December 2014 Teacher's Guide for
How Toxic is Toxic?
Table of Contents
About the Guide 2
Student Questions 3
Answers to Student Questions 4
Anticipation Guide 5
Reading Strategies 6
Background Information 8
Connections to Chemistry Concepts 19
Possible Student Misconceptions 19
Anticipating Student Questions 19
In-Class Activities 20
Out-of-class Activities and Projects 21
References 21
Web Sites for Additional Information 22
About the Guide
Teacher’s Guide editors William Bleam, Regis Goode, Donald McKinney, Barbara Sitzman and Ronald Tempest created the Teacher’s Guide article material. E-mail:
Susan Cooper prepared the anticipation and reading guides.
Patrice Pages, ChemMatters editor, coordinated production and prepared the Microsoft Word and PDF versions of the Teacher’s Guide. E-mail:
Articles from past issues of ChemMatters can be accessed from a DVD that is available from the American Chemical Society for $42. The DVD contains the entire 30-year publication of ChemMatters issues, from February 1983 to April 2013.
The ChemMatters DVD also includes Article, Title and Keyword Indexes that covers all issues from February 1983 to April 2013.
The ChemMatters DVD can be purchased by calling 1-800-227-5558.
Purchase information can be found online at www.acs.org/chemmatters.
Student Questions
1. According to the article, what two factors should be considered to determine if a substance is toxic?
2. Explain why the Paracelsus quote “The dose makes the poison” is important in this article.
3. In the term LD50, what does the “LD” represent?
4. And what is the definition of the entire term “LD50”?
5. What are the units in which LD50 is measured?
6. The article mentions methods of administering poison other than ingestion. What are they?
7. How does toxicity vary with LD50 values?
8. The LD50 value of substance A = 5 mg/kg. The LD50 value of substance B = 23.5 mg/kg. Which substance is more toxic?
9. What is the LD50 value for caffeine?
10. What application of the botulinum toxin demonstrates that in very small concentrations a deadly poison can be harmless?
11. List three concerns about the use of LD50 values to measure toxicity in humans.
Answers to Student Questions
1. According to the article, what two factors should be considered to determine if a substance is toxic?
The two factors identified in the article are how much of the substance enters the body and how it enters the body. See “More on the factors affecting how poisonous a substance is” for a more detailed discussion.
2. Explain why the Paracelsus quote “The dose makes the poison” is important in this article.
The quote stresses the point made in the article that any substance can be a poison if a person is exposed to enough of it.
3. In the term LD50, what does the “LD” represent?
The “LD” is the abbreviation for the term “lethal dose.”
4. And what is the definition of the entire term “LD50”?
“LD50” is the amount of a given substance required to kill 50% of a test population (lab rats or other animals).
5. What are the units in which LD50 is measured?
The units for LD50 are mg substance / kg body mass or mg / kg.
6. The article mentions methods of administering poison other than ingestion. What are they?
The article lists these variations: administration through the skins (dermal LD50), injection (intravenous LD50) and inhalation (LC50), or lethal concentration.
7. How does toxicity vary with LD50 values?
They have an inverse relationship. The lower the LD50 value, the greater the toxicity of the substance.
8. The LD50 value of substance A = 5 mg/kg. The LD50 value of substance B = 23.5 mg/kg. Which substance is more toxic?
Substance A is more toxic because less of it is required to kill 50% of the test subjects.
9. What is the LD50 value for caffeine?
According to the table on page 7, it is 192 mg/kg for rats.
10. What application of the botulinum toxin demonstrates that in very small concentrations a deadly poison can be harmless?
It can be used in very small doses to treat cerebral palsy, multiple sclerosis and Parkinson’s disease. And used in even smaller doses, it removes wrinkles in the skin; this product is Botox.
11. List three concerns about the use of LD50 values to measure toxicity in humans.
Three concerns about the use of LD50 values to measure human toxicity are:
a. Citing ethics concerns, animal rights groups object to using lab animals, even rats, in laboratory tests to determine toxicity.
b. Despite rats having anatomies similar to humans (making them good test subjects for toxicity tests), their anatomies still differ significantly from those of humans, thus possibly making the results of these LD50 tests far less accurate than we assume them to be.
c. LD50 values measure acute toxicity, not chronic effects. This means that these tests only measure immediate effects of doses, not long-term (acute) effects (e.g., lead poisoning has a cumulative effect on the body).
