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Toxicity of Pesticides

Toxicity of Pesticides

A pesticide is any substance used to control pests. Pests may be target insects, vegetation, fungi, etc. Most control the pests by poisoning them. Unfortunately, pesticides can be poisonous to humans as well. Some are very poisonous, or toxic, and may seriously injure or even kill humans. Others are relatively non-toxic. Pesticides can irritate the skin, eyes, nose, or mouth. The most important thing to remember is that you should always use caution whenever you work with any pesticide!

Goals of This Module

  • Understand what toxicity is and how it affects humans.
  • Learn the three routes of entry (how pesticides enter the body) and the importance of each.
  • Be familiar with how toxicity is measured and what is meant by label warning statements.

Toxicity: What is it?

The toxicity of a substance is its capacity to cause injury to a living system. A living system can be many things: a human body, or parts of the body (such as the lungs or the respiratory system); a pond, a forest and those creatures that live there. Toxicity represents the kind and extent of damage that can be done by a chemical. In other words, if you know the toxicity of a pesticide, you know "how poisonous" it is.
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Dose-Time Relationship

The effect of a pesticide, or any substance for that matter, is dependent on a number of factors. The most important factor is the dose-time relationship. Dose is the quantity of a substance that a surface, plant, or animal is exposed to. Time means how often the exposure occurs. Thus, the dose- time relationship is how much of the substance is involved and how often the exposure to the substance occurs. This relationship gives rise to two different types of toxicity that pesticide applicators must know and understand. They are acute and chronic toxicity.
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Kinds of Toxicity

Acute vs.Chronic Acute toxicity refers to how poisonous a pesticide is to a human, animal, or plant after a single short-term exposure. Acute toxicity is used to describe effects which appear promptly, or within 24 hours of exposure. A pesticide with a high acute toxicity is deadly even when a very small amount is absorbed. Acute toxicity levels are used as a way to assess and compare how poisonous pesticides are. The acute toxicity of a pesticide is used as the basis for the warning statements on the label. Acute toxicity may be measured as acute oral toxicity, acute dermal toxicity, and acute inhalation toxicity.

Chronic toxicity is the delayed poisonous effect from exposure to a substance. Chronic toxicity of pesticides concerns the general public, as well as those working directly with pesticides because of potential exposure to pesticides on/in food products, water, and the air. It is measured in experimental conditions after three months of either continuous or occasional exposure.

A material that has high acute toxicity does not necessarily have high chronic toxicity. Nor does a chemical with low acute toxicity necessarily have low chronic toxicity. For many pesticides, the toxic effects following single acute exposures are quite different from those produced by chronic exposure. If, for example, large amounts of the pesticide cryolite are eaten by rats at one time little or no harmful effects will be observed. It quickly passes through the intestinal tract and is eliminated without harmful effects. However, if rats are fed small amounts of cryolite every day in their feed, they become ill and die. Cryolite is a very insoluble compound, meaning that it does not readily dissolve. The small amount of chemical that is absorbed from a one-time exposure is not sufficient to cause illness, but absorption of the same small amount every day, day after day, can cause chronic illness and death. The effects of both acute toxicity and chronic toxicity are dose-related; the greater the dose, the greater the effect.

While you cannot change the inherent toxicity of pesticides, you can limit the possibility of poisoning by preventing and/or limiting exposure. In other words, the risk of harm from pesticide exposure is equal to how poisonous the pesticide is, multiplied by the amount and route of exposure to the pesticide, or:


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Types of Pesticide Exposures

A pesticide exposure is defined as coming in contact with a pesticide. There are two types of exposure that may occur, acute and chronic.

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Acute exposure refers to a one-time contact with a pesticide. When experimental animals are exposed to a pesticide to study its acute toxicity, acute exposure is defined as contact for 24 hours or less. Acute effects can be readily detected and more easily studied than chronic effects. Immediate toxic effects are more likely to be produced by those pesticides that are rapidly absorbed.

