowned cohort studies, have been collecting data from more than 300,000 nurses for many years.
The Sister Study, conducted by the National Institute of Environmental Health Sciences, followed more than 50,000 women whose sisters had breast cancer. Studying sisters, who share environment, genes and experiences, provides a greater chance of identifying risk factors that may help us find ways to prevent breast cancer.
Environmental epidemiological studies can provide powerful tests of specific hypotheses, particularly since these studies reflect what occurs in real life. Using a technique called meta-analysis, scientists also can statistically combine findings from several studies to afford more precise estimates of potential cause-effect relationships. The results are often critical starting points for developing hypotheses that lead to testing specific relationships between exposures and disease using experimental models designed to demonstrate causal relationships and underlying mechanisms (Moeller, 2005; Friis, 2012).
The advantages of using an environmental epidemiological approach to exploring relationships between exposures and disease include the ability to ask questions and obtain results that are framed in real-world situations and in contexts that are therefore meaningful to the lives of the study participants. As study participants are increasingly included in many aspects of research design and data analysis, the process and the outcome of these studies can lead to more demand for such mechanistic studies and to community-based action to alter exposures (Brown, 2012; Friis, 2012).
There are so many factors in our real-world environments that may affect health outcomes, yet that cannot be controlled or even identified. Direct associations between particular exposures and health outcomes are difficult to ascertain. And while the particularities of the exposure history of a given community may give a powerful indicator of possible associations between particular environmental toxicants and health outcomes, similar associations in other communities or with different participant profiles cannot be assumed.
Perhaps the most serious limitations of environmental epidemiological studies are the difficulties in determining the most appropriate means and time for assessing exposures to environmental toxicants. These determinations are particularly critical in thinking about relationships between environmental exposures and the development of a disease like breast cancer that has a long latency – often decades – between exposure and disease diagnosis. A substantial body of research demonstrates that prenatal and early-life exposures to environmental chemicals and radiation may have significant effects on adult development of breast cancer and other diseases. However, consistently measuring exposures during these key developmental time periods is quite difficult, limiting the complete picture.
In understanding the links between environmental factors and breast cancer, it is therefore important to be able to examine exposure levels at many times across the lifespan. The challenges of exploring these relationships in our mobile, industrial society require the use of a variety of complementary approaches to examine environmental exposures and development of breast cancer. These approaches span the gamut and include measurement of present and past exposures in our environment; measurement of chemical contaminants in our bodies (biomonitoring) at different times over the lifespan; development of community-based studies finding intersections between these approaches; and new information from genomic studies that examine predisposing vulnerabilities to disease progression in different groups and individuals.