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PDF Files for PrintingLesson 5-What is the Risk? 

At a Glance


Students apply their growing understanding of the concepts of toxicology (dose, response, individual susceptibility, potency, and threshold) to their discussion of the 1950s tragedy in Minamata, Japan. They learn how to assess the risk of people to specific chemical hazards and make decisions about how to manage that risk.

Major Concepts

People can make some choices about chemical exposure; however, some exposure is controlled at a level other than an individual one. Collective groups of people, such as communities and governments, seek to control chemical exposure on a community or global level.


After completing this lesson, students will

Background Information

map of JapanThe Minamata Case Study

When people living in Minamata, Japan, in the 1950s began slurring their speech occasionally or dropping their chopsticks at a meal, no one thought much of it. Some people cruelly laughed, claiming their clumsy friends were acting like the cats that were "dancing" strangely in the street and falling to their death in the sea. When it seemed like more and more people were suffering from the mysterious lack of coordination, the community began to realize that something was seriously wrong. But, people did not know that they were seeing the first signs of a debilitating nervous condition caused by ingesting mercury.1

We now know the tragic story of Minamata. The Minamata Bay was polluted with the industrial waste from the Chisso Corporation, which manufactured acetaldehyde used to make plastics. The mercury that the company used in the production process was discharged into the bay, incorporated into bacteria, and passed through the food chain to people living in the area. The people in the town were slowly being poisoned by their most important food source: fish.

polluting Minamata Bay with industrial waste
Photo: W. Eugene Smith and Aileen M. Smith

The consequences of such blatant polluting seem obvious to people today. But at the time, science had not yet documented the hazards of mercury, and environmental awareness was not pervasive. In fact, the Minamata case has become a classic lesson in the tragedy of industrial pollution and the need to anticipate the unexpected consequences of introducing chemicals into the environment. Although the story is now half a century old (and "ancient history" for today's middle school students), it has a well-documented cause and effect, as well as a resolution. In this way, it provides a good model for teaching about risk assessment and management that students can apply to their analysis of current exposures to chemicals.

Risk Assessment

Today, when toxicologists study the extent and type of negative effects associated with a particular level of chemical exposure, they can use what they learn to assess the threat of that chemical to people's health. To do this, toxicologists measure a person's risk of exposure to the chemical. For example, even though dioxin is considered the most toxic synthetic chemical known, it does not pose the greatest risk to humans because the potential for significant dioxin exposure is quite small. In addition, while the lethal dose of a chemical is an important measurement to make, it is quite possible that a chemical will produce a very undesirable toxic effect at doses that cause no deaths at all. These lower doses may be the amount to which people are regularly exposed.

How a person is exposed to a chemical also determines the factor of risk. In the case of a single exposure, the amount of chemical and the way the body is known to respond to the chemical determine the severity of the toxic response. In the case of repeated exposures to a chemical, it is not only the amount of chemical that counts, but also the frequency of exposure. If the body is able to rid itself entirely of the chemical before the next exposure, it is possible that each exposure is akin to a single exposure to the chemical. If, however, the body still retains some of the chemical from the previous exposure, accumulation of the chemical can occur and eventually can reach toxic levels, even if each exposure is small.

Many of the measurements that guide toxicologists in their assessment of human risk are based on studies of animals other than humans. This fact, coupled with the individual susceptibility of different members of the human population, makes it difficult to know with absolute certainty the level of risk to which each individual is exposed. With adequate information, however, toxicologists can predict the health risks associated with specific chemical exposures and help the human population make informed decisions about how to limit those exposures.

Managing Risk

The built-in uncertainty of risk assessment makes it essential for people to possess enough knowledge to make decisions about their own exposures to chemicals. With adequate knowledge, individuals can make decisions concerning their exposure to tobacco smoke, pollutants in water, and chemicals in food. By modifying their individual behavior, people can have some control over the chemicals they absorb into their body.

