Ministry of Agriculture

Toxicity & Hazard

General Information


Pesticide applicators should understand the hazards and risks associated with the pesticides they use. Pesticides vary greatly in toxicity. Toxicity depends on the chemical and physical properties of a substance, and may be defined as the quality of being poisonous or harmful to animals or plants. Poisons have many different modes of action, but in general cause biochemical changes which interfere with normal body functions.

The toxicity of any compound is related to the dose. A highly toxic substance causes severe symptoms of poisoning with small doses. A substance with a low toxicity generally requires large doses to produce mild symptoms. Even common substances like coffee or salt become poisons if large amounts are consumed.

Toxicity can be either acute or chronic. Acute toxicity is the ability of a substance to cause harmful effects which develop rapidly following absorption, i.e. a few hours or a day. Chronic toxicity is the ability of a substance to cause adverse health effects resulting from long-term exposure to a substance.

There is a great range in the toxicity of pesticides to humans. The relative hazard of a pesticide is dependent upon the toxicity of the pesticide, the dose received and the length of time exposed.  A hazard can be defined as a source of danger. The hazard in using a pesticide is related to the likelihood of exposure to harmful amounts of the pesticide. Exposure can be influenced by the amount of pesticide used, concentration of the pesticide and how the pesticide and application equipment are handled. A pesticide can be highly toxic as a concentrate, but pose little hazard to the user if:

  1. used in a very dilute formulation,
  2. used in a formulation not readily absorbed through the skin or inhaled,
  3. used only under conditions of no human exposure, or
  4. used by experienced applicators who are equipped to handle the pesticide safely.

In contrast, a pesticide may have a relatively low toxicity but present a hazard because it is used in the concentrated form which may be readily absorbed or inhaled.

Risk is defined as the likelihood that a worker or the environment may be harmed during a particular operation. The toxicity or hazard of the pesticide can't be changed, but the risk can be managed through use of appropriate protective gear and proper handling and application.

The hazards associated with pesticide exposure and toxicity are discussed in the Exposure heading of the Toxicity & Hazard section of this website.

Pesticide Toxicity

The toxicity of a pesticide can be measured several ways, but generally human toxicity is estimated based on test results on rats and other animals. Toxicity studies are only guidelines. A pesticide that is poisonous to rats is not necessarily equally poisonous to people or other animals. Some pesticides are dangerous after one large dose (acute toxicity). Others can be dangerous after small, repeated doses (chronic toxicity).

Measuring Acute Toxicity (LD50 And LC50 Values)

Acute toxicity of a pesticide refers to the effects from a single dose or repeated exposure over a short time (e.g. one day), such as an accident during mixing or applying pesticides. A pesticide with a high acute toxicity can be deadly even if a small amount is absorbed. Acute exposures may be referred to as acute dermal, acute oral or acute inhalation poisoning. Usually the effects of acute exposure, if any, occur within 24 hours.

LD50 or Lethal Dose 50

The  LD50 is the dose of a pesticide that will kill half of a group of test animals from a single exposure by either the dermal, oral or inhalation routes.  The larger an animal, the greater the lethal dose required to kill it. The  LD50 is expressed in milligrams per kilogram of body weight of the test animal.  For example, if a pesticide has an oral  LD50 value of 10 mg/kg, and the test animals each weigh 1 kg, 50% of the animals would die of poisoning if each ate 10 mg of the pesticide. If the test animals weigh 25 kg each, the lethal dose to kill 50% of these animals would be 10 mg/kg X 25 kg = 250 mg each.

A pesticide with a lower  LD50 is more toxic than a pesticide with a higher number because it takes less of the pesticide to kill half of the test animals. The smaller the  LD50 value, the more toxic the pesticide.

LC50 or Lethal Concentration 50

The LC50 value is a measure of the toxicity of a pesticide when test animals breathe air mixed with pesticide dust, vapours or spray mist. The LC50 is the concentration of pesticide which is lethal to 50% of a population of test animals and is usually determined for a specific exposure period (e.g. inhalation for 4 hours). The length of exposure is important because shorter exposure periods generally require higher pesticide concentrations to produce toxic effects. LC50 values for pesticides in air are expressed as the ratio of pesticide to air, in parts per million (ppm) or parts per billion (ppb). LC50 values are also determined for fish and aquatic organisms based on the concentration of pesticide in water for exposure periods of 24 to 96 hours.

Important characteristics to note about LD50 and LC50 values:

  • they are based on a single dose (LD50) or short exposure (LC50);
  • they do not indicate cumulative effects of small doses;
  • they are an indicator of the amount of chemical required to kill or severely injure animals, and do not indicate the amount of chemical causing less severe toxic effects; and
  • the smaller the LD50 or LC50 value, the greater the toxicity (i.e. a pesticide with an LD50 of 5 mg/kg is 100 times more toxic than a pesticide with an LD50 of 500 mg/kg).

LD50 and LC50 values are determined from tests using the pure active ingredient of a pesticide. Formulated pesticides (the products on the store shelf) are generally less toxic than the active ingredient because they are diluted with other ingredients such as carriers.  A pesticide further diluted with water in the spray tank would be even less toxic.

The acute oral LD50 value of a pesticide can be used to roughly estimate the amount of pesticide which might cause severe symptoms of acute poisoning to someone accidentally swallowing it.

Example: If an insecticide active ingredient has an acute oral LD50 to rats of 300 mg per kilogram of body weight, a 70 kg person would be severely poisoned if he consumed 70 kg X 300 mg/kg = 21,000 mg = 21 g of active ingredient.

If a formulated product contained 125 g/L active ingredient, a 70 kg person would be severely poisoned if he consumed (21 g) / (125 g/L)= 0.168 L = 168 mL of product.

