preceding sections, we outlined the complex intertwined ways in which
environmental conditions and agents and animal health co-evolve. In this
section we describe the frameworks used to characterize the effects of
various environmental agents on health. “Health effects” is
the general term that describes the effect of an agent (microbe, chemical,
environmental conditions) on health. These effects are described and characterized
in terms of disease rates (morbidity) or death rates (mortality).
effects are discussed in terms of population health. Thus we may talk
of the occurrence of so many diseases per 100,000 people. The science
of studying public health was first developed to study epidemics of infectious
diseases such as typhoid, cholera and diseases among certain occupational
groups, such as cancer among chimney sweeps in London. This field of study
is called epidemiology.
are generally studied systematically using a framework shown in Figure
2. It is a cause and effect model where one looks at the effects as arising
from exposure to an agent, being influenced by various life factors (genetic
and lifestyle), and manifesting in disease. As we are talking of public
health, the factors in the figure and the total model are treated statistically.
interactions affect each stage of
the exposure-effect process. The “agent in a medium”
refers to the primary agent (chemical, virus, infections) that lives in
or is carried by a medium. Examples are the malaria sporidia in the mosquito
vector (or in the water); or, the carbon monoxide or sulfur dioxide in
air. Various conditions – swampy water close by, weather and wind
patterns – of the natural environment affect the exposure. Environment
here includes the socio-economic and overpopulated conditions. The activity
profile includes the activities that bring people within reach of the
agent and those that might enhance the action of the agents. These could
be positive activities such as exercise or negative ones such as smoking
and other health-related aspects such as diet and previous exposure. They
also include occupational activities that may be associated with exposure
directly or indirectly.
is measured or characterized in ways that include the amount of agent
(e.g., the amount of CO per liter of air) and the pattern that causes
the agent to enter or affect the body – breathing or intake rates,
as well as genetic or other predisposing factors.
exposures” refers to those from lifestyle (alcohol, tobacco, hormones
and pesticides in food, vitamins in food) and from the environment that
may interact with the primary exposure and aggravate or ameliorate the
then causes cellular or tissue level effects, which may or may not be
cumulative. If the body can metabolize and excrete the agent, the effects
may not occur or may be different than if the agent accumulates, especially
in organs such as lungs and liver, which are “cleansing” organs.
The cellular level effects then may build over time either with continuous
exposure or with intermittent exposure.
foreign chemical enters the body, it gets absorbed and processed in different
ways. Figure 3, adapted from Human Physiology: The Mechanisms of Body
Function by Vander, Sherman, and Luciano (McGraw Hill, 2004), shows a
scheme of the metabolic pathways of a foreign chemical.
3: Metabolic pathway of a chemical.
one agent you might be exposed to because of your habits,
residence, or work. Adapt Figure 3 for that agent.
shown in Figure 3 cumulatively contribute to disease. The action of the
agent may be site-specific. For example, lead, which is chemically similar
to calcium, deposits in the bones; iodine deposits in the thyroid gland,
which is the body’s iodine processor. It has been found that certain
synthetic chemicals such as the pesticide DDT (mentioned earlier in the
section on malaria) and the drug DES (diethylstilbestrol, given to pregnant
women to prevent miscarriage) are hormone mimics or endocrine disruptors.
The body metabolizes them as if they are the female hormone estrogen.
This causes serious damage to developmental and reproductive processes.
DDT has been shown to make eggshells fragile, and DES causes vaginal cancer
in children when they become teen-agers. As the effect was delayed significantly
from the time of administration, and happened in a person other than the
one who took it, the discovery of the effects and the banning of DES happened
years after its use began.
body does not metabolize a chemical, it may end up deposited in fatty
tissue or the liver. PCB’s (polychlorinated biphenyls) an industrially
useful synthetic chemical, is one that does not break down in the environment
or in the body. This family of “persistent” chemicals were
introduced in 1929 and used for numerous applications – cooling
fluids in transformers, as lubricants, cutting oil, and in paints, varnishes,
inks and pesticides – and became ubiquitous in the environment.
It has even spread to very remote “pristine” areas.
years after its use began, it was everywhere. PCB’s tend to deposit
in the liver and fatty tissue. Scientist Theo Colborn says that they “might
be found virtually anywhere imaginable: in the sperm of a man tested at
a fertility clinic in upstate New York, in the finest caviar, in the fat
of a newborn baby in Michigan, in penguins in Antarctica, in the bluefin
tuna served in a sushi bar in Tokyo, in the monsoon rains falling in Calcutta,
in the milk of a nursing mother in France, in the blubber of a sperm whale
cruising in the South Pacific, in a wheel of ripe brie cheese, in a handsome
striped bass landed off Martha’s Vineyard on a summer weekend. Like
most persistent synthetic chemicals, PCB’s are world travelers.”
(Our Stolen Future, p. 91-92)
Persistent chemicals can also bioaccumulate and biomagnify as they work
their way up the food chain. Figure 4, from Our Stolen Future
(p.27) shows how the small amounts in plankton cumulate up through the
food chain, becoming a million times more concentrated in the bird.
4: As PCBs work their way up the food chain, their concentrations
in animal tissue can be magnified up to 25 million times. Micorscopic
organisms pick up persistant chemicals from sediments, a continuing
source of contamiantation, and water and are consumed in large
numbers by filter feeding tint animals called zooplankton. Latger
species like mysids then consume zooplankton, fish eat the mysids,
and so on up the food wev to the herring gull. (Figure and caption
from Our Stolen Future, p. 27)
transformation, persistence, and bioaccumulation are therefore three ways
in which synthetic chemicals that we have not evolved with and become
part of our ecology cause their effects.
and various metabolic processes and depositions thus become steps that
result in disease. For example, liver cancer is a long-term effect of
PCB exposure. In order to be able to characterize and manage the effects,
various frameworks have been evolved. One used most often is risk assessment.
We now discuss the basic principles of risk assessment and risk analysis.