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Introduction to Toxicology

A Brief Introduction to the Concepts of Toxicology We Will Be Using Throughout This Course

Units Used to Measure Chemicals in the Environment

  • PPM – Parts per million
  • PPB – Parts per billion
  • PPT – Parts per trillion

A simple cube 1 cubic meter in volume

Is formed of 1,000,000 cubes, 1 cubic centimeter each

One part per million is

  • 1 inch in 16 miles
  • 1 minute in two years
  • 1 cent in $10,000
  • 1 ounce of salt in 31 tons of potato chips
  • 1 bad apple in 2,000 barrels of apples

Divide each 1cc block into 1,000 blocks 0.1cm on a side

One part per billion is

  • 1 inch in 16,000 miles
  • 1 second in 32 years
  • 1 cent in $10,000,000
  • 1 pinch of salt in 10 tons of potato chips
  • 1 lob in 1,200,000 tennis matches
  • 1 bad apple in 2,000,000 barrels of apples

And divide each 0.1cc block into 1,000 blocks 0.01cm on a side

One part per trillion is

  • 1 postage stamp in the area of the city of Dallas
  • 1 inch in 16 million miles (more than 600 times around the earth)
  • 1 second in 320 centuries
  • 1 flea on 360 million elephants
  • 1 grain of sugar in an Olympic sized pool
  • 1 bad apple in 2 billion barrels

Important Concept

  • On the left side of the decimal point (ß. )
    • 1 trillion is bigger than 1 billion
    • 1 billion is bigger than 1 million
    • 1 million is bigger than 1 thousand
  • On the right side of the decimal point ( . à)
    • 1 part per trillion is smaller than 1 part per billion
    • 1 part per billion is smaller than 1 part per million
    • 1 part per million is smaller than 1 part per thousand…

Important Relationship

For water at STP (standard temperature [23oC] and pressure [15 psi])

1 cc = 1ml = 1g

Which means that

1 liter of water = 1 kg.

1 mg / kg = 1 ppm.

1mm3 / liter = 1 ppm.

1 mg / liter = 1 ppm.

Measures of Toxicity

  • Toxicity of chemicals is determined in the laboratory
  • The normal procedure is to expose test animals
    • By ingestion, application to the skin, by inhalation, gavage, or some other method which introduces the material into the body, or
    • By placing the test material in the water or air of the test animals’ environment
  • Toxicity is measured as clinical "endpoints" which include
    • Mortality (death)
    • Teratogenicity (ability to cause birth defects)
    • Carcinogenicity (ability to cause cancer), and,
    • Mutagenicity (ability to cause heritible change in the DNA)
  • At this time we will discuss 2 measures of mortality – the LD50 and the LC50.

Measures of Toxicity: The Median Lethal Dose

  • LD50 The amount (dose) of a chemical which produces death in 50% of a population of test
               animals to which it is administered by any of a variety of methods
  • mg/kg – Normally expressed as milligrams of substance per kilogram of animal body weight

Measures of Toxicity: The Median Lethal Concentration

  • LC50 The concentration of a chemical in an environment (generally air or water) which produces
               death in 50% of an exposed population of test animals in a specified time frame
  • mg/L – Normally expressed as milligrams of substance per liter of air or water (or as ppm)

Primary Routes of Exposure to Pesticides

There are three primary routes by which organisms are exposed to pesticides.

  • Oral
  • Dermal
  • Inhalation

Primary Routes of Exposure: Oral Exposure

  • Any exposure to pesticide which occurs when the chemical is taken in through the mouth and passes through the gastrointestinal tract
  • During oral exposure, although carried within the body, the pesticide is still outside of the body cavity

Primary Routes of Exposure: Dermal Exposure

  • Exposure of the skin to a pesticide
  • Most common route of human exposure
  • With proper hygiene this type of exposure is generally not serious unless there is a specific, rapid toxicological effect (often eye effects) which is of concern

Primary Routes of Exposure: Inhalation Exposure

  • Occurs when a pesticide is breathed into the lungs through the nose or mouth
  • Significant route of exposure for aquatic organisms
  • Not of toxicological concern until it crosses from the lung into the body (unless the chemical is corrosive)

Duration of Exposure

Three terms are commonly used to describe the duration of dose(s)

  • Acute
  • Chronic
  • Subchronic

Duration of Exposure: Acute Exposure

  • Application of a single or short-term (generally less than a day) dosing by a chemical
  • If toxic symptoms are expressed, they are referred to as symptoms of "acute toxicity"

Duration of Exposure: Chronic Exposure

  • Expression of toxic symptoms only after repeated exposure to a chemical in doses regularly applied to the organism for a time greater than half of its life-expectancy
  • If toxic symptoms are expressed, they are referred to as symptoms of "chronic toxicity"

Duration of Exposure: Subchronic Exposure

  • Toxic symptoms are expressed after repeated applications for a timeframe less than half the life expectancy of the organism – but more often than a single dose or multiple doses applied for only a short time
  • If toxic symptoms are expressed, they are referred to as symptoms of "subchronic toxicity"

Remember –

  • For pesticides – less is more when dealing with toxicity
  • The less you need to cause a toxic effect – the more toxic the substance is
  • Thus an LD50 of 25 mg/kg is more toxic than is one of 7,000 mg/kg

