- •CONTENTS
- •Preface
- •Contributors
- •1 Introduction to Toxicology
- •1.1 Definition and Scope, Relationship to Other Sciences, and History
- •1.1.2 Relationship to Other Sciences
- •1.1.3 A Brief History of Toxicology
- •1.3 Sources of Toxic Compounds
- •1.3.1 Exposure Classes
- •1.3.2 Use Classes
- •1.4 Movement of Toxicants in the Environment
- •Suggested Reading
- •2.1 Introduction
- •2.2 Cell Culture Techniques
- •2.2.1 Suspension Cell Culture
- •2.2.2 Monolayer Cell Culture
- •2.2.3 Indicators of Toxicity in Cultured Cells
- •2.3 Molecular Techniques
- •2.3.1 Molecular Cloning
- •2.3.2 cDNA and Genomic Libraries
- •2.3.3 Northern and Southern Blot Analyses
- •2.3.4 Polymerase Chain Reaction (PCR)
- •2.3.5 Evaluation of Gene Expression, Regulation, and Function
- •2.4 Immunochemical Techniques
- •Suggested Reading
- •3.1 Introduction
- •3.2 General Policies Related to Analytical Laboratories
- •3.2.1 Standard Operating Procedures (SOPs)
- •3.2.2 QA/QC Manuals
- •3.2.3 Procedural Manuals
- •3.2.4 Analytical Methods Files
- •3.2.5 Laboratory Information Management System (LIMS)
- •3.3 Analytical Measurement System
- •3.3.1 Analytical Instrument Calibration
- •3.3.2 Quantitation Approaches and Techniques
- •3.4 Quality Assurance (QA) Procedures
- •3.5 Quality Control (QC) Procedures
- •3.6 Summary
- •Suggested Reading
- •4 Exposure Classes, Toxicants in Air, Water, Soil, Domestic and Occupational Settings
- •4.1 Air Pollutants
- •4.1.1 History
- •4.1.2 Types of Air Pollutants
- •4.1.3 Sources of Air Pollutants
- •4.1.4 Examples of Air Pollutants
- •4.1.5 Environmental Effects
- •4.2 Water and Soil Pollutants
- •4.2.1 Sources of Water and Soil Pollutants
- •4.2.2 Examples of Pollutants
- •4.3 Occupational Toxicants
- •4.3.1 Regulation of Exposure Levels
- •4.3.2 Routes of Exposure
- •4.3.3 Examples of Industrial Toxicants
- •Suggested Reading
- •5 Classes of Toxicants: Use Classes
- •5.1 Introduction
- •5.2 Metals
- •5.2.1 History
- •5.2.2 Common Toxic Mechanisms and Sites of Action
- •5.2.3 Lead
- •5.2.4 Mercury
- •5.2.5 Cadmium
- •5.2.6 Chromium
- •5.2.7 Arsenic
- •5.2.8 Treatment of Metal Poisoning
- •5.3 Agricultural Chemicals (Pesticides)
- •5.3.1 Introduction
- •5.3.3 Organochlorine Insecticides
- •5.3.4 Organophosphorus Insecticides
- •5.3.5 Carbamate Insecticides
- •5.3.6 Botanical Insecticides
- •5.3.7 Pyrethroid Insecticides
- •5.3.8 New Insecticide Classes
- •5.3.9 Herbicides
- •5.3.10 Fungicides
- •5.3.11 Rodenticides
- •5.3.12 Fumigants
- •5.3.13 Conclusions
- •5.4 Food Additives and Contaminants
- •5.5 Toxins
- •5.5.1 History
- •5.5.2 Microbial Toxins
- •5.5.3 Mycotoxins
- •5.5.4 Algal Toxins
- •5.5.5 Plant Toxins
- •5.5.6 Animal Toxins
- •5.6 Solvents
- •5.7 Therapeutic Drugs
- •5.8 Drugs of Abuse
- •5.9 Combustion Products
- •5.10 Cosmetics
- •Suggested Reading
- •6 Absorption and Distribution of Toxicants
- •6.1 Introduction
- •6.2 Cell Membranes
- •6.3 Mechanisms of Transport
- •6.3.1 Passive Diffusion
- •6.4 Physicochemical Properties Relevant to Diffusion
- •6.4.1 Ionization
- •6.5 Routes of Absorption
- •6.5.1 Extent of Absorption
- •6.5.2 Gastrointestinal Absorption
