Basic Terminology

Toxicology Defined

Toxicology is an evolving medical science and toxicology terminology is evolving with it. Most terms are very specific and will be defined as they appear in the tutorial. However, some terms are more general and used throughout the various sections. The most commonly used terms are introduced in this section.

  • Toxicology is the study of the adverse effects of chemicals or physical agents on living organisms.
  • toxicologist is a scientist who determines the harmful effects of agents and the cellular, biochemical, and molecular mechanisms responsible for the effects.
  • Toxinology, a specialized area of study, looks at microbial, plant and animal venoms, poisons, and toxins.

Terminology and definitions for materials that cause toxic effects are not always consistently used in the literature. The most common terms are toxicant, toxin, poison, toxic agent, toxic substance, and toxic chemical.

Toxicant, toxin, and poison are often used interchangeably in the literature but there are subtle differences as shown below:

Person spraying a chemical onto a field of grass

Figure 1. Pesticide chemicals are toxicants
(Image Source: iStock Photos, ©)


Figure 2. Amanita muscaria mushroom contains a neurotoxin
(Image Source: iStock Photos, ©)

Black widow spider with a distinctive red hourglass shape on its abdomen

Figure 3. Black Widow spiders produce a poison that is a toxin
(Image Source: Texas Parks & Wildlife, ©)


  • Substances producing adverse biological effects of any kind.
  • May be chemical or physical in nature.
  • Effects may be acute or chronic.


  • Peptides or proteins produced by living organisms.
  • Venoms are toxins injected by a bite or sting.


  • Toxins produced by organisms.

Toxic Agents

toxic agent is anything that can produce an adverse biological effect. It may be chemical, physical, or biological in form. For example, toxic agents may be:

Chemical (such as cyanide)

Physical (such as radiation)

Biological (such as snake venom)

Chemical structure of cyanide

Figure 4. Cyanide is a chemical toxic agent
(Image Source: NLM IRIS and ChemIDplus)

Radioactive materials

Figure 5. Radioactive materials are physical toxic agents
(Image Source: iStock Photos, ©)


Figure 6. Snake venom is an example of a biological toxic agent
(Image Source: iStock Photos, ©)

The toxicity of the agent is dependent on the dose.

A distinction is made for diseases people get from living organisms. Organisms that invade and multiply within another organism and produce their effects by biological activity are not classified as toxic agents but as biological agents. An example of this is a virus that damages cell membranes resulting in cell death.

Microscopic view of a virus

Figure 7. Microscopic view of a virus
(Image Source: iStock Photos, ©)

If the invading organisms excrete chemicals which are the basis for their toxicity, the excreted substances are known as biological toxins. In that case, the organisms are called toxic organisms. A specific example is tetanus. Tetanus is caused by a bacterium, Clostridium tetani. The bacteria C. tetani itself does not cause disease by invading and destroying cells. Rather, a toxin (neurotoxin) that the bacteria excrete travels to the nervous system and produces the disease (Figure 8).

Micrograph of a group of Clostridium tetani bacteria

Figure 8. Micrograph of a group of Clostridium tetani bacteria
(Image Source: CDC)

Toxic Substances

toxic substance is simply a material that has toxic properties.  It may be a discrete toxic chemical or a mixture of toxic chemicals.  For example, lead chromate, asbestos, and gasoline are all toxic substances. More specifically:

  • Lead chromate is a discrete toxic chemical.
  • Asbestos is a toxic material that does not have an exact chemical composition but comprises a variety of fibers and minerals.
  • Gasoline s a toxic substance rather than a toxic chemical in that it contains a mixture of many chemicals. Toxic substances may not always have a constant composition. The composition of gasoline varies with octane level, manufacturer, time of season, and other factors.
Lead chromate, asbestos, and gasolene

Figure 9. Examples of toxic substances: lead chromate (left), asbestos (center), and gasoline (right)
(Image Source: iStock Photos, ©)

Systemic Toxicants and Organ Toxicants

Toxic substances may be systemic toxicants or organ toxicants.

systemic toxicant affects the entire body or many organs rather than a specific site. For example, potassium cyanide is a systemic toxicant in that it affects virtually every cell and organ in the body by interfering with the cells’ ability to use oxygen.

Toxicants may also affect only specific tissues or organs while not producing damage to the body as a whole. These specific sites are known as the target organs or target tissues.

  • Benzene is a specific organ toxicant in that it is primarily toxic to the blood-forming tissues.
  • Lead is also a specific organ toxicant; however, it has three target organs: the central nervous system, the kidneys, and the hematopoietic system.

A toxicant may affect a specific type of tissue (such as connective tissue) that is present in several organs. The toxic site is then considered the target tissue.

A systemic toxicant affects the entire body or several organs (left); an organ toxicant affects a specific organ (right)

Figure 10. Systemic toxicant and organ toxicant
(Image Source: iStock Photos, ©)

Types of Cells

The body is composed of many types of cells, which can be classified in several ways. Table 1 shows examples of one classification of one type of cells.

Cell Types Examples
Basic structure cuboidal cells
Tissue type hepatocytes of the liver
Germ cells ova and sperm
Somatic cells non-reproductive cells of the body

Table 1. Types of cells

Germ cells are involved in reproduction and can give rise to a new organism. They have only a single set of chromosomes peculiar to a specific sex. Male germ cells give rise to sperm and female germ cells develop into ova. Toxicity to germ cells can cause effects in a developing fetus that lead to outcomes such as birth defects or miscarriage.

Somatic cells are all body cells except the reproductive germ cells. (Somatic cells include the "basic structure" and "tissue type" cells listed in Table 1). They have two sets (or pairs) of chromosomes. In an exposed individual, toxicity to somatic cells causes a variety of toxic effects, such as dermatitis, death, and cancer.

Figure 11 illustrates the differences between germ cells and somatic cells.

Germ cells (sperm and egg cells) have a single set of chromosomes. Somatic cells (all other body cells) have two sets, or pairs, of chromosomes.

Figure 11. Germ cells and somatic cells
(Image Source: National Human Genome Research Institute,

Natural and Man-Made Chemicals

Often, people mistakenly assume that all man-made chemicals are harmful and natural chemicals are beneficial. In reality, natural chemicals can be just as harmful to human health as man-made chemicals, and in many cases, more harmful. Figure 12 compares the toxicity of several natural and man-made chemicals.

Infographic titled 'Natural and Man-made chemicals.' A common misconception is that all man-made chemicals are harmful and all natural chemicals are good for us. However, many natural chemicals are just as harmful to human health, if not more so, than man-made chemicals. Examples of chemicals that result in toxic effects at 1000 mg/kg of body weight include natural chemicals such as muscimol (found in fly agaric mushrooms), solanine (found in green potatoes), and amygdalin (found in apple seeds), and man-made chemicals such as ethylene glycol (used in anti-freeze), aspirin, and sodium thiopental (formerly used for lethal injections). Examples of checmials with no toxic effects seen at 1000 mg/kg of body weight include natural chemicals such as citric acid (found in lemons and limes), sucrose (table sugar), and water; and man-made chemicals such as MSG (a food additive), teflon (used in non-stick pans), and aspartame (an artificial sweetener). Any substance, if given in large enough amounts, can cause death. Some are lethal after only a few nanograms, whilst others require kilograms to achieve a lethal dose. Chemical toxicity is a sliding scale, not black and white - and whether a chemical is naturally occuring or man-made tells us nothing about its toxicity.

Figure 12. Natural and man-made chemicals
(Image Source:, Creative Commons license)