Chronic exposure refers to a repeated contact with a pesticide. The study of chronic toxicity is accomplished by repeatedly exposing test animals for more than three months. In addition to producing long-term low-level effects, chronic exposure to pesticides may result in immediate, "acute" effects after each exposure. In other words, frequent exposure to a chemical can produce acute and chronic symptoms. The potential for a chronic effect is related to the level and frequency of exposure received.

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Routes of Entry

There are three specific ways in which pesticides may enter your body. You may be poisoned no matter how they enter. Sometimes you can even be poisoned without knowing it, especially if the pesticide enters through the skin or lungs.

Dermal Route

Wet, dry, or gaseous forms of pesticides can be absorbed through the skin. This may occur if pesticides are allowed to get on the skin while mixing or applying, or if pesticide-contaminated clothing is not removed promptly and properly cleaned before being worn again. Oil or paste forms allow greater absorption through the skin than water-based pesticides. Some pesticides do not pass through the skin very readily. Others are quickly absorbed through the skin and can be as dangerous as if they were swallowed. Skin varies in its capacity to act as a barrier to pesticide absorption. The eyes, ear drums, scalp and groin area absorb pesticides more quickly than other areas on the body. Damaged or open skin can be penetrated by a pesticide much more readily than healthy, intact skin. Once they are absorbed through skin, pesticides enter the blood stream and are carried throughout the body.

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Inhalation Route

Whether as dusts, spray mist, or fumes, pesticides can be drawn into your lungs as you breathe. Inhalation of pesticides can occur during the mixing of wettable powders, dusts, or granules. Poisoning can also occur while fumigating or spraying without a self contained breathing apparatus or a proper respirator in enclosed or poorly ventilated areas such as greenhouses, apartments, or grain bins. The largest particles that are inhaled tend to stay on the surface of the throat and nasal passages, and do not enter the lungs. Smaller particles can be inhaled directly into the lungs. The number of particles needed to poison by inhalation depends upon the concentration of the chemical in the particles. Even inhalation of dilute pesticides can result in poisoning. Once they are absorbed through the surfaces of the lungs, chemicals enter the blood stream and are distributed to the rest of the body.
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Oral Route

Pesticides can enter the body through the mouth (also called ingestion). This can occur when hands are not properly washed before eating or smoking. They may be swallowed by mistake, if they are improperly stored in food containers. Ingested materials can be absorbed anywhere along the gastrointestinal tract; the major absorption site is the small intestine. Once absorbed, they eventually enter the blood stream by one of several means, and circulate throughout the body.
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Which Route Is More Important?

You can be poisoned no matter which way pesticides enter your body. While there are few chemicals that are equally poisonous by all routes of entry, some pesticides can enter all three ways and poison you. (For example, parathion is toxic regardless of how it is absorbed).

The dermal and inhalation routes of pesticide entry are likely to be the most important routes of pesticide applicator exposure. It is unlikely that you would purposely eat or drink the chemicals you are using, but you may breathe them in, splash them on your skin, or expose yourself to pesticide "fallout."

Healthy skin can slow the absorption of a pesticide when dermal contact occurs. Liquid pesticides containing solvents and oil based pesticides are absorbed quickly compared to dry pesticides. The applicator must know that damaged skin (chapped, cut, or abraded) has lost its ability to slow the entry of a pesticide into the body.
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The Qualities of the Exposed Individual

The Qualities of the Exposed IndividualThe qualities of the exposed individual influence the toxicity of a pesticide since different individual characteristics will affect how the person responds to a pesticide. Some examples of these individual qualities include:

  • health conditions: heredity, pregnancy, and disease may cause individuals to respond differently.
  • age: youngest and oldest individuals tend to be most sensitive.
  • gender/sex: male and female individuals may respond very differently.
  • environment: exposure to other toxic substances in food, air, water, etc.
  • health behaviors: customs or habits such as smoking, dietary practices, drug use, personal hygiene, etc.
  • body size: the effect of a dose is closely related to body weight. The heavier the individual, the more poison needed to cause an effect.
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    Effects of Toxicity

    In addition to being acute or chronic, toxic effects can be any of the following:

    *Local or systemic (Both effects can occur with some pesticides.)