- •6.5.3 Dermal Absorption
- •6.5.4 Respiratory Penetration
- •6.6 Toxicant Distribution
- •6.6.1 Physicochemical Properties and Protein Binding
- •6.7 Toxicokinetics
- •Suggested Reading
- •7 Metabolism of Toxicants
- •7.1 Introduction
- •7.2 Phase I Reactions
- •7.2.4 Nonmicrosomal Oxidations
- •7.2.5 Cooxidation by Cyclooxygenases
- •7.2.6 Reduction Reactions
- •7.2.7 Hydrolysis
- •7.2.8 Epoxide Hydration
- •7.2.9 DDT Dehydrochlorinase
- •7.3 Phase II Reactions
- •7.3.1 Glucuronide Conjugation
- •7.3.2 Glucoside Conjugation
- •7.3.3 Sulfate Conjugation
- •7.3.4 Methyltransferases
- •7.3.7 Acylation
- •7.3.8 Phosphate Conjugation
- •Suggested Reading
- •8 Reactive Metabolites
- •8.1 Introduction
- •8.2 Activation Enzymes
- •8.3 Nature and Stability of Reactive Metabolites
- •8.4 Fate of Reactive Metabolites
- •8.4.1 Binding to Cellular Macromolecules
- •8.4.2 Lipid Peroxidation
- •8.4.3 Trapping and Removal: Role of Glutathione
- •8.5 Factors Affecting Toxicity of Reactive Metabolites
- •8.5.1 Levels of Activating Enzymes
- •8.5.2 Levels of Conjugating Enzymes
- •8.5.3 Levels of Cofactors or Conjugating Chemicals
- •8.6 Examples of Activating Reactions
- •8.6.1 Parathion
- •8.6.2 Vinyl Chloride
- •8.6.3 Methanol
- •8.6.5 Carbon Tetrachloride
- •8.6.8 Acetaminophen
- •8.6.9 Cycasin
- •8.7 Future Developments
- •Suggested Reading
- •9.1 Introduction
- •9.2 Nutritional Effects
- •9.2.1 Protein
- •9.2.2 Carbohydrates
- •9.2.3 Lipids
- •9.2.4 Micronutrients
- •9.2.5 Starvation and Dehydration
- •9.2.6 Nutritional Requirements in Xenobiotic Metabolism
- •9.3 Physiological Effects
- •9.3.1 Development
- •9.3.2 Gender Differences
- •9.3.3 Hormones
- •9.3.4 Pregnancy
- •9.3.5 Disease
- •9.3.6 Diurnal Rhythms
- •9.4 Comparative and Genetic Effects
- •9.4.1 Variations Among Taxonomic Groups
- •9.4.2 Selectivity
- •9.4.3 Genetic Differences
- •9.5 Chemical Effects
- •9.5.1 Inhibition
- •9.5.2 Induction
- •9.5.3 Biphasic Effects: Inhibition and Induction
- •9.6 Environmental Effects
- •9.7 General Summary and Conclusions
- •Suggested Reading
- •10 Elimination of Toxicants
- •10.1 Introduction
- •10.2 Transport
- •10.3 Renal Elimination
- •10.4 Hepatic Elimination
- •10.4.2 Active Transporters of the Bile Canaliculus
- •10.5 Respiratory Elimination
- •10.6 Conclusion
- •Suggested Reading
- •11 Acute Toxicity
- •11.1 Introduction
- •11.2 Acute Exposure and Effect
- •11.3 Dose-response Relationships
- •11.4 Nonconventional Dose-response Relationships
- •11.5 Mechanisms of Acute Toxicity
- •11.5.1 Narcosis
- •11.5.2 Acetylcholinesterase Inhibition
- •11.5.3 Ion Channel Modulators
- •11.5.4 Inhibitors of Cellular Respiration
- •Suggested Reading
- •12 Chemical Carcinogenesis
- •12.1 General Aspects of Cancer
- •12.2 Human Cancer
- •12.2.1 Causes, Incidence, and Mortality Rates of Human Cancer
- •12.2.2 Known Human Carcinogens
- •12.3 Classes of Agents Associated with Carcinogenesis
- •12.3.2 Epigenetic Agents
- •12.4 General Aspects of Chemical Carcinogenesis
- •12.5 Initiation-Promotion Model for Chemical Carcinogenesis
- •12.6 Metabolic Activation of Chemical Carcinogens and DNA Adduct Formation
- •12.7 Oncogenes
