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Students divide substances into categories: made of chemicals/not made of chemicals, synthetic/naturally occurring, and toxic/nontoxic. When the teacher reveals that all the substances are made of chemicals, students discuss how their concept of what a chemical is might differ from the scientific definition. Students observe a mystery chemical and determine what precautions they might need to take when handling an unknown substance. Then, students read case studies of real exposures to chemicals.
Everything in the environment is made of chemicals. Both naturally occurring and synthetic substances are chemical in nature. People are exposed to chemicals by eating or swallowing them, breathing them, or absorbing them through the skin or mucosa, and they can protect themselves from harmful chemicals by blocking these routes of exposure.
After completing this lesson, students will
Simply stated, a chemical is any substance that has a defined molecular composition. Molecules, which are the smallest units into which a compound can be divided and still be that compound, can be made up of one or more elements. Sometimes, the elements are the same, such as in oxygen: Two oxygen atoms are chemically bonded together to form the gas, oxygen, or O2. Sometimes, the elements that form molecules are of different types, such as those in water: Two hydrogen atoms combine with one oxygen atom to form a molecule of water, or H2O. All forms of matter are made of one or more of the more than 100 elements combined in many different molecular combinations. This means that all forms of matter are made of chemicals.
 Paracelsus Reproduced with the permission of the Albertina Wien (Vienna). |
Long ago, humans observed that some chemicals derived from nature were poisonous. Poisonous chemicals produced naturally by living organisms (such as plants, animals, and fungi) are called toxins. Historically, knowledge of toxins was a powerful tool to use against enemies: Many murderers in ancient Greece and later throughout Europe used toxins.1 A significant contribution to the field of toxicology was made by the scientist Paracelsus (1493–1541). He recognized that the same chemical could have both therapeutic (medicinal) and toxic (poisonous) properties depending on how much of it was used. His work paved the way for the concept of the dose-response relationship (see Lesson 3 for more information about dose and response).1
With the onset of the industrial revolution and the emergence of the science of synthetic chemistry, a variety of new chemicals was made by humans. It is estimated that more than 65,000 chemicals have been manufactured for commercial use in industrialized countries.1 Whether on purpose or not, humans come into contact with these chemicals during manufacture, handling, or consumption. Exposure to a vast array of synthetic chemicals can occur when a person ingests food or drink, works in an agricultural setting with pesticides, or lives in a home among solvents, paints, plastics, and fuels. Although many of the chemicals greatly benefit us, some can have a toxic effect on human systems. These substances are called toxicants, a broad category that includes naturally occurring toxins.
How do people know if a chemical is toxic? The science of toxicology informs them of the nature of poisons. A toxicologist is a scientist who is trained to study the harmful effects of chemicals on living organisms. These harmful effects can include death, but not all toxicants are lethal. Some other harmful effects that toxicologists study are disease, tissue damage, genetic alterations, and cancer. Because there are so many ways that toxicants can affect living things and there are so many different kinds of chemicals in the environment, toxicology is a very broad science and there are many different kinds of toxicologists.2
What Do Toxicologists Do?Descriptive toxicologists evaluate the toxicity of drugs, food additives, and other products. They ask the question, What happens if …? about the amount of a toxicant and the response that a living system has to the toxicant. The descriptive toxicologist might work in a pharmaceutical laboratory or in an academic setting doing data analysis, animal testing, and/or human clinical trials.3
Mechanistic toxicologists study how a chemical causes toxic effects on living organisms. They study biomedical research, biochemistry, and physiology to understand how a chemical is absorbed, distributed, and excreted. In order to develop antidotes, a mechanistic toxicologist uses information about how a chemical harms an organism. This kind of toxicological work is often done in an academic setting or in private industry.3 Clinical toxicologists usually are physicians interested in the prevention, diagnosis, and treatment of poisoning cases. Clinical toxicologists specialize in toxicology issues concerning drugs used for treatment, such as side effects and overdoses; drugs of abuse, such as alcohol and cocaine; and accidental poisonings. These toxicologists have specialized training in emergency medicine and poison management. Veterinarians can be clinical toxicologists who study poisons in animals.3