    Local effects refer to those that take place at the site of contact with a material. Examples of this include: skin inflammation on the hand, in response to hand contact with a pesticide; or irritation of the mucous membrane lining the lungs, due to inhalation of toxic fumes.

    Systemic effects are quite different because they occur away from the original point of contact. Systemic effects may occur when pesticides are distributed throughout the body, or "system". An example of a systemic effect is the blocking of an essential chemical of the nervous system, called "cholinesterase" (pronounced ko-li-nes-ter-ace), upon exposure to some types of pesticides.

    *Immediate or delayed (Both effects can occur with some materials.)

    Immediate toxic effects are those which are experienced upon or shortly after exposure. (For example, a sneezing attack in response to inhaling pesticides during mixing).

    Delayed effects occur after some time has passed. While they may not be obvious, such as long term reproductive effects, delayed effects can result from a single exposure. Tumors may not be observed in chronically exposed people for 20 to 30 years after the original exposure to a cancer-causing or "carcinogenic" chemical.

    *Reversible or irreversible

    Reversible effects are not permanent and can be changed or remedied. Skin rash, nausea, eye irritation, dizziness, etc. are all considered reversible toxic effects. Injury to the liver is usually reversible since this organ has an ability to regenerate itself.

    Irreversible effects are permanent and cannot be changed once they have occurred. Injury to the nervous system is usually irreversible since its cells cannot divide and be replaced. Irreversible effects include birth defects, mutations, and cancer.

    *Additive, antagonistic, or synergistic

    An additive effect is one in which the combined effect of two pesticides is equal to the sum of the effects of each (ie. 2 + 2 = 4.)

    An antagonistic effect occurs when the toxic effect of the combination of pesticides is less than what would be predicted from the individual toxicities. Antagonism is like adding 2 + 2 and getting 3 as the result.

    A synergistic effect occurs when the combined toxic effect of two pesticides is much greater, or worse, than the sum of the effects of each by itself. Synergism is similar to adding 2 + 2 and getting 5 as the result.

    Exposure to pesticides may also result in the following:

    • Reproductive effects: effects on the reproductive system or on the ability to produce healthy offspring.
    • Teratogenic effects: effects on unborn offspring, such as birth defects.
    • Carcinogenic effects: produces cancer in living animal tissues.
    • Oncogenic effects: tumor-forming effects (not necessarily cancerous.)
    • Mutagenic effects: permanent effects on genetic material that can be inherited.
    • Neurotoxicity: poisoning of the nervous system, including the brain.
    • Immunosuppression: blocking of natural responses of the immune system responsible for protecting the body.

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    Measuring Toxicity

    It is quite difficult to figure out the exact toxicity of a pesticide for humans. Animal testing is the primary way we measure toxicity. Many types of animals are used to test pesticide toxicity, including rats, rabbits, mice, guinea pigs and dogs. However, due to some differences between the way our bodies and the bodies of animals work, results of animal tests cannot always be applied or "extrapolated" to humans. In other words, a pesticide may be more or less toxic to humans than to the animals in which it was tested. Similarly, something that appears to be extremely toxic to test animals may not necessarily be poisonous in humans. Toxicity studies are just guidelines for estimating and comparing toxic effects of pesticides. The word "detected" is important when talking about measuring toxic effects. We can only talk about what we can see or observe. The term "No Observable Effect Level", or NOEL, means that at the stated dose, no effects were observed in test animals.

    Acute Toxicity


    To figure out how acutely toxic a pesticide is, scientists give laboratory animals short-term exposure to doses of the pesticide being tested. Experimental doses are given orally, as well as put on the eyes, skin, and in the air that the test animals breathe. The animals are then observed carefully for changes.