- •12.8 Tumor Suppressor Genes
- •12.8.1 Inactivation of Tumor Suppressor Genes
- •12.8.2 p53 Tumor Suppressor Gene
- •12.9 General Aspects of Mutagenicity
- •12.10 Usefulness and Limitations of Mutagenicity Assays for the Identification of Carcinogens
- •Suggested Reading
- •13 Teratogenesis
- •13.1 Introduction
- •13.2 Principles of Teratology
- •13.3 Mammalian Embryology Overview
- •13.4 Critical Periods
- •13.5 Historical Teratogens
- •13.5.1 Thalidomide
- •13.5.2 Accutane (Isotetrinoin)
- •13.5.3 Diethylstilbestrol (DES)
- •13.5.4 Alcohol
- •13.6 Testing Protocols
- •13.6.1 FDA Guidelines for Reproduction Studies for Safety Evaluation of Drugs for Human Use
- •13.6.3 Alternative Test Methods
- •13.7 Conclusions
- •Suggested Reading
- •14 Hepatotoxicity
- •14.1 Introduction
- •14.1.1 Liver Structure
- •14.1.2 Liver Function
- •14.2 Susceptibility of the Liver
- •14.3 Types of Liver Injury
- •14.3.1 Fatty Liver
- •14.3.2 Necrosis
- •14.3.3 Apoptosis
- •14.3.4 Cholestasis
- •14.3.5 Cirrhosis
- •14.3.6 Hepatitis
- •14.3.7 Oxidative Stress
- •14.3.8 Carcinogenesis
- •14.4 Mechanisms of Hepatotoxicity
- •14.5 Examples of Hepatotoxicants
- •14.5.1 Carbon Tetrachloride
- •14.5.2 Ethanol
- •14.5.3 Bromobenzene
- •14.5.4 Acetaminophen
- •14.6 Metabolic Activation of Hepatotoxicants
- •Suggested Reading
- •15 Nephrotoxicity
- •15.1 Introduction
- •15.1.1 Structure of the Renal System
- •15.1.2 Function of the Renal System
- •15.2 Susceptibility of the Renal System
- •15.3 Examples of Nephrotoxicants
- •15.3.1 Metals
- •15.3.2 Aminoglycosides
- •15.3.3 Amphotericin B
- •15.3.4 Chloroform
- •15.3.5 Hexachlorobutadiene
- •Suggested Reading
- •16 Toxicology of the Nervous System
- •16.1 Introduction
- •16.2 The Nervous system
- •16.2.1 The Neuron
- •16.2.2 Neurotransmitters and their Receptors
- •16.2.3 Glial Cells
- •16.3 Toxicant Effects on the Nervous System
- •16.3.1 Structural Effects of Toxicants on Neurons
- •16.3.2 Effects of Toxicants on Other Cells
- •16.4 Neurotoxicity Testing
- •16.4.1 In vivo Tests of Human Exposure
- •16.4.2 In vivo Tests of Animal Exposure
- •16.4.3 In vitro Neurochemical and Histopathological End Points
- •16.5 Summary
- •Suggested Reading
- •17 Endocrine System
- •17.1 Introduction
- •17.2 Endocrine System
- •17.2.1 Nuclear Receptors
- •17.3 Endocrine Disruption
- •17.3.1 Hormone Receptor Agonists
- •17.3.2 Hormone Receptor Antagonists
- •17.3.3 Organizational versus Activational Effects of Endocrine Toxicants
- •17.3.4 Inhibitors of Hormone Synthesis
- •17.3.5 Inducers of Hormone Clearance
- •17.3.6 Hormone Displacement from Binding Proteins
- •17.4 Incidents of Endocrine Toxicity
- •17.4.1 Organizational Toxicity
- •17.4.2 Activational Toxicity
- •17.4.3 Hypothyroidism
- •17.5 Conclusion
- •Suggested Reading
- •18 Respiratory Toxicity
- •18.1 Introduction
- •18.1.1 Anatomy
- •18.1.2 Cell Types
- •18.1.3 Function
- •18.2 Susceptibility of the Respiratory System
- •18.2.1 Nasal
- •18.2.2 Lung
- •18.3 Types of Toxic Response
- •18.3.1 Irritation
- •18.3.2 Cell Necrosis
- •18.3.3 Fibrosis
- •18.3.4 Emphysema
- •18.3.5 Allergic Responses
- •18.3.6 Cancer
- •18.3.7 Mediators of Toxic Responses
