One of the major problems with chemical use is accidental poisoning, for instance when the chemical is being applied, or from residues in food, air or water. Deposition on skin often results in a higher rate of absorption than inhalation. There can be permanent effects from an acute episode, such as lung or kidney damage.
Chronic effects can be a result of exposure to multiple low-level doses.
An LD50 below 300 is considered toxic, below 50 very toxic. Even though animals are the subjects, the results are then extrapolated to humans. In addition,some are persistent so they can keep on killing other organisms in the future.