Epidemiology faces its limits.

T h e news about health risks comes thick and fast these days, and it seems almost constitutionally contradictory. In January of last year, for instance, a Swedish study found a significant association between residential radon exposure and lung cancer. A Canadian study did not. Three months later, it was pesticide residues. The Journal of the National Cancer Institute published a study in April reportingcontrary to previous, less powerful studies-that the presence of DDT metabolites in the bloodstream seemed to have no effect on the risk of breast cancer. In October. it was on the press for its reporting of epidemiology, and even on the public “for its unrealistic expectations” of what modern medical research can do for their health. But many epidemiologists interviewed by Science say the problem also lies with the very nature of epidemiologic studies-in particular those that try to isolate causes of noninfectious disease, known variously as “observational” or “risk-factor” or “environmental” epidemiology. The predicament of these studies is a simple one: Rothman, editor of the journal Epidemiology: “We’re pushing the edge of what can be done with epidemiology. ” With epidemiology stretched to its limits or beyond, says Dimitrios Trichopoulos, head of the epidemiology department at the Harvard School of Public Health, studies will inevitably generate false positive and false negative results “with disturbing frequency. ” Most epidemiologists are aware of the problem, he adds, “and tend to avoid causal inferences on the basis of isolated studies or even groups of studies in the absence of compelling biomedical evidence. However, exceptions do occur, and their frequency appears to be increasing. ” As Trichopoulos explains, “Objectively the problems are not more than they used to be, but the pressure is greater on the profession, Anxiety epidemic. Protesting risks that may-or may not-be real. Over the past 50 years, epiand thenumber who practice it is greater. ” demiologists have succeeded As a result, journals today are full of studin identifying the more conies suggesting that a little risk is not nothing soicuous determinants of at all. The findines are often touted in mess abortions and breast cancer. Maybe yes. Maybe no. In January of this year it was electromagnetic fields (EMF) from power lines. This time a study of electric utility workers in the United States suggested a possible link between EMF and brain cancer but-contrary to a study a year ago in Canada and France-no link between EMF and leukemia. These are not isolated examples of the conflicting nature of epidemiologic studies; they’re just the latest to hit the newspapers. Over the years, such studies have come up with a mind-numbing array of potential disease-causing agents, from hair dyes (lymphomas, myelomas, and leukemia) to coffee (pancreatic cancer and heart disease) to oral contraceptives and other hormone treatments (virtually every disorder known to woman). The pendulum swings back and forth, subjecting the public to an “epidemic of anxiety,” as Lewis Thomas put it over a decade ago. Indeed, last July, the New England Journal of Medicine (NEJM) published an editorial by editors Marcia Angell and Jerome Kassirer asking the pithy question, “What Should the Public Believe? ” Health-conscious Americans, wrote Angell and Kassirer, “increasingly find themselves beset by contradictory advice. No sooner do they learn the results of one research study than they hear of one with the opposite message. ” Kassirer and Angell place responsibility