Environmental Health Hazards 77mimic those of other chronic diseases associated with risky
lifestyle patterns, poor nutrition, and aging. Also, it is dif-
ficult to isolate a causative agent from among the multiple
toxicants we are routinely exposed to.Disease-Causing Agents
in the Environment
Disease-causing agents are infectious organisms, such
as bacteria, viruses, protozoa, and parasitic worms that
cause diseases. Typhoid, cholera, bacterial dysentery, po-
lio, and infectious hepatitis are some of the most com-
mon bacterial or viral diseases that are transmissible
through contaminated food and water. Diseases such as
these are considered environmental health hazards. We
do not discuss other human diseases, such as acquired
immunodeficiency syndrome (AIDS), that are not trans-
missible through the environment.
The vulnerability of water supplies to waterborne
disease-causing agents was dramatically demonstrated
in 2000, when the first waterborne outbreak in North
America of a deadly strain of Escherichia coli occurred in
Ontario, Canada. Several people were killed, and several
thousand became sick. Prior to this outbreak, this deadly
E. coli strain had been transmitted almost ex-
clusively through contaminated food.
The largest outbreak of a waterborne
disease ever recorded in the United States
occurred in 1993, when a microorganism
(Cryptosporidium) contaminated the water
supply in the greater Milwaukee area. About
370,000 people developed diarrhea. These
and similar outbreaks raise concerns about
the safety of our drinking water. Because
sewage-contaminated water is an environ-
mental threat to public health, periodic tests
are made for the presence of sewage in our
drinking water supplies. The best indicator of
sewage-contaminated water is the presence of
the common intestinal bacterium E. coli be-
cause it doesn’t appear in the environment ex-
cept from human and animal feces. Tests such
as those for E. coli are used to indicate the possi-
ble presence of various disease- causing agents- Define toxicology and epidemiology.
- Explain why public water supplies are monitored
for fecal coliform bacteria despite the fact that
most strains of E. coli do not cause disease. - Describe the link between environmental changes
and emerging diseases, such as swine flu.
T
he human body is exposed to many kinds of
chemicals in the environment. Both natural
and synthetic chemicals are in the air we
breathe, the water we drink, and the food we
eat. All chemicals, even “safe” chemicals such as sodium
chloride (table salt), are toxic if exposure is high enough.
For example, a 1-year-old child will die from ingesting
about 2 tablespoons of table salt; table salt is also harmful
to people with heart or kidney disease. Chemicals with
adverse effects are known as toxicants.
Toxicology is one of two main approaches we use to
understand threats to human health. Toxicologists (a)
study the effects of toxicants on living organisms, (or parts
of organisms, such as cells in a test tube), (b) evaluate the
mechanisms that cause toxicity, and (c) develop ways to
prevent or minimize adverse effects. (Developing appropri-
ate handling or exposure guidelines for specific
toxicants is one of these ways.)
Epidemiology involves studying how
chemicals (toxicants), biological agents (dis-
ease), and physical hazards (accidents, radia-
tion) affect the health of human populations.
Epidemiologists study large groups of people
and investigate a range of possible causes and
types of diseases and injuries.
The effects of toxicants following exposure
can be immediate (acute toxicity) or prolonged
(chronic toxicity). Symptoms of acute toxic-
ity range from dizziness and nausea to death.
Acute toxicity occurs immediately to several
days following a single exposure. In comparison,
chronic toxicity generally produces damage fol-
lowing long-term, low-level exposure to a toxi-
cant. Toxicologists know far less about chronic
toxicity than they do about acute toxicity, partly
because the symptoms of chronic toxicity often
Environmental Health Hazards
LEARNING OBJECTIVES
toxicology The study
of toxicants, chemicals
with adverse effects on
health.
epidemiology The
study of the effects of
chemical, biological,
and physical agents
on the health of
human populations.
acute toxicity
Adverse effects that
occur within a short
period after high-level
exposure to a toxicant.
chronic toxicity Ad-
verse effects that oc-
cur after a long period
of low-level exposure
to a toxicant.