Not all decisions about chemical exposure and control can be made at an individual level, however. Local, national, and global communities of people are exposed to chemicals over which they have very little individual control. People are exposed to air pollution from factories and cars or chemicals used by farmers on crops without any individual consent. To manage a community's risk from chemicals in the environment, organizations and agencies set standards to protect human health.

chemical exposure and the individualD

There are choices about chemical exposure over which individuals have control (represented by the inner circle in the adjacent diagram). Individuals are also affected by their immediate environment (their friends and family, as well as the air, soil, and water around their homes and workplaces); the middle circle of the diagram describes influences on an individual over which he or she has less control. Finally, the outer circle describes the world that surrounds individuals over which they have little control but that can have an impact on individuals. The arrows between each concentric circle indicate that individuals, their environment, and the world at large all affect each other.

One step in community risk management is to determine how much risk is acceptable to people. If the chance that exposure to a particular chemical causes cancer is only 1 in 1 million, people often are less concerned than if the chance is 1 in 10. The picture becomes more complicated when societal issues weigh in. Is the exposure voluntary (as in smoking cigarettes) or involuntary (as in pollution from a factory)? Does it occur in the workplace or at home? Are there acceptable alternatives to the use of the toxic chemical? How would use of a safer chemical change the economic picture?2

To establish some individual control over community management of chemical exposure, people can choose to be involved with organizations and agencies that are concerned with the prevention of toxic chemical exposure on a community level.

Notes about Lesson 5

In this lesson, students have the opportunity to apply many of the concepts of toxicology to a scenario that involved toxic chemicals in Minamata, Japan. By looking at a situation from the 1950s, students can recognize how far scientists and the general public have come in their understanding of chemical hazards and their knowledge of how to minimize risk from these hazards. Students can begin to identify situations in their own lives in which they make conscious decisions to limit their chemical exposure and those over which they have little control.

In Advance

Web-Based Activities
Activity Number Web Version

Activity 1


Activity 2


Extension Activity


Activity Number Master Number Number of Copies

Activity 1

Master 5.1, Risk Assessment and Management
Master 5.2, Minamata Disease

1 transparency
1 for each student

Activity 2

Master 5.1, Risk Assessment and Management
Master 5.2, Minamata Disease

1 transparency
1 for each student

Extension Activity



Activity 1 Activity 2 Activity 3

For the class:

  • Web site address
  • computer with Internet access
  • overhead projector
  • transparency of Master 5.1, Risk Assessment and Management
  • plain paper

For each student:

  • 1 copy of Master 5.2, Minamata Disease

For the class:

  • overhead projector
  • transparency of Master 5.1, Risk Assessment and Management
  • plain paper

For each student:

  • 1 copy of Master 5.2, Minamata Disease

For the class:

  • current event stories students began collecting in Lesson 1, Extension Activity


Master 5.2, being folded to partially conceal the informationActivity 1

Arrange for students to have access to computers.

Make a transparency of Master 5.1, Risk Assessment and Management.

Duplicate Master 5.2, Minamata Disease, one for each student. To allow students to read only small amounts of the information at a time, fold along the dashed lines.

Activity 2

Gather the same materials used in Activity 1.

Extension Activity

Remind students to bring in the current event stories they began collecting in Lesson 1.

Be sure to have a transparency of Master 5.1, Risk Assessment and Management.



1. Remind students that there are chemicals in the environment that cause health problems for humans. Tell students that toxicologists study the extent and type of health problems associated with a particular level of chemical exposure and use what they learn to assess the threat of that chemical to the health of people in particular situations. This kind of analysis is called a risk assessment. Display the top half of a transparency of Master 5.2, Risk Assessment and Management.

National Science Education Standards icon Content Standard F:
Students should develop understanding of personal health, natural hazards, and risks and benefits.

2. Distribute the folded sheets made from Master 5.2, Minamata Disease. Tell students that they are going to practice the steps to making a risk assessment by using a well-known case from Japan in the 1950s. Instruct students to read Part I of Master 5.2. Then, discuss the answers to the questions in Step 1 on the Risk Assessment and Management transparency.

fishermen in Minamata Bay
Photo: W. Eugene Smith and Aileen M. Smith

Once students have answered the questions on the transparency, ask them to offer ideas about what they think was contaminating the fish.

3. Instruct students to unfold the first fold, revealing Part II. Ask them to read the paragraphs and then answer the questions in Step 2 of the Risk Assessment on the transparency.