Anticipation Guide
Anticipation guides help engage students by activating prior knowledge and stimulating student interest before reading. If class time permits, discuss students’ responses to each statement before reading each article. As they read, students should look for evidence supporting or refuting their initial responses.
Directions: Before reading, in the first column, write “A” or “D,” indicating your agreement or disagreement with each statement. As you read, compare your opinions with information from the article. In the space under each statement, cite information from the article that supports or refutes your original ideas.
Me / Text / Statement1. All chemicals are toxic.
2. How a substance enters your body affects its toxicity.
3. You should be more concerned about inhaling mercury vapors than touching metallic mercury.
4. Lethal doses (LD50) of chemicals are determined using hamsters.
5. The lethal dose (LD50) of a substance is less for children than adults.
6. The higher the LD50 of a substance, the more toxic it is.
7. Vitamin D is added to milk sold in U. S. stores to help prevent cataracts.
8. Theobromine (found in chocolate) is more toxic than caffeine.
9. The most toxic substance known (botulinum toxin) is used in small amounts to treat the symptoms of certain diseases.
10. LD50 values can be used to measure chronic effects of a toxin.
Reading Strategies
These graphic organizers are provided to help students locate and analyze information from the articles. Student understanding will be enhanced when they explore and evaluate the information themselves, with input from the teacher if students are struggling. Encourage students to use their own words and avoid copying entire sentences from the articles. The use of bullets helps them do this. If you use these reading strategies to evaluate student performance, you may want to develop a grading rubric such as the one below.
Score / Description / Evidence4 / Excellent / Complete; details provided; demonstrates deep understanding.
3 / Good / Complete; few details provided; demonstrates some understanding.
2 / Fair / Incomplete; few details provided; some misconceptions evident.
1 / Poor / Very incomplete; no details provided; many misconceptions evident.
0 / Not acceptable / So incomplete that no judgment can be made about student understanding
Teaching Strategies:
1. Links to Common Core Standards for writing:
- ELA-Literacy.WHST.9-10.2F: Provide a concluding statement or section that follows from and supports the information or explanation presented (e.g., articulating implications or the significance of the topic).
- ELA-Literacy.WHST.11-12.1E: Provide a concluding statement or section that follows from or supports the argument presented.
2. Vocabulary and concepts that are reinforced in this issue:
- Lethal dose (LD)
- Amino acid
- Enzyme
- Organic molecular structure
- Metric system
- Electromagnetic radiation
- Redox reaction
- Pheromones
- Volatility
3. To help students engage with the text, ask students which article engaged them most and why, or what questions they still have about the articles.
Directions: As you read, complete the graphic organizer below to describe evidence for the scientific or engineering practices required to determine toxicity of substances.
Asking questions or defining problems
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Obtaining, evaluating, and communicating information
Background Information
(teacher information)
More on toxicity
Poisonous. Hazardous. Toxic. Dangerous. All are adjectives we use, often interchangeably, to describe chemicals that may cause harm to living organisms by reacting chemically with substances in the organisms so as to do some degree of harm, including death. Do the terms all mean the same thing? By explaining the concept of toxicity the article suggests that the terms have different meanings. The question involving the terms mentioned above is, “Is this substance poisonous?” The Merck Index provides some basics:
Poisoning is contact with a substance that results in toxicity. Symptoms vary, but certain common syndromes may suggest particular classes of poisons. Diagnosis is primarily clinical, but for some poisonings, blood and urine tests can help. Treatment is supportive for most poisonings; specific antidotes are necessary for a few. Prevention includes labeling drug containers clearly and keeping poisons out of the reach of children.
Most poisonings are dose-related. Dose is determined by concentration over time. Toxicity may result from exposure to excess amounts of normally nontoxic substances. Some poisonings result from exposure to substances that are poisonous at all doses. Poisoning is distinguished from hypersensitivity and idiosyncratic reactions, which are unpredictable and not dose-related, and from intolerance, which is a toxic reaction to a usually nontoxic dose of a substance.
