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What is toxicology?. toxic compounds. physical agents. `. toxicology … ……… • Is the study of the harmful effects of chemicals and physical agents on living organisms • Examines adverse effects ranging from acute to long-term

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  • toxicology …………
    • • Is the study of the harmful effects of chemicals and physical agents on living organisms
    • • Examines adverse effects ranging from acute to long-term
    • • Is used to assess the probability of hazards caused by adverse effects
    • • Is used to predict effects on individuals, populations and ecosystems

These adverse effects may occur in many forms, ranging from immediate death to subtle changes not realized until months or years later.

They may occur at various levels within the body, such as an organ, a type of cell, or a specific biochemical.


Sources of toxic compounds

  • Synthetic organic compound
  • 1. Air, water, and food pollutants
    • Air-CO, oxides of nitrogen, oxides of sulfur, hydrocarbons and particulates
    • Water-agricultural chemicals including pesticides, herbicides, fugicides, nematocides, rodenticides, fertilizer
    • Halogenated hydrocarbons- chloroform, dichloroethane, tetrachloride
    • Clorinated aromatics-PCB, TCDD
    • Detergents-alkyl benzene sulfonates

2. Chemical additives in food

As preservatives-antibacterial, antifungal, or antioxidant

To change physical characteristics, taste, color, odor


3.Chemicals in work placeInorganics-metals and flurides, CO, etc.Organic compounds-aliphatic hydrocarbons (hexene)

aromatic hydrocarbons (eg. benzene, toluene)

halogenated hydrocarbons




amino compounds

4. Drugs of abuseCNS depressants-ethanol,secobarbitalCNS stimulants-cocaine, methamphatamine,

nicotine, caffeine

Opioids-heroin, morphine

Hallucinogens-PCP, LSD,THC


5.Therapeutic drugs

The danger to the individual depends on :

the nature of the toxic response

the dose necessary to produce the toxic response

the relationship between the therapeutic dose and the toxic dose

eg, anticancer drugs are carcinogens

Diethylstilbestrol (DES)


Chloroquinol-SMON-subacute myelo-optic neuropathy

Methyldopa, chloropromazine, methotrexate

In general, toxic side effects are not common and may occur only in susceptible individuals or populations.


6. Pesticides

7. Solvents

8. Polycyclic aromatic hydrocarbons (PAH)多環芳香

Incomplete combustion of organic materials, in smoke from wood, coal, oil, tobacco, in tar and broiled foods


9. Cosmetics

Allergic reactions and contact dermatitis

Bromate, cold-wave neutralizer

Thioglycolates and tioglycerol-cold-wave lotion and depilatories

Sodium hydroxide-hair straighteners


Naturally occurring toxins

  • 1. Mycotoxins
  • 2. Microbial toxins
  • 3. Plant toxins
  • 4. Animal toxins
  • "Toxin"=refers to toxic substances that are produced naturally
  • "Toxicant"=substance that is produced by anthropogenic origin

An interdisciplinary field…………

Descriptive Toxicology: The science of toxicity testing to provide information for safety

evaluation and regulatory requirements.

Mechanistic Toxicology: Identification and understanding cellular, biochemical and

molecular basis by which chemicals exert toxic effects.

Regulatory Toxicology: Determination of risk based on descriptive and mechanistic

studies, and developing safety regulations.


Clinical Toxicology: Diagnosis and treatment of poisoning; evaluation of methods of detection and intoxication, mechanism of action in humans (human tox, pharmaceutical tox) and animals (veterinary tox). Integrates toxicology, clinical medicine, clinical biochemistry/pharmacology.


Occupational Toxicology: Combines occupational medicine and occupational hygeine.

Environmental Toxicology: Integrates toxicology with sub-disciplines such as ecology,

wildlife and aquatic biology, environmental chemistry.


The scope of toxicity

  • Mechanisms of Toxic Action
    • 1. Biochemical toxicology
    • 2. Behavioral toxicology-behavior is the final integrated expression of nervous function
    • 3. Nutritional toxicology-the effects of diet
    • 4. Carcinogenesis-cell growth
    • 5. Teratogenesis-developmental process
    • 6. Mutagenesis-genetic material
    • 7. Organ toxicity-organ function

B. Measurement of toxicants and toxicity

1. Analytic toxicology

2. Toxicity testing

3. Toxicologic pathology

4. Structure-activity study

5. Biomathematics and statistics

6. Epidemiology

C. Applied Toxicology

1. Clinical toxicology

2. Veterinary toxicology

3. Forensic toxicology

4. Environmental toxicology

5. Industrial toxicology


D. Chemical use classes

  • 1. Agricuture chemicals
  • 2. Clinical drugs
  • 3. Drugs of abuse
  • 4. Food additives
  • 5. Industrial chemicals
  • 6. Naturally occurring substances- phytotoxin, mycotoxin, inorganic minerals
  • 7. Combustion products

E. Regulatory Toxicology

1. Legal aspects-formulation of laws and regulations and their enforcement

2. Risk assessment-

the definition of risks, potential risks and risk- benefit equations

F. Development of antidotes


Toxic Effects

  • Immediate effect and delayed effect
  • CO, cyanide
  • 2. Local effect and systemic effect
  • target organ
  • 3. Reversible and irreversible effect
  • 4. Anaphylactic reaction (allergic reaction)
  • 5. Idiosyncratic reaction 特異體質反應

DoseDose by definition is the amount of a substance administered at one time.