- •18.4 Examples of Lung Toxicants Requiring Activation
- •18.4.1 Introduction
- •18.4.2 Monocrotaline
- •18.4.3 Ipomeanol
- •18.4.4 Paraquat
- •18.5 Defense Mechanisms
- •Suggested Reading
- •19 Immunotoxicity
- •19.1 Introduction
- •19.2 The Immune System
- •19.3 Immune Suppression
- •19.4 Classification of Immune-Mediated Injury (Hypersensitivity)
- •19.5 Effects of Chemicals on Allergic Disease
- •19.5.1 Allergic Contact Dermatitis
- •19.5.2 Respiratory Allergens
- •19.5.3 Adjuvants
- •19.6 Emerging Issues: Food Allergies, Autoimmunity, and the Developing Immune System
- •Suggested Reading
- •20 Reproductive System
- •20.1 Introduction
- •20.2 Male Reproductive Physiology
- •20.3 Mechanisms and Targets of Male Reproductive Toxicants
- •20.3.1 General Mechanisms
- •20.3.2 Effects on Germ Cells
- •20.3.3 Effects on Spermatogenesis and Sperm Quality
- •20.3.4 Effects on Sexual Behavior
- •20.3.5 Effects on Endocrine Function
- •20.4 Female Reproductive Physiology
- •20.5 Mechanisms and Targets of Female Reproductive Toxicants
- •20.5.1 Tranquilizers, Narcotics, and Social Drugs
- •20.5.2 Endocrine Disruptors (EDs)
- •20.5.3 Effects on Germ Cells
- •20.5.4 Effects on the Ovaries and Uterus
- •20.5.5 Effects on Sexual Behavior
- •Suggested Reading
- •21 Toxicity Testing
- •21.1 Introduction
- •21.2 Experimental Administration of Toxicants
- •21.2.1 Introduction
- •21.2.2 Routes of Administration
- •21.3 Chemical and Physical Properties
- •21.4 Exposure and Environmental Fate
- •21.5 In vivo Tests
- •21.5.1 Acute and Subchronic Toxicity Tests
- •21.5.2 Chronic Tests
- •21.5.3 Reproductive Toxicity and Teratogenicity
- •21.5.4 Special Tests
- •21.6 In vitro and Other Short-Term Tests
- •21.6.1 Introduction
- •21.6.2 Prokaryote Mutagenicity
- •21.6.3 Eukaryote Mutagenicity
- •21.6.4 DNA Damage and Repair
- •21.6.5 Chromosome Aberrations
- •21.6.6 Mammalian Cell Transformation
- •21.6.7 General Considerations and Testing Sequences
- •21.7 Ecological Effects
- •21.7.1 Laboratory Tests
- •21.7.2 Simulated Field Tests
- •21.7.3 Field Tests
- •21.8 Risk Analysis
- •21.9 The Future of Toxicity Testing
- •Suggested Reading
- •22 Forensic and Clinical Toxicology
- •22.1 Introduction
- •22.2 Foundations of Forensic Toxicology
- •22.3 Courtroom Testimony
- •22.4.1 Documentation Practices
- •22.4.2 Considerations for Forensic Toxicological Analysis
- •22.4.3 Drug Concentrations and Distribution
- •22.5 Laboratory Analyses
- •22.5.1 Colorimetric Screening Tests
- •22.5.2 Thermal Desorption
- •22.5.6 Enzymatic Immunoassay
- •22.6 Analytical Schemes for Toxicant Detection
- •22.7 Clinical Toxicology
- •22.7.1 History Taking
- •22.7.2 Basic Operating Rules in the Treatment of Toxicosis
- •22.7.3 Approaches to Selected Toxicoses
- •Suggested Reading
- •23 Prevention of Toxicity
- •23.1 Introduction
- •23.2 Legislation and Regulation
- •23.2.1 Federal Government
- •23.2.2 State Governments
- •23.2.3 Legislation and Regulation in Other Countries
- •23.3 Prevention in Different Environments
- •23.3.1 Home
- •23.3.2 Workplace
- •23.3.3 Pollution of Air, Water, and Land
- •23.4 Education
- •Suggested Reading
- •24 Human Health Risk Assessment
