Introduction to Toxicology
Objectives: Upon completion of this module, the learner will be able to:
- Define toxicology and commonly associated terms
- Differentiate the sub-disciplines of toxicology
- Describe the classifications of toxic agents
- Describe the field of toxicology and its application when assessing health effects associated with toxic agents commonly found at hazardous waste sites
- Understand the roles and responsibilities of various agencies
- Identify potential sources for additional information
I. Introduction to Toxicology
A. What is Toxicology?
Toxicology is the study of how natural or man-made poisons cause undesirable effects in living organisms.
Harmful or Adverse Effects: Those that are damaging to either the survival or normal function of the individual.
Toxicity: Describes the degree to which a substance is poisonous or can cause injury. Toxicity depends on: dose, duration and route of exposure, shape and structure of the chemical, and individual human factors.
Toxic: Relates to poisonous or deadly effects on the body by inhalation, ingestion, absorption, or direct contact with a chemical.
Toxicant: Any chemical that can injure or kill humans, animals, or plants; a poison. Used for toxic substances produced by or as a by-product of human-made activities (e.g., dioxin).
Toxin: Usually refers to toxic substances produced naturally (e.g., microbial, vegetable, or synthetic chemical origin that reacts with cellular components).
Toxic Symptom: Any feeling or sign indicating the presence of a poison in the system.
Toxic Effects: The health effects that occur due to exposure to a toxic substance.
Selective Toxicity: A chemical produces injury to one kind of living matter without harming another form of life.
Dose: The actual amount of a chemical that enters the body. Can be due to acute (short-term, usually ≤24 hours) or chronic (long-term, weeks to years) exposure.
Dose-Response: Relationship between exposure and health effect, established by measuring response relative to increasing dose. Core concept: "the dose makes the poison."
Threshold Dose: The dose or exposure level below which harmful effects are not seen in a population. Also called No Observed Adverse Effect Level (NOAEL) or No Effect Level (NEL). Note: For carcinogens, no safe threshold is assumed.
Individual Susceptibility: Differences in types of responses to hazardous substances between people.
Sensitive Sub-population: Persons more at risk from illness due to exposure than the average healthy person (e.g., very young, chronically ill, very old, pregnant women).
B. The Field of Toxicology
Toxicology addresses questions in agriculture (pesticide effects), animal testing for dose-response, and how chemicals/waste affect individual health.
C. Sub-disciplines of Toxicology
Environmental Toxicology: Study of chemicals contaminating food, water, soil, or atmosphere; effects on plants, animals, humans.
Occupational (Industrial) Toxicology: Health effects from exposure to chemicals in the workplace. Aims to protect workers and ensure safe environments.
Regulatory Toxicology: Gathers and evaluates toxicological information to establish concentration-based standards of "safe" exposure.
Food Toxicology: Ensures safe, edible food supply. Studies additives, allergies, and contaminants like pesticides or heavy metals in food.
Clinical Toxicology: Concerned with diseases from short- or long-term exposure to toxic chemicals. Involves emergency physicians familiar with toxic substance symptoms.
Descriptive Toxicology: Gathers toxicological information from animal experimentation to establish illness/death doses. Used by EPA, OSHA, FDA to set exposure limits.
Forensic Toxicology: Establishes cause-effect relationships between exposure to a drug/chemical and toxic/lethal effects.
Analytical Toxicology: Identifies toxicants through analysis of body fluids, stomach content, excrement, or skin.
Mechanistic Toxicology: Observes how toxic substances cause their effects, considering factors like molecule size, tissue type, solubility.
Also Read Clinical Toxicology - Short Notes
II. Classification of Toxic Agents
Toxic substances are classified into:
- A. Heavy Metals: Not created/destroyed by humans. Health effects via increased presence in environment or chemical structure changes (e.g., Chromium III ↔ VI).
- B. Solvents and Vapors: Widespread exposure (occupational, domestic). Evaporated solvents pose inhalation threats.
- C. Radiation and Radioactive Materials: Energy release as waves/particles (e.g., nuclear reactions).
- D. Dioxin/Furans: By-products of chlorine processing (e.g., paper industries), originally found in Agent Orange.
- E. Pesticides: Substances intended to prevent, destroy, repel, or mitigate pests (insecticides, herbicides, etc.).
- F. Plant Toxins: Lethal chemicals produced by plants (e.g., taxol from yew plant for chemotherapy).
- G. Animal Toxins: From venomous (bite/sting delivery) or poisonous (toxic tissues) animals.
H. Subcategories of Toxic Substance Classifications
Substances can also be classified by:
- Effect on target organs (liver, kidney, hematopoietic system)
- Use (pesticide, solvent, food additive)
- Source (animal, plant toxins)
- Effects (cancer mutation, liver injury)
- Physical state (gas, dust, liquid)
- Labeling requirements (explosive, flammable, oxidizer)
- Chemistry (aromatic amine, halogenated hydrocarbon)
- Poisoning potential (extremely toxic, very toxic, slightly toxic)
I. General Classifications of Interest to Communities
- Air pollutants
- Occupation-related exposures
- Acute and chronic poisons
Note: All chemicals may be poisonous at a given dose and via a particular route (e.g., too much oxygen, water, or salt).
