
Purpose
In this exercise,
students explore the meaning of risk in terms of a simple
exercise in probability. They explore the idea that not all
risks have the same consequences and are not likely to occur
at the same rate. The exercise helps students to evaluate
the impact of risk on the basis of probabilities, benefits,
and their perceptions.


Background
Nothing in life is a "sure
thing." While it is unlikely that it will snow in Texas in April, it
is possible. In the same way, it is unlikely that it will be 80 degrees
in Massachusetts in December, but it is possible. Saying something is
"unlikely" or that it "probably will happen" is an indication of the
probability that this particular thing will occur. Every day
you weigh probabilities even without realizing it.
For example, when you take
a test, you think about the chances of getting an "A." How likely you
think it is that you'll get an "A" depends on how well you studied and
how hard you think the test will be. Or, if you want to be picked for
a team, the chances that you'll make it depends on how many other people
want to play and how good you are in comparison to everyone else.
When it comes to the environment,
the chance of something dangerous happening is called risk. Each
type of hazardous substance, hazardous waste, or dangerous situation
involves a different amount of risk. The risk is made up of two parts:
(1) the chance that people will be exposed to the substance, and (2)
the chance that exposure will injure or harm them. Environmental
risk measures the probability that the environment will be damaged
by a particular hazardous situation. Decisions on cleaning up a Superfund
site are based on the risks the site poses to people and the environment.
There are generally many
factors involved in determining the risks surrounding a particular hazardous
situation. These factors include the potential for damage each particular
substance can cause, the chances that the substance is going to spread
from the original site through water, wind, or some other means, and
the chance that people will come in contact with the substance.
In this Warmup, students
perform a simple exercise in probability to gain a beginning understanding
of how chance operates. For additional information on probability, and
risk, and environmental risk assessment, see the Suggested
Reading list found at the end of the HazEd materials. Other HazEd
materials that are related to this topic include Activity
7: Identifying Risks at a Superfund Site and Activity
9: Making Decisions about Hazardous Waste Cleanup.
Preparation

Gather the following
materials:
 Several coins
 Several sheets of
paper
 Several pencils
 Copies for each student
of the Student Worksheet, Risks and
Benefits.
Procedure

Explain to students that
the class is going to conduct an exercise in probability by tossing
a coin. Explain that, for one brief moment, when a coin is tossed
into the air, it assumes a state of unpredictability. We know that
it will either be heads or tails, but we cannot know which one while
the coin is in the air. Even so, in repeated trials under similar
conditions, we do know that heads will come up half the time and tails
the other half. This illustrates the theory of probabilityhow likely
it is that a particular result will occur in a given situation.

Organize the class into
groups of about 2 or 3 students each. Give each group a penny, a piece
of paper, and a pencil. If necessary, demonstrate how to toss a coin
to determine heads or tails.

Instruct each group to
flip the coin 50 times, recording the results of each toss. Have the
students record the total number of heads and total number of tails
that occurred after:
 5 coin flips
 10 coin flips
 25 coin flips
 50 coin flips.
Record each group's
results on the chalkboard.

Explain that few events
are as predictable as a coin toss. No matter how many times a coin
comes up heads, there is only a 5050 chance that the next toss will
be tails. While the ratio of heads to tails may vary with only a few
repetitions, the ratio stabilizes at or near onehalf after many repetitions.
(For example, theoretically it would take a million people tossing
coins 10 times a minute 40 hours a week for 9 centuries for a coin
to fall on heads 50 consecutive times.)

Have the class compare
the results of the coin toss exercise for each group. Did the ratio
of heads to tails vary after 5 tosses? After 10 tosses? Was the final
ratio about onehalf? If not, why?

Explain that although
the coin toss demonstrates the fundamental principle of probability,
determining the risks of injury, disease, or death from a particular
hazard is far more complex. This is true mainly because these risks
are dependent on the occurence of other factors, and the interaction
of multiple factors, such as contact (or exposure) to the hazard that
causes the effect.
 Distribute a copy of
the Student Worksheet, Risks and Benefits,
to each student and review the instructions for both parts. Give students
about 10 minutes to complete the worksheet (individually or in small
groups). Have students discuss their answers to the questions.
Extensions (Optional)

Assign students to go
to the library and look up details to support the answers they gave
in Part B of the worksheet (for example, what gasoline is made of
and why it is harmful, the number of automobile or airplane accidents
that occur each year, or where PCBs come from).

Have the students write
a short story about how one of the risks listed in the worksheet was
harmful to people or the environment. This may require students to
conduct some research. The resources in the Suggested Reading list
are helpful. Allow the students to base their story on a true event
if they wish. Select the best stories, and ask the students who wrote
them to summarize them for the class. Discuss the stories in terms
of the students' personal willingness to accept voluntary and involuntary
risks.