- •24.1 Introduction
- •24.2 Risk Assessment Methods
- •24.2.2 Exposure Assessment
- •24.2.3 Dose Response and Risk Characterization
- •24.3 Noncancer Risk Assessment
- •24.3.1 Default Uncertainty and Modifying Factors
- •24.3.2 Derivation of Developmental Toxicant RfD
- •24.3.3 Determination of RfD and RfC of Naphthalene with the NOAEL Approach
- •24.3.4 Benchmark Dose Approach
- •24.3.5 Determination of BMD and BMDL for ETU
- •24.3.6 Quantifying Risk for Noncarcinogenic Effects: Hazard Quotient
- •24.3.7 Chemical Mixtures
- •24.4 Cancer Risk Assessment
- •24.5 PBPK Modeling
- •Suggested Reading
- •25 Analytical Methods in Toxicology
- •25.1 Introduction
- •25.2 Chemical and Physical Methods
- •25.2.1 Sampling
- •25.2.2 Experimental Studies
- •25.2.3 Forensic Studies
- •25.2.4 Sample Preparation
- •25.2.6 Spectroscopy
- •25.2.7 Other Analytical Methods
- •Suggested Reading
- •26 Basics of Environmental Toxicology
- •26.1 Introduction
- •26.2 Environmental Persistence
- •26.2.1 Abiotic Degradation
- •26.2.2 Biotic Degradation
- •26.2.3 Nondegradative Elimination Processes
- •26.3 Bioaccumulation
- •26.4 Toxicity
- •26.4.1 Acute Toxicity
- •26.4.2 Mechanisms of Acute Toxicity
- •26.4.3 Chronic Toxicity
- •26.4.5 Abiotic and Biotic Interactions
- •26.5 Conclusion
- •Suggested Reading
- •27.1 Introduction
- •27.2 Sources of Toxicants to the Environment
- •27.3 Transport Processes
- •27.3.1 Advection
- •27.3.2 Diffusion
- •27.4 Equilibrium Partitioning
- •27.5 Transformation Processes
- •27.5.1 Reversible Reactions
- •27.5.2 Irreversible Reactions
- •27.6 Environmental Fate Models
- •Suggested Reading
- •28 Environmental Risk Assessment
- •28.1 Introduction
- •28.2 Formulating the Problem
- •28.2.1 Selecting Assessment End Points
- •28.2.2 Developing Conceptual Models
- •28.2.3 Selecting Measures
- •28.3 Analyzing Exposure and Effects Information
- •28.3.1 Characterizing Exposure
- •28.3.2 Characterizing Ecological Effects
- •28.4 Characterizing Risk
- •28.4.1 Estimating Risk
- •28.4.2 Describing Risk
- •28.5 Managing Risk
- •Suggested Reading
- •29 Future Considerations for Environmental and Human Health
- •29.1 Introduction
- •29.2 Risk Management
- •29.3 Risk Assessment
- •29.4 Hazard and Exposure Assessment
- •29.5 In vivo Toxicity
- •29.6 In vitro Toxicity
- •29.7 Biochemical and Molecular Toxicology
- •29.8 Development of Selective Toxicants
- •Glossary
- •Index
470 BASICS OF ENVIRONMENTAL TOXICOLOGY
Table 26.3 Measured and Predicted Bioaccumulation Factors in Fish of Chemicals That Differ in Susceptibility to Biotransformation
|
Susceptibility to |
|
Bioaccumulation Factor |
|
|
|
|
|
|
Chemical |
Biotransformation |
Predicted |
Measured |
|
|
|
|
|
|
Chlordane |
Low |
47,900 |
38,000 |
|
PCB |
Low |
36,300 |
42,600 |
|
Mirex |
Low |
21,900 |
18,200 |
|
Pentachloro-phenol |
High |
4,900 |
780 |
|
Tris(2,3-dibromo-propyl)phosphate |
High |
4,570 |
3 |
Source: Predicted bioaccumulation factors were based upon their relative lipophilicity as described by, D. Mackay, Environ. Sci. Technol. 1982, 16: 274–278.
