PRINCIPLES OF TOXICOLOGY - Biology East Borneo

520 EPIDEMIOLOGIC ISSUES IN OCCUPATIONAL AND ENVIRONMENTAL HEALTHby each subpopulation. For example, different rate ratios would be reported for different age groupsif age is considered a confounding variable. The most common method of controlling for confoundingis the standardization of the rates of disease. An external or internal reference or standard populationis used to derive standard proportions for the confounding variable; these proportions are applied tothe different subpopulations so that these subpopulations are equivalent with respect to the confoundingvariable. Finally, if statistical modeling such as multivariate analysis is to be done, then confoundingvariables can be controlled by including them in the model.21.10 OTHER ISSUESDisease clusters are apparent increases of disease or death in time and/or space. Epidemics are diseaseclusters. In order to be epidemiologically meaningful, the individuals in the disease cluster must havea historic shared or similar exposure (also known as an aggregation). Many accepted occupationaldiseases were discovered originally as disease clusters. In environmental clusters, sometimes these diseaseincreases are due to the same and/or shared toxic exposure in a given community, but more often the increasesare due to chance. In other words, as a result of randomness, a group of individuals who live in the samegeographic area has the same disease for different reasons. Therefore, drawing conclusions from a singleepidemiologic study that indicates a positive disease–exposure connection is difficult.With increased public and agency awareness, disease clusters are being reported more and morefrequently. The natural assumption of individuals in communities is that a disease cluster is due to somelocal toxic environmental exposure. Not only is this rarely the situation, but it can be very difficult toinvestigate the environmental disease cluster due to issues of the small numbers of cases, poor exposuremeasurement, and the lack of comparability of study populations. These limitations and the possibility thatthe cluster is due purely to chance are difficult concepts to explain to worried community residents.A further problem plaguing epidemiologic research of human health effects in communities withenvironmental exposures is the relatively small populations studied. The small numbers render validstatistical analysis and generalizability of the study conclusions questionable. Not only are the numbersof people small, but again these populations are often studied in crisis (such as with Love Canal, NewYork) with considerable anxiety as well as legal involvement; this atmosphere makes objectivescientific investigation very difficult.Nevertheless, as exposures decrease in occupational settings and as environmental pollution isidentified and its possible human health effects questioned, more epidemiologic studies will beperformed in communities in the future, despite the limitations.21.11 SUMMARYEpidemiology is the study of the distribution and determinants of disease or death in humanpopulations. Since epidemiology is predominantly an observational rather than experimental science,it relies heavily on data collected in records and questionnaires. In the case of the environment or theworkplace, epidemiology attempts to determine associations between a chemical exposure andparticular human health effects. Using measures of risk, comparing unexposed and exposed populationsfor disease risk or comparing diseased and well populations for exposure risk, epidemiologicstudies can associate a disease risk with a particular exposure.REFERENCES AND SUGGESTED READINGBaker, E. L., and T. P. Matte, “Surveillance for Occupational Hazards and Disease,” in Textbook of ClinicalOccupational and Environmental Medicine, L. Rosenstock and M. R. Cullen, eds., Saunders, Philadelphia,1994, pp. 61–67.