Chemicals can be released into the soil, water or air from spilled
or leaking containers, leaking landfills or dumps, through spraying,
or released from smokestacks. These chemicals follow a pathway,
or a route from the time of release to the point of human contact.
When a substance enters the ecosystem
where it is not normally found, it is called a contaminant. Humans
become exposed to these contaminants by touching, breathing, or
ingesting substances that contain the chemical (ATSDR, 1993).
To determine if human health is at risk of disease from contaminants,
two things must occur: there must be an exposure to a contaminant;
and the contaminant must be toxic. A complete exposure pathway must
be present for disease to occur. The elements of an exposure pathway
- The source - how the contaminant got into the environment
- The transport media - how the contaminant moves through the
- The exposure point - how people came in contact with the contaminant
- The exposure route - how the contaminant entered the body
- The receptor population - how susceptible the population is
to the contaminant (adapted from ATSDR, 1994).
Additionally, whether the exposure causes disease
depends on the dose (how much), the duration (how long you are
exposed), the route of exposure (breathing, eating, or through
the skin), other chemicals to which you are exposed, and individual
characteristics such as age, sex, nutritional status, genetic
traits, lifestyle and general health condition (ATSDR, 1993).
Analyzing the exposure pathway at a contaminated
site is an important challenge. Consideration must be given to
how the chemical behaves in various kinds of environments (soil,
air, water, sediment, biota). Some contaminants disperse quickly
and others are very tenacious. For example, if drinking water
is contaminated, individuals can be exposed to the contaminant
not only by drinking but by showering through skin absorption
and through inhaling droplets in the air. Or, if surface water
is contaminated with a chemical that clings to soil particles,
individuals may be exposed as the chemical makes its way through
the food chain when animals ingest contaminants in soil and sediment.
Learning what chemicals are present and how they "behave"
are critical steps in understanding risk.
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