Poisoning is commonly due to ingestion but can result from injection, inhalation, or exposure of body surfaces (eg, skin, eye, mucous membranes). Many commonly ingested nonfood substances are generally nontoxic (see Table 1:Substances Usually Not Dangerous When Ingested*); however, almost any substance can be toxic if ingested in excessive amounts.
(http://www.merckmanuals.com/professional/injuries_poisoning/poisoning/general_principles_of_poisoning.html) (Note that Table 1 referred to in this quote can be accessed by using the Merck Index link in “More on poisons”, below)
Another way to answer the “Is it poisonous?” question is to consider two things—the toxicity of the substance and the health hazard of the substance. And if we decide that the substance is toxic or hazardous, or both, we may simply tell our students that the substance is dangerous, a non-technical term that warns students to be cautious around the substance in question.
The context for these ideas is the process of analyzing the risks and benefits involved with chemicals. Risk analysis is an important function of many endeavors, no more so than in our everyday interaction with chemicals. The risk assessment for human health has grown in importance in recent years due to new understandings about how chemicals affect human health.
The World Health Organization (WHO) estimates that more than 25% of the global burden of disease is linked to environmental factors, including exposures to toxic chemicals. Lead exposure, for example, accounts for 3% of the cerebrovascular disease burden and 2% of the ischaemic heart disease burden worldwide. Some 9% of the global burden of lung cancer is attributed to occupational exposure to toxic substances, and 5% to outdoor air pollution. Lung cancer and mesothelioma are caused by exposure to asbestos, which remains in use in some countries. Unintentional poisonings kill an estimated 355 000 people each year, two thirds of them in developing countries, where such poisonings are strongly associated with excessive exposure to, and inappropriate use of, toxic chemicals, including pesticides.
(http://www.inchem.org/documents/harmproj/harmproj/harmproj8.pdf)
The ubiquitous nature of chemicals in our lives and the potential risks they may cause, as well as the potential benefits they may produce, requires a careful analysis of those risks and benefits. Again, according to the World Health Organization:
Human health risk assessment is a process intended to estimate the risk to a given target organism, system or (sub)population, including the identification of attendant uncertainties, following exposure to a particular agent, taking into account the inherent characteristics of the agent of concern as well as the characteristics of the specific target system (IPCS, 2004).
It is the first component in a risk analysis process that also includes risk management and risk communication. Human health risk assessment of chemicals refers to methods and techniques that apply to the evaluation of hazards, exposure and harm posed by chemicals, which in some cases may differ from approaches used to assess risks associated with biological and physical agents.
The risk assessment process begins with problem formulation and includes four additional steps: 1) hazard identification, 2) hazard characterization, 3) exposure assessment and 4) risk characterization (IPCS, 2004).
(http://www.inchem.org/documents/harmproj/harmproj/harmproj8.pdf)
We should consider toxicity, then, as one key factor in the more general risk-benefit analysis of chemicals in our lives.
As the article describes, the toxicity of a substance is a property that can be measured in a laboratory and is a fixed number for a given substance. It is commonly measured by determining the 50% lethal dose, LD50, of the substance, using lab animals as test subjects. It is a direct measure of how poisonous a substance is. (See below for “More on LD50”). However, the degree to which a substance is a hazard can vary. Whether the substance is a hazard depends on circumstances—how the substance is stored, how it is transported or how it is used. This is where you can inject lab safety into the discussion because hazards can be controlled. Lab safety rules are often designed to minimize hazards. See below for “More on lab safety”.
An important note of context for the article—the article discusses only those substances that react chemically to cause potential harm to people. It is the chemical mechanism that distinguishes this group of substances from other substances that are dangerous. For example, an explosion is dangerous and can injure or kill a person, but the cause of the injury is not chemical. In this case the injury is caused by physical pressure of shock waves that result from the explosion or by being struck by foreign objects propelled by the blast. There is no LD50 in this situation. This article does not deal with these kinds of physical trauma.