Pesticides are poisons designed to destroy unwanted life forms. Used properly, modern pesticides can perform their functions without causing significant hazards to humans or the environment. Pesticides have many uses in homes, gardens, farms, forests and public health. It is difficult to imagine what life would be like without modern pesticides. This discussion is continued from BioTalk 12.

Pesticide Properties

Properties that affect a pesticides potential to pollute water include formulation, toxicity, persistence, volatility, solubility in water and soil adsorption. The risk of pollution is also affected by soil characteristics application methods, weather and other factors.

Formulation

Pesticides come in several physical forms and formulations that make them easy to store, transport and apply. Common formulations include water dispersable granules, wettable powders, dusts, aerosols, solid or liquid baits, granules, emulsifiable and flowable concentrates and solutions. Less common formulations include microcapsules, plastic beads, plastic membranes, plastic ropes and controlled release dispensers. While most environmental hazards come from the active ingredient in a pesticide, the way its formulation interacts with the environment determines the overall hazard of a pesticide. Spray formulations can drift with the wind or vaporise into the air. Other formulations can leach into ground water or be carried into surface water by rainfall or irrigation runoff. Even pesticides that bind to soil particles can find their way into surface water if erroded by wind or water.

Toxicity
The active ingredient is the chemical compound in a pesticide that kills or otherwise affects the target pest. Other substances in a pesticide formulation are inert ingredients that act as carriers and preservatives for active ingredients, and also make mixing and application easier. The toxicity of a pesticide is determined by the amount that is required to produce biological effects.

Pesticide Dose and Effective Dose
The dose is the amount of a pesticide used at one time. The effective dose is the amount of pesticide needed to kill or otherwise affect the target pest. Amounts less than the effective dose will likely not kill the target pest and may lead to increased resistance. Amounts greater than the effective dose will not necessarily be more effective in killing the target pest. Instead, this larger dose may kill more non-target organisms, cost more, and pollute the environment. The LD₅₀ is the dose of a particular material, taken by mouth, skin or inhaled, that is lethal to 50 percent of a group of test animals. The higher an LD₅₀ is, the lower the toxicity of the substance. Pesticides with low LD₅₀ are extremely toxic. The basic measuring unit  used is milligrams of toxin per kilograms of body weight or "mg/kg". The following table shows the LD₅₀ values for rats for various pesticides and other familiar chemicals and natural products.