PRINCIPLES OF TOXICOLOGY

520 EPIDEMIOLOGIC ISSUES IN OCCUPATIONAL AND ENVIRONMENTAL HEALTH
by each subpopulation. For example, different rate ratios would be reported for different age groups
if age is considered a confounding variable. The most common method of controlling for confounding
is the standardization of the rates of disease. An external or internal reference or standard population
is used to derive standard proportions for the confounding variable; these proportions are applied to
the different subpopulations so that these subpopulations are equivalent with respect to the confounding
variable. Finally, if statistical modeling such as multivariate analysis is to be done, then confounding
variables can be controlled by including them in the model.
21.10 OTHER ISSUES
Disease clusters are apparent increases of disease or death in time and/or space. Epidemics are disease
clusters. In order to be epidemiologically meaningful, the individuals in the disease cluster must have
a historic shared or similar exposure (also known as an aggregation). Many accepted occupational
diseases were discovered originally as disease clusters. In environmental clusters, sometimes these disease
increases are due to the same and/or shared toxic exposure in a given community, but more often the increases
are due to chance. In other words, as a result of randomness, a group of individuals who live in the same
geographic area has the same disease for different reasons. Therefore, drawing conclusions from a single
epidemiologic study that indicates a positive disease–exposure connection is difficult.
With increased public and agency awareness, disease clusters are being reported more and more
frequently. The natural assumption of individuals in communities is that a disease cluster is due to some
local toxic environmental exposure. Not only is this rarely the situation, but it can be very difficult to
investigate the environmental disease cluster due to issues of the small numbers of cases, poor exposure
measurement, and the lack of comparability of study populations. These limitations and the possibility that
the 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 with
environmental exposures is the relatively small populations studied. The small numbers render valid
statistical analysis and generalizability of the study conclusions questionable. Not only are the numbers
of people small, but again these populations are often studied in crisis (such as with Love Canal, New
York) with considerable anxiety as well as legal involvement; this atmosphere makes objective
scientific investigation very difficult.
Nevertheless, as exposures decrease in occupational settings and as environmental pollution is
identified and its possible human health effects questioned, more epidemiologic studies will be
performed in communities in the future, despite the limitations.
21.11 SUMMARY
Epidemiology is the study of the distribution and determinants of disease or death in human
populations. 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 the
workplace, epidemiology attempts to determine associations between a chemical exposure and
particular human health effects. Using measures of risk, comparing unexposed and exposed populations
for disease risk or comparing diseased and well populations for exposure risk, epidemiologic
studies can associate a disease risk with a particular exposure.
REFERENCES AND SUGGESTED READING
Baker, E. L., and T. P. Matte, “Surveillance for Occupational Hazards and Disease,” in Textbook of Clinical
Occupational and Environmental Medicine, L. Rosenstock and M. R. Cullen, eds., Saunders, Philadelphia,
1994, pp. 61–67.