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Lesson 3 Plan

Biological Effects of Ionizing Radiation

Purpose:

This lesson presents the current understanding of the biological effects of ionizing radiation. Students will learn which factors are important in determining the biological effects of radiation and what those effects can be. They will also learn why it is difficult to be sure about the effects of low-level radiation.

Concepts:

  1. Ionizing radiation in the form of charged alpha and beta particles can damage molecules and thus cells, tissues, and organs. Such damage can lead to tissue and organ disease. Gamma rays and X-rays can have the same effects.

  2. The biological effects of ionizing radiation depend on the radiation dose or energy absorbed, the type of radiation, and the type and volume of biological cells exposed.

  3. There are two general categories of biological effects from ionizing radiation: somatic effects and genetic effects.

  4. Biological effects from exposures to low levels of radiation are usually estimated from effects observed from high radiation doses.

  5. It’s not possible to reduce our exposure level to zero, yet at least some risk is usually assumed for any exposure greater than zero.

  6. Some individual scientists, however, now believe there is evidence suggesting that a slightly higher than average exposure may actually make people healthier by stimulating their immune systems.

  7. Shielding protects individuals and the environment from exposure to ionizing radiation from radioactive materials or gamma rays and X-rays.

  8. Exposure can also be reduced by shortening exposure time and increasing distance from the source of radiation.

Duration of Lesson:

Two 50-minute class periods

Objectives:

After participating in this lesson, the student will be able to

  1. name the principal factors that determine the biological effects of ionizing radiation;

  2. explain the difference between somatic and genetic effects;

  3. discuss some effects of acute exposures to high levels of radiation;

  4. explain why we don’t know exactly what effects low levels of radiation may have;

  5. discuss evidence interpreted by some to suggest that radiation levels slightly higher than average may actually help keep a population group healthier.

Skills:

Reading, interpreting, drawing conclusions, using data from tables, calculating

Vocabulary:

Absorbed dose, acute exposure, alpha particle, beta particle, chronic exposure, DNA (Deoxyribonucleic acid), gamma ray, genetic effect, germ cell, gonad, ionizing radiation, millirem, molecule, mutation, ovary, ovum, rad, radiation, radiation sickness, radiologist, rem, somatic effect, sperm, testis, ultraviolet light, X-ray

Materials:

Reading Lesson: “Biological Effects of Ionizing Radiation”
Activity Sheets: “Calculating Your Personal Annual Dose”

“Biological Effects of Ionizing Radiation: Key Concepts”

“Internal Exposure from Radioactivity in Food”
Answer Keys: “Biological Effects of Ionizing Radiation”

“Internal Exposure from Radioactivity in Food”
Background Notes: “Radiation Exposure”

Suggested Procedure:

  1. The vocabulary introduced in this lesson is extensive. While many words are defined in the text, it may be helpful to preview the vocabulary words before beginning the reading.

  2. Assign the reading entitled “Biological Effects of Ionizing Radiation.” You may wish to supplement the reading lesson by having students study sections of the following Web sites:

    Nuclear Medicine Technology Resource Web site: www3.sympatico.ca/lgoodin/rad-phy.htm

    The University of Michigan Health Physics Web site: www.umich.edu/~radinfo/

    “Cancer Causes and Risk Factors” National Cancer Institute Web site: www.cancer.gov/cancertopics/prevention-genetics-causes/causes

  3. Lead a class discussion of the reading.

For Class Discussion:

  1. What areas of the human body are more sensitive to radiation than others? Besides the type and volume of cells exposed to radiation, what other factors influence the biological effects of radiation?

    (Organs with rapidly dividing cell systems, such as the bone marrow, gonads, and intestines are more sensitive to radiation.) (Radiation dose and type of radiation)

  2. What terms are used to describe received doses of radiation? What are acute exposures? What symptoms might be experienced in the hours, days, and weeks after an individual receives an acute exposure of radiation?

    (Rad — radiation absorbed dose. Rem — roentgen- equivalentman — usually in one-thousandths, or millirem.)

    (Large doses received over short periods of time are acute exposures. Doses received over long periods of time are chronic exposures.)

    (Symptoms: hours — nausea, headache, appetite loss, diarrhea; weeks — changes in blood cell populations, hemorrhaging, hair loss, infertility and/or sterility.)

  3. The main types of ionizing radiation we are concerned with are alpha and beta particles, gamma rays, and X-rays. Use the illustration showing deposition of energy to discuss how these various types of ionizing radiation interact with human tissue. What does the term “per unit path” mean?

    (Alpha particles have the shortest paths in human tissue and deposit the most energy per unit path. Beta particles travel much farther than alpha particles and deposit less energy per unit path. They deposit their energy over a much longer pathway. Gamma rays and X-rays, being waves of pure energy and having no charge or mass, travel very long distances in human tissue, and deposit the least energy per unit path.)

  4. It is estimated that natural sources of radiation could account for only 1 to 3 percent of the cancer deaths normally expected in the U.S. population. Using the table “Cancer Deaths Attributed to Various Sources” in the reading lesson entitled “Biological Effects of Ionizing Radiation,” discuss the contribution of the various sources to cancer deaths in the United States.

