The greatest argument against the use of pesticides is dangerous, unintended side effects. However, without the adequate control of pests, life for us humans would be pretty miserable. EPA's Citizen's Guide to Pest Control and Pesticide Safety is an excellent reference manual. Acute toxicity results from a single exposure to a high level of toxicant, the result of which can be fatal. This is generally the intention of pesticides.
Chronic toxicity results from long-term exposure of low levels of toxicant.
This is generally an unintended side effect of pesticides. Chronic exposure can lead to resistant strains of the pests that were to be eliminated. Some materials are exempt from this if they are essential in small doses for life and health. The dose is normally listed as the amount of substance in milligrams and is compared to the body weight of the organism in kilograms. The last column extrapolates the LD50 for a 150 pound human.