National Science Education Standards iconContent Standard E:
Students should develop understandings about science and technology. Perfectly designed solutions do not exist. All technological solutions have tradeoffs, such as safety, cost, efficiency, and appearance. . . . Technological solutions have intended benefits and unintended consequences. Some consequences can be predicted, others cannot.

Once students have answered the questions on the transparency, ask them to suggest answers to the question at the end of Part II: What made this contamination of the fish so dangerous to humans?

4. Instruct students to unfold the next fold, revealing Part III. Ask students to read the paragraph and then answer the questions in Step 3 of the risk assessment.

5. Ask students not to unfold the last fold until directed to do so during the next activity. Discuss the information from the reading and answer the concluding question on the risk assessment: How great is the risk to people?

dead fish
Photo: Corel

Because of their dependence on fish as a primary source of food, the potential risk of mercury poisoning from contaminated fish for people living in Minamata was very high.

6. Play the video segment on the Web site that describes the Minamata story.

Web activity iconOpen the Web site in your browser (see instructions for using the Web site). From the main page, click on Web Portion of Student Activities, then select Lesson 5—What Is the Risk? Play the video documentary for the students.

Because the time period and geographic location of the Minamata tragedy are so far removed from students' experiences, the visual representation of the story on the Web site helps it come alive for students.


seagulls on rocks by the water
Photo: Corel

1. Remind students that mercury is used today in thermometers and batteries. (Although newer thermometers now use red alcohol, many old ones contain mercury.) Tell students that although they do not live in Minamata in the 1950s, inappropriate disposal of items containing mercury poses a threat to their environment, even today. Since garbage either is incinerated or covered up in landfills, mercury can make its way into the environment through emission of burning gases into the air or groundwater contamination. Fish contaminated with mercury can make their way into the food supply.

2. Ask students how they think they can avoid mercury poisoning from contaminated fish.

Most students will say that they could stop eating fish, thereby eliminating their risk just by avoiding exposure to the mercury-contaminated fish. Some students may indicate that the risk of mercury poisoning provides a great excuse to avoid a less-than-favorite food: fish.

Ask students if it is always possible to avoid a chemical in order to eliminate possible exposure. What about a chemical in the air? Could students choose not to breathe in order to avoid exposure to an air pollutant?

This question brings up the issue of control. If your food supply is varied enough, you can choose not to eat fish and still remain healthy. (This might not be an option for an island population that depends on fish for protein.) You cannot, however, choose not to breathe as a way to avoid exposure to an air pollutant. You would need to find other ways to limit your exposure to the air pollutant, like staying inside, not exercising outside, or wearing a mask that filters the air.

3. Tell students that one of the reasons for understanding the role of toxicology in human health is to empower the students to make choices that decrease their risk of becoming ill due to exposure to harmful chemicals. Once they know the risk from a chemical exposure, they can manage their risk by deciding how to deal with the risk. Walk the students through the steps of Risk Management on the bottom half of the transparency of Master 5.1, Risk Assessment and Management. Contrast the situation in Minamata, Japan, in the 1950s with the life of today's typical U.S. middle school student.

First, ask the students to think about risk assessment:

assessment iconBefore discussing the current event with the class, ask students to do a risk assessment individually. Collect students' written summaries and evaluate them for an understanding of the process of assessing risk. Then discuss the students' ideas for managing the risk.

Middle school students can eat fish sensibly, dispose of mercury-containing products safely, and support organizations that provide hazardous waste cleanup in their communities. Regulatory agencies can measure mercury contamination in fish and regulate fishing or sales of fish from contaminated waters.

Extension Activity

1. Review a local or current situation in which people are being exposed to a hazardous chemical. Use the Risk Assessment and Management transparency to discuss students' ideas about the level of risk for the community and ways to manage that risk.

Tip from the field test: This is a good time to go back to the current event articles the students have been collecting since Lesson 1. Choose one or two of the most interesting situations and assess risk for the population and decide how to manage the risk.

National Science Education Standards iconContent Standard G:
Students should develop an understanding of the nature of science and the history of science.

Because a current situation most likely will be unresolved, you will need to lead an open-ended discussion and help students recognize that there might not be answers for some of their questions at this time. This process of asking questions and not knowing the "right" answers is representative of the nature of science and scientific inquiry.


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