Linking health problems to an exposure to environmental contaminants in the environment might seem straightforward at first. Often, community members suggest that a health survey be conducted in their community. A health survey is a count of the number of people with illnesses (or health outcomes) in their community. A health survey provides information about the health status of the community. However, in order to suggest a link between exposure and illness, one would need to carry out a scientific health study. A scientific health study requires careful methods of measuring exposure and illness. To understand these studies, it is necessary to know the essentials about the fields of epidemiology and environmental epidemiology.

DESCRIPTIVE STUDIES Descriptive studies are generally used at the beginning of an investigation. They are used to gather evidence and describe what is happening. In a descriptive study, an epidemiologist collects information to look for unusual trends or patterns of disease in a group of people.

ANALYTICAL STUDIES Analytical studies are used in later stages of an investigation to see what connections exist between exposures and health outcomes. In an analytical study, an epidemiologist compares different groups of people to help understand how different risk factors may play a role in causing a health outcome.

In epidemiology, the target population is the group of people that researchers want to study. The characteristics of the target population depend on the type of study that is being conducted.

In order to do so, environmental epidemiologists must carefully measure disease in the target population and the amount of exposure to that population.

Environmental epidemiologists ask the questions:

• Do people exposed to a contaminant have greater numbers of disease than people who were not exposed to a contaminant?
• Did people with disease experience a higher level of exposure than people without disease?

This website focuses on studies that try to answer the first question. In order to decide whether or not a health study can answer these questions, scientists consider various factors before conducting this type of study. It is also important for environmental epidemiologists to consider environmental justice principles when working with a community that has been affected by contamination.

An epidemiological health study is only one of many tools that communities may choose to explore when seeking to understand and advocate for their community's health. Other options include community organizing and policy advocacy. The intentions of a community will determine what tools and strategies can be used to meet their goals.

Biomarkers can connect genetic factors and environmental exposure events to different points along the exposure-disease continuum. For example, biomarkers might be measured at the time of exposure, at the time that a chemical interacts with a cell membrane, and at the time when the first changes begin to take place in the cell or organ.

Biomarkers can be used in health studies to determine whether someone was exposed to a contaminant.

Classic examples of biomarkers include:

• Measuring blood lead levels as an indicator of exposure to lead
• DNA adducts in urine as an indicator of exposure to mold in peanut butter (aflatoxin)
• Urinary metabolites as an indicator of exposure to benzene

Epidemiologists can use GIS to assign these items to geographic locations on a map:

• Addresses of a study population
• Source of contamination and its distribution into the environment
• Health outcome data for different geographical areas

Using GIS, scientists can construct different maps to describe how environmental contamination, human exposure, and health outcomes vary over a geographical region to:

• Identify potential exposure pathways by utilizing location of the study population and source of contamination
• Estimate how levels of contaminants in the natural environment and human exposure to contaminants vary in relationship to geographic location. For example, GIS has been used in environmental epidemiology to assess exposure to contaminants in drinking water and to reconstruct historic residential exposure to large-scale pesticide application.
• Analyze how rates of health outcomes (such as asthma hospitalization) vary in relationship to geographic location (such as census tract)

Thus, environmental epidemiologists can analyze the relationship between contamination and health outcomes in a region. If the data is available, these relationships can be analyzed over a period of time.

GIS is a complex and rapidly evolving field. Currently, there are many challenges to using GIS widely in environmental epidemiology. For example, careful methods must be used to code information accurately; otherwise, the resulting analysis can be far from precise. In addition, statistical methods for GIS to conduct spatial analysis are still being developed.