2. Environmental Toxicology
Chapter 3
Environmental Toxicology

3. Learning Objectives
By the end of this chapter the reader will be able to:
Define the term toxicology
Define three important terms used in the field of toxicology
State five factors that affect responses to a toxic chemical
Discuss the steps involved in risk assessment
Describe methods for human exposure assessment

4. Basic Assumption of Toxicology
“All substances are poisons; there is none that is not a poison. The right dose differentiates a poison from a remedy.”

5. Definition of Toxicology
Toxicology is defined as “the study of the adverse effects of chemicals on living organisms.”

6. History of Toxicology: Paracelsus
Paracelsus was one of the founders of modern toxicology.
Active during the time of da Vinci and Copernicus (early 16th century).
His contributions included the concept of the dose-response relationship and the notion of target organ specificity of chemicals.

7. History of Toxicology: Mathieu Orfila
In the 1800s, he authored a number of significant works, among them Trait des poisons (1813).
This work described various types of poisons and their bodily effects, a development that contributed to the foundations of forensic toxicology.

8. What is a Toxicologist?
A scientist who has received extensive training in order to investigate in living organisms “the adverse effects of chemicals (including their cellular, biochemical, and molecular mechanisms of action) and assess the probability of their occurrence.”

9. Fields within Toxicology
Regulatory
Forensic
Clinical
Environmental
Reproductive
Developmental

10. Environmental Toxicology
Environmental toxicology examines how environmental exposures to chemical pollutants may present risks to biological organisms, particularly animals, birds, and fish.

11. Poison
“defined as any agent capable of producing a deleterious response in a biological system.”

12. Toxicity Refers to “the degree to which something is poisonous.”
Related to a material’s physical and chemical properties

13. Toxicants
Toxic substances that are man-made or result from human (anthropogenic) activity.

14. Toxin
Usually refers to a toxic substance made by living organisms including reptiles, insects, plants and microorganisms.

15. Examples of Toxic Plants
Some mushrooms (e.g., Amanita phalloides, “death cap”)
Poison hemlock
Foxglove
Poison oak/poison ivy
Rhubarb, especially the leaves, which have high levels of oxalates
Some houseplants such as dieffenbachia

16. Dose
Refers to “the amount of a substance administered at one time.”

17. Several Ways of Describing a Dose
Exposure dose
Absorbed dose
Administered dose
Total dose
External dose
Internal dose
Biologically effective dose

18. Lethal Dose 50 (LD50)
To describe toxic effects, toxicologists use the symbol LD50, which is “the dosage (mg/kg body weight) causing death in 50 percent of exposed animals.”
Used to compare the toxicities of different chemicals.

19. Dose-Response Relationship
A type of correlative relationship between “the characteristics of exposure to a chemical and the spectrum of effects caused by the chemical.”

20. Dose-Response Curve
A type of graph used to describe the effect of exposure to a chemical or toxic substance upon an organism such as an experimental animal.

21. Two Types of Dose-Response Curves
One for the responses of an individual to a chemical
One for a population

22. Figure 3-2 Individual dose–response curve.
Source: Reprinted with permission from AE Marczewski, M Kamrin. Toxicology for the Citizen. 2nd ed. East Lansing, Mich: Michigan State University, Center for Integrative Toxicology; 1991:5. .

23. Figure 3-3 A population dose–response curve.
Source: Adapted from Guidelines for Ecological Risk Assessment. US Environmental Protection Agency, Risk Assessment Forum. Washington, DC, EPA/630/R095/002F; 1998:81.

24. Figure 3-4 The threshold of a dose–response curve.
Refers to the lowest dose at which a particular response may occur.
Source: Reprinted from National Institutes of Health, National Institute of Environmental Health Sciences. Chemicals, the Environment and You: Explorations in Science and Human Health, p. 63. Available at: Accessed February 25, 2010.

25. Factors That Affect the Concentration and Toxicity of a Chemical
Route of entry into the body
Received dose of the chemical
Duration of exposure
Interactions that transpire among multiple chemicals
Individual sensitivity

26. Most Frequent Sites of Exposure to Environmental Chemicals
Gastrointestinal tract
Respiratory System
Skin

27. Routes of Exposure
Ingestion (e.g., consumption of contaminated food or drink)
Injections into the bloodstream
Contact with the surface of the skin (topical mode)
Inhalation

28. Effects of Chemical Mixtures
Additive means that the combination of two chemicals produces an effect that is equal to their individual effects added together.
Synergism indicates that the combined effect of exposures to two or more chemicals is greater than the sum of their individual effects.

29. Effects of Chemical Mixtures (continued)
Potentiation happens when one chemical that is not toxic causes another chemical to become more toxic.
Antagonism means that “two chemicals administered together interfere with each other’s actions or one interferes with the action of the other.”

30. Terms That Describe Duration of Exposure
Acute - usually a single exposure for less than 24 hours
Subacute - exposure for one month or less
Subchronic - exposure for one to three months
Chronic - exposure for more than three months

31. Direct Adverse Effects of Exposure to Chemicals
Local effects - damage at the site where a chemical first comes into contact with the body.
Systemic effects - generalized distribution of the chemical throughout the body by the bloodstream to internal organs.
Target organ effects - some chemicals may confine their effects to specific organs.

32. Latency
The time period between initial exposure and a measurable response.
The latency period can range from a few seconds (in the case of acutely toxic agents) to several decades for agents that may be carcinogenic.

33. Carcinogen
A chemical (or substance) that causes or is suspected of causing cancer, a disease associated with unregulated proliferation of cells in the body.

34. Testing for Toxicity
The subjects used for testing the toxicity of chemicals include the following:
Volunteers who have had normal or accidental exposures
Animals exposed purposively (in vivo experiments)
Cells derived from human, animal, or plant sources (in vitro experiments)

35. Risk Assessment
Provides a qualitative or quantitative estimation of the likelihood of adverse effects that may result from exposure to specified health hazards or from the absence of beneficial influences.

36. Process of Risk Assessment
Hazard identification
Dose-response assessment
Exposure assessment
Risk characterization

37. Hazard Identification
Definition: hazard
“Inherent capability of an agent or a situation to have an adverse effect.”
Hazard identification: “examines the evidence that associates exposure to an agent with its toxicity and produces a qualitative judgment about the strength of that evidence…”

38. Dose-Response Assessment
Measures “the relationship between the amount of exposure and the occurrence of the unwanted health effects.”

39. Exposure Assessment
The procedure that “identifies populations exposed to the toxicant, describes their composition and size, and examines the roots, magnitudes, frequencies, and durations of such exposures.”

40. Exposure Assessment (continued)
Attempts to answer the following questions:
Who or what is exposed (e.g., people, aquatic ecosystems)?
Does the exposure occur through breathing air, drinking water, skin contact or any other routes?
How much exposure occurs?
How often and for how long does exposure occur, that is, what is its frequency and duration?

41. What is an Exposure Assessment?
Estimating concentrations in the environment using models
Assessing exposures among populations
Assessing exposures using monitoring data
Assessing exposures using models

42. Risk Characterization
Develops “estimates of the number of excess unwarranted health events expected at different time intervals at each level of exposure.”

43. Risk Management
Oriented toward specific actions and “consists of actions taken to control exposures to toxic chemicals in the environment.
Exposure standards, requirements for premarket testing, recalls of toxic products, and outright banning of very hazardous materials are among the actions that are used by governmental agencies to manage risk.”