which a chemical bioaccumulates is dictated by the concentration present in the environment. Contaminants that are readily eliminated from the environment will generally not be available to bioaccumulate. An exception would be instances where the contaminant is continuously introduced into the environment (i.e., receiving water of an effluent discharge).
As discussed above, lipophilicity is a major determinant of the bioaccumulation potential of a chemical. However, lipophilic chemicals also have greater propensity to sorb to sediments, thus rendering them less available to bioaccumulate. For example, sorption of benzo[a]pyrene to humic acids reduced its propensity to bioaccumulate in sunfish by a factor of three. Fish from oligotrophic lakes, having low suspended solid levels, have been shown to accumulate more DDT than fish from eutrophic lakes that have high suspended solid contents.
Once absorbed by the organism, the fate of the contaminant will influence its bioaccumulation. Chemicals that are readily biotransformed (Chapter 7) are rendered more water soluble and less lipid soluble. The biotransformed chemical is thus less likely to be sequestered in lipid compartments and more likely to be eliminated from the body. As depicted in Table 26.3, chemicals that are susceptible to biotransformation, bioaccumulate much less than would be predicted based on lipophilicity. Conjugation of xenobiotics to glutathione and glucuronic acid (Chapter 7) can target the xenobiotic for biliary elimination through active transport processes thus greatly increasing the rate of elimination (Chapter 10). Differences in chemical elimination rates contribute to species differences in bioaccumulation.
26.4TOXICITY
26.4.1Acute Toxicity
Acute toxicity is defined as toxicity elicited as a result of short-term exposure to a toxicant. Incidences of acute toxicity in the environment are commonly associated with accident (i.e., derailment of a train resulting in leakage of a chemical into a river) or imprudent use of the chemical (i.e., aerial drift of a pesticide to nontarget areas). Discharge limits placed upon industrial and municipal wastes, when adhered to, have been generally successful in protecting against acute toxicity to organisms in waste-receiving areas. As discussed in Chapter 11, the acute toxicity of a chemical is commonly quantified as the LC50 or LD50. These measures do not provide any insight
TOXICITY |
471 |
Table 26.4 Ranking Scheme for Assessing the Acute Toxicity of Chemicals to Fish and Wildlife
Fish LC50 |
Avian/Mammalian |
|
Example |
|
(mg/L) |
LD50 (mg/kg) |
Toxicity Rank |
Contaminant |
|
|
|
|
|
|
>100 |
>5000 |
Relatively nontoxic |
Barium |
|
10–100 |
500 |
–5000 |
Moderately toxic |
Cadmium |
1–10 |
50 |
–500 |
Very toxic |
1,4-Dichlorobenzene |
<1 |
<50 |
Extremely toxic |
Aldrin |
|
|
|
|
|
|
into the environmentally acceptable levels of contaminants (a concentration that kills 50% of the exposed organisms is hardly tolerable). However, LC50 and LD50 values do provide statistically sound, reproducible measures of the relative acute toxicity of chemicals. LC50 and LD50 ranges for aquatic and terrestrial wildlife, respectively, and their interpretation are presented in Table 26.4.
