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What Is Toxicity?

Toxicity is the ability of a chemical substance to injure a person, animal, plant, or other organism. Pesticides aren’t the only substances that can be toxic. Bee stings, poisonous plants, and other natural substances can be toxic to sensitive individuals.

All chemicals are toxic to some degree to certain organisms, but pesticides are specifically designed to be toxic to the organisms they target. Pesticide toxicity can vary depending on the organism exposed. For example, a pesticide that has low toxicity for humans and other mammals can be highly toxic to fish, bees, or birds.

There are two types of toxicity: acute and chronic.

Acute (short-term) toxicity refers to the ability of a pesticide or other chemical to cause harm in a short period of time. Acute toxicity of a pesticide is indicated by its signal word, which can be found on the product label.

The signal words "CAUTION," "WARNING," and "DANGER" (in order of increasing toxicity) indicate the relative acute toxicity, or short-term effects, of the active ingredients to humans. They do not refer to long-term effects to humans nor do they indicate the effect on aquatic invertebrates.

Signal words are based on measurements of 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.

Chronic (long-term) toxicity describes the harm caused by a pesticide or other chemical from repeated exposure over a long period of time. Chronic toxicity can result in tumors, sterility, birth defects, permanent nerve damage, blood disorders, and other conditions.

What Is Half-Life?

A half-life is the length of time it takes for a pesticide to lose half of its toxicity in a given environment. Generally, pesticides that have a short half-life, meaning that they break down rapidly, are less likely to cause harm. Half-life can vary depending on the environmental conditions and the type of pesticide.

What Is an Indicator Species?

Indicator species, or bioindicators, are organisms or groups of species used to indicate the quality or health of an environment. They are often used to measure the toxicity of chemicals or to assess changes in an environment. Indicator species may include plants, animals, or microorganisms.
Some indicator species are more sensitive to pollutants in water than other aquatic organisms. When sensitive species are exposed to pollutants like pesticides, their populations decline, while populations of pollutant-tolerant species thrive. This allows for water quality to be assessed by looking at the types and abundances of different organisms. Low numbers of aquatic organisms like Ceriodaphnia indicate poor water quality.

Ceriodaphnia dubia (water flea) is a tiny aquatic invertebrate found in ponds, creeks, lakes, and rivers. It is an important link in the aquatic food chain and a food source for several predators, including fish and various invertebrates. C. dubia is used as an indicator organism to measure toxicity in fresh water.

Diazinon and chlorpyrifos s have been identified as impairing water quality because they are occurring at levels toxic to C. dubia.