If the product was mixed in a spray solution of 100 mL/10L of spray, then a person would be severely poisoned if he consumed (168 mL) / (10 ml/L) = 16.8 L of spray.

Research shows the susceptibility of mammals to pesticides varies with the species, age, sex and health of the animals being tested. Therefore data on the toxicity of pesticides to test animals can only serve as a guide to the probable toxicity of a pesticide to an individual person. When pesticides are registered, safety factors are included to account for the differences between species and between individuals.

As a general guide, the approximate dose which would cause severe injury or death to an average human adult is illustrated in the table below for various LD50 ranges.

Relation of oral LD50 to approximate lethal dose in adult humans.
Oral LD50 of Active Ingredient (a.i.) Approximate lethal dose to average size adult*
(70 kg or 155 lb.)
less than 5 mg/kg less than 0.3 mL
(one to two drops)
5 to 50 mg/kg 0.3 to 3 mL
(a few drops to half a teaspoon)
50 to 500 mg/kg 3 mL to 30 mL
(half a teaspoon to two tablespoons)
500 to 5,000 mg/kg 30 mL to 300 mL
(1 to 10 fluid ounces)
5,000 to 15,000 mg/kg 300 mL to 900 mL
(10 to 30 fluid ounces)
*Note that a larger amount of pesticide would have to be consumed to obtain a lethal dose if the active ingredient was diluted as in most formulated products and spray solutions. Note also that a child who is one-fifth the weight of an adult would require only one-fifth the amount of pesticide to suffer the same toxic effects as the adult.

On this basis, pesticides are classified into groups of low, moderate and high acute toxicity 

Relative toxicity of pesticides to humans based on acute oral and dermal LD50's.
Acute Toxicity Oral LD50 Dermal LD50
High/Very less than 50 mg/kg less than 200 mg/kg
Moderate 50 to 500 mg/kg 200 to 1,000 mg/kg
Low/Slight over 500 mg/kg over 1,000 mg/kg

The more toxic the pesticide, the less needed to kill an animal. Therefore, very toxic pesticides have smaller LD50 values and slightly toxic pesticides have larger LD50 values.

Here are examples of LD50 values for four pesticides.

Pesticide Active Ingredient (a.i.) LD50 (mg/kg)
Oral Dermal
aldicarb 0.8 3
diazinon 300 2,150
malathion 1,000 4,100
atrazine 1,780 7,500

Here are examples of LD50 values for three common household compounds. They have a low acute toxicity but could cause toxic reactions if consumed in sufficient quantities.

CompoundOral LD50 (mg/kg)
acetylsalicylic acid (Aspirin) 1,000
sodium chloride (table salt) 3,320
ethylene glycol (antifreeze) 460

Measuring Chronic Toxicity

Chronic toxicity refers to the effects of long-term or repeated lower level exposures to a toxic substance, such as when a pesticide applicator is frequently wetted with spray during unsafe spray practices. The effects of chronic exposure do not appear immediately after first exposure and may take years to produce symptoms. Pesticides which have a tendency to accumulate, or which break down slowly in body tissues, usually represent the greatest chronic exposure hazard. Someone who is frequently exposed to low doses of such pesticides may develop symptoms of poisoning long after the first exposure. Chronic exposure may include chronic oral, chronic dermal or chronic inhalation poisoning.

The symptoms of chronic toxicity develop slowly and persist for a long time after exposure. Such symptoms may occur in three ways:

  1. As a complication of acute poisoning. For example, a severe exposure to a pesticide may cause acute effects such as nausea, chest pain and vomiting as well as chronic effects resulting from kidney, liver and lung damage.
  2. As a slowly progressive condition, without any incident of acute poisoning. For example, increased breathing difficulty or skin sensitization (allergy) after repeated use of a pesticide.
  3. As the occurrence of a disease or condition initiated by previous exposure. For example, the development of cancer years after a period of exposure.

Very few pesticides now in use are known to cause chronic effects, if used according to label directions. However, a few pesticides are suspected or known to cause chronic illness in test animals or humans when exposure levels are high. The registration of some pesticides has been cancelled because the suspected or identified chronic effects represented a significant health hazard.

Chronic toxicity of a pesticide is assessed differently from acute toxicity (LD50 or LC50) A number of different tests are performed on animals which help to predict whether a pesticide will cause long-term effects. Test animals are exposed to sublethal levels of pesticides for periods ranging from about 90 days to several years. They are examined for a wide variety of toxic effects from dermal, oral and respiratory exposure. Such effects include:

  • Carcinogenicity - ability to produce cancer or to assist carcinogenic chemicals.
  • Mutagenicity - ability to cause genetic changes.
  • Teratogenicity - ability to cause birth defects.
  • Oncogenicity - ability to induce tumor growth (not necessarily cancers).
  • Liver damage - death of liver cells, jaundice (yellowing of the skin), fibrosis and cirrhosis.
  • Reproductive disorders - such as reduced sperm count, sterility, and miscarriage.
  • Nerve damage - including accumulative effects on cholinesterase depression associated with organophosphate insecticides.
  • Allergenic sensitization - development of allergies to pesticides or chemicals used in formulation of pesticides.

Evidence of chronic effects is usually associated with long-term exposure of test animals to relatively high doses. Thus people who run the greatest risk of developing any chronic effects are workers involved in the manufacture of chemicals or applicators exposed to high levels of pesticides over many years.

Chronic toxicity symptoms may not always be recognized as having been caused by exposure to a toxic chemical months or years in the past. The levels of exposure which might cause chronic effects in a human individual are very difficult to predict. Thus anyone handling pesticides should attempt to minimize their exposure and eliminate the possibility of chronic effects.

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