Acute toxicity of environmental chemicals is determined experimentally with select species that serve as representatives of particular levels of trophic organization within an ecosystem (i.e., mammal, bird, fish, invertebrate, vascular plant, algae). For example, the US Environmental Protection Agency requires acute toxicity tests with representatives of at least eight different species of freshwater and marine organisms (16 tests) that include fish, invertebrates, and plants when establishing water quality criteria for a chemical. Attempts are often made to rank classes of organisms by toxicant sensitivity; however, no organism is consistently more or less susceptible to the acute toxicity of chemicals. Further the use of standard species in toxicity assessment presumes that these species are “representative” of the sensitivity of other members of that level of ecological organization. Such presumptions are often incorrect.
26.4.2Mechanisms of Acute Toxicity
Environmental chemicals can elicit acute toxicity by many mechanisms. Provided below are example mechanisms that are particularly relevant to the types of chemicals that are more commonly responsible for acute toxicity in the environment at the present time.
Cholinesterase Inhibition. The inhibition of cholinesterase activity is characteristic of acute toxicity associated with organophosphate and carbamate pesticides (see Chapter 11 for more detail on cholinesterase inhibition). Forty to 80% inhibition of brain cholinesterase activity is typically reported in lethally poisoned fish. Acute toxicity resulting from cholinesterase inhibition is relatively common among incidents of acute poisoning of fish and birds due to the high volume usage of organophosphates and carbamates in applications such as lawn care, agriculture, and golf course maintenance. Cholinesterase inhibition in fish may occur following heavy rains in aquatic habitats adjacent to areas treated with the pesticides and subject to runoff from these areas. Acute toxicity to birds commonly occurs in birds that feed in areas following application of the pesticides.
472 BASICS OF ENVIRONMENTAL TOXICOLOGY
Narcosis. A common means by which industrial chemicals elicit acute toxicity, particularly to aquatic organisms, is through narcosis. Narcosis occurs when a chemical accumulates in cellular membranes interfering with the normal function of the membranes. Typical responses to the narcosis are decreased activity, reduced reaction to external stimuli, and increased pigmentation (in fish). The effects are reversible, and nonmoribund organisms typically return to normal activity once the chemical is removed from the organism’s environment. Prolonged narcosis can result in death. Approximately 60% of industrial chemicals that enter the aquatic environment elicit acute toxicity through narcosis. Chemicals that elicit toxicity via narcosis typically do not elicit toxicity at specific target sites and are sufficiently lipophilic to accumulate in the lipid phase or the lipid-aqueous interface of membranes to sufficient levels to disrupt membrane function. Chemicals that induce narcosis include alcohols, ketones, benzenes, ethers, and aldehydes.
Physical Effects. Perhaps most graphic among recent incidents of environmental acute toxicity is the physical effects of petroleum following oil spills. Slicks of oil on the surface of contaminated waters results in the coating of animals, such as birds and marine mammals, that frequent the air-water interface. Such a spill of unprecedented magnitude and consequence in the United States occurred on March 24, 1989, when the hull of the Exxon Valdez was ruptured on Bligh Reef in Prince William Sound, Alaska. Nearly 11 million gallons of crude oil spilled onto the nearshore waters killing more wildlife than any prior oil spill in history. Thousands of sea birds and mammals succumbed to the acute effects of the oil.
Hypothermia is considered a major cause of death of oiled marine birds and mammals. These organisms insulate themselves from the frigid waters by maintaining a layer of air among the spaces within their coat of fur or feathers. The oil penetrates the fur/feather barrier and purges the insulating air. As a result the animals rapidly succumb to hypothermia. In addition to hypothermia, these animals can also experience oil toxicosis. Inhalation of oil, as well as ingestion through feeding and preening, can result in the accumulation of hydrocarbons to toxic levels. Toxicity to sea otters has been correlated to degree of oiling and is characterized by pulmonary emphysema (bubbles of air within the connective tissue of the lungs), gastric hemorrhages, and liver damage.