However, other parameters are needed to characterize the exposure to xenobiotics.  The most important are the number of doses, frequency, and total time period of the treatment.For example:

650 mg Tylenol as a single dose

500 mg Penicillin every 8 hours for 10 days

10 mg DDT per day for 90 days


A common dose measurement is mg/kg body weight.

The commonly used time unit is one day and thus, the usual dosage unit is mg/kg/day.

Environmental exposure units are expressed as the amount of a xenobiotic in a unit of the media. mg/liter (mg/l) for liquids

mg/gram (mg/g) for solids

mg/cubic meter (mg/m3) for airOther commonly used dose units for substances in media are parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt).


Fractionatinga total dose usually decreases the probability that the total dose will cause toxicity.  The reason for this is that the body often can repair the effect of each subtoxic dose if sufficient time passes before receiving the next dose.  In such a case, the total dose, harmful if received all at once, is non-toxic when administered over a period of time.  For example, 30 mg of strychnine swallowed at one time could be fatal to an adult whereas 3 mg of strychnine swallowed each day for ten days would not be fatal.


vinyl chloride,

high dose-hepatotoxicant

long latent period at lower doses-carcinogen

very low dose-no effect


chronic use-deleterious effects on the gastric mucosa

fatal dose 0.2-0.5 g/kg


dietary essentials eg. Iron, copper, magnesium, cobalt, manganese, and zinc

toxic at higher dose


"All substances are poisons; there is none which is not a poison.  The right dose differentiates a poison and a remedy.“

Paracelsus (1493-1541)


Knowledge of the dose-response relationship:

establishes causality that the chemical has in fact induced the observed effects

establishes the lowest dose where an induced effect occurs - the threshold effect

determines the rate at which injury builds up - the slope for the dose response.


Dose Response

Individual, or graded, dose-response relationship

results from an alteration of a specific biochemical process

Quantal dose-response relationship

in a population-”all or none”

determination of the LD50

LD (lethal dose)50-the dose required to kill 50% of a population of an organism under stated conditions


Normal equivalent deviations(NEDs)

NED for 50% response is 0

NED for 84.1% response is 1

Probit (probability unit)=NED+5


LD50 is used to:

  • classify substances or products for regulatory purposes including safe transportation and labeling,
    • B) provide information for treatment of acute intoxications
    • C) standardize certain biological products,
    • D) set dose levels for subsequent toxicity studies
    • E) provide comparative information on the chemical's dose response curve
    • F) provide data for evaluation and validation of alternative test methods.

Classical LD50

  • The Classical LD50 test is used to determine the lethal dose (LD50) of a substance that will kill 50% of test animals. Typically, this method can use 100 or more animals. The test material is administered in increasing doses, usually 5 or more, to groups of 10 male and 10 female animals. Mortalities are recorded within a given period, and the LD50 is determined with the aid of statistical calculations.

2. Limit Test

  • Acute toxicity test in which, if no ill-effects occur at a pre-selected maximum dose, no further testing at greater exposure levels is required.
  • Five to ten animals of each sex or 10 animals of the susceptible sex are administered a dose specified by regulations. Toxic responses occurring within a given period are recorded. Based on the results, a regulatory action or additional testing may be required.

The LD50 tests have become controversial among toxicologists, animal welfare organizations, legislators and the public primarily due to the ethics of using a large number of animals and evaluating only mortality.



  • Food and Drug Administration:
  • Does not require the use of the Classical LD50 test.
  • Accepts alternatives.
  • Refers to the Limit test.

Organization for Economic Cooperation and Development:

  • Discourages the use of Classical LD50 test.
  • Recommends the Limit test (2 g/kg dose).
  • When compound related mortality occurs in the limit test, then 5 animals per dose, at least 3 dose levels are used to produce a range of toxic effects and mortality rates; clinical observations and pathological investigations are conducted.
  • A fixed dose procedure, which uses morbidity instead of mortality as the end point, is also recommended.

British Toxicology Society:

    • The LD50 should only be determined with any accuracy where scientifically and ethically justified. Such cases are relatively rare.
    • Examination of few animals in detail rather than many for statistical purposes.
    • Limit tests could be used, provided animals in distress are killed humanely, if this would not interfere with the objectives.
    • For classification of substances and preparations, a fixed-dose procedure targeted to acute signs could replace the current practice of LD50 determination.

Descriptive animal toxicity tests

  • Two main principle
    • The effects produced by a compound in laboratory animals, when properly qualified, are applicable to humans.
    • The exposure of experimental animals to toxic agents in high dose is necessary and valid method of discovering possible hazards in human.

Descriptive animal toxicity tests

  • Acute Single dose with effects occurring for a short period of time (usually up to 96 hr)
    • Acute lethality (die in a 14-day period)
    • LD50 (Median Lethal Concentration)
    • Skin & eye irritation
    • sensitization
  • Subacute Multiple doses administered for up to 14 days

Subchronic Continuous dosing for up to 90 days

NOAEL-no observed adverse effect level

Chronic Continuous dosing for up to 6 months to 2 years

carcinogenic potential

• Acute effects do not predict chronic effects

• Doses causing chronic effects may not cause acute or sub-acute effects

• Chronic effects of a chemical exposure may manifest themselves as a common disease and go unnoticed


Course Objectives


• mechanisms by which chemicals cause cell injury and cell death

• the mode of action of specific organic and inorganic chemicals

• how to interpret results of in vitro tests for the evaluation of in vivo toxicity

• How multiple chemical exposures and other stressors can alter toxicity


• What factors influence individual susceptibility

• the importance of dose in determining adverse effects

of chemicals

• what factors influence the target organ dose of a chemical

• mechanisms by which chemicals affect specific organ system functions