    Lethal Dose Fifty (LD50)

    "Lethal Dose Fifty" (LD50) is one way the toxicity of chemicals are measured. LD50 is the amount of a pesticide that has killed half of the animals in a laboratory test. The LD50 is found for both dermal and oral routes of exposure. For example, an acute oral LD50 indicates the amount of pesticide swallowed that has killed half of the animals tested.

    The smaller the LD50 value, the less chemical required to kill half of the test animals, and the more poisonous the pesticide. So, a pesticide with a dermal LD50 of 25 (rabbit) is more poisonous than a pesticide with a dermal LD50 of 2000 (rabbit).

    LD50's do not tell us how a chemical acts, nor do they tell us how sensitive different organs within an animal or human might be. They simply tell us how much of the chemical it takes to kill half of the test animals. LD50's for different chemicals can only be compared if the same test animal was used, and even then it cannot be taken as an indication of the ful toxic potential of either chemical.

    Milligrams per kilogram (mg/kg)

    Pesticide LD50 values are measured in units of weight called "milligrams" per "kilogram" (mg/kg). A single paper clip weighs about one gram. Cutting the clip into 1000 equal parts will make pieces that weigh one milligram each. There are approximately 28,000 milligrams in an ounce. A kilogram is about equal to 2.2 pounds. The LD50 value refers to the number of milligrams of pesticide that was needed to kill half of the test animals for each kilogram of the animal's body weight. For example, an acute oral LD50 of 5 mg/kg for pesticide A (rats) indicates that it is toxic when there are 5 mg of this chemical given orally for every kilogram (or 2.2 pounds) of the animal's weight.

    Parts per million (ppm)

    Another way of expressing how much pesticide is involved in toxic doses is referred to as "parts per million", abbreviated "ppm". One part per million means that for every million parts of a solution or mixture, there is one part of the substance being measured. The measures mg/kg and ppm are used interchangably since a milligram is one millionth of a kilogram. Other measures that you might come across when looking at the toxicity of a pesticide include: "parts per billion" (ppb) and "parts per trillion" (ppt). The following list may help you remember how small these concentrations are:

    • *parts per million (ppm) = 1 milligram (mg)/kilogram (kg)
    • 1 inch in 16 miles
    • 1 minute in 2 years
    • *parts per billion (ppb) = 1 inch in 16,000 miles
    • 1 second in 32 years
    • *parts per trillion (ppt ) = 1 inch in 16,000,000 miles
    • 1 second in 32,000 years

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    Lethal Concentration Fifty (LC50)

    To figure out the "acute inhalation toxicity" of a pesticide, scientists add a known amount of the pesticide to air. The amount that causes half of the animals to die is the "Lethal Concentration Fifty" (LC50) of the pesticide. The lower the LC50 value, the more poisonous the pesticide. Lethal Concentration Fifty is measured in milligrams per liter (mg/l) or ppm and sometimes in milligrams per cubic meter (mg/m3).

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    Chronic Toxicity Measures

    There is no standard measure like LD50 for chronic toxicity studies. Often the length of the experiment is in days, months, or years and the amount of each dose is stated. For example, a study of chronic oral toxicity might look like this: " 8 milligrams of pesticide were fed to rats daily for two years. No symptoms of poisoning appeared."

    Two classes of pesticides, the organophosphates and carbamates, can slowly poison by attacking an essential body chemical called "cholinesterase". The chronic exposure to organophosphate pesticides can be measured by monitoring changes in blood cholinesterase levels. In humans, decreased blood cholinesterase levels are a sure sign that exposure to these types of pesticides should be avoided until the level is measured as being normal again. (For more on this subject, see Chapter VIII on cholinesterase tests).