26.4.3Chronic Toxicity
Chronic toxicity is defined as toxicity elicited as a result of long-term exposure to a toxicant. Sublethal end points are generally associated with chronic toxicity. These include reproductive, immune, endocrine, and developmental dysfunction. However, chronic exposure also can result in direct mortality not observed during acute exposure. For example, chronic exposure of highly lipophilic chemicals can result in the eventual bioaccumulation of the chemical to concentrations that are lethal to the organisms. Or as discussed previously, mobilization of lipophilic toxicants from lipid compartments during reproduction may result in lethality. It is important to recognize that, while theoretically, all chemicals elicit acute toxicity at a sufficiently high dose, all chemicals are not chronically toxic. Chronic toxicity is measured by end points such as the highest level of the chemical that does not elicit toxicity during continuous, prolonged exposure (no observed effect level, NOEL), the lowest level of the chemical that elicits
TOXICITY 473
Table 26.5 Acute and Chronic Toxicity of Pesticides Measured from Laboratory Exposures of Fish Species
|
LC50 |
Acute |
Chronic |
|
|
Pesticide |
(µg/L) |
Toxicity |
Value (µg/L) |
ACR |
Chronic Toxicity |
|
|
|
|
|
|
Endosulfan |
166 |
Extremely toxic |
4.3 |
39 |
Yes |
Chlordecone |
10 |
Extremely toxic |
0.3 |
33 |
Yes |
Malathion |
3,000 |
Very toxic |
340 |
8.8 |
No |
Carbaryl |
15,000 |
Moderately |
378 |
40 |
Yes |
|
|
toxic |
|
|
|
|
|
|
|
|
|
toxicity during continuous, prolonged exposure (lowest observed effect level, LOEL), or the chronic value (CV) which is the geometric mean of the NOEL and the LOEL. Chronic toxicity of a chemical is often judged by the acute : chronic ratio (ACR), which is calculated by dividing the acute LC50 value by the CV. Chemicals that have an ACR of less than 10 typically have low to no chronic toxicity associated with them (Table 26.5).
The following must always be considered when assessing the chronic toxicity of a chemical: (1) Simple numerical interpretations of chronic toxicity based on ACRs serve only as gross indicators of the potential chronic toxicity of the chemical. Laboratory exposures designed to establish chronic values most often focus upon a few general endpoints such as survival, growth, and reproductive capacity. Examination of more subtle end points of chronic toxicity may reveal significantly different chronic values. (2) Laboratory exposures are conducted with a few test species that are amenable to laboratory manipulation. The establishment of chronic and ACR values with these species should not be considered absolute. Toxicants may elicit chronic toxicity in some species and not in others. (3) Interactions among abiotic and biotic components of the environment may contribute to the chronic toxicity of chemicals, while such interactions may not occur in laboratory assessments of direct chemical toxicity. These considerations are exemplified in the following incidence of chronic toxicity of chemicals in the environment.
26.4.4Species-Specific Chronic Toxicity
Tributyltin-Induced Imposex in Neogastropods. Scientists noted in the early 1970s that dogwhelks inhabiting the coast of England exhibited a hermaphroditic-like condition whereby females possessed a penis in addition to normal female genitalia. While hermaphroditism is a reproductive strategy utilized by some molluscan species, dogwhelks are dioecious. This pseudohermaphroditic condition, called imposex, has since been documented worldwide in over 140 species of neogastropods. Imposex has been implicated in reduced fecundity of neogastropod populations, population declines, and local extinction of affected populations.
The observation that imposex occurred primarily in marinas suggested causality with some contaminant originating from such facilities. Field experiments demonstrated that neogastropods transferred from pristine sites to marinas often developed imposex. Laboratory studies eventually implicated tributyltin, a biocide used in marine paints, as the cause of imposex. Tributyltin is toxic to most marine species evaluated in the
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