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Environment
and Disease
Epidemiology
 Epidemiology, or the study of the occurrence
of disease in people, is the most common method of determining
a cause of cancer. This technique is easily accepted and certainly
the most easily understood. Associating exposure to a chemical
with a large number of people either dead or ill with an identified
disease often can be very simple and straightforward. In fact,
most health regulations today are based on epidemiological studies.
 Epidemiological studies are very old and have
a glorious and grand tradition. The report, in 1775, by Sir
Percival Pott of England, that chimney sweeps had a very high
incidence of scrotal cancer may have been the first epidemiological
study.
However, there are
many problems with epidemiology. Usually people do not know
the chemicals they have been exposed to and the amount of their
exposure. To produce a good epidemiological study it is critical
to know chemical exposure and quantity, not only for the workers,
but also for consumers of a product. In addition, people find
it very difficult to remember the important exposures of 5,
10, or 15 years ago as well as the exposures of today or yesterday.
 Another major problem is that this method
cannot detect small differences in the rates of common diseases.
Although a very large number of people may be affected by a
common disease, relatively small differences cannot be picked
up by epidemiology. For example, it would be very difficult
to determine epidemiologically if a chemical increased the rate
of a common disease.
Perhaps the most
significant problem is that epidemiology is entirely an after-the-fact
science. Diseases can be related to causes only after people
have experienced exposures of sufficient intensity and duration
to produce illness and death.
 This delay is less important when the effect,
such as an adverse drug reaction, happens very rapidly. However,
chronic diseases, particularly cancer, have a latent, or silent,
period of decades. Most cancers do not develop until 20 or 30
years after exposure has occurred. Therefore, it may be necessary
for an entire generation of people to be exposed to a chemical
before epidemiological studies can prove that a particular compound
does cause cancer or some other chronic disease.
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National
Institute of Environmental Health Sciences
National
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