III. Toxicological Information Sources
A. Agency for Toxic Substances and Disease Registry (ATSDR)
Part of U.S. DHHS. Provides health-based information for chemical waste site cleanups (CERCLA/Superfund). Assesses health hazards at Superfund Sites. Products include:
- Toxicological Profiles (specific chemicals & health effects)
- Case Studies in Environmental Medicine
- Public Health Statements
- ToxFAQs™ (summaries on hazardous substances)
- Medical Management Guidelines for Acute Chemical Exposures
Hazardous Substances and Health Effects Database (HazDat): Contains info on substances at NPL/non-NPL sites and associated health effects.
B. United States Environmental Protection Agency (EPA)
Enforces federal laws protecting human health/environment. 10 regional offices. Key laws regulating chemicals:
- Clean Air Act (CAA) - controls air emissions
- Clean Water Act (CWA) - regulates pollutant discharge to surface waters
- Safe Drinking Water Act (SDWA) - sets drinking water contaminant standards
- Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA/Superfund) - hazardous waste site cleanup
- Resource Conservation and Recovery Act (RCRA) - hazardous waste management
- Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) - pesticide regulation
- Toxic Substances Control Act (TSCA) - chemical testing/reporting
- Emergency Planning and Community Right to Know Act (EPCRA) - emergency response planning
C. Centers for Disease Control and Prevention (CDC)
Promotes health by preventing disease, injury, disability. Includes environmental/occupational hazards. Key centers:
- National Center for Environmental Health (NCEH): Addresses chemical exposure hazards inside/outside workplace.
- National Institute for Occupational Safety and Health (NIOSH): Researches and recommends prevention of work-related illnesses/injuries. Functions: investigate hazardous work conditions, evaluate chemical hazards, create prevention methods, conduct research, provide education/training.
D. Nuclear Regulatory Commission (NRC)
Regulates nuclear materials for commercial, industrial, academic, medical purposes (power plants, medical isotopes, etc.). Evaluates acceptable exposure levels for workers.
E. Food and Drug Administration (FDA)
Promotes/protects public health by ensuring safety and efficacy of products (food, drugs, medical devices).
F. American Conference of Governmental Industrial Hygienists (ACGIH)
Professional organization publishing Threshold Limit Values (TLV) and Biological Exposure Indices (BEI) for hundreds of chemicals (updated annually). Guidelines for occupational exposure evaluation.
G. Electronic Databases
- TOXNET (www.toxnet.nlm.nih.gov): Multiple databases including Hazardous Substances Data Bank (HSDB) and Registry of Toxic Effects of Chemical Substances (RTECS).
- CHEMTREC (www.chemtrec.org): Emergency information/assistance for chemical incidents. Provided by American Chemistry Council.
- Material Safety Data Sheets (MSDS): Available on OSHA website (www.osha.gov). Provide substance properties, first aid, emergency response info.
- HazDat: On ATSDR website (www.atsdr.cdc.gov). Info on hazardous substances at waste sites and related health effects.
IV. Test Your Knowledge Quiz
1. Which group is most sensitive to toxic substance exposures?
a) Adult women b) Infants c) Adult men d) Adolescents
2. Working 10 years at a chemical facility without protective equipment, developing serious health effects over 7 years indicates:
a) Chronic exposure b) Acute exposure
3. Concerned about vegetables grown in contaminated soils. Which toxicologist to contact?
a) Descriptive b) Environmental c) Regulatory d) Food
4. Risks from vegetables grown in soil with high lead/arsenic concentrations involve which substance type?
a) Toxin b) Toxicant
5. Larger exposure amount/dose leads to greater observed response/effect.
a) True b) False
6. Which toxicologist takes blood, urine, hair samples for testing?
a) Descriptive b) Analytical c) Mechanistic d) Forensic
7. Toxic agents can be classified by physical state, effects, and source.
a) True b) False
8. Which agency deals with health effects from environmental exposure to toxic chemicals?
a) EPA b) CDC c) ATSDR d) NRC
9. Which database has emergency handling procedures, environmental data, regulatory status, and human exposure info?
a) TOXNET b) HazDat c) IRIS d) CHEMTREC
10. HazDat contains info on hazardous substances at NPL and non-NPL waste sites and emergency events.
a) True b) False
11. NOAEL is the same as NEL.
a) True b) False
12. "Toxicant" refers to toxic substances from human-made activities.
a) True b) False
V. Activity Lab & Key Terms
Crossword Puzzle Key Terms:
- Toxin: Toxic substances produced naturally
- HazDat: Contains information on hazardous substances found at NPL and non-NPL sites
- Threshold Dose: The dose/exposure level below which harmful effects are not seen
- Individual Susceptibility: Differences in response to exposure between persons
- Clean Air Act: Implements regulations controlling air emissions
- Exposure: The act of coming into contact with a hazardous substance
- Chronic: Long-term exposure
- NOAEL: No observed adverse effect level
- Acute: Short-term exposure (usually
