PRINCIPLES OF TOXICOLOGY

502 OCCUPATIONAL AND ENVIRONMENTAL HEALTH
United States whereas in 1998 they accounted for 65 percent of all reported occupational illness. The
percent distribution of reported occupational illnesses by category of illness for private industry in the
United States is presented for years 1982–1998 in Table 3.
20.2 CHARACTERISTICS OF OCCUPATIONAL ILLNESS
Health care providers often overlook the occupational cause of human illness. This is due to several
special characteristics of occupational disease that may obscure its occupational origin.
1. The clinical and pathological presentation of occupational disease is often identical to that of
nonoccupational disease. For example, asthma (excessive airways narrowing in the lungs) due to airborne
exposure to toluene diisocyanate is clinically indistinguishable from asthma due to other causes.
2. Occupational disease may occur after the termination of exposure. An extreme example would
be asbestos-related mesothelioma (a cancer affecting the lining of the lung and abdomen) that can
occur 30–40 years after the exposure. Even relatively acute illness can occur after the exposure episode.
Some forms of occupational asthma manifest at night, several hours after the end of the exposure.
3. The clinical manifestations of occupational disease can vary with the dose and timing of
exposure. For example, at very high airborne concentrations, elemental mercury is acutely toxic to the
lungs and can cause pulmonary failure. At lower levels of exposure, elemental mercury has no
pathologic effect on the lungs but can have chronic adverse effects on the central and peripheral nervous
systems.
4. Occupational factors can act in combination with nonoccupational factors to produce disease.
A classic example is the interaction between exposure to asbestos and exposure to tobacco smoke.
Long-term exposure to asbestos alone increases the risk of lung cancer about fivefold. Long-term
smoking of cigarettes increases the risk of lung cancer about 10–20-fold. When exposed to both,
however, the risk of lung cancer is increased about 50–70-fold.
20.3 GOALS OF OCCUPATIONAL AND ENVIRONMENTAL MEDICINE
Occupational and environmental medicine is both a preventive and a clinical specialty. Prevention
activities are often divided into three categories, primary, secondary, and tertiary. Primary prevention
is accomplished by reducing the risk of disease. In the occupational setting, this is most commonly
done by reducing or eliminating exposure to hazardous substances. As exposure is reduced, so is the
risk of adverse health consequences. Such reductions are typically managed by industrial hygiene
personnel and are best accomplished by changes in production process or associated infrastructure.
Such changes might include substitution of a safer substance for a more hazardous one, enclosure or
special ventilation of equipment, as well as rotation of workers through areas in which hazards are
present to reduce the dose to each worker. (Note that this method does increase the number of workers
exposed to the hazard.)
Secondary prevention is accomplished by identifying health problems before they become clinically
apparent (i.e., before workers report feeling ill) and making interventions to limit the resulting
disease. This is a major goal of occupational health surveillance, which is discussed in greater detail
below. The underlying assumption is that such early identification will result in a more favorable
outcome. An example of secondary prevention in occupational health is the measurement of blood
lead levels in workers exposed to lead. An elevated blood lead level indicates a failure of primary
prevention but can allow for corrective action before clinically apparent lead poisoning occurs.
Corrective action would be to improve the primary prevention activities listed above.
Tertiary prevention is accomplished by minimizing the adverse clinical effects on health of an illness
or exposure. Treatment of lead poisoning (headache, muscle and joint pain, abdominal pain, anemia,
kidney dysfunction) by administration of chelating medication is an example of tertiary prevention.