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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


Agriculture and Natural Resources, University of California

Accessibility   /WATER/U/toxicology.html revised: July 10, 2014. Contact webmaster.