PRINCIPLES OF TOXICOLOGY

516 EPIDEMIOLOGIC ISSUES IN OCCUPATIONAL AND ENVIRONMENTAL HEALTH
years prior to the diagnosis of the cancer. This span of time from initial exposure to disease onset is
called latency. In general, the longer the latency, the more difficult it is to establish a disease–exposure
connection. In addition, chronic diseases such are heart disease may be due to different exposures in
different individuals, yet appear to be the same disease. This can be particularly difficult to evaluate
unless large studies of large populations are used. For example, lung cancer can be associated with
exposure to smoking alone, asbestos alone, and to a combination of the two; for many years, lung
cancer due to asbestos exposure without smoking was not accepted until sufficiently large studies were
performed.
21.6 POPULATION ISSUES
As was discussed above, many disease–exposure connections in humans were established by the
evaluation of workers and their occupational diseases. However, working populations are usually
young and healthy while communities are composed of young and old people, healthy and sick.
Another important issue is that the health effects of chemicals on fetuses and growing children can
be devastating at levels which are relatively tolerated by adults (as with the example of the neurologic
effects of lead and mercury). Other sensitive populations identified include persons with immunosuppression,
asthma, and even multiple-chemical sensitivity.
All the populations described above raise the issue of the generalizability of the results of an
epidemogic study in a particular population to another population. Can epidemiologic studies that
suggest a disease–exposure connection, or more to the point, a “safe” level of exposure in one human
population, be extrapolated to another population?
21.7 MEASUREMENT OF DISEASE OR EXPOSURE FREQUENCY
As noted above, a basic function of epidemiology is to measure the rates or risk of disease in exposed
populations (or risk of exposure in diseased populations) for comparison purposes. Rates consist of a
numerator and a denominator in which the numerator is the number of people with the disease or
exposure and the denominator is the total number of people at risk for this disease or exposure over a
set period of time. Without evaluating the number of deaths or cases of a particular disease by the total
number of people at risk in that particular population, it would be impossible to compare the risk of
death or disease in that population with the risk of another population. Comparing only the number of
deaths or cases between two populations can be misleading, if one population is much larger or
substantially different than the other.
To increase the comparability of rates between populations, these measures of risk have been further
refined by adjusting them for various population characteristics. For example, typical rates will be
adjusted for age, sex, socioeconomic class, and/or race and/or ethnic group. The reason for this is that
various subpopulations can experience differing risks. For example, the risk of breast cancer is at least
10 times higher in women than in men, or the risk of cancer is higher in older than younger populations.
The basic measure of disease or exposure frequency in populations is the prevalence of a particular
disease in the population. The prevalence is defined as the number of cases of a particular disease in
a specific population at a single point in time. Prevalence is expressed as percent because it is a
proportion not a rate. For example, if five asbestos workers have lung cancer in a worker population
of 1000 people, the prevalence of lung cancer in this worker population is 5:1000 or 0.5 percent.
Prevalence includes all persons who have the particular disease at a given point in time, regardless of
when they developed the disease. Therefore, the prevalence of prolonged chronic diseases such as
arthritis are usually much higher than the prevalence of acutely fatal diseases such as meningitis.
Prevalence is the measure of disease and exposure frequency for cross-sectional studies.
Most measurements of disease or exposure frequency in epidemiologic studies are expressed as
rates: numbers of persons who developed a disease in a given population at risk during a period of