    Acute Toxicity Label Warning Statements

    Based on the LD50 and the results of other acute tests, each pesticide is classified into a "toxicity category" and given an associated "signal word". A signal word must appear on every product label so that pesticide users are alerted to the pesticide's acute toxicity. Toxicity categories are based on the acute oral, dermal, and inhalation toxicities, as well as eye and skin irritation effects of each pesticide. A pesticide is categorized by its highest level of toxicity. For example, if the acute oral toxicity and acute dermal toxicity of a pesticide are in the slightly toxic category, but its acute inhalation toxicity is in the highly toxic category, the pesticide label will have the signal words for a highly toxic pesticide.

    The following table indicates the four categories of pesticide toxicity:

    Categories of Acute Toxicity
    Category Signal Word Required on Label Oral Ld50 Mg/kg Dermal LD50 mg/kg Inhalation LC50 mg/l Approximate Oral dose that can Kill an Average Person
    I Highly toxic DANGER-*[Poison! Skull Crossbones] From 0 to 50 From 0 to 200 From 0 to 0.2 A few drops to 1 teaspoon full [or a few drops on the skin]
    II Moderately Toxic WARNING! From 50 to 500 From 200 to 2000 From 0.2 to 2 Over 1 teaspoonful to 1 ounce
    III Slightly Toxic CAUTION!! From 500 to 5000 From 2000 to 20,000 From 2.0 to 20 Over 1 ounce to 1 pint or 1 pound
    IV Relatively Non-toxic CAUTION!! More than 5000 More than 20,000 Greater than 20 Over 1 pint or 1 pound

    * Not used for skin and eye irritation effects.

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    Hazard is the risk of danger. It is the chance that harm will come from the use of a pesticide to the applicator, bystanders, livestock, wildlife, crops, consumers, water, etc. Hazard is often confused with toxicity, but they are not necessarily the same. The hazard of a toxic chemical is always based on two things; its ability to harm (i.e. its toxicity, corrosiveness) and the ease with which a person can come in contact with the chemical. For example, a highly toxic pesticide is usually considered "hazardous" because of the risk that it poses to the public or the environment. However, with proper handling, a highly toxic pesticide can actually pose a low risk or low hazard. Many factors besides a pesticide's actual toxicity can make it hazardous. These include: the skill of the applicator; the target pest involved; the type of pesticide; the formulation chosen; the other chemicals involved in the formulation; and the concentration and dosage used.

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    Concentration and Dosage. Usually, the more concentrated a formulation is the more hazard it poses. Dilute the concentrated pesticide and you reduce the hazard. For example, one ounce of pesticide A contains a lethal oral dose. If the same one ounce of pesticide A is diluted in ten gallons of water, each ounce of the dilute mixture will contain 0.0008 ounces of pesticide A. The handling of the dilute mixture is thus reduced when compared to the concentrate. Use good judgment when figuring out the concentration and dosage of a pesticide; try to use the lowest concentration and/or dosage that is necessary to control the target species.

    Applicator. A skilled, experienced applicator using a highly toxic material will be less of a hazard to himself and others than perhaps a homeowner who applies pesticides on his/her property. A certified applicator should have the skill and knowledge to handle all pesticides safely.

    Target. The site of application is called the target. It can consist of plants, soil, insects, animals, structures and many other things. The intended use of a pesticide on a target is to control specific target pests without harming fragile "nontarget species." The ideal pesticide controls the target pest and poses little or no hazard to nontarget species, as well as the target area itself.

    Formulation. The hazard of a pesticide is also influenced by the way a pesticide is put together, or made into a formulation for use. Depending on the original toxicity of the pesticide, formulations that are easily absorbed or inhaled may pose more of a hazard than those that are less easily absorbed or inhaled. Keeping in mind all the factors that influence the toxicity of the pesticide, formulations generally pose the following toxicity hazard in decreasing order: emulsifiable concentrate > oil solution > water emulsion > water solution > wettable powder/flowable (in suspension) > dust > granular. Choose the safest formulation available to do the job. (See Chapter XV for a more detailed explanation on formulations).

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    All pesticides can be hazardous. Use caution whenever you handle them!

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