Words again

Low Risk

Signal Words

The relative acute toxicity of a pesticide is reflected on the label in the form of a "signal word." The (toxicologically) appropriate signal word MUST appear on every pesticide label. The three possible signal words are:

  • Caution
  • Warning
  • Danger

Signal Words: Caution

  • "Caution" reflects the lowest degree of relative toxicity
  • All pesticides with an LD50 of greater than 500 mg/kg must display this word on their label
  • Actually includes two groups of pesticides – those classed by the EPA as "Relatively nontoxic (>5,000 mg/kg) and those classed as "slightly toxic" (500 – 5,000 mg/kg)

Signal Words: Warning

  • "Warning" reflects an intermediate degree of relative toxicity
  • All pesticides with an LD50 of greater than 50 and less than 500 mg/kg must display this word on their label
  • Pesticides in this category are classed as "moderately toxic"

Signal Words: Danger

  • "Danger" reflects the highest degree of relative toxicity
  • All pesticides with an LD50 of less than 50 mg/kg must display this word on their label
  • Pesticides here are classed as "highly toxic"


  • Legally defined term – not just anything you don’t like
  • Any pesticide with an LD50 of 50 mg/kg or less
  • Labels must reflect this classification
  • Label must have the signal word "DANGER" plus the word "POISON"
  • Label also must display the skull and crossbones icon

Relative toxicity

  • Organisms can’t differentiate between "natural" and "synthetic" chemicals
  • "Synthetic" does not mean toxic or poisonous
  • "Natural" does not mean safe or even low risk
  • Chemicals must be evaluated in their biological context of behavior in organisms
  • Mode of action, not source, is the concern of toxicologists and informed users of pesticides
  • Most herbicides act on biological pathways not present in humans
  • Those approved for use in the Region (much more about these later!) have LD50s of 50 mg/kg or greater – they have intermediate or relatively low toxicity
  • Some of the insecticides used are highly toxic
  • Some examples of pesticides and other chemicals are given to show relative risk of pesticides in the environment in which we live
  • This is NOT to trivialize the pesticides
  • Always treat them with caution and respect
  • But, have a realistic recognition of their relative risk in a world of risks

Relative toxicity: Insecticides

  • TCDD (Dioxin) – 0.1 mg/kg
  • Parathion – 13.0 mg/kg
  • Nicotine – 50.0 mg/kg
  • Carbaryl – 270.0
  • Malathion – 370.0 mg/kg

Relative Toxicity: Herbicides and Additives

  • Paraquat – 95mg/kg
  • 2,4-D – 375mg/kg
  • 2,4-DP – 532mg/kg
  • Triclopyr – 630mg/kg
  • Tebuthiuron – 644mg/kg
  • Dicamba – 757mg/kg
  • Hexazinone – 1,690mg/kg
  • Glyphosate – 4,320mg/kg
  • Limonene – 5,000mg/kg
  • Clopyralid – >5,000mg/kg
  • Sulfometuron Met.. – >5,000mg/kg
  • Imazapyr – >5,000mg/kg
  • Diesel oil – 7,380mg/kg
  • Picloram – 8,200mg/kg
  • Fosamine am.. – 24,400mg/kg
  • Kerosene – 28,000mg/kg

Relative Toxicity: Comparative information

Highly toxic chemicals – 0 – 50 mg/kg range (taste 1 teaspoonful)

  • Botulinus toxin – 0.00001mg/kg
  • Dioxin – 0.1mg/kg
  • Parathion – 13.0mg/kg
  • Strychnine – 30.0mg/kg
  • Nicotine – 50.0mg/kg

Moderately toxic chemicals – 50 - 500 mg/kg range (teaspoonful 1 ounce)

  • Paraquat – 95mg/kg
  • Caffeine – 200mg/kg
  • Carbaryl – 270mg/kg
  • Malathion – 370mg/kg
  • 2,4-D – 375mg/kg

Slightly toxic chemicals – 500 – 5,000 mg/kg range (1 ounce 1 pint)

  • 2,4-DP – 532mg/kg
  • Triclpoyr – 630mg/kg
  • Tebuthiuron – 644mg/kg
  • Dicamba – 757mg/kg
  • Formaldehyde – 800mg/kg
  • Hexazinone – 1,690mg/kg
  • Asprin – 1,700mg/kg
  • Vitamin B3 – 1,700mg/kg
  • Household bleach – 2,000mg/kg
  • Table salt – 3,750mg/kg
  • Glyphosate – 4,320mg/kg

Relative Toxicity: Are all substances toxic?


  • All are toxic to some quantifiable degree
  • Sugar has an LD50 of 30,000 mg/kg
  • The foresters favorite – ethanol has an LD50 of only 13,700 mg/kg
  • Even water has a recognized LD50 of slightly greater than 80,000 mg/kg

Relative Toxicity: The Last Word

  • Pesticides are chemicals introduced into the environment to perform a function
  • The source of a chemical (synthetic vs. natural) is irrelevant when considering its toxicity
  • Pesticides should be treated with care and proper respect – but so should household cleaners, gasoline and kerosene, bleaches,paints and all other chemicals

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Last updated on Thursday, November 07, 2002 at 01:14 PM
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