368EPIDEMIOLOGY
ORIGINS AND DEFINITIONSEpidemiology (Waterhouse, 1998) is a science that basically
borrows from the other sciences to form its own area of exper-
tise. The actual word epidemiology can be broken down into
three parts: fi rst epi, which means “upon”; then demo, which
is population; and fi nally ology, which refers to studying. So
we can in a simple form say epidemiology is the study of
events that occur upon or on populations or groups. Overall,
epidemiology is not interested in the individual, but rather the
population; however, these data are often used to relate and
infer risks to an individual. The fi eld of epidemiology inter-
acts with other science areas and rarely functions on its own.
For example, in the study of occupational diseases, there
may be an interaction of occupational exposure and health
effects in determining the risk of a specifi c disease (Stern,
2003). Biostatistics, the study of statistical relationships for
biological systems, is an area often in close association with
epidemiologists. It could even be argued that epidemiolo-
gists cannot easily function without using basic biostatistics.
Thus, epidemiologists are routinely trained in the basics of
biostatistics as well. In addition, it is not uncommon for some
epidemiologists to have been originally trained or cotrained
in other disciplines (e.g., environmental health).
The fi eld of epidemiology can be broken down into dif-
ferent subject areas. In the simplest form it can be grouped
as acute (e.g., accidents), chronic (e.g., type II diabetes), and
infectious (e.g., malaria). However, it can also be grouped by
subject name, such as occupational epidemiology, environ-
mental epidemiology, cardiovascular epidemiology, and so
forth. The other way of classifying epidemiology is by dis-
ease name, such as malaria epidemiology, epidemiology of
heavy metals, and so forth. Thus, like most scientifi c fi elds
of study, this area can be categorized in many different ways
depending on one’s prospective. In this chapter, we are con-
cerned with the area of epidemiology that is most closely
associated with environmental science and engineering.
Traditionally environmental and occupational epidemiology
were related to those in environmental science and engineer-
ing, but as the world changes and the concept of global epi-
demiology emerges, most if not all subfi elds or subjects of
epidemiology are becoming interspersed among previously
distinct and separate scientifi c and other fi elds of study (e.g.,
sociology). However, due to the necessity of brevity in this
chapter, the focus will stay on the traditional subject areas of
environmental and occupational epidemiology.One of the biggest problems with environmental epi-
demiology is that studies rarely fi nd a strong association
for cause and effect. This is commonly thought to be a
result of confounders and problems in conducting studies
of this nature. These problems include the lack of a clear
study population, low-level exposures, inaccurate exposure
doses, and related confounding factors. Some of these con-
cerns can be overcome in occupational studies where the
population is better defi ned and exposures have been better
documented, although the same issues can also occur in
this area of epidemiology as well. However, these problems
should not discourage us from conducting or evaluating
epidemiological investigations. Readers should be aware
of general texts on this subject, and a few are mentioned
here as potential references (Lilienfeld and Stolley, 1994;
Timmreck, 1998; Friis and Sellers, 1998), although this list
is not complete.
Epidemiology begins with the application of numbers
to a disease, set of cases, or event (like accidents), primar-
ily in the sense of counting rather than measurement. Some
can even say that counting is at the heart of epidemiology,
because it provides us with how many of the cases or events
exist or occurred (Lange et al., 2003a). Disease, which is
used here to include all events or occurrences that may be
identifi ed in an epidemiological study, are identifi ed as
either incidence or prevalence. These two terms are rates of
occurrence or existence for the disease. The term disease,
in this chapter, will also mean and include any event or case
that is measured, such as cancer, injury, disorder, or a simi-
lar occurrence. Incidence is the number of cases that arose
during a specifi c time period, usually a year; prevalence is
the number of cases that exist at some point in time or within
a time period of interest, again, usually a year. In most cases,
prevalence will be a larger numerical value than incidence.
This is true when people with the disease survive for a long
period of time, which would be a time period longer than the
time period established for the incidence rate. However, if the
disease event is very short or can occur multiple times over
a short period of time, incidence and prevalence can be simi-
lar. If the same disease event can occur more than once in the
same person, it is possible that the incidence can be greater
than prevalence. An example of this would be infl uenza (the
fl u, which is a viral disease) in a small population, say 15
people in an isolated location (e.g., a research station in the
Arctic). If prevalence is counted as anyone having the dis-
ease during the time period and incidence of the occurrenceC005_011_r03.indd 368C005_011_r03.indd 368 11/18/2005 10:25:38 AM11/18/2005 10:25:38 AM