    1. Is natural radiation a big cause of cancer compared with other causes?

    2. Are food additives a big cause of cancer?

    3. What are some things we can do in terms of lifestyle to minimize our risk of getting cancer?

      (The two most important things according to this chart would be to watch diet and avoid smoking. Others relate to sexual lifestyle, occupation, exposures related to our natural environment and pollution.)

  5. Some risk of a genetic effect is assumed for even low levels of avoidable radiation exposure. In light of this, discuss the following:

    1. A woman who is 50 years old has two children, ages 17 and 22. Her doctor told her that it is important for her to have a series of X-rays. She is concerned about possible genetic effects of radiation. Discuss whether she should be concerned.

      (The woman is past ordinary child-bearing age, and her children are already born. She is not likely to have any more children. And she cannot pass on a genetic effect to already born children from an event that occurs after they are born.)

    2. Suppose the woman is 20 years old and has no children. Discuss whether she should be concerned about genetic effects to her children.

      (Scientists generally agree that exposure to ordinary diagnostic X-rays is not likely to be harmful. Nevertheless, it is wise for a young woman and her doctor to discuss risks and to weigh the risks and the benefits to be gained from them.)

  6. Discuss the following statement: “It is wise for a woman of childbearing age to determine whether or not she is pregnant before being exposed to ionizing radiation above background levels.”

    (Again, scientists generally agree that exposure to ordinary diagnostic X-rays is not likely to be harmful. Rapidly dividing cells are especially sensitive to radiation. So prenatal exposures to the developing infant carry some risk. It is important to balance the benefits of the exposure against the risk.)

  7. Discuss the role we think the DNA molecules play in determining the effect of radiation on humans. See “DNA: Mighty Molecules” and the diagram of the molecule in “Biological Effects of Ionizing Radiation.”

    (There is some indication that ionizing radiation causes cancer or a genetic disorder by radiation damage to the DNA that is passed on to a great many cells when the cell containing the damaged DNA divides. Exactly how radiation or other agents cause such effects is not completely understood. We know that even at low levels, radiation can damange DNA. But the body protects itself against routine radiation by repairing most damaged cell DNA. This happens all the time. Also, cells with damaged DNA typically die without reproducing and passing on the damaged DNA. These are some reasons we cannot prove health or genetic effects from low doses of radiation. In addition, radiation exposure has never been demonstrated to cause hereditary effects in human populations.)

  8. Some effects of exposure to ionizing radiation are delayed and don’t show up for many years. Why does this make it difficult to determine exactly what the effects of exposures to low levels of ionizing radiation are?

    (The negative health effects of low exposures to ionizing radiation, if there are any, take decades to develop. There are also no observable differences between the negative health effects from radiation and those from many other agents. Therefore, a causeand- effect relationship between low radiation exposures and a delayed health effect in humans is hard to establish definitively.)

  9. There is not much data on the effects of exposures to low levels of radiation to examine. Instead, scientists rely on information gained from data about high exposures or on data from studies with animals, chiefly mice. Discuss how these two facts affect uncertainty about effects of low exposures.

    (There is also a question to what extent the data from lowexposure studies with animals can be applied to humans. As in Question 10 above, for humans the delay of many years between exposures and effects leads to uncertainties as to a causal relationship.

    Scientists agree, however, that low exposures spread over weeks or months or years have a much lower effect than the same total exposure given all at once.)

  10. The human species, like all life on our planet, has always been exposed to natural radiation. Certain scientists speculate that since we’ve always been exposed to ionizing radiation, some degree of radiation exposure may even be good for us. Some scientists have researched data on human population groups that receive somewhat higher than average radiation levels (often due to their occupations or the location where they live). They have interpreted the data to suggest that human populations exposed to such slightly to moderately higher than normal levels of radiation actually live longer, and even get fewer cancers overall, than do other population groups. The term for the theoretical benefits to human health from showewhat higher levels of radiation is hormesis, or adaptive response. What do you think of this idea?

    (Answers will vary. The concept remains controversial. As noted in the reading lesson, there are many possible causes of cancer, and researchers may not be aware that for a given population, some other, non-radiation cause is lower than expected or even completely absent. For a variety of reasons, two different people who receive the same additional radiation dose in the same situation may have different responses to that extra dose. The age of persons receiving a dose can make a difference as well.

    The National Research Council of the National Academy of Sciences publishes and updates a special report on health risks from low levels of radiation. The report, now in its seventh edition, is under the direction of the Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation. The committee was formerly know as the Committee on the Biological Effects of Ionizing Radiation [BEIR], and the report itself is still commonly referred to by the acronym BEIR and the edition number. The official title of the latest edition [2005] is Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII, Phase 2.

    After analyzing the relevant research published since 1990, the BEIR VII Committee still concluded that “the higher the dose, the greater the risk; the lower the dose, the lower the likelihood of harm to human health.” The committee reported that “current scientific evidence is consistent with the hypothesis that ther is a linear dose-response relationship between exposure to ionizing radiation and the development of radiation-induced solid cancers in humans.” In short, “the preponderance of information indicates that there will be some risk, even at low doses” [BEIR VII, pages 18, 20].)

Teacher Evaluation of Student Understanding:

Student participation in class discussion and completion of activity will indicate understanding.

Enrichment:

Reading Lesson: “Using Radioisotopes to Date Materials”

Activity Sheet: “Using Radioisotopes to Date Materials”

Answer Key: “Using Radioisotopes to Date Materials”

 
Ionizing Radiation
Ionizing Radiation