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Pesticides and Water Quality

General Toxicology


  • Toxicology is the science that deals with poisons and their effect on living organisms.


  • Toxicity is the inherent ability of a pesticide to cause harm to a specific organism.
  • Some pesticides are more toxic than others and present higher risks to users, nontarget organisms, and the environment.
  • The effect of the pesticide (the toxic reaction) is related to the amount of pesticide (the dose) applied.
  • Toxicity is often measured in LD-50, the Lethal Dose to kill 50% of exposed individuals, or LC-50, the Lethal Concentration in water or air to kill 50% of exposed individuals of a species.
  • LD-50 does not provide information about long-term (or chronic) effects such as cancer, sterility, and permanent nerve damage that can result from repeated low level exposures.
  • The Extension Toxicology Network provides more information on toxicity of specific pesticides.


  • The half-life is the length of time it takes for a pesticide to lose half of its toxicity in a given environment.
  • Pesticides that break down rapidly are less likely to cause harm.

Indicator organisms

  • An indicator organism is a test species that is used in experiments to measure the toxicity of poisonous materials.
  • Indicator organisms are exposed to various levels of toxic materials over a period of time in order to determine their potential toxicity to the organism and to evaluate their effects on survival and reproduction.
  • Ceriodaphnia and Hyalella are tiny aquatic invertebrates used as indicator organisms to measure toxicity in water.

Drop of diazinon in volume of 2 pools is toxic to organism
Click on image to enlarge

Illustration by Celeste Rusconi

Statewide IPM Program, Agriculture and Natural Resources, University of California
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