Environmental Factor, September 2009, National Institute of Environmental Health Sciences
Summer Students Explore Basics of Epidemiology
By Laura Hall
In the last interactive Summers of Discovery seminar of the year on August 5, NIEHS Epidemiology Branch Chief Dale Sandler, Ph.D., and Senior Investigator Allen Wilcox, M.D., Ph.D., worked as a team to give the students "a taste of epidemiology." With their team approach and small group discussion format, the duo kept the pace varied and the students engaged.
Wilcox opened the seminar by telling the audience that they would explore "how in a broad sense epidemiology can be applied to environmental questions" and learn about the "nuts and bolts aspects" of how epidemiologists make observations from their studies. Pointing out that epidemiology is an observational science, Wilcox told the students that epidemiological observations can be hard to interpret because scientists cannot control the conditions as they do in experimental science. Epidemiology studies can establish associations, but researchers must always keep in mind that there could be alternate explanations for the results.
Sandler explained that epidemiologists study populations not individual patients or cells. Epidemiologic studies are not necessarily about mechanisms, although some studies do lead to insights about mechanisms or test, in the real world, theories that derive from mechanistic studies. Sometimes, epidemiology studies can show that environmental health problems exist before any mechanism is known, she noted. As part of the discussion, the students mentioned some of the environmental agents causing health problems that they recognized, such as asbestos, endocrine disruptors, tobacco smoke, drinking water pollutants and pesticides.
Sandler continued with an historical perspective on pesticide use and how awareness of deleterious health effects gradually developed. She talked about the Agricultural Health Study (http://www.niehs.nih.gov/research/atniehs/labs/epi/studies/ahs/index.cfm) (AHS) she is presently involved with here at NIEHS. The AHS is a study of licensed pesticide applicators and their spouses and is the largest study of farmers in the world.
In explaining why epidemiologists wanted to study the effects of pesticide use on farmers, Sandler told the students, "You and I have no clue what amount and what kind of pesticide we're exposed to." She stated that it is much easier to study health effects of pesticides or other chemicals in workers rather than in the general population.
For farmers, it is possible to collect extensive data on pesticide exposures and work practices. Since farmers apply pesticides only some days of the year, their overall exposures are more similar to general population exposures than if they were handling chemicals every day. Their family members have potential exposures even when they are not engaged in farming.
Using the AHS as an example, Sandler discussed the types of epidemiology studies that can be performed, such as mortality, case-control and cohort studies. She observed that epidemiology studies can take a long time, particularly when the outcome of interest is a rare disease. Political and economic forces can contribute to resistance to acknowledging a health hazard indicated by epidemiology studies.
After this overview, the audience divided into two groups led by Sandler and Wilcox to discuss a recent news report that the Navajo population living near formerly active uranium mines are concerned about possible health effects caused by radiation exposure. Lung cancer and other lung diseases are a well-established consequence of uranium mining but health effects among persons who do not work in the mines have not been established. The goal was to determine what factors should be considered in designing an epidemiology study on this case.
The students came up with ideas about what kind of information could be gathered, how to identify exposed people and route of exposure, and how to find comparison controls that are unexposed or have lower levels of exposure. Group members talked about ethical issues and the need to consider cultural issues when recruiting potential study participants. They also pondered how to document health outcomes.
In all, the students came away with a fresh and more detailed understanding of the value of epidemiology and the factors that must be considered when designing and interpreting the studies.
(Laura Hall is a biologist in the NIEHS Laboratory of Pharmacology currently on detail as a writer for the Environmental Factor.)