Forensic toxicologists study the application of toxicology to the law. They work with pathologists and law enforcement officers at a crime scene. The forensic toxicologist uses chemical analysis to help establish the cause of death and determine the circumstances of death in a postmortem investigation.2 Environmental toxicologists study the effects of pollutants on organisms, populations, ecosystems, and the biosphere. Toxicologists concerned with the effects of environmental pollutants on human health fit into this group. Most commonly, however, environmental toxicologists study the impacts of chemicals on nonhuman organisms such as fish, birds, terrestrial animals, and plants.2 Regulatory toxicologists use scientific data to decide how to protect humans and animals from excessive risk. Regulatory toxicologists aim to protect the public from chemical exposure by establishing regulatory standards for food, drugs, water, air, and insecticides, to name only a few. Government bureaus such as the U.S. Food and Drug Administration (FDA) and the U.S. Environmental Protection Agency (EPA) employ regulatory toxicologists.2,3 |
Toxicants can harm an organism only if they are absorbed by the organism and reach the organs that are the target of their toxicity. This can happen through three routes:
In humans and other animals, toxicants usually affect one or more target organs such as the lungs, skin, or gastrointestinal tract. For example, if a person inhales asbestos fibers, the fibers get stuck in the airways of the lungs and irritate the lung lining, causing lung impairment over time. Dermatitis can result if the asbestos fibers irritate skin cells.
Sometimes the toxicant crosses from the external environment of the lung, skin, or gastrointestinal tract into the bloodstream.1 Many parts of the human body are designed to absorb chemicals quickly and effectively. The stomach, intestines, and colon absorb nutrients from our diet. These organs easily absorb nutrients and other chemicals because of their large surface area, thin diffusion distance, and high blood flow. The lungs also are designed for rapid absorption. Chemicals that are inhaled are quickly absorbed into the bloodstream through the thin walls of the air sacs in the lungs. The skin protects the body from harmful agents in the environment. However, the skin is in direct contact with the environment. While the dense outer layer of skin cells is a good barrier to chemical absorption, it is not perfect, even when intact. When the skin is cut or abraded, it absorbs chemicals very rapidly.4
Students often harbor misconceptions about chemicals. When asked what a chemical is, rather than define the word, students tend to give examples of synthetic, toxic chemicals like pesticides. When asked to name some things made of chemicals, students list items such as shampoo, window cleaner, processed foods, and "fake sugar" (aspartame). Students believe that chemicals pollute rivers and air. Students often do not realize that natural substances in the world around them also are made of chemicals. When asked if it would be better if there were fewer chemicals in the world, one student replied that fewer human-made chemicals would mean less pollution. When pressed, students will agree that some synthetic chemicals, like a pain reliever, can be good; however, students also recognize that even "good" chemicals like pain relievers can be toxic if a person takes too much.5
The purpose of this lesson is to help move students from the view that chemicals are toxic, synthetic substances that are bad for human health and the environment to the more inclusive view that all things in the environment, including their bodies, are made of chemicals. Some naturally occurring and synthetic chemicals can have a detrimental effect on human health and the environment, but many do not. Those that have a harmful effect on human health do so because they get into the body through inhalation, ingestion, and absorption.
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Master 1.4, Questions for Case Studies |
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| a Because everything in the
environment is made of chemicals, any item will work, such as salt, sugar,
lemon, soft drink, liquid soap, window cleaner, shampoo, apple, rock,
leaf, chair, and water. Use the chemicals students test in Lesson 2 (see
Preparation for
Activity 3 on next page), plus others that do and do not fit students'
concept of chemicals. b Item cards depict objects that are too big for the materials table or are potentially dangerous substances that students should consider when they choose items made of chemicals. c Make sure that the glass jar fits inside the shoe box. d Collect clothing such as elbow pads, knee pads, shorts, T-shirt, long-sleeved shirt, pants, different kinds of hats, hip waders, boots, sandals, sneakers, socks, sunglasses, protective goggles, ear and nose plugs, paper mask, mittens, gloves, and latex gloves. |
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